The Application of Calcium-Based Expansive Agents in High-Strength Concrete: A Review

Abstract

In this study, comprehensive investigation of the shrinkage compensation mechanisms of calcium-based expansive agents (CEAs), their effects on the properties of (ultra) high-strength concrete (HSC/UHSC), and the existing problems in applying this methodology was conducted. Analyses showed that the rational use of CEAs under certain conditions could greatly or completely inhibit the development of autogenous shrinkage of HSC/UHSC and significantly reduce the risk of associated cracking. However, it was found that the hydration of the CEAs affected the hydration process of other binders, thereby altering the microstructure of concrete. This, in turn, led to a reduction in mechanical properties such as compressive strength, flexural strength, and elastic modulus, with the rate of reduction increasing as the amount of CEA used increased. Moreover, when attempting to improve the shrinkage compensation effects, increasing the amount of CEA presented a risk of delayed expansion cracking of the HSC/UHSC. Neither the expansion mechanism, expansion conditions, nor the inhibition methods have yet been fully clarified in the current stage. Lastly, newly proposed Ca–Mg composite EAs were outlined, and the research prospects of Ca–Mg composite EAs in HSC/UHSC were explored.

1. Introduction

High-strength/ultra-high-strength concrete (HSC/UHSC) has been increasingly used in construction, including super high-rise buildings, long-span structures, and critical infrastructure projects, due to its significant advantages such as a dense internal structure, high compressive strength, and excellent durability [1,2]. However, HSC/UHSC is highly prone to autogenous shrinkage cracking due to its very low water–binder ratio, high unit binder content, significant heat of hydration, and weak aggregate restraint [3,4]. Autogenous shrinkage cracking has become a major limiting factor in the engineering application of HSC/UHSC [5,6,7], as such cracks are nearly impossible to repair post-construction without adversely affecting the structural performance. Therefore, controlling the development of autogenous shrinkage during the mix design stage is crucial for promoting the use of HSC/UHSC.

Numerous studies have demonstrated that the rational application of expansive agents (EAs) [8,9,10], which compensate for shrinkage through expansion-induced stress, and/or shrinkage-reducing admixtures (SRAs) [10,11], which decrease the surface tension of the pore solution, along with internal curing agents [12,13], can effectively control the autogenous shrinkage of HSC/UHSC and reduce the risk of cracking. Among these strategies, the utilization of EAs is the most direct and economically feasible approach [14,15,16]. These have therefore become widely used in construction, in particular calcium-based expansive agents (CEAs) with calcium oxide, sulfate, sulfoaluminate, and so on, as active ingredients, which represented a series of expansive products developed in the early stages of research and development, with relatively mature production and application technology, as well as wide usage.

However, at present, there are still several key issues to be addressed regarding the application of CEAts in HSC/UHSC. These include: (1) A lack of a unified understanding of the shrinkage compensation mechanism of CEAs, making it difficult to regulate the synergy between EAs and cement hydration processes [14,17]; (2) The use of CEAs may alter the hydration process of cement, leading to adverse effects on the development of the concrete’s mechanical properties [18,19]; (3) Higher demands on curing conditions when using CEAs, increasing construction difficulty [20]; and (4) Improper control of the dosage of CEAs which may lead to delayed expansion damage [17,21]. Regarding these issues, extensive research has been conducted in recent years, resulting in progress. However, most individual studies have focused on specific topics for in-depth longitudinal research, which may not directly provide reference or guidance for practical engineering applications.

Based on the above background, this study aims to consolidate the latest research findings concerning the application of CEAs in crack control for HSC/UHSC, offering guidance for practical engineering applications and fostering the advancement and refinement of the technical framework for CEAs in HSC/UHSC. Initially, we conducted a comprehensive survey of common calcium-based expansive agents, including their types, expansion conditions, and influencing factors. Subsequently, by integrating the hydration reaction characteristics of CEAs, we conducted an in-depth analysis of the shrinkage compensation (expansion) mechanism. Furthermore, a detailed examination was undertaken to assess the impact of CEAs on the performance of the HSC/UHSC, coupled with a re-view of current research status regarding delayed expansion associated with these agents. Finally, considering the constraints of CEAs, we summarized the present research status of novel magnesium oxide-based expansive agents (MEAs) and provided insights into the research direction for optimizing CEAs’ performance. This study offers a phased summary of CEAs’ application technology in HSC/UHSC, significantly contributing to the advancement of research and application technology in this field.

2. Calcium-Based Expansive Agents

2.1. Classification

CEAs can be primarily divided into three types based on their hydration products: the calcium sulfoaluminate type (CSA), the CaO type and the composite type [22,23]. The main chemical compositions, expansive hydration products and the hydration reactions for each type are summarized in

Table 1. Main chemical hydration reactions of CEAs [23,24,25,26].

Type	Main Components	Expansive Hydrates	Main Chemical Reactions
CSA	C4A3Ŝ, C3A, CŜ	AFt	C4A3Ŝ + 6C + 8CŜ + 96H→3(C3A∙3CŜ∙32H) C3A + 3CŜ + 32H→C3A∙3CŜ∙32H
CaO	C	CH	C + H→CH
CSA–CaO Composite	C4A3Ŝ, CaO, CŜ	AFt, CH	C4A3Ŝ + 6C + 8CŜ + 96H→3(C3A∙3CŜ∙32H) C + H→CH

C: CaO; A: Al₂O₃; Ŝ: SO₃; H: H₂O.

. As shown in Table 1, CSA type Eas mainly include C₄A₃Ŝ and C₃A, with ettringite (Aft) as the main expansive hydration product. CaO type Eas primarily consist of free lime, with calcium hydroxide (CH) as the hydration product. The composite type combines the above two systems, consisting of sulfoaluminate, free lime, and gypsum, and its main expansive hydration products include both Aft and CH.

Normally, CaO type EAs hydrate very fast in the initial stage and the hydration rate can reach more than 90% within 24 h at room temperature (20 °C) [27,28], whereas the hydration rate of sulfoaluminate is only about 70% in 72 h [29], but the latter has been shown to contribute to the shrinkage compensation effect for a prolonged period (about 7 days). Therefore, it is considered that CSA type EAs are more conducive to achieving long-term expansion and effectively controlling autogenous shrinkage after 7 days.

Furthermore, based on the hydration process shown in Table 1, it can be theoretically inferred that the solid phase volume increases by about 3.1 times when C₄A₃Ŝ is fully hydrated and generates AFt, while the volume of solid only increases by about 1.7 times when CaO is fully hydrated and generates CH [23]. This signifies that ettringite-generating EAs have higher expansion efficiency compared to CaO type EAs.

However, it can be confirmed from Table 1 that the hydration of CSA and composite type EAs requires a significantly higher amount of water compared to that of CaO type agents. In particular, the complete hydration of 1 mole of C₄A₃Ŝ consumes 96 moles of water. Therefore, previous studies have suggested that EAs generating ettringite are not suitable for ultra-high-performance concrete (UHPC) with a low W/B ratio due to their substantial water requirement in the hydration process and the instability of the expansive product [14,30]. Concrete augmented with CEAs requires thorough moisture curing to prevent the occurrence of more severe cracking issues.

2.2. Expansion Mechanism

Currently, there is still some controversy about the expansion mechanism of CEAs. For CaO type EAs, the expansion mechanism is relatively clear and can be described as follows: the reaction between CaO and water produces CH, and in situ crystallization leads to an increase in the volume of the solid phase [23,31]. This results in significant expansion stress and achieves a shrinkage compensation effect [32,33]. In the early days, there was also a viewpoint that the voids formed inside the cement matrix with hydration products also contributed to the volumetric expansion [34].

For CSA type EAs, several controversial expansion mechanisms have been proposed [23,35,36]. The traditional mainstream views mainly include the crystallization pressure hypothesis [37,38,39,40,41,42], the water absorbent swelling hypothesis [30,43], the in situ hydration hypothesis [30,44,45], and the osmotic pressure hypothesis [34,46,47]. The main viewpoints of these hypotheses are summarized in

Table 2. Classic hypotheses on the expansion mechanism of CSA type EAs.

Hypothesis	Main Content	Ref.
Crystallization pressure hypothesis	Anisotropic ettringite crystals continuously generate and cross-grow, causing crystallization pressure inside the paste matrix and resulting in expansion.	[37,38,39,40,41,42]
Water absorption swelling hypothesis	Colloidal ettringite is able to attract a large number of water molecules, which causes interparticle repulsion, resulting in an overall expansion of the system.	[30,43]
In situ hydration hypothesis	The ettringite generated by the reaction of sulfate and aluminate in the solid phase has expansion properties, while the reaction in the liquid phase does not.	[30,44,45]
Osmotic pressure hypothesis	By the disintegration of expansive substances during hydration or osmotic pressure, coexisting pores are generated, causing expansion.	[34,46,47]

. It can be confirmed from Table 2 that the crystallization pressure hypothesis emphasizes the influence of the morphology and location of ettringite crystals. The water absorption swelling hypothesis highlights the physical properties of the molecular structure. The in situ hydration hypothesis focuses on the formation process of ettringite. Meanwhile, the osmotic pressure hypothesis emphasizes the role of micropore changes during the formation of ettringite.

In addition, some scholars have also proposed that the morphology [48] and location [32] of ettringite are also related to its expansive properties. Mehta et al. believed that colloidal ettringite is the main cause of expansion [30], while later studies suggested that only prismatic needle-like crystals with a large aspect ratio can cause expansion [38,39,48]. Deng’s study proposed that when ettringite crystals are formed in situ in high alkaline pore solutions, they can cause large expansion, while partial or complete precipitation in the pores can only cause small expansion or no expansion [32]. At the same time, the studies of Bizzozero et al. [42] and Chaunsali et al. [49] also pointed out that the expansion mechanism of CSA type EAs is closely related to the supersaturation degree of SO₄²⁻ and Ca²⁺ ions in the pore solution, as well as the distribution of ettringite crystals and the constraint conditions.

2.3. Expansion Conditions and Affecting Factors

This study categorizes the conditions and factors affecting the expansion behavior of CEAs into three main categories: (1) environmental conditions, (2) hydration characteristics, and (3) microstructural characteristics. These three primary factors directly influence the expansion properties of CEAs and interact indirectly with each other, as shown in Figure 1.

As shown in Figure 1, the basic physicochemical properties, such as chemical composition and particle size distribution of CEAs directly determines the hydration characteristics, thus affecting the expansion properties. The chemical composition can change the types and characteristics of hydration products, while the particle size distribution directly affects the rate of hydration. For CSA type EAs, when gypsum is insufficient, C₄A₃Ŝ will hydrate, through equation 1 below, to generate the non-expansive monosulfate (C₃A∙CŜ∙12H), referred to as “AFm”, which prevents the development of expansion performance [50,51,52,53].

C₄A₃Ŝ + 6C + 2CŜ + 36H→3(C₃A∙CŜ∙12H) (AFm)

(1)

Mehta’s research also found that the presence or absence of CaO results in different crystal sizes of ettringite generated by the hydration of C₄A₃Ŝ (As shown in Figure 2). Only in the presence of CaO does the formation of small-sized colloidal ettringite (as shown in Figure 2A) cause significant expansion [30,54]. When CaO is insufficient, large rod-shaped crystalline ettringite (as shown in Figure 2B) forms instead, which does not cause significant expansion [30]. Cohen et al. [55] conducted a study on Type K expansive cement and found that cement with greater fineness has a faster reaction between C₄A₃Ŝ and gypsum due to more expansive sites. This characteristic can shorten the expansion duration and reduce sensitivity to abnormal behavior. Furthermore, the authors of this study also considered that with the diversified development of raw materials’ composition in HSC/UHSC, the interaction between CEAs and mineral additives in HSC/UHSC should also be gradually valued and deeply studied, which is currently insufficient.

It can also be seen in Figure 1, that the temperature, humidity, and restraint conditions of the environment directly affect the expansion properties of CEAs. Adequate moisture supply is necessary for the complete hydration of substances such as CaO, C₄A₃Ŝ, CA, and C₃A, especially for HSC/UHSC with extremely low water–cement ratios. The humidity during external/internal curing influences the hydration process and degree of CEAs, thereby changing their expansion behavior [19,55,56] (as shown in Figure 1).

A typical case study is illustrated in Figure 3. Under continuous underwater curing, the expansion strain shows a significant and continuous increase (solid blue line). However, under prolonged dry conditions, the expansion strain does not fully develop and remains at a relatively low value (solid yellow line). When specimens are transferred to a dry environment after 28 days of underwater curing, a noticeable decrease in expansion occurs (solid red line). During initial dry curing for 100 days, the expansion is minimal and stabilizes. However, when subjected to underwater/drying cycles, the expansion rapidly increases, ultimately reaching a level like that of continuous underwater curing (dot-ted blue line). This indicates that the CSA-type EAs, which did not fully react during the dry period, complete their hydration and achieve full expansion during subsequent underwater curing. This interesting experimental result demonstrates that the performance of the EAs is highly sensitive to curing humidity conditions. Adequate water curing is essential for the agent to fully hydrate and achieve its expansive (shrinkage compensating) effect [20].

Regarding temperature conditions, a certain degree of thermal curing can improve the hydration rate and degree of the EAs [20,28] and promote the development of expansive performance. However, a curing temperature above 70 °C will cause the decomposition of ettringite [57,58], resulting in a decrease in expansive performance, when thermal curing is imposed on ettringite-based CEA systems.

The microstructure of cementitious materials will also indirectly influence the expansion behavior of CEAs. Deng concluded that the alkaline environment in internal pore locations provides favorable conditions for the in situ nucleation of ettringite crystals, leading to expansion through in situ reactions [32]. The oversaturation pressure generated by the enrichment of ions such as Ca²⁺ and SO₄²⁻ in the interfacial transition zone (ITZ), microcracks, and other voids promotes the generation and precipitation of expansive hydration products, resulting in expansion [42,59].

Especially in the complex environment of cement-based composites, which is a heterogeneous system, the development of expansion behavior often occurs due to the coupling of multiple mechanisms and the combined effects of multiple factors. Further in-depth discussions are needed on the development of shrinkage compensation functions of CEAs in low water–cement ratio HSC/UHSC.

3. Autogenous Shrinkage Compensating Mechanism of CEAs

The hydration of the CEAs generates expansive hydrates, the quantity of which increases and the morphology of which changes with the progress of hydration. This causes an increase in the volume of the solid phase, which in turn compensates for the volume reduction arising from the hydration of the cement paste [60].

Figure 4 below is a conceptual diagram of the morphological changes during various hydration stages of cement paste with CEAs, based on existing research achievements [25,61,62,63,64].

As shown in Figure 4, the shrinkage compensation process of CEAs in cement paste can be roughly divided into four stages. Before mixing (Stage I), the solid particles are relatively dispersed and can exist independently from the other stages. After adding water, the system enters the pre-hydration stage (Stage II). At this point, the Clínker Portland (mainly C₃A) and CEA particles begin to react, with hydration products forming on the surface of the clinker particles. However, during this stage, there are few hydration products, the microstructure of the solid phase has not yet formed, and the volume increase in the AFt or CH does not cause expansion stress, meaning its expansive properties have not yet been activated.

As hydration progresses, the hydration products of the Clínker Portland gradually increase, forming the skeleton of the solid phase and gradually losing plasticity. The cement paste begins to set, and the elastic modulus starts to increase. This stage is referred to as the critical hydration stage (Stage III). During this stage, the further hydration of CEA generates more AFt and/or CH, and the growth of crystals exerts expansion pressure on the cement paste matrix, initiating its shrinkage-compensating role. As the hydration continues, the amount of AFt and CH generated by CEAs further increases, inducing expansion stress that effectively reduces the shrinkage of the cement paste, fully utilizing the shrinkage-compensating properties of the EAs (Stage IV).

When observing the microstructure of cement paste with the added CSA type EA, Morioka et al. [61] and Yamamoto et al. [62] found that the increase in volume of expansive hydrates had not only compensated for the reduced volume of cement paste, but that the capillary voids generated during the hydration of the EA may in addition constitute one of the reasons for the shrinkage compensation. This finding was like that from experiments with calcium silicate hydrate (C-S-H) gels, where the voids (gel pores) formed within the C-S-H solid phase were also shown to increase the volume of C-S-H to a certain extent [63,64].

Figure 5 below, is a conceptual diagram showing the change in cement paste volume after an EA had been added during the hydration process. I, II, III, and IV in Figure 5 correspond to the four stages in Figure 4.

When considering the effect of the CEAs, the process could be divided into two main stages, namely, an effective expansion stage and a delayed expansion stage. The effective expansion referred to the increase in volume generated by the hydration of the EAs, which effectively reduced (compensated for) the volume shrinkage of the cement paste [18,25]; when the degree of expansion equals that of the cement paste shrinkage, or when the expansion rate exceeds the shrinkage rate, the macroscopic volume change manifested as expansion. If the EAs failed to complete their reaction in the early stage of hydration, then as the reaction continued, the expansion stress caused in the cement paste after hardening (once the elastic modulus had fully developed) was found to exceed the tensile strength of the cement paste, causing the failure of the cement paste expansion. This phenomenon is known as the delayed expansion stage of the EAs. Under normal circumstances, the amount of EAs added should be reasonably determined through experimental research to ensure that the shrinkage compensation effect is mainly exerted in the effective expansion stage, but some studies have shown that excessive use of EAs resulted in delayed expansion failure of concrete [17,21].

Considering the interaction between CEAs and the hydration of cement components, Morioka et al. [18,65] showed that the early hydration of C₃A was delayed when a CSA type EA was used. When C₃A was combined with anhydrite (CaSO₄), it was observed to promote the reaction of haüyne (C₄A₃Ŝ) crystals in the EAs. Additionally, a study by Sagawa and Nawa [27] also clarified that the mixing of a CSA type EA reduced the reaction rate of C₃S in cement clinker but significantly increased the reaction rate of slag components. It can also be seen from Figure 5 that the hydration rate of the EAs, specifically the coordination of the development timing of expansion and the hydration process of the cement clinker, was the key point for the CEAs to effectively fulfill the role of shrinkage compensation [24]. Lastly, it was found that in HSC/UHSC with a low water–binder ratio, controlling the balance between the hydration rate of the CEAs and that of the components (C₃S, C₂S), which was shown to be related to the development of the elastic modulus of the cement paste, was critical to effectively controlling the cracking problems caused by autogenous shrinkage.

4. Influence of CEAs on the Properties of HSC/UHSC

A series of studies have demonstrated that the application of CEAs is an important measure to control the risk of cracking caused by autogenous shrinkage, improve the durability of HSC/UHSC, and promote the development of HSC/UHSC technology [14,16,66]. However, the application of CEAs has direct or indirect impacts on the development of properties such as the hydration process, mechanical strength, and microstructure in HSC/UHSC. In order to promote the scientific application of CEAs for controlling autogenous shrinkage in HSC/UHSC, this chapter conducts a comprehensive analysis and summary of the previous findings related to the influence of CEAs on the properties of HSC/UHSC.

4.1. Hydration Characteristics of Binders

The addition of CEAs significantly influences the hydration process of binders in HSC/UHSC, and this influence varies depending on the type of CEAs. Figure 6 presents a comparison of hydration heat release curves for binders with 0%, 1%, and 2%wt of CaO type EAs added [67]. It can be observed that the addition of CaO type EAs significantly increases the main peak and the cumulative heat release at around 120 h. Additionally, within a certain range, the main peak and cumulative heat release show a positive correlation with the amount of CaO type EAs added. This conclusion aligns completely with the findings of others [56,67,68,69]. Zhou’s research indicates that when the amount of CaO type EAs exceeds a certain threshold (8%wt), the main peak of heat flow is delayed, and both the peak and cumulative heat release are reduced compared to a lower dosage of EAs. Therefore, it is speculated that the excessive use of CaO type EAs can delay the hydration of cementitious materials [69].

The research conducted by Li et al. [70] found that the use of CSA type EAs can shorten the induction period of the OPC–CSA system to a certain extent and significantly increase the heat flow and cumulative heat release. Therefore, it is concluded that CSA type EAs promote the hydration of the OPC–CSA system [70]. In contrast, Shen et al. [24] studied the hydration heat characteristics of UHPC with 10%wt CSA–CaO composite type EAs added and found that CSA–CaO led to a delay in the early hydration reaction of the binders. Regarding this issue, the authors of this paper speculate that this delay may be due to an excessive dosage of the EAs, as mentioned in Zhou’s research [69].

4.2. Autogenous Shrinkage

In order to confirm the compensation effects of EAs on autogenous shrinkage of HSC/UHSC, the present authors synthesized the data from previous studies and summarized the relationship between the compensation rate of autogenous shrinkage of HSC/UHSC and the amount of CEAs used, as shown in Figure 7 below. The autogenous shrinkage compensation rate was calculated by Formula (2) [71]:

α = |ε_EX(t) − ε_PL(t)/ε_PL(t)| × 100%

(2)

where α refers to the autogenous shrinkage compensation rate (%); ε_EX(t) refers to the autogenous shrinkage strain of concrete with added expansive material at age t; and ε_PL(t) refers to the autogenous shrinkage strain of concrete without any CEAs at the same age of t.

As shown in Figure 7, within a certain range, regardless of the hydration time, the autogenous shrinkage compensation rate increased with the augmentation in CEAs used, that is, the greater the amount of CEAs added, the higher the reduction effect in autogenous shrinkage [8,9,72,73,74]. At the same time, it was found that even when other conditions remained the same, the shrinkage compensation effects varied according to the type of EAs. A significant further finding from the present study, however, was that as the content of CEAs increased, the strain on the concrete gradually developed into expansion deformation (that is, when the shrinkage compensation rate exceeded 100%), presenting a potential risk of cracking as the expansion continued to develop.

Based on the above result, when using CEAs to control autogenous shrinkage cracks in HSC/UHSC, the type of EAs should be correctly selected and their appropriate ratio to the amount of concrete determined. In addition, with the view to optimizing the autogenous shrinkage compensation effect, it was also found necessary to control the development trend of concrete deformation in the later stage to prevent long-term expansion and damage to concrete.

As discussed before, the hydration heat of the binders is known generally to cause a high temperature rise within the HSC/UHSC, so that the influence of thermal conditions on the shrinkage compensation effects of the CEAs cannot be ignored. Figure 8 below summarizes the autogenous shrinkage compensation rate of CEAs under different thermal conditions [75,76].

Figure 8 shows that at the hydration age of 40 days, regardless of the water–binder ratio, the shrinkage compensation rate under normal temperature curing conditions (20 °C) was significantly higher than that under high temperature curing conditions (45 °C, 80 °C, or 90 °C, respectively), and that the higher the curing temperature was, the more the shrinkage compensation rate decreased. Regarding this phenomenon, Mitani et al. [75] inferred that, on the one hand, the early reaction rate of the CaO type EAs was higher under high temperature conditions, whereby a large amount of EAs were found to be consumed in the early stage, so that there was not enough EAs remaining by the effective expansion stage, resulting in a reduced long-term shrinkage compensation effect. On the other hand, they reported that a mineral admixture (e.g., silica fume) consumed the expansive hydration product (CH) through pozzolanic reactions at a rate that increased with the rise in temperature [75,77], resulting in a decrease in the amount of expansive hydration products and of the shrinkage compensation effect under high temperature curing.

From the above study findings, temperature variations directly affected the shrinkage compensation effect of the CEAs. Regarding this point, not only was it hypothesized to be due to the consumption of the expansive hydration product (CH) by pozzolanic reaction, but was also ascribed, in the case of ettringite-based EAs, to the fact that AFt decreased due to thermal decomposition under high temperature conditions [78,79,80,81], which in turn was also believed to directly affect the shrinkage compensation effects of EAs. The present authors therefore concluded that when using CEAs, not only the influence of binders on the hydration process of the EAs, but also the influence of temperature on the shrinkage compensation effect, should be considered, especially for concrete with a low water–binder ratio. Especially in the latter case, effective temperature control and maintenance measures should thus be taken to ensure that the shrinkage compensation effect of CEAs can be fully exerted during the effective expansion stage.

4.3. Compressive and Flexural Strength

Previous studies [69,82,83,84], consistently concluded that the addition of an appropriate amount of CEAs can enhance the early compressive strength of USC/UHSC (usually within 7 days). However, it may lead to a reduction in the medium to long-term compressive strength values, and either the increase rate in early strength or the decrease rate in medium to long-term strength escalate with the dosage of the CEAs. Liu considered that the heat released during the hydration of CEAs can accelerate the early hydration of cement, increase the degree of hydration, and improve the compactness and homogeneity of the UHPC microstructure due to the formation of expansive hydrates, thereby promoting the development of early strength [85].

Based on previous research data [12,65,72,73,86,87,88], the relationship between the reduction in compressive strength and the addition rate of CEAs was investigated in the present study, and the results are shown in Figure 9 below.

As can be observed from Figure 9, although the data from the different studies considered were relatively diverse, it was generally established that (i) when CEAs were used, the compressive strength of HSC/UHSC decreased; and (ii) the reduction rate in compressive strength rose with the increase in the ratio of added CEAs. From the above review data, it was found that when the ratio of CEAs used exceeded 3.0% of the cementitious material, the compressive strength of concrete decreased by over 15%. Regarding the reasons for the negative effects of CEAs on the compressive strength of HSC/UHSC, the present authors considered that, firstly, when cement was replaced by the same amount of EAs, the actual water–binder ratio increased; secondly, they ascertained that the hydration of the EAs consumed the free water in the concrete mix, thereby affecting the hydration rate and extent of other binders, consequently affecting the development of strength of the concrete; and thirdly, they determined that the expansion stress generated by the CEAs destroyed the microstructure of the cement paste or the interfacial transition zone (ITZ) and caused micro-cracks in the cement paste or the ITZ, resulting in a decline in the mechanical properties of the concrete [15].

The influence of CEAs on the flexural strength development of HSC/UHSC is similar to that on compressive strength. Within the first 7 days of hydration, the flexural strength of UHSC gradually increases with increasing dosage of CEAs, whether it is a CaO type EA [69,85], CSA type EA [86,89], or CSA–CaO composite type EA [24]. However, after 7 days, the trend reverses, and the flexural strength decreases as the dosage of CEAs increases. Furthermore, the flexural strength of the specimens with added CEAs is significantly lower than that of the control group without addition. The authors attribute this result to the same explanation as the previous discussion on compressive strength.

4.4. Elastic Modulus

The elastic modulus is an important parameter for the development of mechanical performance and deformation control of HSC/UHSC, and several studies have shown that the addition of CEAs can directly or indirectly vary the elastic modulus of HSC/UHSC. Experimental results by Meddah et al. [90] demonstrated that the addition of 1.5%, 2.0%, and 2.5% CSA type EAs to HPC with water–binder ratios of 0.15, 0.23, and 0.30 resulted in a certain degree of decrease in the elastic modulus. Zhou et al. [69] indicated that the addition of CaO–MgO composite type EA decreased the elastic modulus of UHPC, and the magnitude of reduction increased with the increase in dosage of the EA. This is mainly because the EAs affect the hydration of cement in low water–binder ratio systems, as mentioned above, and weaken the internal structure, consequently leading to a decrease in the elastic modulus.

Similarly, experiments conducted by Wyrzykowski et al. showed a certain decrease in the elastic modulus with the addition of 20% CSA type EAs [20]. Wyrzykowski et al., pointed out that this reduction is influenced by curing conditions. When compared to underwater conditions, specimens exposed to prolonged dry or wet–dry cycles showed more significant decreases in the elastic modulus, as shown in Figure 10. This is considered to be due to the supply of moisture affecting the continuous hydration of the CEAs, thereby impacting the development of the internal structure and microcracks.

4.5. Pore Structures

The porosity and pore structure characteristics have a significant effect on the autogenous shrinkage, strength, and durability of HSC/UHSC. According to the capillary tension theory [91,92,93,94,95], when a liquid phase is present in the pores, a shrinkage stress is generated on the liquid–solid interface of the matrix due to capillary pressure. Furthermore, the shrinkage stress rapidly increases as the pore size decreases. Studies by Mehta et al. [84] and Li et al. [85] have indicated a positive correlation between the volume ratio of pores within the range of 5–50 nm and the autogenous shrinkage of cement paste. Research conducted by Wang et al. [87] confirmed that the micromechanical properties of C(-A)-S-H are mainly influenced by the pore structure. Based on these findings, research conducted by Shen et al. [24] demonstrated that adding 10% CSA–CaO composite type EAs to UHPC did not significantly change the total porosity but led to a significant decrease in the quantity of fine pores within the 5–50 nm range and an increase in the quantity of larger pores above 100 nm, as shown in Figure 11. Chen also confirmed in his study on the pore structure characteristics of UHPC paste, that the addition of 2% CaO type EAs increased the quantity of larger pores above 100 nm, resulting in a decrease in strength and elastic modulus [96]. Therefore, the application of CEAs can alter the pore distribution characteristics of HSC/UHSC, reducing the quantity of harmless or less harmful pores below 50 nm [88,97]. This effectively compensates for autogenous shrinkage but increases the quantity and average pore size of harmful pores, thus adversely affecting the development of mechanical properties.

Based on the analysis in this chapter, it is known that the use of CEAs in low water–binder ratio HSC/UHSC systems has an influence on the hydration process of binders, thereby altering the microstructure and pore distribution characteristics of HSC/UHSC. While playing a role in compensating for autogenous shrinkage, CEAs can have an adverse effect on the mechanical properties, such as compressive strength, flexural strength, and elastic modulus of HSC/UHSC to some extent. However, it is important to emphasize that the impact on the performance of HSC/UHSC is directly related to the type and dosage of CEAs. Therefore, in practical engineering applications, the reasonable selection of the type and the scientific design of the dosage of CEAs are significant in optimizing the autogenous shrinkage compensation effects and reducing the adverse effects on other properties. It is the key to achieving the large-scale application of CEAs in HSC/UHSC.

5. Delayed Expansion of CEAs in HSC/UHSC

To scientifically, safely, and efficiently exert the shrinkage compensation effects of CEAs, extensive research on the dosage of CEAs has been conducted in recent years [24,98,99]. These studies showed that the shrinkage compensation effects of CEAs correlated positively with their incremental addition. While this finding suggested that increasing the dosage of CEAs could be a conventional means of improving shrinkage compensation effects, some studies revealed that this strategy brought about a destructive impact on HSC/UHSC due to delayed expansion when an excessive ratio of CEAs were used. Figure 12 illustrates the typical delayed expansion phenomena caused by CEAs. The left side of the figure shows the measurement results of expansion strain, while the right side displays the corresponding expansion cracks.

5.1. Delayed Expansion Mechanism of CEAs

The expansion mechanism of CEAs is not yet fully understood. Previous studies have established that the amount of water required for the full hydration of cement is about 20–22 wt% [100,101], and more water is needed when CEAs are added in a low water–binder ratio system (HSC/UHSC). Therefore, Kaku et al. considered that, in cases of extremely low water–binder ratios (16.5%), large amounts of unreacted EA remained in the early stage of hydration, and then during the long-term curing, the unhydrated EAs absorbed water from aggregates and the hydration continued, resulting in expansion stress and cracks within the hardened concrete [9,67]. Similarly, Liu et al. [83] described the delayed expansion phenomenon of CaO type EAs, which they attributed to the formation of CH within the hardened cement paste during the long-term curing stage [102]. Elsewhere, Morioka et al. [102] observed the long-term, continuous expansion in low water–binder ratio mortar, which was also attributed to the ongoing hydration of unreacted EA particles within the mortar in the early stage. Based on this finding, Kaku et al. [9] proposed that CEAs with larger specific surface areas had a relatively high hydration rate compared to those with smaller surface areas, which suggested that they could fully react in cases of low water–binder ratios, which in turn might to some extent avoid the problem of delayed expansion.

In contrast, Hirao [103] considered that the delayed expansion of CSA type EAs with AFt as the main expansion source was similar to that of delayed ettringite formation (DEF), that is to say that the early AFt generated by the hydration of CSA type EAs became decomposed by heat curing in the early stage, and reformed again when the temperature returned to normal during long-term curing, which caused delayed expansion. This point has been the subject of discussions among various researchers in the past, who found that the conditions for delayed expansion of CSA type EAs and DEF to occur were not entirely consistent, concluding that it could not be confirmed whether the delayed expansion mechanism of CSA type EAs was the same as that of DEF [104].

In the present authors’ view, the delayed expansion mechanism of CEAs cannot be fully explained by a single theory. Instead, they believe that it is more reasonable to consider, in the case of normal temperature curing, that the remaining CEAs have not achieved full hydration within the early stage, causing delayed expansion from slow hydration after hardening. While they consider this to apply to both CaO and CSA type EAs, the delayed expansion mechanism in the case of CSA type EAs under early thermal curing may in their view be more similar to that of DEF expansion.

5.2. Delayed Expansion Conditions of CEAs

Some studies have reported cases of unexplained abnormal expansion when CEAs were used in HSC/UHSC. According to Maruyama and Sato [21], when the water–binder ratio was 15%, and the unit dosage of CSA type EAs was 35 kg, rapid secondary expansion occurred after 100 days of sealed curing. Meanwhile, Niximoto et al. [105] recounted that when the water–binder ratio was 16% and the unit dosage of CSA type EAs was 30 kg, the tendency towards re-expansion also appeared after 60 days of sealed curing. Compared to the sealed curing conditions applied in the above two studies, Zhang et al. [98] reported that when the water–binder ratio was 16.5% and the unit dosage of CSA type EAs was 70 kg, significant secondary expansion-induced damage was observed in the specimen after it was switched from sealed to air curing after 7 days.

Table 3. Research cases on delayed expansion failure of HSC/UHSC using CEAs.

W/B Ratio	Type	Amount	Curing Conditions	Ref.
0.165	CaO, CSA	40 kg/m3	(a) 20 °C constant temperature, sealed curing; (b) 20 °C constant temperature, sealed curing for 7 days →20 °C air curing;	[9]
0.15	CSA	35 kg/m3	(a) 20 °C constant temperature, sealed curing; (b) 40 °C constant temperature for 7 days →20 °C constant temperature, sealed curing;	[21]
0.18	CaO	80 kg/m3	20 °C constant temperature, sealed curing;	[96]
0.25	CSA	30 kg/m3	60 °C constant temperature curing for 8 h→ 20 °C constant temperature, sealed curing;	[61]
0.15	CSA	40 kg/m3	80 °C constant temperature curing for 48 h →20 °C constant temperature, sealed curing for 7 days →20 °C 60% RH curing;	[72]
0.165	CSA	70 kg/m3	(a) 20 °C constant temperature, sealed curing for 7 days →20 °C 60% RH curing; (b) 20 °C constant temperature, sealed curing for 7 days →20 °C 100% RH curing; (c) 20 °C constant temperature, sealed curing for 7 days →20 °C water curing;	[98]
0.16	CSA	30 kg/m3	60 °C constant temperature curing for 8 h →20 °C constant temperature, sealed curing;	[105]

below summarizes the findings of the research on delayed expansion caused by CEAs in HSC/UHSC investigated in the present study.

Table 3 thus shows that:

(1)

The problem of delayed expansion occurred in UHSC with extremely low water–binder ratios.

(2)

When delayed expansion occurred, an excessive dosage of CEAs had essentially been used compared to the recommended dosage of 2–4% [106,107].

(3)

When the content in CEAs was high, delayed expansion-induced failure could occur regardless of the type of EAs.

(4)

Irrespective of the early curing conditions, namely whether room temperature or heat curing was applied, cases of delayed expansion occurred.

(5)

Even with a constant EA dosage, the incidences in delayed expansion did not directly correlate with the curing conditions (temperature and/or humidity).

From the five points above, it proved difficult in the present study to identify and determine a unifying basis for the expansion characteristics and dosage conditions of CEAs.

To date, the mechanism of delayed expansion of CEAs has not been clarified nor has a consensus been formed on the subject, while the conditions for its occurrence have yet to be fully determined. Therefore, careful consideration should continue to be afforded towards improving the shrinkage compensation effects to increase the added amounts of CEAs. In addition, most of the existing research on the delayed expansion of CEAs has seldom considered the influence of constraints on expansion behavior [39,71,82,94], even though in any actual engineering context, concrete is invariably subject to conditions of internal and external constraints.

6. Outlook

As autogenous shrinkage compensation admixtures for HSC/UHSC, CEAs are the main products backed by relatively mature research and benefiting from industrial application technology. However, the implementation methodology of adding CEAs to HSC continues to face major problems such as rapid expansion and development in the early stage of hydration, insufficient compensation effects in the later stage, and poor stability of hydration products, high maintenance and curing requirements, and difficult control of expansion properties. Thus, the engineering application of CEAs remains limited.

6.1. Mg–Ca Composite Type EAs

In recent years, as a new type of shrinkage compensation product, MgO-based EAs have been extensively studied in performance evaluation and production process design [108,109,110,111,112]. However, due to some disadvantages of MgO-based EAs, such as the shrinkage compensation performance and the calcination activity being difficult to control [113], the production cost is high, and there are certain defects in the actual application. The hydration rate of MgO-based EAs is slower than that of CEAs, and their reactivity is easier to control. Although the shrinkage compensation effect of MgO-based EAs on HSC/UHSC in the early stage is relatively low, long-term hydration and the continuous development of shrinkage compensation have been shown to be able to be maintained [114]. Based on this characteristic, some researchers proposed a new type of Ca–Mg composite, type EA, which uses CaO components to achieve early expansion, uses high-activity MgO components to achieve mid-term expansion, and uses low-activity MgO components to achieve late expansion, therefore realizing the shrinkage compensating effects in the whole process [115,116,117,118].

6.2. Expansive Hydraulic Cement

With the diversified development of raw materials’ composition in HSC/UHSC, the interaction between CEAs and mineral admixtures in HSC/UHSC should also be gradually valued and deeply studied. R. Talero having demonstrated and experimentally verified that the reactive alumina (Al₂O₃^r− or Al tetra- and/or penta-coordinated [119]) from natural pozzolans (with natural pozzolan from the Canary Islands—Spain [120,121,122], Al₂O₃^r− content 11.41% [123]) and artificial:

-

fly ash type, siliceous in nature, or V type [124,125,126,127,128,129] or Class F [124,125,126,127,128,129] (Al₂O₃^r−: 13.60%) [123],

-

metakaolin type (MK = thermally activated clay, Al₂O₃^r−: 14.86% [123]),

• which, replacing Portland Cement (PC) up to 40% by mass, not only manage to form all of them rapid forming ettringite (ett-rf) [120,121,122,123,124], being in plaster-bearing solution [130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146], but, in addition, said Al₂O₃^r− content of the MK used (its original kaolin doped with 50% quartz) stimulates it to hydrate to the C₃A (%) content of the PC fraction with which it was mixed, at the same rapid speed as its Al₂O₃^r−. That is, C₃A (%) content of said PC fraction does not form slow forming ettringite (ett-lf) [133,134,135,136,137,138,139,140,141,142,143,144], as when it is without MK, but rather ett-rf [133,134,135,136,137,138,139,140,141,142,143,144] also like that of its Al₂O₃^r−.

This stimulation of rapid sulfatic hydration of the C₃A of the PC is produced, according to R. Talero, by direct [135,136], non-direct [147] and indirect [135,136,137,138,139,140,141,142,143,144] routes above all and very especially, considering it to be more specific than generic because their corresponding C₃S or C₂S contents were not stimulated to hydrate at the same rate as C₃A under the same sulfatic circumstances: being both, Al₂O₃^r− and C₃A, in a common plaster-bearing solution. And to such an extent that the Al₂O₃^r− content of the MK stimulates the C₃A content of the PC with which it was mixed to hydrate more and more quickly, that the resulting final expansion must be regarded as being more synergic than additive, and, in any case, being much greater than that of both, the Al₂O₃^r− of the MK and the C₃A (%) of the PC, would have originated separately in the same plaster-bearing solution, which is why R. Talero called it the Expansive Synergic Effect (ESE) [137,138,139]. For this purpose, it is necessary to remember that the molar volume ratio of C₃A and of ett-lf or AFt phase is 8.0 (actual value [148]), while that of Al₂O₃^r− is 28.3 (estimated value) when considering that its molar volume is that of g-Al₂O₃ [149].

In addition, and as a consequence of its very different formation rate, the size of the ett-lf acicular prismatic crystals is ten times larger than those of ett-rf because their precipitation speed is ten times lower, as logical [150]. All of this justifies, therefore, that the 2nd conclusion of reference [141] was the following: “The 15.05% gypsum amount added to each PC with MK did not behave as aggressive but as setting regulation or setting control. This gypsum amount could be considered as being scarcely lower than the optimum amount for such a goal, determined using the ASTM C452-68 method [151] adapted by R. Talero” (for this other purpose: to dose natural gypsum stone type setting regulation for different cement types [126,152,153]). And in the same way and with the same method, another different amount of excess plaster can also be dosed to create the necessary and precise amount of expansion:

(1)

That counteracts its own autogenous shrinkage without said excess of plaster, thus giving rise to a “hydraulic cement without shrinkage”, and for that of an HSC/UHSC, K type, M type or S type expansive cements also only that its autogenous shrinkage will be different, of course, so your amount of excess plaster needed will be different as well, or

(2)

That it is greater even to the point of being also considered “expansive hydraulic cement” according to the ASTM C 845-90 standard [152], which, in any case, of the two, may also be considered CEA, different from the K, M and S types described above and, in addition, much more economical because the thermal activation temperature of the original kaolin or matrix kaolin is approximately half of the clinkerization temperature for any of the latter three. And if the clay used is a random mixture of illite, montmorillonite and kaolinite, which is the most common, whose kaolinite content is also <40% and even insignificant or testimonial [154], instead of using kaolin alone, its economic cost will be significantly even less. The only requirement will be that the Al₂O₃^r− content of said random mixture must be >8% if all its Al has coordination 4, and >10% if the coordination of its Al is also a random mixture of 4 and 5. Or, for the contrary,

(3)

That it does not have any excess amount of gypsum but only that of the OPC with which it was mixed to be a pozzolanic cement type IV/A or IV/B of the EN 197-1:2011 standard [126]. But, in any case, the higher Al₂O₃^r− (%) content of the thermally activated clay and C₃A (%) content of the OPC are the more expansive the resulting expansive hydraulic cement will be and the more economical it will be as well because, in addition, a greater gypsum amount will be required like setting regulator as it is the most economical cement component of all in Spain and in many other Mediterranean countries.

In contrast, the corresponding reactive silica (SiO₂^r−) content of the MK did not stimulate hydration equal to the C₃A (%) content but was instead quite the opposite: it made it difficult even to the point of preventing it, with hydration time and SiO₂^r− content [155] of the pozzolan enough that, based on the behavior of said MK, it did not have it: 38.30% only [140] (>25% [126]). On the other hand, SF and diatomite did: 88.5% and 89.2%, respectively [156,157,158].

7. Conclusions

This research aims to analyze and summarize the research progress and application status of CAEs in the control of autogenous shrinkage of HSC/UHSC. The expansion mechanism, factors influencing expansion, autogenous shrinkage compensation mechanism of CEAs, and the effects of CEAs on the performance of HSC/UHSC were comprehensively discussed. Additionally, the phenomenon of delayed expansion of CEAs and future research directions were also discussed. Based on the analysis, the main conclusions are summarized as follows:

(1)

The addition of CEAs is positively correlated with the compensation rate for autogenous shrinkage of HSC/UHSC within a certain range. Within the data range investigated in this study, incorporating more than 4% of a calcium-based EA can nearly achieve zero shrinkage or slight expansion in HSC/UHSC.

(2)

The addition of CEAs leads to a significant decrease in the compressive strength (over 15% in this research), flexural strength, and elastic modulus due to the adverse effect on the hydration process and pore structures of HSC/UHSC. In engineering applications, it is necessary to comprehensively consider the balance between the compensation effect of autogenous shrinkage and the negative influence on the mechanical properties of incorporating CEAs.

(3)

Increasing the amount of CEA can lead to the risk of delayed expansion cracking in concrete, regardless of curing conditions (temperature and humidity). Additionally, the mechanism, occurrence conditions, and preventive measures of delayed expansion remain unclear and require further study.

(4)

In addition to the CEA, K, M and S types, there is another one, based on the optimal mixture of OPC, thermally activated clay and gypsum that is much more economical and easier to dose and implement in HSC/UHSC.

In general, in the formulation design stage of HSC/UHSC, it is necessary to conduct a reasonable design of the EAs’ use scheme according to the actual engineering conditions. It is furthermore imperative to ensure that the purpose of effectively controlling autogenous shrinking cracks is achieved, while at the same time ensuring that any other damage is avoided, to achieve a scientific and reasonable application of CEAs.

The key issues in fully exploiting the shrinkage compensation effect of CEAs are the rational design of application methods and the appropriate control of their expansion performance. Firstly, an appropriate dosage is the fundamental measure to control expansion, ensuring shrinkage compensation while preventing damage caused by excessive expansion. Secondly, proper curing conditions are necessary as the hydration of CEAs requires a significant amount of water. Superabsorbent polymers (SAPs), used as internal curing agents, can deliver internal curing water and maintain internal relative humidity [159,160]. When used in conjunction with CEAs, they can control the expansion behavior to some extent [161,162,163,164]. Thirdly, the combination of CaO type and CSA type EAs can fully exploit their respective characteristics at different stages of hydration, providing an effective means to regulate the expansion performance of CEAs [24]. Fourthly, the composite use of CEAs with shrinkage-reducing admixture (SRA) [15,96], MgO-based EAs [108,165], and other similar admixtures represent an alternative to achieving the expression and control of expansion performance.