The Effects of Crystalline Admixtures on Concrete Permeability and Compressive Strength: A Review

Abstract

The durability and strength of concrete in construction can be significantly compromised by permeability issues, which pose considerable challenges to its long-term effectiveness and reliability. By analyzing six selected articles from the Scopus database, this study meticulously synthesizes findings on the effectiveness of CAs in improving these essential properties of concrete. The research meticulously documents and analyzes key variables such as the CA dosage, water–cement ratio, evaluation duration, and treatment conditions, providing a thorough understanding of the factors that influence the performance of CAs in concrete. The results robustly indicate that CAs significantly reduce concrete permeability, thereby enhancing its resistance to water and other detrimental substances, and simultaneously boosts the compressive strength, leading to stronger and more durable concrete structures. However, the study also reveals that the impact of CAs can vary considerably depending on the specific conditions and methodologies employed in the individual studies. This underscores the importance of standardized testing procedures to ensure consistent and comparable results across different studies. This research provides valuable insights for optimizing the use of CAs in concrete formulations, ultimately aiming to improve the durability, performance, and sustainability of concrete in construction applications.

1. Introduction

Concrete is one of the most widely used construction materials due to its versatility, durability, and cost-effectiveness [1,2,3,4]. However, its performance may frequently be hindered by permeability issues that enable the infiltration of water and other liquids, ultimately undermining the durability and structural integrity of concrete structures [5,6,7]. Also, improving the impermeability of concrete while keeping or increasing its compressive strength is very important.

One promising solution involves the use of crystalline admixtures in concrete [8,9,10,11]. Applied during mixing, these admixtures react with cement hydration byproducts to form insoluble crystalline structures within the concrete matrix [11,12,13,14], effectively filling pores and reducing permeability [15]. Furthermore, crystalline admixtures have shown to enhance compressive strength across various applications [9,16,17]. The development and application of crystalline admixture technologies have significantly advanced over time [18,19]. Initially, the primary concern in concrete construction was addressing water ingress, a pervasive issue that often led to structural deterioration, reinforcement corrosion, and costly repairs [20,21]. Concrete, by its nature, is a porous material, which allows water and other harmful substances to infiltrate its structure, compromising its integrity and longevity [22,23,24]. In the early stages, researchers and engineers focused on identifying chemical compounds that could react with water within the concrete matrix to form crystalline structures [25,26]. These compounds, often based on specific types of silicates, were designed to react with the byproducts of cement hydration, particularly calcium hydroxide [27,28,29]. The reaction would result in the formation of insoluble, needle-like crystals that could grow within the micro-cracks and capillary pores of the concrete, effectively sealing them and preventing further water ingress [30,31,32]. As research in materials science and chemistry advanced, the formulations of crystalline admixtures were refined. Scientists gained a better understanding of the crystallization process and how it could be optimized to enhance concrete’s properties. This led to the development of more sophisticated admixtures that not only improved impermeability but also enhanced other critical properties of concrete, such as the compressive strength and durability [33,34,35]. These advancements made crystalline admixtures more reliable and efficient, leading to their widespread acceptance in the construction industry [8,36,37]. Crystalline admixtures proved especially valuable in projects subjected to harsh weather conditions or underground constructions [38,39,40,41]. The ability of these admixtures to provide long-term protection against water damage and chemical attack made them a standard component in many concrete formulations [42,43]. Furthermore, the application techniques for crystalline admixtures evolved. Initially, these admixtures were primarily used in new concrete constructions, which were mixed into the concrete during batching [44,45,46]. However, as the technology progressed, they were also developed for use in existing structures as surface-applied treatments [47,48]. These treatments could penetrate the concrete surface and initiate the crystallization process within the existing matrix, providing an effective means of retrofitting and enhancing the durability of older structures [38,49,50,51].

Despite many studies into the impact of crystalline admixtures on concrete, the findings have been inconsistent. This inconsistency can be attributed to variations in experimental conditions, the types of admixtures utilized, and the assessment methods employed. For example, Harry Hermawan et al. investigated the effect of crystalline admixtures on concrete and found a significant reduction in permeability alongside a noticeable increase in compressive strength. Similarly, Wang, Fan [52] reported that the incorporation of crystalline admixtures resulted in a marked improvement in both concrete impermeability and compressive strength, demonstrating the effectiveness of these additives in improving the concrete properties [53]. In contrast, Munn et al. determined that the addition of a 0.8% PRA crystal mixture to concrete containing Type-GB cement (with 20% fly ash) resulted in a reduction in the compressive strength from 34 MPa to 32 MPa after 7 days [54]. Understanding the mechanisms by which crystalline admixtures improve concrete properties is crucial for advancing construction materials. The primary action of these admixtures involves a chemical reaction between their components and the calcium hydroxide and other byproducts of cement hydration [55]. This reaction produces insoluble needle-shaped crystals that grow within the capillary pores and micro-cracks of the concrete [56]. These crystalline formations effectively block the pathways for water and other deleterious substances, significantly reducing the permeability [42]. Moreover, these crystals continue to grow in the presence of moisture, providing a self-sealing capability that can repair minor cracks and further enhance the durability of the concrete [57,58].

This review aims to evaluate the effectiveness of crystalline admixtures on concrete permeability and compressive strength by analyzing the methodologies used in selected studies. It also explores environmental considerations and potential sources of variation in reported effects, advocating for standardized testing. By synthesizing data from multiple trials, this review seeks to provide critical guidance for researchers, engineers, and construction practitioners in optimizing the use of crystalline admixtures to enhance concrete performance while promoting environmental sustainability.

2. Materials and Methods

2.1. Data Collection

To create a strong basis for this research, an extensive literature review was conducted using the Scopus database. The search keywords “crystalline admixture” and “concrete” initially yielded 5004 papers published between 1973 and 2024. This number was narrowed down to 252 papers when the search was limited to the article title, abstract, and keywords. The distribution of these articles over the years provides valuable insights, as seen in Figure 1, for the past 10 years. Over the past 20 years, the number of articles has steadily increased, with a big jump in recent years. In 2023, the number of publications reached 41 articles, up from just 10 in 2016. This rapid growth suggests increasing awareness and research on crystalline admixtures in concrete.

To ensure a focused analysis on the impact of crystalline admixtures on concrete, a further selection process was undertaken. Articles whose titles and abstracts did not explicitly discuss the effect of crystalline admixtures on concrete were excluded. This rigorous selection resulted in a final dataset of 114 articles encompassing the period from 1973 to 2024. These articles form the foundation for the investigation into the influence of crystalline admixtures on concrete properties and performance. Figure 2 shows the type of each study, the year, and the source.

Based on the citation data for various publication years, a clear trend emerges regarding the impact and reception of research articles over time. Notably, recent publications in 2024 show a range of citation counts, with some receiving no citations yet, while others have garnered significant attention. For instance, articles published in 2022 and 2023 also exhibit varied citation numbers, indicating the ongoing relevance and interest in those topics. Older publications, such as those from 2015 and earlier, show a mix of highly cited articles alongside others with fewer citations, suggesting enduring influence for some studies while highlighting the evolution of scholarly impact over the years. This analysis underscores the dynamic nature of academic influence, influenced by both the timeliness and enduring relevance of research findings across different publication years, as shown in Figure 3.

2.2. Bibliometric Analysis

VOSviewer is a highly versatile software tool designed specifically for constructing and visualizing bibliometric networks, making it an essential resource for researchers engaging in scientometric and bibliometric analyses [59,60,61]. It is renowned for its capacity to process and represent complex relationships within large datasets, particularly those drawn from bibliographic sources such as academic publications, patents, and conference proceedings [62,63]. These networks can include journals, researchers, or individual publications, and they can be based on citations, bibliographic couplings, co-citations, or co-authorship relations [64,65,66,67]. The tool’s capacity to handle large datasets is particularly beneficial in fields with extensive research output, such as materials science or civil engineering [68,69,70,71]. The size of each node typically correlates with its frequency or prominence in the dataset, while the distance between nodes indicates the strength of their relationship or the degree of their co-occurrence [72,73,74,75]. By examining co-authorship networks, researchers can identify the most prolific authors and institutions, as well as the collaborations that have driven significant advancements in the field [60,76,77,78]. In addition, VOSviewer offers a text mining functionality that can be used to construct and visualize co-occurrence networks of important terms extracted from a body of scientific literature [79,80,81,82].

To gain deeper insights into research trends on crystalline admixtures in concrete, a bibliometric analysis was conducted using VOSviewer on the 114 selected articles from the Scopus database. These articles were chosen from an initial pool of 252, with 138 being excluded due to their irrelevance to the topic. This analysis concentrated on two primary aspects: mapping the geographic distribution of research interest and identifying the most frequently occurring keywords. For the geographic distribution, visual maps were created to represent the relationships and collaborations between different countries involved in this research domain, highlighting the global landscape of the research activity [83,84]. For the keyword analysis, co-occurrence networks of keywords were constructed to identify the most frequently occurring terms and their interrelations, providing a comprehensive overview of the main topics and themes in the literature [85].

2.3. Inclusion and Exclusion Criteria

Following the initial search, we applied exclusion criteria to refine the selection of studies. We focused on studies published after 2016, written in English, and categorized as either articles or conference papers, ensuring they contained all necessary data for thorough analysis. This approach was intended to ensure that only relevant and recent studies were included for evaluation. As a result, six studies met these criteria and were included in our review, as illustrated in Figure 4. To provide a comprehensive overview, we sought to gather essential data from these studies, including the dosage of crystalline admixtures (CA %), the compressive strength of both the control and treated samples (MPa), the duration of evaluation (days), the water-to-cement ratio (w/c), the treatment conditions, the permeability control and treatment, the concrete type, and the type of crystalline admixture used. Additionally, we ensured access to the full texts of these studies and selected experiments to avoid redundancy while maintaining a focused and concise presentation in our article.

A total of 87 documents were excluded from the review for various reasons.

Table 1. Outlines of the reasons for excluding documents from the review.

N	Authors Names	Title and Reference	Reason for Exclusion
1	Park B.; Choi Y.-C.	Self-healing products of cement pastes with supplementary cementitious materials, calcium sulfoaluminate and crystalline admixtures [86]	The authors do not discuss the CA effects on the concrete permeability and compressive strength.
2	Zhang G.-Z.; Liu C.; Ma X.; Yu X.-K.	The Effects of Crystalline Admixture on the Self-Healing Performance and Mechanical Properties of Mortar with Internally Added Superabsorbent Polymer [87]	The authors discuss the CA effects on permeability and strength only for mortar, but not concrete.
3	Dufka Á.; Žižková N.; Brožovský J.	An analysis of crystalline admixtures in terms of their influence on the resistance of cementitious composites to aggressive environments [88]	The paper does not mention compressive strength results for standard conditions, only for specimens stored in aggressive environments.
4	Lo Monte F.; Repesa L.; Snoeck D.; Doostkami H.; Roig-Flores M.; Jackson S.J.P.; Alvarez A.B.; Nasner M.; Borg R.P.; Schröfl C.; Giménez M.; Alonso M.C.; Serna P.; De Belie N.; Ferrara L.	Multi-performance experimental assessment of autogenous and crystalline admixture-stimulated self-healing in UHPFRCCs: Validation and reliability analysis through an inter-laboratory study [89]	The paper does not discuss the CA effects on the concrete permeability and compressive strength.
5	Tsampali E.; Stefanidou M.	The role of crystalline admixtures in the long-term healing process of fiber-reinforced cementitious composites (FRCC) [90]	The paper primarily focuses on the effects of crystalline admixtures on fiber-reinforced cementitious composites.
6	Yang Y.-C.; Li H.-B.; Yang X.-G.; Chen S.-Q.; Zhou J.-W.	Experimental Study on the Influence of a Cementitious Permeable Crystallization Admixture (CPCA) in Improving Concrete Durability [91]	While this paper provides all the relevant data needed, it cannot be included at this stage due to space constraints or the current focus of the review. The current selection process prioritizes a balance across different types of crystalline admixtures and evaluation methods, necessitating a limitation on the number of studies incorporated in this version. This paper is planned for inclusion in future updates when the analysis is expanded to incorporate more detailed studies on specific crystalline admixtures.
7	Ding Y.; Wu Z.; Huang Q.; Wang Q.; Ren Q.; Zhang Z.; Zhang J.; Huang K.	Research on crystalline admixtures for low carbon buildings based on the self-healing properties of concrete [92]	Although this paper provides all the necessary data, including crystalline admixture percentages, compressive strength, and other parameters, its primary focus is on the microstructural analysis of self-healing mechanisms using advanced techniques like X-ray diffraction (XRD), thermogravimetric analysis (TG-DTG), and scanning electron microscopy (SEM). This review, however, emphasizes the macro-level effects of crystalline admixtures on concrete’s permeability and compressive strength, without delving into detailed microstructural analysis. As a result, this paper will not be included at this stage to maintain consistency with the broader focus of the current analysis.
8	Dobrovolski M.E.G.; Trisotto A.V.; Santos N.C.S.; Trentin P.O.; Medeiros-Junior R.A.	Effect of Crystalline Admixtures in the Mass Transport of Concrete with Polypropylene Microfibers [93]	The authors do not discuss the CA effects on the concrete permeability and compressive strength.
9	Azarsa P.; Gupta R.; Azarsa P.; Biparva A.	Durability and self-sealing examination of concretes modified with crystalline waterproofing admixtures [94]	This paper primarily focuses on freeze–thaw resistance, self-sealing, and corrosion resistance.
10	Mottl M.; Reiterman P.; Pazderka J.	The Influence of Aggressive Environmental Conditions on the Adhesion of Applied Crystalline Materials [95]	The paper does not discuss the CA effects on concrete permeability and compressive strength.
11	Manhanga F.C.; Khmurovska Y.; Rudžionis Ž.	Determination of Crack Healing Efficiency of Concrete Containing Crystalline Admixture in Experimental Procedures Using Image Analysis [96]	The authors do not discuss the CA effects on the concrete permeability and compressive strength.
12	Lauch K.-S.; Desmettre C.; Charron J.-P.	Self-healing of concrete containing different admixtures under laboratory and long-term real outdoor expositions based on water permeability test [97]	The paper focuses on self-healing in different exposure conditions rather than a focused analysis of the permeability and compressive strength, which limits its relevance in this review.
13	Kong K.H.; Lye C.Q.	Crystalline Admixtures for Autonomous Healing in Concrete: The Past, Present and Future [98]	This paper provides a broad overview on self-healing in concrete but lacks the specific experimental data on permeability and compressive strength.
14	Ravitheja A.; Reddy T.C.S.; Sashidhar C.	Improvising the Self-Healing Capabilities of Concrete Using Different Pozzolanic Materials and Crystalline Admixtures [99]	The paper focuses on self-healing capacities and the effects of pozzolanic materials alongside crystalline admixtures, with less emphasis on detailed experimental data specifically related to permeability and compressive strength.
15	Wang M.; Yang X.; Zheng K.; Chen R.	Properties and Microstructure of a Cement-Based Capillary Crystalline Waterproofing Grouting Material [100]	The paper focuses on high-performance grout materials for tunnel applications, which diverges from the focus on crystalline admixtures in general concrete for permeability and compressive strength analysis.
16	Dario A.; Suwondo R.	The influence of crystalline technology as concrete admixture on compressive strength and permeability [16]	The paper is relevant, as it discusses the effects of crystalline admixtures on both compressive strength and permeability. However, it does not specify the duration of the evaluation.
17	Suwondo R.; Suangga M.; Dario A.; Cunningham L.	Enhancing concrete durability through crystalline waterproofing admixtures: a comprehensive performance evaluation [101]	The paper focuses on crystalline waterproofing admixtures (CWAs), which differ from the crystalline admixtures (CAs) used in this review.
18	Zhang G.-Z.; Ma X.; Liu Y.	Study on the Synergistic Effect of Superabsorbent Polymer and Crystalline Admixture on Self-Healing Performance of Mortar Based on Image Binarization Method [102]	This study has emphasis on mortar and its specific self-healing mechanisms, which differs from the broader focus on concrete and its general properties.
19	Feng Z.; Shen D.; Zhang J.; Tang H.; Jiang G.	Effect of crystalline admixture on early-age residual stress and cracking potential of high-strength concrete [103]	The paper focuses on the impact of crystalline admixtures on compressive strength and shrinkage, but does not provide detailed information on durability or long-term performance, and the full text is inaccessible for further evaluation.
20	Cappellesso V.G.; Petry N.S.; Longhi M.A.; Masuero A.B.; Dal Molin D.C.C.	Reduction of concrete permeability using admixtures or surface treatments [104]	The paper evaluates the effectiveness of crystalline admixtures on permeability and compressive strength, but does not provide specific details on the duration of evaluation or a comprehensive analysis of the long-term durability.
21	Li H.-F.; Ma X.; Zhang G.-Z.	Optimization of mortar self-healing performance-based on response surface methodology: A multifactor analysis of zeolites, crystalline admixtures, and water-to-binder ratio [105]	This study meets the criteria, but a similar study is already included in the review. To avoid redundancy and keep the review concise, this article will not be included.
22	Sameera V.K.; Keshav L.	Mechanical And Durability Behaviour Of GGBS, M-sand Based Concrete With Varying Percentages Of Two Crystalline Admixtures—An Experimental Study [106]	The primary focus on GGBS substitution introduces a variable not aligned with the core aims of this review.
23	Suwondo R.; Ozzie V.; Alzhrani T.	Enhancing Concrete Durability with Crystalline Admixtures: An Experimental Study [101]	This study meets the criteria, but a similar study is already included in the review. To avoid redundancy and keep the review concise, this article will not be included.
24	Khomwan N.; Sujjavanich S.; Kheaw-on T.	Effect of crystalline waterproofing materials on corrosion potential of steel in concrete [107]	While the study provides valuable insights into the performance of two types of crystalline waterproofing materials (CWPMs) in improving concrete durability and corrosion protection, its primary focus on NaCl exposure and steel corrosion may not fully align with the scope of this review.
25	Li H.-F.; Yu Q.-Q.; Zhang K.; Wang X.-Y.; Liu Y.; Zhang G.-Z.	Effect of types of curing environments on the self-healing capacity of mortars incorporating crystalline admixture [58]	This study focuses on mortar rather than concrete, which differs from the scope of studies in this review.
26	de Souza Oliveira A.; Toledo Filho R.D.; de Moraes Rego Fairbairn E.; de Oliveira L.F.C.; da Fonseca Martins Gomes O.	Microstructural characterization of self-healing products in cementitious systems containing crystalline admixture in the short- and long-term [108]	This paper lacks specific data on permeability and compressive strength, focusing more on microstructural changes and healing products.
27	Cuenca E.; Criado M.; Giménez M.; Alonso M.C.; Ferrara L.	Effects of Alumina Nanofibers and Cellulose Nanocrystals on Durability and Self-Healing Capacity of Ultrahigh-Performance Fiber-Reinforced Concretes [109]	This study is exclused because it discusses ultra-high-performance fiber-reinforced concrete with crystalline admixtures, alumina nanofibers, and cellulose nanocrystals.
28	Kumar K.P.; Rao M.K.	A study on the Water Proofing Behaviour of Fly Ash, M-sand and Dust based concrete with varying percentages of different Crystalline Admixtures [110]	This study is excluded because it focuses its nvestigations into the mechanical and long-term durability characteristics of concrete made with fly ash and M-Sand.
29	Mahmoodi S.; Sadeghian P.	Effect of different exposure conditions on the self-healing capacity of engineered cementitious composites with crystalline admixture [111]	The focus of the study is on engineered cementitious composites (ECCs) rather than traditional concrete, and the specific crack widths and exposure conditions deviate from the primary interest in general concrete durability and permeability improvements.
30	Shetiya R.K.; Elhadad S.; Salem A.; Fülöp A.; Orban Z.	Investigation into the Effects of Crystalline Admixtures and Coatings on the Properties of Self-Healing Concrete [8]	The focus of this study is on self-healing concrete and its specific properties, which deviates from the main scope of traditional concrete durability and permeability improvements under various environmental conditions.
31	Lopes R.C.; Bacarji G.W.; Bacarji E.; Oliveira A.M.	Influence of crystallizing type chemical admixture on precast micro concretes: a statistical analysis and holistic engineering overview; [Influencia de un aditivo químico tipo cristalizador en el microhormigón prefabricado: un análisis estadístico y una visión holística de la ingeniería] [112]	Since the study does not provide clear evidence of the effectiveness of crystalline admixtures, particularly in the context of improving the compressive strength or durability, it was not relevant for inclusion in the current analysis of crystalline admixture performance.
32	Wang X.; Qiao H.; Zhang Z.; Tang S.; Liu S.; Niu M.; Li G.	Effect of fly ash on the self-healing capability of cementitious materials with crystalline admixture under different conditions [113]	The study is excluded because it focuses on the self-healing capability of mortar with a crystalline admixture and a high fly ash content, which negatively impacted the performance.
33	Cuenca E.; D’Ambrosio L.; Lizunov D.; Tretjakov A.; Volobujeva O.; Ferrara L.	Mechanical properties and self-healing capacity of Ultra High Performance Fibre Reinforced Concrete with alumina nano-fibres: Tailoring Ultra High Durability Concrete for aggressive exposure scenarios [114]	The study is excluded because it focuses on the impact of alumina nanofibres on ultra-high-performance fibre-reinforced cementitious concrete under aggressive conditions.
34	Cuenca E.; Tejedor A.; Ferrara L.	A methodology to assess crack-sealing effectiveness of crystalline admixtures under repeated cracking-healing cycles [115]	While this study investigates the effects of crystalline admixtures, its scope is specific to the interaction of steel fibers and repeated cracking–healing behavior. The research does not provide detailed insights on permeability and compressive strength, which are crucial factors for inclusion.
35	Manvith Kumar Reddy C.; Ramesh B.; Macrin D.	Effect of crystalline admixtures, polymers and fibers on self healing concrete—a review [116]	The study focuses on a broad review of self-healing concrete approaches, both natural and artificial, without providing specific quantitative data on permeability or compressive strength.
36	Chandra Sekhara Reddy T.; Ravitheja A.	Macro mechanical properties of self healing concrete with crystalline admixture under different environments [117]	The study does not specify the type of crystalline admixture (CA) used.
37	Li G.; Liu S.; Niu M.; Liu Q.; Yang X.; Deng M.	Effect of granulated blast furnace slag on the self-healing capability of mortar incorporating crystalline admixture [118]	The study focuses on the combination of the crystalline admixture (CA) with granulated blast furnace slag (GBFS), which introduces a variable (GBFS) that is not consistent with the scope of the studies focusing solely on the effect of the CA on concrete properties.
38	Doostkami H.; Roig-Flores M.; Negrini A.; Mezquida-Alcaraz E.J.; Serna P.	Evaluation of the Self-healing Capability of Ultra-High-Performance Fiber-Reinforced Concrete with Nano-Particles and Crystalline Admixtures by Means of Permeability [119]	Since the focus includes multiple materials and modifications beyond crystalline admixtures, the results may not specifically isolate the effect of the CA, which could limit its relevance to studies solely on CAs in conventional concrete.
39	Lo Monte F.; Ferrara L.	Self-healing characterization of UHPFRCC with crystalline admixture: Experimental assessment via multi-test/multi-parameter approach [120]	The study lacks specific numerical data for permeability and compressive strength, both for the control and treatment groups.
40	Byoungsun P.; Young C.C.	Investigating a new method to assess the self-healing performance of hardened cement pastes containing supplementary cementitious materials and crystalline admixtures [121]	The study does not provide specific numerical data for permeability and compressive strength measurements. It focuses more on heat production and the chemical analysis of self-healing products rather than mechanical properties, making it less relevant for comparing the impact of crystalline admixtures on the physical durability of concrete.
41	Cappellesso V.G.; dos Santos Petry N.; Dal Molin D.C.C.; Masuero A.B.	Use of crystalline waterproofing to reduce capillary porosity in concrete [122]	This study does not provide detailed or specific numerical data for permeability control related to crystalline admixtures. Additionally, the study emphasizes comparing crystalline waterproofing with silica fume admixtures, which shifts the focus away from the direct evaluation of crystalline admixtures on permeability and compressive strength in isolation.
42	Indira M.; Vignesh D.	Study on the performance of crystalline silica as a substitute for fine aggregate in concrete	The research focuses primarily on the partial replacement of fine aggregate with crystalline silica, rather than on the direct use of crystalline admixtures in concrete. It emphasizes the role of crystalline silica as a microfiller, not specifically its effects on permeability or compressive strength recovery. Additionally, no numerical data related to permeability or mechanical strength control with treatment using crystalline admixtures are provided.
43	DE SÁ PETRUCCI R.; Hastenpflug D.	Evaluation of crystalline waterproofing admixture on portland cement concrete [123]	The study evaluates the influence of crystalline waterproofing admixture specifically on the porosity and compressive strength of Portland cement concrete.
44	Geraldo R.H.; Guadagnini A.M.; Camarini G.	Self-healing concrete with crystalline admixture made with different cement content [124]	The paper provides comprehensive data on the mechanical properties and porosity of self-healing concrete with crystalline admixtures, covering various cement contents. However, for this study, a similar paper has been selected to maintain a more focused scope and streamline the analysis.
45	de Souza Oliveira A.; Dweck J.; de Moraes Rego Fairbairn E.; da Fonseca Martins Gomes O.; Toledo Filho R.D.	Crystalline admixture effects on crystal formation phenomena during cement pastes’ hydration [125]	This paper provides comprehensive data on the effects of crystalline admixtures on cementitious pastes. However, to maintain a more manageable scope for the current study, a similar paper with a more focused dataset was selected instead.
46	Ravitheja A.; Reddy T.C.S.; Sashidhar C.	Self-Healing Concrete with Crystalline Admixture—A Review [33]	This paper effectively investigates the impact of crystalline admixtures on the self-healing capacity of cementitious composites, particularly noting that additives to the admixtures tend to enhance healing for larger cracks.
47	Guzlena S.; Sakale G.	Self-healing concrete with crystalline admixture—A review [126]	This study does not provide specific numerical values for permeability or compressive strength in both the control and treated samples, which limits the ability to make precise comparisons or quantify the performance enhancements.
48	Park B.; Choi Y.C.	Prediction of Self-Healing Potential of Cementitious Materials Incorporating Crystalline Admixture by Isothermal Calorimetry [127]	The paper does not provide specific numerical values for permeability or compressive strength, which are crucial for a detailed comparison and analysis in the review.
49	Gojević A.; Ducman V.; Grubeša I.N.; Baričević A.; Pečur I.B.	The effect of crystalline waterproofing admixtures on the self-healing and permeability of concrete [17]	The study focuses on a specific crystalline waterproofing admixture (CWA) rather than crystalline admixtures (CAs) in general.
50	Pazderka J.	Concrete with crystalline admixture for ventilated tunnel against moisture [38]	This study does not provide adequate general data.
51	De Sá Petrucci R.; Hastenpflug D.	Evaluationof crystalline waterproofing admixtureon portland cementconcrete	The paper specifically addresses the crystalline waterproofing admixture (CWA) and its impact on porosity and compressive strength. It does not cover the broader spectrum of crystalline admixtures (CAs) or their general effects on concrete’s self-healing properties. This limits its applicability if the study aims to explore CAs more comprehensively.
52	Reiterman P.; Davidová V.; Pazderka J.; Kubissa W.	Reduction of concrete surface permeability by using crystalline treatment [128]	The paper primarily discusses the use of a repair mortar with a crystallizing admixture for improving concrete surface permeability. It does not cover broader aspects of crystalline admixtures (CAs) in general or their effects on the self-healing and durability of concrete as a whole.
53	Ferrara L.; Cuenca E.; Tejedor A.; Brac E.G.	Performance of concrete with and without crystalline admixtures under repeated cracking/healing cycles [129]	The paper discusses self-healing capacity and the behavior of fibre-reinforced concretes, but does not provide specific numerical data related to the permeability and compressive strength for crystalline admixtures.
54	Cobos R.B.; Pinto F.T.; Moreno M.S.	Analysis of the influence of crystalline admixtures at early age performance of cement-based mortar by electrical resistance monitoring [130]	This paper primarily explores the early-age performance of cement-based mortars and the effects of crystalline admixtures on the setting time and hydration through electrical resistance monitoring.
55	Wang X.; Fang C.; Li D.; Han N.; Xing F.	A self-healing cementitious composite with mineral admixtures and built-in carbonate [131]	The paper does not specifically address the broader impacts of crystalline admixtures on concrete permeability or compressive strength, which are key factors for this review.
56	Pazderka J.	The crystalline admixture effect on concrete and cement mortar compressive strength [132]	The paper does not specify the type of crystalline admixture (CA).
57	Al-Kheetan M.J.; Rahman M.M.; Chamberlain D.A.	A novel approach of introducing crystalline protection material and curing agent in fresh concrete for enhancing hydrophobicity [133]	The paper provides comprehensive data on various aspects of concrete performance, including both crystalline and curing agent interactions under different conditions. However, it covers a broad range of details and methodologies that may contribute to the length of the article, making it challenging to fit within the desired scope and length constraints.
58	Chandra Sekhara Reddy T.; Ravitheja A.; Sashidhar C.	Micromechanical Properties of Self-Healing Concrete with Crystalline Admixture and Silica Fume [134]	This study is not selected because it provides detail data on the self-healing capabilities of crystalline admixtures (CAs) and silica fume (SF) under various exposure conditions, but integrating these specific data could make the article longer than intended.
59	Sideris K.K.; Chatzopoulos A.; Tassos C.; Manita P.	Durability of concretes prepared with crystalline admixtures [135]	The paper does not provide the CA type and the dose.
60	Roig-Flores M.; Pirritano F.; Serna P.; Ferrara L.	Effect of crystalline admixtures on the self-healing capability of early-age concrete studied by means of permeability and crack closing tests [136]	The paper is not freely accessible, limiting the ability to review all of the relevant data and details. This lack of accessibility prevents a thorough evaluation of the paper’s findings and makes it less suitable for inclusion, especially if specific numerical data and detailed analyses are crucial for the review.
61	Guzlena S.; Sakale G.	Self-healing of glass fibre reinforced concrete (GRC) and polymer glass fibre reinforced concrete (PGRC) using crystalline admixtures [137]	The paper is not freely accessible, limiting the ability to review all of the relevant data and details. This lack of accessibility prevents a thorough evaluation of the paper’s findings and makes it less suitable for inclusion, especially if specific numerical data and detailed analyses are crucial for the review.
62	Žáková H.; Pazderka J.; Reiterman P.	Textile reinforced concrete in combination with improved self-healing ability caused by crystalline admixture [138]	The study is not selected because, while it provides valuable insights into the interaction between crystalline admixture (CAs) and textile-reinforced concrete (TRC), it focuses specifically on this combination and its effects on autogenous healing.
63	García-Vera V.E.; Tenza-Abril A.J.; Saval J.M.; Lanzón M.	Influence of crystalline admixtures on the short-term behaviour of mortars exposed to sulphuric acid [139]	The study focuses on the performance of crystalline admixtures in mortars rather than concrete.
64	Cuenca E.; Cislaghi G.; Puricelli M.; Ferrara L.	Influence of self-healing stimulated via crystalline admixtures on chloride penetration [140]	The study primarily focuses on chloride penetration resistance and crack healing in concrete exposed to marine environments, which is a specific scenario.
65	Al-Kheetan M.J.; Rahman M.M.; Chamberlain D.A.	Development of hydrophobic concrete by adding dual-crystalline admixture at mixing stage [141]	This study focuses on hydrophobicity rather than permeability or compressive strength; admixture performance related to water absorption is considered more than structural improvements, which may not align with the primary focus on durability and strength.
66	Park B.; Choi Y.C.	Effect of healing products on the self-healing performance of cementitious materials with crystalline admixtures [142]	The paper has no specific data for permeability and compressive strength.
67	Ziegler F.; Masuero A.B.; Pagnussat D.T.; dal Molin D.C.C.	Evaluation of internal and superficial self-healing of cracks in concrete with crystalline admixtures [143]	Although the study provides valuable insights into the effects of crystalline admixtures on self-healing and chloride diffusion, the findings focus more on internal healing rather than surface-level improvements.
68	Mircea C.; Toader T.-P.; Hegyi A.; Ionescu B.-A.; Mircea A.	Early age sealing capacity of structural mortar with integral crystalline waterproofing admixture [46]	The study focuses on the performance of crystalline admixtures in mortar.
69	Abro F.R.; Buller A.S.; Lee K.-M.; Jang S.Y.	Using the steady-state chloride migration test to evaluate the self-healing capacity of cracked mortars containing crystalline, expansive, and swelling admixtures [144]	This study is on mortar.
70	Lim S.; Kawashima S.	Mechanisms Underlying Crystalline Waterproofing through Microstructural and Phase Characterization [145]	While this study provides a thorough analysis of the microstructural changes in cement-based materials due to crystalline waterproofing agents, it focuses primarily on microscopic and chemical analyses rather than on the broader mechanical properties like strength, permeability, or durability.
71	Nasim M.; Dewangan U.K.; Deo S.V.	Autonomous healing in concrete by crystalline admixture: A review [30]	It does not provide data for permeability and compressive strength.
72	Ferrara L.; Krelani V.; Moretti F.	On the use of crystalline admixtures in cement based construction materials: From porosity reducers to promoters of self healing [146]	While it provides valuable insights into the effectiveness of crystalline admixtures in various concrete types and their impact on self-healing, it may be too specialized in its scope and experimental setup. The focus on advanced composites and specific test conditions may not align with the broader context of standard concrete applications and the general performance of crystalline admixtures in typical field conditions.
73	Li D.; Chen B.; Chen X.; Fu B.; Wei H.; Xiang X.	Synergetic effect of superabsorbent polymer (SAP) and crystalline admixture (CA) on mortar macro-crack healing [147]	While it provides valuable insights into the synergetic effects of these materials and the role of citric acid in promoting chemical precipitation, the focus on specific types of admixtures and additives may limit its applicability to more general studies of crystalline admixtures alone.
74	Takagi E.M.; Lima M.G.; Helene P.; Medeiros-Junior R.A.	Self-healing of self-compacting concretes made with blast furnace slag cements activated by crystalline admixture [148]	This study focuses on the effect of a crystalline admixture combined with different types of cements and blast furnace slag (BFS] percentages on self-healing in concrete. While it provides detailed results on mechanical recovery and permeation reduction, the specific focus on varying BFS content and its interaction with AR glass fiber and crystalline admixtures may not align with more general research on crystalline admixtures alone.
75	Takagi E.M.; Lima M.G.; Helene P.; Medeiros R.A., Jr.	Self-healing of self-compacting concretes made with blast furnace slag cements activated by crystalline admixture [148]	Multiple entries in the database.
76	Nasim M.; Dewangan U.K.; Deo S.V.	Effect of crystalline admixture, fly ash, and PVA fiber on self-healing capacity of concrete [149]	The study focuses mainly on early-age cracks and specific admixtures, with limited exploration of long-term durability or broader applications. This narrow scope may not provide comprehensive insights into the overall effectiveness of self-healing technologies in concrete.
77	Mohammadreza Hassani E.; Vessalas K.; Sirivivatnanon V.; Baweja D.	Influence of permeability-reducing admixtures on water penetration in concrete [12]	The study primarily evaluates the effectiveness of permeability-reducing admixtures in preventing water penetration rather than their impact on self-healing capabilities. Since the focus is on water resistance and mixture design rather than crack healing, it does not align with the criteria for assessing self-healing performance.
78	Durga C.S.S.; Ruben N.	Assessment of various self healing materials to enhance the durability of concrete structures [150]	This paper provides a broad overview of self-healing materials and their mechanisms in concrete, including various types of agents and their applications. It lacks specific experimental results, detailed comparisons, or focused analysis on the performance of particular self-healing admixtures, which are crucial for targeted assessments. Therefore, it does not align well with the requirement for detailed, focused studies on specific self-healing capabilities.
79	Azarsa P.; Gupta R.; Biparva A.	Assessment of self-healing and durability parameters of concretes incorporating crystalline admixtures and Portland Limestone Cement [151]	While the paper provides valuable insights into the effects of crystalline admixtures on concrete properties such as strength, self-healing, and durability, it is not selected, since this paper covers a broad range of tests and results, which might not align with the requirement for studies focusing specifically on self-healing mechanisms or detailed analyses of single variables.
80	Pazderka J.; Hájková E.	The speed of the crystalline admixture’s waterproofing effect in concrete [152]	The paper’s focus on the timing of waterproofing might not offer sufficient comprehensive information needed for a detailed evaluation of self-healing or durability characteristics.
81	Kheaw-on T.; Khomwan N.; Sujjavanich S.	The Effect of Crystalline Waterproofing Materials on Accelerated Corrosion of Steel Reinforcement in Concrete [153]	This study investigates the effects of crystalline waterproofing materials (CWPMs) on corrosion behavior, comparing surface coating and admixture types.
82	Cuenca E.; Mezzena A.; Ferrara L.	Synergy between crystalline admixtures and nano-constituents in enhancing autogenous healing capacity of cementitious composites under cracking and healing cycles in aggressive waters [154]	This study points out the development of ultra-high-durability concrete (UHDC) and the incorporation of nanoscale constituents to enhance self-healing and durability. The focus is on nano-constituents such as alumina nanofibers and cellulose-based materials rather than on crystalline admixtures (CAs) specifically.
83	Kannikachalam N.P.; Marin Peralta P.S.; Snoeck D.; De Belie N.; Ferrara L. “Kannikachalam, Niranjan Prabhu (58070938300)	Assessment of impact resistance recovery in Ultra High-Performance Concrete through stimulated autogenous self-healing in various healing environments [155]	The paper does not present specific numerical data on permeability or compressive strength.
84	Cappellesso V.; Ferrara L.; Gruyaert E.; Van	Resilient crystalline admixture in ultra-high performance self-healing concrete under cyclic freeze-thaw with de-icing salts [156]	While this study explores the resilience of UHPC with crystalline admixtures (CAs) under freeze–thaw (FT) conditions and their healing capabilities, it does not provide specific quantitative data on key parameters such as permeability and compressive strength. The focus on the qualitative aspects of self-healing and durability under FT conditions limits its applicability for studies requiring detailed numerical values related to permeability and compressive strength improvements.
85	Doostkami H.; Formagini S.; Serna P.; Roig-Flores M.;”Doostkami;” Hesam	Effects of healing start time and duration on conventional and high-performance concretes incorporating SAP [157]	The paper does not mention the CA type.
86	Zhang Y.; Wang Q.; Chen J.; Tang J.; Zhou H.; Zhou W.; Chang X.; Cheng Y.;”Zhang;” Yongzhen	Preparation and performance study of active chemicals in cementitious capillary crystalline waterproofing materials [158]	The paper is on mortar.
87	Li, Desheng; Lai, Yuanming Wen, Zhi; Gao, Qiang; Ding, Zhaowei; Zheng, Hao; Chen, Bing	A synergetic self-sealing model for cement-based composite using granular expansive agent and crystalline admixture [159]	The paper does not present specific numerical data on permeability or compressive strength.

provides a detailed explanation of these reasons, outlining the specific criteria that led to their exclusion.

3. Results and Discussion

3.1. Literature Analysis Using VOSviewer

A bibliometric analysis was performed using VOSviewer on 114 selected articles from the Scopus database to gain a deeper understanding of research trends related to crystalline admixtures in concrete. This analysis focused on the following two main aspects:

1.

Geographic distribution of research interest: Setting a minimum document threshold of one within a country revealed a total of 31 countries actively researching this topic. Among these, Italy, with 20 documents and 1235 citations, Spain, with 18 documents and 617 citations, Brazil, with 14 documents, and India all emerged as leading contributors, having the highest number of publications in the dataset. While the Netherlands, the United Kingdom, and Malta had a lower publication volume, their presence indicates a global interest in crystalline admixtures, as seen in Figure 5.

2.

Highly frequent keywords: VOSviewer identified the most relevant keywords by setting a minimum occurrence threshold of six. This analysis yielded 49 keywords, with “crystalline admixture” and “crystalline admixtures” unsurprisingly appearing as the largest circles in the VOSviewer map. This centrality underscores the core focus of research in this field. Interestingly, the analysis further revealed connections between crystalline admixtures and various concrete properties, including the compressive strength, self-healing, permeability, and water absorption. These connections highlight the potential of crystalline admixtures to influence a range of crucial concrete performance parameters. Given this, this study will delve deeper into the impact of crystalline admixtures, specifically on the compressive strength and permeability, as shown in Figure 6.

When exploring the impact of crystalline additives on concrete, researchers commonly investigate their influence on several key properties: self-healing, compressive strength, permeability, and corrosion resistance. A bibliometric analysis of 114 articles revealed that self-healing and corrosion resistance were frequently discussed, with keywords related to self-healing appearing 51 times and durability 30 times. Compressive strength was mentioned 29 times, while permeability appeared 15 times. Figure 7 shows the connections between keywords with a minimum occurrence of 15 times on VOSviewer.

Given the substantial focus on self-healing and corrosion resistance in the literature, this research will prioritize an in-depth examination of permeability and compressive strength. Understanding how crystalline additives affect these properties is crucial for enhancing the durability and performance of concrete structures.

3.2. Methodologies Employed in Selected Studies

The methodologies of the selected key studies on the effectiveness of crystalline admixtures (CAs) in concrete are summarized below. Each study provides valuable insights into different aspects of CA application, including permeability and compressive strength.

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Wang, Fan [53]: This comprehensive study focused on the development of Waste Recycled Concrete (WRC) using 42.5 N Ordinary Portland Cement (OPC), waste glass powder (WGP), a crystalline admixture (CA) from XYPEX, and recycled coarse aggregates (RCAs). Materials were sourced from various suppliers, including Tongling Shangfeng Cement Company Limited for the OPC and Taiyuan Hengtai Mineral Materials Co. for the WGP. Twelve different mixing ratios of the WRC were tested, with WGP replacement rates of 0%, 10%, 20%, and 30%, and CA addition ratios of 0%, 1%, and 2%. Permeability testing involved conserving specimens for 28 days and subjecting them to 1.2 MPa water pressure for 24 h. The permeability coefficient (K) was calculated based on the average seepage height, water pressure, and constant pressure time, following the Chinese standard GB-T50082-2009 [160]. Compressive strength tests were conducted according to Chinese standard GB/T-50081-2019 [161] using six parallel specimens to ensure reliable results.

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Escoffres, Desmettre [162]: This study explored the effects of crystalline admixtures on concrete’s permeability and compressive strength by preparing the following three types of concrete mixtures: high-performance concrete (HPC), high-performance fiber-reinforced concrete (HPFRC) with a 0.75% volume of steel macrofibers, and HPFRC enhanced with a 2% crystalline admixture (HPFRC-CA) using Sika’s WT-250. All of the mixes had a consistent water-to-binder ratio (w/b) of 0.43. Permeability tests were conducted on tie specimens containing Grade 400 W steel rebar under monotonic tensile loading conditions after 50 days of curing. Additional permeability tests under a 7-day constant loading condition assessed the self-healing capabilities. The permeability coefficient (Kw) was calculated using Darcy’s law. Compressive strength and mechanical properties were assessed at 28 and 50 days, following ASTM standards, with the HPFRC and HPFRC-CA tensile strengths evaluated using dog-bone specimens. Preliminary tests on unreinforced prisms provided insights into the mechanical behavior and self-healing potential under bending and load conditions.

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Antón, Gurdián [163]: This investigation focused on the compressive strength of concrete by preparing the following two types of mixtures: a reference mix, denoted as C, and another incorporating a commercial crystalline admixture (CCADM), labeled as C*. The compositions were adjusted to maintain similarity, with the CCADM added at a dosage of 0.29% relative to the cement mass in C*. Concrete C had a water/cement (w/c) ratio of 0.60, slightly higher than C*, to ensure comparable consistencies. Specimens, including steel-reinforced samples, were fabricated and cured for 28 days in a humid chamber at 90% RH and 23 °C, followed by compressive strength testing in accordance with UNE-EN 12390-3 standards. Each concrete type underwent evaluation using three cubic samples of a 150 mm side length, ensuring consistency in the testing conditions.

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Pazderka and Hájková [164]: This study investigated the impact of crystalline admixtures on the compressive strength using cube-shaped specimens (150 × 150 × 150 mm³) with C20/25 concrete. The study evaluated Penetron Admix and Xypex Admix C-1000 NF according to EN 12390-3 standards 28 days after preparation, with each type of material undergoing compressive strength testing. The findings highlighted significant enhancements in the compressive strength with the incorporation of crystalline admixtures, offering valuable insights into their application for enhancing concrete durability and performance.

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Nataadmadja, Setyandito [165]: This study focused on preparing concrete specimens targeting a compressive strength of 25 MPa, utilizing aggregates, cement, water, and a Xypex C-1000 NF crystalline admixture. The suitability of the aggregates was assessed according to Standard Nasional Indonesia (SNI) 7656:2012 standards. Both wet- and dry-mix methods were employed to ensure thorough mixing and to evaluate the structural integrity of the concrete. This approach provided critical insights into the performance and durability of the concrete under load, which was essential for assessing the structural resilience and material sustainability.

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Hermawan, Wiktor [166]: This study evaluated water permeation through cracked concrete disks sealed in PVC tubes with SikaFlex^® sealant, ensuring water flow exclusively through the cracks. The author performed experiments on the permeability and water flow, but did not mention specific numbers.

For the compressive strength, formulations incorporated CEM III/A 42.5 N cement, with 49% clinker, 46% blast furnace slag, and 5% minor constituents. The Penetron Admix Crystalline Admixture (Penetron 2024) was used at dosages ranging from 1% to 2% by weight of cement (bwoc). Mix designs (M1 to M7) tailored for exposure to XC4 and XF3 conditions targeted the self-healing properties. Testing procedures included the initial slump, air content, and density assessments, followed by compression tests at 7, 28, and 91 days. Statistical analysis via ANOVA provided insights into the significant parameters influencing the concrete performance, contributing to understanding CA’s role in enhancing the durability and structural integrity.

Table 2. Effectiveness of crystalline admixtures (CAs) on concrete permeability.

Authors	Year	CA (%)	Permeability (Control) (m.s−1)	Permeability (Treatment) (m.s−1)	Duration of Evaluation (day)	w/c	Treatment Conditions	Concrete Type	Crystalline Admixture Type
Wang, Fan [52]	2024	1	6.4 × 10−12	5.8 × 10−12	28	0.53	0% WGP	Recycled aggregate concrete (RAC) with Portland cement	Commercial crystalline admixture (CCADM)
		1	6.4 × 10−12	3.5 × 10−12	28	0.59	10% WGP
		1	6.4 × 10−12	2.5 × 10−12	28	0.66	20% WGP
		1	6.4 × 10−12	1.8 × 10−12	28	0.76	30% WGP
		2	6.4 × 10−12	5.4 × 10−12	28	0.53	0% WGP
		2	6.4 × 10−12	1.5 × 10−12	28	0.59	10% WGP
		2	6.4 × 10−12	1.9 × 10−12	28	0.66	20% WGP
		2	6.4 × 10−12	0.5 × 10−12	28	0.76	30% WGP
Escoffres, Desmettre [162]	2018	2	2.11 × 10−6	8.12 × 10−7	7	0.42	constant loading 250 MPa	HPFRC-CA	WT-250 commercialized by Sika

summarizes the effectiveness of crystalline admixtures (CAs) on the permeability in concrete specimens from two studies. Yang, Chen [167] investigated the impact of the CA dosage and water–cement (w/c) ratio on the permeability in recycled aggregate concrete. Their findings showed that a 1% CA dosage reduced the permeability from 6.4 × 10⁻¹² m/s to 1.8 × 10⁻¹² m/s at a w/c ratio of 0.76. Increasing the CA dosage to 2% further decreased the permeability to 0.5 × 10⁻¹² m/s with a 30% replacement of cement with waste glass powder (WGP). Escoffres, Desmettre [162] examined the effect of CA under constant loading, demonstrating a significant permeability reduction from 2.11 × 10⁻⁶ m/s to 8.12 × 10⁻⁷ m/s with a 2% CA dosage at a w/c ratio of 0.42. The duration of the permeability evaluation also varied across the studies. Short-term evaluations ranged from 7 days to 21 days, highlighting the importance of assessing both the short-term and long-term effects of CAs. The w/c ratio also played a role, with Wang, Fan [52] observing decreased permeability with higher CA dosages at varied w/c ratios (0.53 to 0.76), while Escoffres, Desmettre [162] showed effective permeability reduction with a CA at a constant, lower ratio (0.42). The concrete types and treatment conditions also varied. Wang, Fan [52] used recycled aggregate concrete, demonstrating the applicability of CAs in sustainable construction materials. Escoffres, Desmettre [162] employed high-performance fiber-reinforced concrete (HPFRC-CA) with the admixture, showcasing its effectiveness under high mechanical loading. The studies investigated different commercial CAs, including CCADM [52] and WT-250 [162], highlighting the variety and potential specific applications of different crystalline admixtures.

Table 3. Effectiveness of crystalline admixtures (CAs) on the concrete compressive strength.

Authors	Year	CA (%)	Compressive Strength (Control) (MPa)	Compressive Strength (Treatment) (Mpa)	Duration of Evaluation (day)	w/c	Treatment Conditions	Concrete Type	Crystalline Admixture Type
Antón, Gurdián [163]	2024	0.29	30	31.71	28	0.57	90% RH and 23 °C	Reinforced concrete with Portland cement	Commercial crystalline admixture (CCADM)
Wang, Fan [53]	2024	1	35	33	28	0.53	0% WGP	Recycled aggregate concrete (RAC) with Portland cement	Commercial crystalline admixture (CCADM)
		2	35	34	28	0.53	0% WGP
Escoffres, Desmettre [162]	2018	2	48.2	58.5	28	0.42	Compressive strength (fc), Young’s modulus (Ec), tensile strength (ft), and self-healing capability under load	HPFRC-CA	WT-250 commercialized by Sika
		2	54.7	58.3	50
Pazderka and Hájková [164]	2016	2	36.8	36.2	28	0.5	Water pressure test with 0.5 Mpa for 72 h; waterproofing effect assessed at different times after specimen creation	C16/20 concrete	Penetron Admix
		2	36.8	36.3					Xypex Admix C-1000
Nataadmadja, Setyandito [165]	2020	0.8	25	25	28	Not specified	100% Water	Concrete with Xypex additive	Xypex C-1000 NF
		0.8	25	27			110% Water
		0.8	25	24			120% Water
		0.8	25	23			130% Water
		1	25	25			100% Water
		1	25	26			110% Water
		1	25	29			120% Water
		1	25	24			130% Water
		1.2	25	25			100% Water
		1.2	25	25.5			110% Water
		1.2	25	30			120% Water
		1.2	25	27.5			130% Water
Hermawan, Wiktor [166]	2024	1	46.7	46.4	7	0.49	Cured at 20 °C, >90% RH	Reinforced concrete	Penetron 2024
		1	51.7	53	28	0.49
		1	55	57	91	0.49
		2	46.7	46.2	7	0.49
		2	51.7	56.2	28	0.49
		2	55	59.5	91	0.49

presents compressive strength data from multiple studies, each investigating the effects of different crystalline admixture (CA) dosages on various types of concrete. Antón, Gurdián [163] explored the effect of a 0.29% CA on reinforced concrete using a commercial crystalline admixture (CCADM), reporting a slight increase in the compressive strength from 30 MPa (control) to 31.71 MPa after 28 days under standard conditions (90% RH and 23 °C) with a w/c ratio of 0.57. Wang, Fan [52] examined recycled aggregate concrete (RAC) with Portland cement, incorporating 1% and 2% CA. For a w/c ratio of 0.53, the study found that the compressive strength slightly decreased from 35 MPa to 33 MPa with 1% CA, and increased to 34 MPa with 2% CA over 28 days. Escoffres, Desmettre [162] investigated high-performance fiber-reinforced concrete (HPFRC) with 2% CA, reporting significant gains in compressive strength from 48.2 MPa to 58.5 MPa after 28 days and from 54.7 MPa to 58.3 MPa after 50 days. The study, conducted under conditions of compressive strength (fc), Young’s modulus (Ec), and tensile strength (ft) with self-healing capability under load, suggests that CA is highly effective in high-performance concretes, resulting in notable strength improvements. Pazderka and Hájková [164] focused on C16/20 concrete, using 2% CA and evaluating it under water pressure tests. The results indicated a slight decrease in compressive strength from 36.8 MPa to 36.2 MPa, and from 36.8 MPa to 36.3 MPa with different crystalline admixtures (Penetron Admix and Xypex Admix C-1000). This minor decrease suggests that, while CA may not significantly enhance the compressive strength in all concrete types, it can still provide other benefits, like improved impermeability. Nataadmadja, Setyandito [165] conducted an extensive study on concrete with the Xypex additive across various water conditions (100%, 110%, 120%, and 130%). The compressive strength results varied with the CA dosages of 0.8%, 1%, and 1.2%, showing slight increases and decreases in the strength values. For example, with 1.2% CA under a 120% water condition, the strength increased from 25 MPa to 30 MPa, demonstrating that water conditions can influence the effectiveness of the CA in enhancing the compressive strength. Hermawan, Wiktor [166] investigated the effects of 1% and 2% CA on reinforced concrete, reporting compressive strength improvements at various intervals (7, 28, and 91 days). For instance, at 1% CA, the compressive strength increased from 51.7 MPa (control) to 53 MPa after 28 days. At 2% CA, the strength increased from 46.7 MPa (control) to 56.2 MPa at 28 days and from 55 MPa to 59.5 MPa at 91 days. This study highlights that higher CA percentages and longer evaluation periods can significantly enhance the compressive strength. Overall, the studies collectively suggest that crystalline admixtures can positively impact the compressive strength, with the extent of the improvement varying based on factors such as the CA dosage, concrete type, and evaluation period. While some studies report marginal improvements, others, particularly those involving high-performance concrete, show significant strength gains. The variability in the results underscores the importance of optimizing the CA dosage and treatment conditions to achieve the desired enhancements in concrete performance.

Comparative Analysis

• Permeability: All of the studies indicated that crystalline admixtures generally reduce the permeability. Wang, Fan [52] and Escoffres, Desmettre [162] demonstrated substantial reductions with higher CA dosages, whereas Hermawan, Wiktor [166] focused on other aspects, not providing specific numerical results.
• Compressive Strength: The effect of crystalline admixtures on the compressive strength varied. Antón, Gurdián [163] and Pazderka and Hájková [164] reported minimal increases or slight decreases in strength, whereas Yankai Wang and Nataadmadja, Setyandito [165] observed mixed results depending on the dosage and conditions. Escoffres, Desmettre [162] and Hermawan, Wiktor [166] showed significant improvements, particularly in high-performance and reinforced concretes.
• Dosage and Conditions: The impact of crystalline admixtures on both the permeability and compressive strength is influenced by the dosage, the type of concrete, and the environmental conditions. High dosages and specific conditions can lead to significant improvements, while lower dosages or different conditions may show minimal or variable effects.
• Sustainability and Performance: Studies incorporating recycled materials, such as Wang, Fan [52], demonstrate the potential for CAs to enhance the sustainability while improving the performance. The inclusion of self-healing capabilities, as seen in Escoffres, Desmettre [162], Pazderka and Hájková [164], Hermawan, Wiktor [166], suggests additional benefits for long-term durability.

Overall, while crystalline admixtures generally enhance the concrete performance, including both the permeability and compressive strength, the degree of improvement varies based on several factors. Future research should focus on optimizing the dosage, understanding the interplay with environmental conditions, and exploring the full potential of crystalline admixtures in various concrete applications.

3.3. Chemical and Physical Insights into Crystalline Admixtures’ Test Results

The fundamental chemical and physical phenomena underlying the observed effects of crystalline admixtures in concrete are crucial for understanding the outcomes reported in the selected studies. The study by Antón, Gurdián [163] delves into the chemical and physical interactions within concrete containing crystalline admixtures. They found that these admixtures interact with cement hydration products, particularly calcium hydroxide, leading to the formation of insoluble crystalline structures. These crystals fill capillary pores and micro-cracks, reducing the porosity and enhancing the impermeability and compressive strength of the concrete. This process is critical for improving the concrete’s durability, particularly in environments exposed to moisture, where the continued growth of these crystals promotes a self-healing mechanism that seals micro-cracks over time.

Similarly, Wang, Fan [53] further explored the role of crystalline admixtures in enhancing concrete durability. Their study focuses on the chemical interactions that lead to the precipitation of calcium carbonate and other crystalline products within cracks. These products effectively seal the cracks and reduce the permeability, particularly in the presence of moisture. The study also emphasizes the importance of optimizing mix design parameters, such as the water–cement (w/c) ratio and cement content, to maximize the self-healing properties of the concrete. The findings suggest that adjusting these parameters can significantly improve the effectiveness of crystalline admixtures, leading to better long-term performance and durability.

The study by Escoffres, Desmettre [162] specifically examines the self-healing capabilities of high-performance fiber-reinforced concretes (HPFRCs) incorporating crystalline admixtures. This study highlights how the crystalline admixture facilitates the formation of aragonite, a form of calcium carbonate, within cracks, in contrast to the calcite formation seen in concrete without the admixture. Although aragonite forms at a slower rate, it contributes to a more effective sealing of cracks over time, especially under constant loading conditions. The inclusion of fibers in the concrete enhances crack control by promoting smaller, more uniformly distributed cracks, which are more easily sealed by the self-healing products. The study also notes that the overall mechanical recovery of the concrete is improved due to the stronger bond between the fibers and the concrete matrix, likely aided by the crystalline admixtures, further linking chemical interactions to physical performance outcomes.

The study by Pazderka and Hájková [164] investigates the effects of crystalline admixtures on the water vapor permeability and compressive strength of concrete. Similar to the findings of Antón et al. and Y. Wang et al., this study shows that the crystalline admixtures reduce water vapor permeability by forming insoluble crystals within the concrete’s pore structure. This reduction in permeability not only contributes to the waterproofing effect but also complements the self-healing mechanisms discussed in the other studies. While the admixture may cause a slight deceleration in the initial hardening process, the long-term compressive strength remains unaffected, indicating that the crystalline admixture does not compromise the structural integrity of the concrete.

Nataadmadja, Setyandito [165] contribute foundational insights into the interaction between crystalline admixtures and cement hydration products. Their study highlights the formation of insoluble crystalline structures that fill capillary pores and micro-cracks, leading to a densified concrete matrix with reduced porosity. This densification is crucial for enhancing the impermeability and compressive strength of the concrete. Over time, these crystalline structures continue to grow, promoting a self-healing mechanism that seals micro-cracks as they form, thereby enhancing the long-term durability of the concrete, particularly in moisture-exposed environments.

Finally, Hermawan, Wiktor [166] further extend the understanding of crystalline admixtures by examining their role in stimulating autogenous healing. This study demonstrates that the admixtures not only reduce the porosity but also actively facilitate the precipitation of calcium carbonate and other crystalline products within cracks, further reducing permeability. The presence of moisture triggers the continued hydration of unreacted cement particles, enabling the growth of crystals that contribute to crack closure; Hermawan, Wiktor [166] also emphasize the importance of optimizing mix design parameters to enhance these self-healing properties, echoing the findings of Wang, Fan [53]. Their detailed analyses of these chemical and physical interactions provide a clearer understanding of the outcomes observed in the various studies, reinforcing the overall conclusions of the research.

The findings across these studies consistently highlight the importance of crystalline admixtures in enhancing the impermeability, compressive strength, and durability of concrete. The formation of insoluble crystalline structures within the concrete matrix is a key chemical interaction that reduces the porosity and blocks the ingress of water and harmful substances. These chemical processes are complemented by physical interactions, such as the refinement of the pore structure and the promotion of self-healing mechanisms that seal micro-cracks over time. By understanding these underlying phenomena, we can better appreciate why the crystalline admixtures lead to improved performance in concrete, particularly in terms of impermeability, compressive strength, and resistance to corrosion. These insights provide a robust foundation for the study’s conclusions and demonstrate the critical role of crystalline admixtures in enhancing the long-term durability of concrete structures.

3.4. Impact of Concrete Type, Dosage, and Environmental Conditions on Crystalline Admixture Performance

The performance of crystalline admixtures (CAs) in enhancing both the compressive strength and permeability of concrete is influenced by several factors, including the type of concrete, the admixture dosage, the environmental conditions during testing, and curing methodologies. Inconsistencies in findings across various studies can be attributed to these variables. Different types of concrete respond differently to CAs. For example, Antón, Gurdián [163] used reinforced concrete with Portland cement, while Wang, Fan [53] used recycled aggregate concrete (RAC), which tends to have a weaker and more porous structure. This might explain the slight reduction in compressive strength in the RAC, despite improvements in the permeability. Similarly, Pazderka and Hájková [164] used lower-strength C16/20 concrete, while Escoffres, Desmettre [162] utilized high-performance fiber-reinforced concrete (HPFRC) with steel fibers, showing a significant increase in the compressive strength due to the material’s enhanced mechanical properties. These differences indicate that CA’s effectiveness is highly dependent on the base concrete type, with more advanced concretes like HPC showing more pronounced improvements in both strength and permeability. However, recycled aggregate and lower-strength concretes showed variable performance, particularly in compressive strength, highlighting limitations of CAs in these cases.

The admixture dosage is another crucial factor contributing to the inconsistent results. For example, Nataadmadja, Setyandito [165] explored varying dosages (0.8% to 1.2% Xypex C-1000NF) and found that higher water conditions led to inconsistent compressive strength improvements, suggesting that the effectiveness of the CA is dose-dependent and can be influenced by excess moisture. In contrast, Hermawan et al. (2024) found that a 2% dosage of the Penetron CA resulted in gradual improvements in compressive strength over time, demonstrating that higher dosages, when combined with longer evaluation periods, enhance the material’s self-healing and densification properties. This implies that the appropriate dosing of CAs, in combination with controlled testing durations, is necessary to capture their full potential in improving both permeability and compressive strength.

Environmental conditions also play a critical role in the performance of CAs. Controlled environments, such as those used by Antón, Gurdián [163], with consistent humidity (90%) and temperature (23 °C), allowed the CA to fully activate, promoting thorough hydration and crystal formation. As a result, concrete exhibited enhanced impermeability and improved compressive strength. This suggests that in stable, controlled conditions, the CA performs optimally by creating a dense, impermeable microstructure. However, Pazderka and Hájková [164], who tested concrete under constant water pressure for 72 h, reported that the CA had limited influence on the compressive strength despite the improvements in the permeability. This indicates that, in environments with constant water exposure, the ability of the CA to enhance the compressive strength may be reduced due to excessive moisture interfering with hydration and crystallization processes. Conversely, Hermawan, Wiktor [166] found that CA’s self-healing properties were more pronounced in highly humid conditions (>90% RH) over extended periods (up to 91 days). In such environments, ongoing hydration facilitated continuous crystal growth in micro-cracks, leading to significant improvements in both the impermeability and compressive strength. This underscores the importance of humid environments for CA’s self-healing and densification processes, which enhance the long-term durability.

The study by Nataadmadja, Setyandito [165], which explored various water conditions (100%, 110%, 120%, and 130%), further demonstrated that excess moisture could compromise the effectiveness of CAs in improving compressive strength. Although permeability was consistently improved, the presence of excess water interfered with the densification process, suggesting that CAs may not function optimally in environments with an extremely high moisture content. These findings indicate that the performance of the CA is closely tied to the curing environment. Controlled curing environments, such as those with stable humidity and temperature, provide optimal conditions for CA performance, while variable or extreme moisture conditions can hinder the admixture’s ability to improve compressive strength.

In summary, the effectiveness of crystalline admixtures in concrete is influenced by a combination of factors, including the type of concrete, the dosage, the curing conditions, and environmental exposure. High-performance and fiber-reinforced concretes benefit most from CAs, particularly when exposed to controlled, stable curing environments. In contrast, recycled aggregate and lower-strength concretes, as well as those exposed to extreme moisture conditions, tend to show more variable results. These findings emphasize the need to carefully consider these factors when assessing the performance of CAs in different concrete applications, particularly in structures exposed to fluctuating or harsh environmental conditions.

3.5. Advancements in Crystalline Admixtures

The efficacy of crystalline admixtures has been enhanced through advancements in the understanding of their interaction with different types of cement and supplementary cementitious materials [168]. Researchers have focused on how these admixtures interact with components like slag, fly ash, and silica fume, leading to optimized mixtures that maximize both impermeability and strength [18,169,170]. These tailored formulations help in achieving specific performance targets, such as high early strength or improved durability in marine environments [101]. Moreover, the production process of crystalline admixtures has become more sustainable, with manufacturers focusing on reducing the environmental impact of their products. This includes the use of recycled materials in the production of admixtures and the development of low-energy manufacturing processes [1]. These efforts contribute to the overall sustainability of concrete construction, aligning with the industry’s goals of reducing carbon emissions and conserving natural resources [167]. Another significant advancement is the application of crystalline admixtures in new construction techniques such as 3D printing [171]. The self-sealing properties of these admixtures are particularly beneficial in printed concrete, where they help to address the unique challenges of layer-by-layer construction, such as the potential for increased porosity and reduced bonding between layers [172]. In recent years, the development and application of crystalline admixtures in concrete have seen significant advancements, leading to their widespread adoption in various construction projects [173]. These admixtures represent a cutting-edge approach to enhancing concrete properties beyond traditional methods. One of the notable advancements is the integration of nanotechnology, which has allowed for the production of more refined and effective crystalline particles [174,175]. These nano-sized particles can penetrate deeper into the concrete matrix, ensuring a more comprehensive sealing of micro-cracks and capillaries [176]. Furthermore, crystalline admixtures are being tailored to meet specific environmental and structural requirements. For instance, formulations have been developed to perform optimally in extreme temperatures and aggressive environments, broadening the scope of their applicability [177,178]. This customization ensures that concrete structures can maintain their integrity and functionality under diverse and challenging conditions [30,101].

3.6. Microstructural Effects of Crystalline Admixtures

Crystalline admixtures play a crucial role in modifying the microstructure of concrete, which directly impacts its permeability and compressive strength. These admixtures react with hydration byproducts such as calcium hydroxide to form insoluble crystalline structures within the concrete matrix. This process refines the pore structure by filling capillary voids and micro-cracks, thus reducing the overall porosity of the concrete. This densification of the microstructure enhances the bond between the cement paste and aggregates, which is key to improving the compressive strength.

The study by de Souza Oliveira, Dweck [125] provides additional insights into the reaction mechanisms of crystalline admixtures, particularly their influence on the hydration processes. The study demonstrates that the presence of crystalline admixtures not only increases the formation of calcium hydroxide but also promotes the crystallization of monocarboaluminate, a phase with higher thermal stability than calcium carbonate. This transformation is indicative of the long-term hydration potential of crystalline admixtures, as they continue to facilitate hydration reactions well beyond the initial curing period. Moreover, the study reveals that the addition of crystalline admixtures retards the hydration process, leading to a more gradual and controlled formation of hydration products, which, in turn, enhances the durability and mechanical properties of the concrete.

The recrystallization of calcium hydroxide and the conversion of less stable phases into more stable ones, such as monocarboaluminate, are particularly significant. These processes not only contribute to the reduction in the permeability but also enhance the self-healing capabilities of the concrete. When cracks form, the continued hydration facilitated by the crystalline admixtures leads to the formation of new crystalline structures within these cracks, effectively sealing them over time. This self-healing ability is crucial for maintaining the long-term integrity of concrete structures, especially those exposed to aggressive environmental conditions.

Advanced analytical techniques, such as thermogravimetric analysis (TGA) and X-ray diffraction (XRD), used in the study by de Souza Oliveira et al., have confirmed these microstructural changes. TGA results indicate that the presence of crystalline admixtures increases the amount of calcium hydroxide formed, while XRD analysis confirms the formation of monocarboaluminate and other stable crystalline phases. These findings underscore the importance of crystalline admixtures in refining the microstructure of concrete, thereby enhancing both its impermeability and compressive strength.

3.7. Self-Healing Mechanisms of Crystalline Admixtures

The self-healing properties of crystalline admixtures are a key feature that significantly enhances the durability and longevity of concrete structures. These properties arise from a combination of chemical reactions and physical processes that are activated when cracks form in the concrete. Chemically, crystalline admixtures contain reactive compounds that remain dormant within the concrete matrix until they come into contact with water, typically when cracks develop. When moisture infiltrates these cracks, the crystalline admixtures react with calcium hydroxide, a byproduct of cement hydration, as well as other unhydrated cement particles. This reaction leads to the formation of additional crystalline structures, such as calcium silicate hydrate (C-S-H), which are crucial for filling and sealing the cracks. The newly formed crystals continue to grow over time, expanding within the cracks and reducing the concrete’s overall porosity.

Physically, the admixtures facilitate the growth of needle-like crystals that penetrate deep into the micro-cracks and capillary pores of the concrete. These crystals act as physical barriers that block the pathways through which water and other aggressive agents, such as chlorides and sulfates, might otherwise penetrate the concrete. This crystallization process not only seals existing cracks but also reinforces the concrete’s internal structure, making it more resistant to further damage. The self-healing process is ongoing, as long as moisture is present, ensuring that cracks are continually repaired as they form, which is particularly beneficial in environments exposed to cyclic wetting and drying.

The implications of this self-healing capability are profound. By autonomously repairing cracks, crystalline admixtures help maintain the impermeability of concrete, thereby protecting embedded steel reinforcement from corrosion. This is especially important in marine environments, where concrete structures are constantly exposed to saltwater, or in regions with freeze–thaw cycles, where water ingress can cause significant damage. Furthermore, the ability to self-heal reduces the need for frequent maintenance and repairs, leading to lower lifecycle costs for concrete structures. This makes crystalline admixtures an attractive option for infrastructure projects where long-term durability and minimal maintenance are critical.

In practice, the effectiveness of these self-healing mechanisms has been demonstrated in numerous studies, where concrete treated with crystalline admixtures has shown a marked ability to close cracks and restore its integrity. For example, research has shown that cracks as wide as 0.4 mm can be completely sealed within weeks under optimal conditions, such as consistent exposure to moisture. This not only extends the service life of the structure but also enhances its performance under challenging conditions. The self-healing properties of crystalline admixtures thus represent a significant advancement in concrete technology, providing a sustainable solution for enhancing the durability of concrete in a wide range of applications.

3.8. Recent Advances in Hybrid Fiber-Reinforced Concrete

In addition to the existing studies, recent research provides further insights into the role of hybrid fiber-reinforced concrete (FRC) under impact conditions. Taghipoor and Sadeghian [179] conducted an experimental investigation focusing on the energy absorption and initial strength of single and hybrid fiber-reinforced concretes under impact loading. Their study revealed that the inclusion of fibers such as Basalt, Forta, and Barchip significantly enhanced the concrete’s energy absorption capacity and initial strength, particularly in hybrid formulations. The authors demonstrated that the combination of fibers improved the fracture resistance and overall toughness, essential for concrete structures exposed to dynamic and severe loads.

Similarly, Sadeghian, Moradi Shaghaghi [178] evaluated hybrid fiber-reinforced concrete (FRC) using a range of fibers, including Basalt and polypropylene-based fibers. The study optimized the fiber content using a Box–Behnken method and confirmed that hybrid fiber reinforcement not only increased the energy absorption but also improved mechanical properties such as compressive strength and durability. The research highlighted the crucial role that fiber combinations play in distributing stress and enhancing the longevity of concrete structures under low-speed impact conditions, which can be particularly beneficial in infrastructure projects subjected to repeated mechanical stress.

These findings reinforce the effectiveness of crystalline admixtures and fiber reinforcement in improving the durability, impact resistance, and permeability of concrete, aligning with the current review’s discussion on enhancing concrete properties through various admixture techniques. By incorporating these newer studies, this review now provides a more comprehensive and up-to-date perspective on the subject.

3.9. Recommendations for Future Research Directions

The substantial heterogeneity observed among the included studies highlights the need for further research to identify and manage sources of variability. Addressing these issues will be crucial for achieving more consistent and reliable outcomes in the application of crystalline technology. Future research should focus on standardizing experimental protocols, including maintaining consistent water-to-cement ratios, selecting uniform types of crystalline additives, and controlling environmental conditions during testing. Establishing a standardized timeline for permeability testing will also facilitate the better comparability of the results. By addressing these variables, the benefits of crystalline technology in concrete permeability reduction and structural enhancement can be more effectively realized.

In addition to standardizing experimental approaches, future research should also explore the integration of crystalline admixtures with other sustainable materials. Investigating the use of recycled aggregates or alternative binders alongside crystalline admixtures could further enhance concrete performance while minimizing environmental impact, aligning with broader sustainability goals.

Furthermore, employing advanced analytical techniques, such as microscopy and spectroscopy, will provide a deeper understanding of the microstructural changes induced by crystalline admixtures. These techniques can uncover detailed interactions at the molecular level, offering valuable insights into how these admixtures enhance concrete properties. Combining these approaches with standardized testing protocols will help optimize the use of crystalline technology, leading to more effective and sustainable concrete solutions.

4. Conclusions

Crystalline admixtures have emerged as a promising solution for enhancing concrete durability and performance. This review has demonstrated their effectiveness in reducing permeability while offering varying impacts on the compressive strength. While the literature highlights the potential of these admixtures, inconsistencies in research methodologies and the complexity of concrete systems necessitate further investigation. Future research should prioritize the standardization of experimental protocols, the exploration of synergistic effects with other sustainable materials, and the in-depth characterization of microstructural changes induced by crystalline admixtures. By addressing these areas, the full potential of crystalline technology can be unlocked, leading to more reliable, durable, and sustainable concrete structures. Ultimately, the successful integration of crystalline admixtures into concrete construction requires a comprehensive understanding of their behavior under various conditions. By building upon the existing knowledge base and addressing identified research gaps, the construction industry can harness the benefits of this technology to create more resilient and environmentally friendly infrastructure.