1. Introduction
Cracks and voids are primary types of damage in concrete structures that can weaken their load-bearing capacity and integrity, potentially resulting in structural failure [
1]. Various types of repair materials are available and, in order to identify more effective and functional repair materials, in-depth research and analysis are required.
As mentioned in EN 1504-5 [
2], crack repair materials used for filling cracks, voids, and interstices can be classified into three kinds: force-transmitting (type F), ductile (type D), and swelling-fitted (type S) injection materials. From the chemical reaction aspect, the injection products can be categorized in terms of the use of reactive polymer (P) or hydraulic (H) binders. Epoxy resin is the most widely used injection product in the market. Issa and Debs [
3] conducted a study focused on repairing cracks in concrete using epoxy resin. They showed that, when cracks occurred, the compressive strength of the concrete decreased by 40.93%; meanwhile, after repair with epoxy resin, the strength reduction was restored to only 8.23%. Ekenel and Myers [
4] used both carbon-fiber-reinforced polymer (CFRP) and epoxy resin fibers for crack strengthening. Their results showed that epoxy resin alone only increased the initial stiffness; however, the CFRP fibers not only enhanced the initial stiffness but also contributed to the ultimate strength. If we compare epoxy resin with cement-based repair materials, the epoxy resin has some drawbacks: (1) Higher Cost: epoxy resin is relatively expensive, especially compared to ordinary cement-based repair materials, increasing construction costs. (2) Higher Thermal Expansion Coefficient [
5]: Epoxy resin has a much higher thermal expansion coefficient than concrete. When temperature changes occur, stress may develop between the two materials, leading to delamination or cracking. (3) Poor UV Resistance [
6]: Epoxy resin is prone to discoloration or degradation when exposed to ultraviolet (UV) light for long periods, affecting both its appearance and performance. This is particularly important for outdoor applications. (4) Low Water Vapor Permeability [
7]: Epoxy resin has poor breathability, which may lead to moisture being trapped inside the substrate (e.g., concrete). This can weaken its adhesion performance or cause blistering. (5) Reduced Adhesion on Moist Substrates [
8]: while epoxy resin has strong adhesive properties, its bonding strength decreases on damp or water-saturated substrates, potentially affecting the effectiveness of the repair. (6) Strict Application Requirements [
8]: epoxy resin requires dry and controlled environmental conditions (e.g., proper temperature and humidity) for proper curing, making the application process more demanding. (7) Environmental and Health Concerns: During curing, epoxy resin may release volatile organic compounds (VOCs). Some epoxy resins can be irritating to the skin and respiratory system, requiring good ventilation and proper personal protective measures during their application. (8) Elasticity and Brittleness Issues: Epoxy resin is relatively rigid and lacks the toughness of cement-based materials. This may lead to stress concentration under certain conditions, making it prone to cracking, especially in structures subjected to dynamic loads. Due to these drawbacks, cement-based materials which can avoid the drawbacks of epoxy resin are considered good alternatives for use as repair materials.
Polymer Cement Composite (PCC) is used for concrete structure repair and bridge deck rehabilitation in engineering applications. Allahvirdizadeh et al. [
9] mentioned that the annual maintenance cost of concrete coated with an applied isolation coating is higher than that of polymer concrete, as the isolation coating may need to be reapplied multiple times within the same service life. This is likely because the service life of polymer repairs is much longer than that of traditional concrete repair methods. Teng et al. [
10] mentioned that when the TSL brushing thickness is greater than or equal to 4.2 mm and the brushing frequency exceeds 3 times, a crack repair effect is observed. This is mainly because the TSL completely fills the cracks and forms a bonding effect at the original crack sites. The enhancement of this bonding effect leads to the repair effect. ElGawady et al. [
11] have mentioned that cement-based crack injection products could increase the interfacial shear bond strength of multi-layer stone walls by 25 to 40 times, with the volume ratio of each component influencing the improvement in shear performance. Minoru et al. [
12] found that using cementitious materials for repair enhanced the bonds between cracks and contributed to improvements in compressive strength and elastic modulus. Perret et al. [
13] have demonstrated that using high early-strength cement and partially replacing it with silica fume yielded good economic benefits. Perret et al. [
14] also demonstrated that the interactions between cement and admixtures influenced the rheological behavior of grout as well as its final setting time. Khayat et al. [
15] found that, when the water-to-binder ratio was 0.6 and a high-range water reducer was added at 2%, the cement grout achieved a balance between rheological properties and mechanical performance. Song et al. [
16] pointed out that polymer-modified materials improve flexural strength and reduce shrinkage. The addition of fibers and the application of an interface agent enhance the bond capability between cement-based repair materials and the concrete. Beril et al. [
17] pointed out that there are few established standards or comprehensive methods for the evaluation of injection grouting materials. It is evident that systematic research is needed in terms of the workability and performance characteristics, testing methods, as well as preparation and curing conditions for these materials.
Zhang et al. [
18] found that sulfoaluminate cement (SAC) mortar reduces drying shrinkage and chloride ion permeability compared to Portland cement. The early formation of ettringite provides nucleation sites, promoting hydration and benefiting the early strength development of SAC, while negatively affecting strength at a later stage. Kim et al. [
19] discovered that including SAC accelerated the early hydration of Portland cement. The combination of Portland cement and SAC improved the rapid development of strength, reduced pH levels, and enhanced the crack-healing capability. Huo et al. [
20] demonstrated that SAC is a low-carbon cementitious material, and the addition of CaO has a significant benefit in improving the compressive strength of SAC composites. Khalil et al. [
21] found that, when Portland cement and SAC were mixed, they exhibited a synergistic effect. Compared to 100% Portland cement and 100% SAC paste, the mixture produced 57% and 45% more ettringite, respectively. Their results indicated that the amount of SAC influenced the reactivity of the Portland cement/SAC blend. Pimraksa and Chindaprasirt [
22] have demonstrated that SAC possesses excellent properties, as the calcium sulfoaluminate helped to mitigate cracks caused by the shrinkage of Portland cement. The formation of ettringite allows for rapid setting and early strength development were achieved quickly, with rapid setting being a crucial characteristic in repair work.
Lin [
23] has explained that early curing of reactive ultra-fine fly ash (RUFA) has significant benefits in filling the pores in the concrete structure and accelerating the formation of ettringite. RUFA improved the bond strength of concrete, enhanced both early and late compressive strength, reduced drying shrinkage, and increased durability. It has also been proven that RUFA enhances adhesion, promotes the pozzolanic reaction, and forms C-S-H gel. This effectively filled the pore structure, increasing density and compressive strength while reducing drying shrinkage, improving workability, and lowering permeability. Krishnaraj and Ravichandran [
24] found that adding RUFA to Portland cement improved the performance of the specimens; specifically, the addition of 45% RUFA in construction applications enhanced the overall engineering quality. Maeijer et al. [
25] emphasized that refining the fineness of fly ash improved the workability of the mixture, and partially replacing cement with RUFA enhanced its mechanical strength. Moghaddam et al. [
26] found that finer fly ash paste enhanced the release of hydration heat, resulting in a denser microstructure. As the fineness of the fly ash increased, it improved the fluidity and compressive strength of the cement paste. Delihowski et al. [
27] discovered that the use of finer fly ash increases reactivity, better flowability, and viscosity. Smaller particle sizes lead to higher strength, forming a dense and uniform microstructure, offering advantages in terms of cost, workability, and sustainability. Finer particles will enhance the pozzolanic reactivity, as their larger specific surface area causes particle collisions to happen more frequently, such that the chemical reaction rate increases. Another strategy to enhance the mechanical and durability performance utilizes the Fuller–Thompson formula for particle size distribution [
28].
The main objective of this study was to identify a suitable mixture based on the EN 1504-5 standard. Portland cement (type I), SAC, and RUFA are adopted to produce cement-based injection material. SAC shortens the setting time, increases early strength, and reduces shrinkage. However, when SAC is used together with Portland cement (type I), the interaction between the two cements lead to a decrease in strength. To address the issues caused by the addition of SAC, RUFA is selected because of its strong pozzolanic reactivity and its ability to enhance late-stage strength. Additionally, the spherical structure of RUFA improves the compactness of the paste and provides better workability. For this purpose, injection grout was prepared using SAC (WSAC = 0%, 15%, 35%, 55%, 75%, 95%), Portland cement (type I) (WC = 0%, 20%, 40%, 60%, 80%, 95%), and RUFA (WRUFA = 5%). Based on the requirements of the standard in terms of injection time, bleeding, volume change, and splitting strength, mixtures that met all the conditions were selected. Further tests assessing setting time, compressive strength, slant shear strength, and mortar length change were conducted to evaluate the mixtures and determine those with optimal performance.