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Article

Study on Properties and Hydration Mechanism of Polymer-Modified High-Belite Sulfoaluminate Cement Repair Mortar

by
Liang Wang
1,
Yaning Wu
1,
Chao Guo
2,
Yuanxin Guo
1,
Gongbing Yue
1,* and
Qiuyi Li
1,*
1
College of Civil Engineering & Architecture, Qingdao Agricultural University, Qingdao 266109, China
2
College of Civil Engineering, Qingdao University of Technology, Qingdao 266033, China
*
Authors to whom correspondence should be addressed.
Buildings 2026, 16(12), 2352; https://doi.org/10.3390/buildings16122352
Submission received: 9 May 2026 / Revised: 9 June 2026 / Accepted: 10 June 2026 / Published: 12 June 2026
(This article belongs to the Special Issue Sustainable Approaches to Building Repair—2nd Edition)
Browse Figures
Versions Notes

Abstract

In this study, the rapidly setting and hardening high-belite sulfoaluminate cement (HBSAC) is used as the cementitious material, with natural river sand as the fine aggregate, and a high-performance repair mortar is prepared through the synergistic use of different polymers and admixtures. The influences of two polymers (VAE and HPMC) on the working performance, mechanical properties, and hydration characteristics of HBSAC mortars are systematically studied. The results showed that the two polymers had a significant improvement effect on the setting time, mortar flowability, and water retention rate of HBSAC mortar. Among them, VAE had a significant effect on the mortar flowability, and a 5% content could increase the flowability of HBSAC mortar by 29.8%. HPMC has a significant improvement effect on setting time and water retention rate; at 0.1% content, it can delay the initial setting time by 6.5 min and achieve a water retention rate of over 90%. As the polymer to binder ratio increases, both polymers, except for 2.5% VAE, which can slightly improve the flexural strength of mortar, will reduce the flexural and compressive strength of mortar, with VAE causing greater damage to strength. On the contrary, the polymer significantly enhanced the bond strength of the mortar. Compared with the cement control group, the 28 d bond strength of 5% VAE and 0.1% HPMC groups increased by 56.7% and 15.1%, respectively. Moreover, the addition of polymers delayed the occurrence of the exothermic peaks of HBSAC dissolution and ettringite formation, but the total amount of hydration heat released within 48 h was higher than that of pure cement. The diffraction peaks of AFt in the hydration products of VAE-HBSAC paste at 3d and 28d showed significant enhancement, and the peak intensity increased with higher doping levels, while the diffraction peak intensity of C2S showed a certain decrease. The polymer significantly increased the weight loss peak intensity and mass loss after heating of AFt, AH3, AFm, and C-S-H gel. The SEM images indicate that VAE can form a mesh on the surface of hydration products and refine the crystal size of AFt; HPMC wraps more flocculent substances around the hydration products, thereby improving the compactness of paste. This study can provide scientific reference for improving the performance and promoting the practical application of high-performance rapid repair mortar for concrete structure damage.

1. Introduction

Subjected to the coupled detrimental effects of long-term seawater immersion, sulfate attack, chloride ion penetration, and wet–dry cycles, marine concrete commonly suffers from durability defects such as surface cracking, mortar spalling, and steel reinforcement corrosion, which severely compromise the long-term service safety of marine infrastructures [1,2,3]. The repair of marine concrete structures is frequently constrained by harsh marine environments, limited construction time windows, and complex construction conditions, thereby requiring efficient and timely repair measures. As an essential repairing material for damaged concrete, repair mortar is required to possess comprehensive performances including rapid setting and hardening, high early-age strength, superior interfacial bonding capability, favorable volume stability, and excellent durability [4,5]. Nevertheless, conventional cement repair mortars hardly satisfy the stringent engineering requirements for marine restoration. It is urgent to develop repair mortars with high corrosion resistance and durable performance for marine engineering. In the current preparation of repair mortar, the synergistic regulation of polymer modification and water retention/thickening components is key to enhancing performance. The toughness, adhesion, and durability are primarily improved by incorporating polymers [6], high-strength fibers [7,8], different mineral admixtures [9,10], and various functional additives [11]. Research on polymer-modified repair mortar has reached a relatively mature stage, involving a wide range of polymer types, such as re-dispersible latex powder, styrene–butadiene–acrylic, epoxy resin emulsion, graphene oxide, etc. [12,13,14].
J. Zhang et al. [15] used styrene acrylate copolymer (SAE) as a polymer modifier for repairing mortar and found that with the increase in the polymer to binder ratio, the flexural and compressive strength of the mortar showed a trend of first decreasing and then increasing, and the bonding strength was significantly improved. C. Shi et al. [16] found that SAE powder has a significant impact on the hydration process of cement mortar. The addition of SAE promotes the formation of AFt, while it has an inhibitory effect on the formation of calcium hydroxide. N. Tarannum et al. [17] found that polyvinyl acetate (PVAC) with a content below 13% can effectively delay the setting time of cement, and the bonding strength of repair mortar shows a continuous enhancement with the increase in polymer content. C. Xu et al. [18] pointed out that vinyl acetate ethylene powder (VAE) has a great improvement effect on the workability of concrete repair mortar, and can increase the bonding strength of mortar at a dosage below 2%. Z. Wang et al. [19] found that using VAE to modify cement mortar can reduce the porosity of the mortar by more than 5 times, but it can cause an increase in pore diameter. R. Wang et al. [20] found that the improvement effect of VAE powder on shrinkage performance is more significant. In addition, polymer latex powder can also improve the durability properties of cement mortar [21], such as resistance to chloride ion penetration, carbonation, and frost resistance, mainly due to the optimization of mortar pore structure. Y. Li et al. [22] indicated that hydroxypropyl methylcellulose (HPMC) can significantly enhance the density, consistency, water retention rate, and setting time of mortar, but it has a certain degradation effect on the mechanical strength. C. Guo [23] observed the effects of HEMC and HPMC on the early morphology of AFt through SEM, and found that the presence of HEMC promoted the AFt morphology to become short rod-shaped, while HPMC promoted the AFt morphology to become needle-shaped. Moreover, the epoxy resin plays a prominent role in improving the mechanical and bonding properties of polymer repair mortar [24,25]. W. Huang et al. [26] found that with a 5% content of waterborne epoxy resin, the interfacial flexural strength, direct shear strength, and interfacial tensile strength of mortar increased by 16.7%, 29.8%, and 6.9%, respectively. W. Xia [27] found that epoxy resin can delay the setting time by wrapping ordinary Portland cement (OPC) particles with a resin film and increasing intermolecular repulsion before the cement sets. On the whole, polymer-modified repair mortars are mainly developed with ordinary Portland cement (OPC) as the base system, alongside ordinary sulfoaluminate cement (SAC), magnesium phosphate cement (MPC) and other series of repair mortars. Traditional OPC-based repair mortars suffer from drawbacks such as slow setting, low early strength, weak bonding strength with old concrete substrates, large drying shrinkage and high cracking tendency [28]. Although SAC-based repair mortars feature rapid hardening and high early strength, they experience later strength regression and weak alkalinity, which is unfavorable for steel reinforcement passivation [29]. Magnesium phosphate cement (MPC) is limited by poor water resistance and high production cost [30]. Alkali-activated materials exhibit large drying shrinkage and poor volume compatibility, making it difficult to meet the dual requirements of rapid construction and long-term service performance [31]. As a new low-carbon cementitious material, high-belite sulfoaluminate cement (HBSAC) integrates superior properties, including rapid setting and hardening, high early strength [32], slight expansion, and sulfate corrosion resistance, with belite minerals, which have continuous later strength development, excellent volume stability and low drying shrinkage [33,34,35]. Its clinker is mainly composed of calcium sulfoaluminate and belite. In the early hydration stage, AFt and aluminum gel are rapidly generated to form a dense hydration structure, ensuring excellent early strength. In the later stage, continuous hydration of belite compensates for the deficiency of later strength of conventional sulfoaluminate cement [36]. Furthermore, based on damage mechanics models and numerical simulations [37], the coupling mechanism between the internal and external stresses of the repair mortar interface can be elucidated [38]. However, although some progress has been made in the research of polymer-modified cement-based materials, there is still a lack of systematic exploration of the synergistic mechanism of VAE- and HPMC-modified HBSAC in repairing mortar, as well as its hydration regulation law and engineering adaptability under fast hardening repair conditions. This study clarified the influence of different polymer dosages on hydration process, mechanical properties, and interfacial bonding properties through multi-scale characterization and performance testing, filling the gap in relevant mechanism and engineering adaptability data.
Therefore, in this study, the rapidly setting and hardening high-belite sulfoaluminate cement (HBSAC) is used as the cementitious material, with high-quality natural river sand as the fine aggregate. By incorporating vinyl acetate–ethylene copolymer latex powder (VAE) and hydroxypropyl methylcellulose (HPMC), a high-performance HBSAC-based rapid repair mortar was prepared. The effects of VAE and HPMC on the setting time, workability, mechanical properties, bonding performance, and hydration characteristics of the repair mortar were systematically analyzed. The hydration characteristics and micro-structure evolution mechanisms were revealed, improving the research framework in the field of rapid repair mortars. This provides a theoretical basis and scientific support for the promotion and application of high-performance low-carbon rapid repair mortars for marine concrete structure damage.

2. Experimental Procedure

2.1. Raw Materials

2.1.1. Cementitious Material

In this study, high-belite sulfoaluminate cement (HBSAC) and ordinary Portland cement (OPC) are used, both with a strength grade of 42.5. The HBSAC was provided from Tangshan Polar Bear Building Materials Co., Ltd., Tangshan, China, while the OPC was obtained from Shandong Shanlv Cement Co., Ltd., Qingdao, China. The physical performance parameters of the two series of cements are presented in Table 1, and the XRF chemical composition is detailed in Table 2.

2.1.2. Polymers

Polymers can improve the basic properties of cement, can increase the bonding and durability of cement-based materials, and are good modifiers for preparing repair mortar. In this study, two polymers, vinyl acetate–ethylene copolymer rubber powder (VAE) and hydroxypropyl methylcellulose ether (HPMC), were selected to enhance the performance of repair mortar. The SEM micro-morphology images of VAE and HPMC are shown in Figure 1a,b. The main performance indexes of VAE and HPMC are given in Table 3 and Table 4, respectively.

2.1.3. Fine Aggregate

The repair mortar has high requirements for sand, and a suitable particle size range will improve the repair effect of the repair mortar. In this study, natural river sand was used as fine aggregate. The fine modulus is 2.5, the bulk density is 1450/kg·m−3 the apparent density is 2600/kg·m−3, the micro-powder content is 1.1%, the clay content is 0.7%, and the crush index is 7.2%, which meets the requirement standards for medium sand.

2.1.4. Water-Reducing Admixture

The high-performance polycarboxylate superplasticizer powder was prepared by Shandong Academy of Building Science; the content is 1.5% of the cement content, with a water reduction rate of 40–45% and a solid content of ≥98%.

2.2. Mix Proportion and Preparation of Repair Mortar

In this study, to investigate the effects of VAE and HPMC on the working performance, mechanical properties, and hydration characteristics of HBSAC repair mortar, the content of VAE was set at 0%, 2.5%, 5%, 7.5%, and 10%, and the content of HPMC was set at 0%, 0.05%, 0.1%, 0.15%, and 0.2%, respectively. The working performance of HBSAC mortar under polymer action is characterized by the initial and final setting time, flowability, and water retention of mortar in the presence of different polymer monomers. We characterize the mechanical and bonding properties of HBSAC mortar under polymer action based on its flexural strength, compressive strength, and bonding strength at the curing ages of 3 d, 7 d, 14 d, and 28 d. The cement/sand ratio of HBSAC mortar is 1:3 and the water/cement ratio is set as 0.5. Based on the experimental results of workability and mechanical properties, several representative groups of VAE and HPMC contents for HBSAC mortar were selected for hydration heat, XRD, TG-DTG, FTIR, and SEM analysis, to reveal the composition of hydration products and internal pore structure characteristics of repair mortar. The detailed mix proportion ratio design is shown in Table 5.

2.3. Experimental Methods

In this study, the setting time of cement paste was tested in accordance with the standard GB/T 1346-2024 [39], “Test Methods for Water Requirement of Normal Consistency, Setting Time and Soundness of the Portland Cement”. The fluidity of mortar was determined based on GB/T 2419-2005 [40], “Method for Determination of Fluidity of Cement Mortar”. The water retention rate of mortar was measured with reference to JGJ/T 70-2009 [41], “Test Methods for Basic Properties of Building Mortar”. The specimens used for mechanical properties and microscopic testing are cured under the conditions of curing temperature of (20 ± 2) °C and relative humidity ≥95%. The flexural and compressive strengths of HBSAC mortar were tested according to GB/T 17671-2021 [42], “Test Method of Cement Mortar Strength (ISO Method)”. The bond strength of repair mortar was measured according to DL/T 5126-2001 [43], “Test Procedure for Polymer-Modified Cement Mortar”. Substrate mortar specimens with dimensions of 70 × 70 × 20 mm were prepared using ordinary Portland cement mortar with a fixed mix proportion, and cured under standard conditions for 27 days before being taken out. The specimen surface was polished with 200-mesh sandpaper, followed by soaking in water for 1 day. After the substrate specimens were taken out and air-dried to a surface-dry state, fresh cement mortar to be tested was poured onto the forming surface with a size of 40 × 40 × 10 mm. The specimens were demolded after 24 h and cured until the specified age. The specimens were taken out one day in advance, and pull-out blocks were bonded onto the mortar surface with epoxy resin adhesive, then further cured for 24 h. A universal testing machine was adopted for the bonding strength test at a constant loading rate of 5 mm/min. Loading was continued steadily until interfacial failure occurred. The ultimate load was recorded to calculate the tensile bonding strength. The hydration heat was tested by a TAM Air eight-channel microcalorimeter. The test conditions were set at a temperature of 25 °C and a water-to-cement ratio of 0.5, with a test duration of 48 h. The cumulative hydration heat release and heat release rate of cement within the specified time were recorded. XRD tests on cement hydration products were carried out using a D8 Advance X-ray polycrystalline diffractometer (Rigaku, Takatsukishi, Japan). The test samples were prepared by dehydrating and drying cement blocks, grinding them into powder with a pulverizer, and sieving through a 200-mesh sieve. During the test, the tube voltage was set to 40 kV, the tube current was set to 40 mA, and the 2θ scanning range was 5–60°. TG-DTG tests were performed with a TGA5500 thermogravimetric analyzer manufactured by TA Instruments (NETZSCH, Selb, Germany). For each test, 5–10 mg of the sample was placed in a crucible, which was gently set on the instrument stage. The temperature program was initiated when the indicator showed a stable mass reading, with the maximum temperature set at 800 °C. The FTIR tests were conducted using a Thermo Scientific Nicolet iS50 Fourier transform infrared spectrometer (Thermo Fisher Scientific, Waltham, America). Powder samples were prepared following the same method described above for conventional tablet pressing testing, with the infrared wavenumber range controlled at 400–4000 cm−1. A TESCAN MIRA LMS cold field emission scanning electron microscope (Carl Zeiss Jena, Oberkochen, Germany) was used to observe the micromorphology of HBSAC paste, hydration products and polymers. The detailed experimental procedure in this study is shown in Figure 2.

3. Results and Discussion

3.1. Setting Time of HBSAC Paste

From Figure 3a,b, it can be seen that the addition of VAE and HPMC can significantly delay the initial and final setting time of HBSAC paste. As the VAE content increases, the initial setting time of HBSAC paste continues to extend. The initial setting time of HBSAC paste is 13 min, while it is 14.5, 16.5, 17.5, and 18 min at 2.5%, 5%, 7.5%, and 10% VAE content, respectively. Compared with the control group, it is delayed by 11.5%, 26.9%, 34.6%, and 38.5%. The final setting time shows an increasing trend followed by a decreasing trend, with the most significant effect observed at a 5% VAE content. The overall effect of HPMC on delaying the setting time is better than VAE, with initial setting times of 15, 19.5, 22.5, and 23 min at 0.05%, 0.1%, 0.15%, and 0.2% HPMC content, respectively. Compared with the control group, HPMC delays the setting time by 15.4%, 50%, 73.1%, and 76.9%. The initial and final setting times of HBSAC increase with the increase in HPMC content, with a greater increase in the low dosage range. The delay effect tends to flatten under high content, which can effectively compensate for the shortcomings of rapid setting and short operable time of HBSAC paste. Figure 3c shows that VAE and HPMC can also delay the initial and final setting time of OPC, with a similar variation tendency to HBSAC, and have a relatively small effect on the difference in initial and final setting time between the two. Under the same content conditions, VAE regulates the setting time mainly through chemical actions and particle adsorption, while HPMC exerts a retarding effect by virtue of its high water retention and film-forming effect. There are obvious differences between their action mechanisms.

3.2. Workability of HBSAC Repair Mortar

As shown in Figure 4a, the flowability of HBSAC mortar increases continuously when the VAE content is 0–7.5%, and begins to decline after 7.5%, but is still higher than that of the pure HBSAC mortar group. The flowability at each content is 181, 206, 235, 237, and 230mm, which are 13.8%, 29.8%, 30.9%, and 27.1% higher than control group, respectively. A 5% VAE content has the most significant impact on the flowability of HBSAC mortar. The flowability of OPC mortar mixed with 5% VAE content is 224mm, which is 16.7% higher than that of pure OPC mortar. This improvement is significantly lower than the 29.8% of HBSAC group, indicating that VAE has a more obvious impact on the flowability of HBSAC mortar. From Figure 4b, with the increase in HPMC content, the flowability of HBSAC mortar continuously increases, but the growth rate between adjacent contents gradually decreases, and the curve tends to flatten. Compared with the pure cement group, the different contents increased by 6.1%, 13.8%, 19.9%, and 21.5%, respectively, with the largest increment observed at 0.05–0.1% content. An appropriate amount of HPMC can improve the flowability, but it cannot exert a greater effect beyond the optimal content. Similar to the VAE series, HPMC has a lower effect on improving the flowability of OPC mortar than HBSAC mortar.
From Figure 5a, VAE can effectively improve the water retention rate of HBSAC and OPC mortar. The water retention rate of HBSAC sand increases first and then decreases with the increase in VAE content. The water retention rate continues to increase in the range of 0–7.5% content, with the most significant increase in content between 2.5% and 5%, and the optimal content being 5%. Although the water retention rate decreased slightly after the dosage exceeded 7.5%, it was still higher than the blank control group. VAE can form polymer lotion in water to interweave polymer molecules and water molecules, enhancing the cohesiveness of mixing water and the adsorption capacity of mortar to water. However, excessive mixing can cause polymers to excessively wrap around cement particles and accumulate at the aggregate interface, hindering normal hydration of cement and coating of aggregates with slurry, resulting in slight bleeding and ultimately reducing water retention rate. Compared to HBSAC, VAE has a higher improvement in the water retention rate of OPC mortar, indicating that it has a more prominent effect on improving the water retention of OPC. However, it still has practical application value for HBSAC. In Figure 5b, it can be seen that HPMC can significantly improve the water retention performance, and the improvement effect is even more excellent. The water retention rate of HBSAC adhesive sand continues to increase with the increase in HPMC content, but the increase gradually slows down. A 0.1% content can make the water retention rate exceed 90%, but when the content exceeds 0.15%, the growth in the water retention rate tends to stagnate, and the modification effect basically reaches the upper limit. The increase in water retention rate of OPC mortar with HPMC is similar to that of HBSAC, indicating that HPMC is less affected by cement varieties and has a wide range of applications, making it a high-quality additive for improving the water retention of mortar.

3.3. Flexural Strength of HBSAC Repair Mortar

Figure 6 shows that with the increase in VAE content, the flexural strength of HBSAC mortar first slightly increases and then significantly decreases, with only a slight increase at a low content of 2.5%, and VAE significantly inhibits its later strength growth. The flexural strength of OPC mortar with a 5% VAE content at different curing ages decreased by 18.5%, 14.3%, 14.1%, and 11.8% compared to the blank control group. This decline is 0%, 1.5%, 2.8%, and 4% in the HBSAC mortar group. Compared to OPC mortar, VAE has a smaller negative impact on the flexural strength of HBSAC mortar. After the addition of HPMC, the flexural strength of HBSAC mortar continues to decrease, making it impossible to achieve strength improvement and hindering the development of later strength. At low content, the decrease in strength is relatively gentle. A small amount of HPMC can slightly improve the early strength of OPC mortar, but it shows a decreasing trend in the middle and later stages. The reason is that HPMC can optimize the internal structure of OPC in the early stage and enhance its integrity, but the polymer film in the later stage will weaken the interface bonding between hydration products and aggregates, resulting in strength attenuation.

3.4. Compressive Strength of HBSAC Repair Mortar

Figure 7 shows that the addition of VAE and HPMC will cause a continuous decrease in the compressive strength of HBSAC mortar at various ages with increasing content. The influences of both on compressive strength are basically consistent with the flexural strength. VAE significantly reduces the compressive strength of HBSAC at different ages within the range of 0% to 10% content, and also has a strength degradation effect on OPC mortar. However, at the same content, VAE causes significantly less damage to the compressive strength of HBSAC mortar, which is beneficial for preparing polymer-modified HBSAC repair mortar with lower strength loss. At a 5% VAE content, the compressive strengths of OPC mortar at different ages were 24.7 MPa, 28.5 MPa, 32.4 MPa, and 35.7 MPa, respectively, which were reduced by 16.3%, 16.7%, 18.2%, and 19.6% compared to the pure cement mortar group. In contrast, HBSAC mortar decreased by 12.1%, 13%, 13.8%, and 12.7% respectively. HPMC will also continue to weaken the compressive strength of HBSAC mortar, but the negative impact of the same HPMC content on the compressive strength of OPC and HBSAC mortar is similar.

3.5. Bond Strength of HBSAC Repair Mortar

Figure 8a indicates that the bond strength of HBSAC mortar at different curing ages continuously increases with the rise in VAE content, and the variation between bond strength and content presents an approximately linear relationship, revealing that VAE can steadily enhance the bond strength of HBSAC mortar. At a VAE content of 10%, the bond strengths at 3 d, 7 d, 14 d and 28 d increase by 153%, 111%, 109.9% and 106.8% respectively compared with the blank group, all more than doubling the original bond strength. At a VAE content of 5.0%, the bond strengths at 3 d, 7 d, 14 d and 28 d increase by 80%, 61.9%, 59.1% and 56.7% respectively.
As illustrated in Figure 8b, HPMC exerts a similar improvement effect on the bond strength of HBSAC mortar to VAE. With the increase in HPMC content, the variation trends of bond strength at 3 d and 7 d are roughly consistent, and those at 14 d and 28 d also follow the same pattern. Relative to the blank group sample, the bond strengths of samples with 0.05–0.2% HPMC increase by 8.9%, 22.2%, 37.8% and 53.3% at 3 d, and by 5.5%, 15.1%, 32.9% and 56.2% at 28 d for each corresponding content. The strength growth at early curing ages caused by content variation is relatively uniform; at later ages, the strengthening effect is weaker than that at early ages under low content, while the improvement amplitude of bond strength rises remarkably with the increase in content. Both polymers can improve the bond performance of HBSAC repair mortar, among which VAE exhibits a more prominent enhancement effect, making it an excellent admixture to ensure stable bonding between repair mortar and substrate.

3.6. Microscopic Properties of HBSAC Repair Mortar

3.6.1. Hydration Characteristics

From Figure 9, it can be seen that the first exothermic peak of the only cement group appeared at 0.2 h with a peak heat flow rate of 112 mW/g. After adding VAE and HPMC modifiers, the time of the first exothermic peak was delayed to 0.3 h, and the peak rate increased to 120 mW/g, indicating that the polymers slightly promoted the early hydration reaction. The second exothermic peak of the cement-only group occurred at 1.4 h with a peak rate of 45 mW/g. All modified groups showed a delayed second exothermic peak to some degree, with the 5% VAE + 0.1% HPMC group exhibiting the most significant delay, suggesting that the formation and encapsulation effect of polymer films slowed down the later hydration reaction rate. Furthermore, the total heat release of all modified groups within 48 h was higher than that of the cement group, 210 J/g. The 5% VAE + 0.1% HPMC group had a total heat release of 245 J/g, while the 10% VAE group reached the highest value at 255 J/g. This indicates that the polymers did not inhibit the final hydration degree, but it may have improved the hydration degree by enhancing the dispersion of cement particles. Overall, HPMC has a stronger inhibitory effect on the cement dissolution stage, while VAE has a more prominent blocking effect on the formation of ettringite. The total heat release of hydration showed that the heat release of HBSAC hydration was mainly concentrated in the first 10 h of the intense reaction stage. Although the polymer delayed the coagulation and ettringite formation rate, the total heat release at 48 h was higher than the benchmark group.

3.6.2. Thermogravimetric Analysis (TG/DTG)

Figure 10a,b show the TG-DTG curves of pure HBSAC hydration for 3 days and 28 days. There are three obvious weight loss peaks for HBSAC hydration products, and 80–180 °C is the strongest peak. At this temperature, the water recrystallization process of AFt and a small amount of C-S-H gel mainly occurs. The reaction rate shows that the hydration product is mainly ettringite; the main reactions occurring at 200–300 °C are dehydration of aluminum gel (Al(OH)3) and monosulfur hydrated calcium sulfoaluminate (AFm). The thermal decomposition of Ca(OH)2 mainly occurs at 600–700 °C. Comparing the two weight loss curves, it can be seen that the mass loss corresponding to the first weight loss peak increased from 13.45% to 15.7%, and the mass loss corresponding to the second weight loss peak increased from 2.55% to 2.69%. This indicates that with increasing age, the content of hydration products such as ettringite and aluminum gel increases accordingly. Figure 10c,d show the TG-DTG curves of hydration products of HBSAC with VAE. The mass loss rate corresponding to the first weight loss peak continuously increases, indicating an increase in AFt generation at the same age. In the second weight loss peak, the decomposition of (CH3COO)2Ca is superimposed. VAE contains esters of acetic acid, and the ester group can hydrolyze into CH3COO− in an alkaline environment, which reacts with Ca(OH)2 to form (CH3COO)2Ca. This substance begins to undergo thermal decomposition at around 160 °C, producing acetone and calcium carbonate. The third weight loss peak is mainly the decomposition of Ca(OH)2 and some CaCO3, and with the increase in content and age, the mass loss continues to increase, indicating that VAE promotes the hydration reaction of C2S and increases the content of Ca(OH)2 in the product.
Moreover, Figure 11 indicates the TG-DTG curve of HBSAC hydration products under the action of HPMC. Compared to Figure 10, it can be seen that with the increase in HPMC content, the 3 d and 28 d mass losses of AFt, AFm, and AH3 of mortar all increase, indicating that the addition of HPMC promotes the hydration reaction of anhydrous calcium sulfoaluminate. At the same HPMC content, with the increase in age, the content of AFt continuously increases, while the content of Ca(OH)2 decreases, indicating that HPMC is not conducive to the generation of Ca(OH)2 in the hydration process of HBSAC.

3.6.3. FTIR Analysis

VAE contains functional groups such as carboxyl (-COOH), hydroxyl (-OH) and ester groups (O-C(O)-C), which enable it to participate in cement hydration reactions. The internal environment of cement hydration is alkaline, in which ester groups undergo hydrolysis, while acid radicals can react with alkaline hydration products. HPMC mainly possesses three types of functional groups: -OH, C=O and C-O-C. The ether bond exhibits excellent hydrophilicity and can form hydrogen bonds with water molecules, which accounts for its outstanding water retention performance. As shown in Figure 12a,b, the FTIR spectra of cement incorporating 5% and 10% VAE content at 3 d and 28 d are basically consistent with those of pure cement paste, only differing in the intensity of characteristic absorption peaks. The characteristic absorption bands of VAE functional groups are mainly located in the range of 1360–4000 cm−1, which is defined as the characteristic frequency region. After VAE addition, the absorption peak intensity of ettringite (AFt) increases with content, indicating that VAE promotes the formation of AFt. In addition, only a weak absorption peak of carboxyl groups is observed in VAE-modified specimens, suggesting that COO- reacts with alkaline Ca(OH)2 to form specific salts. According to Figure 13a,b, the FTIR spectra of cement mixed with 0.1% and 0.2% HPMC content at 3 d and 28 d are almost overlapped, with only slight differences in the stretching vibration peak of -OH, and the position and quantity of characteristic peaks are roughly identical to those of pure cement paste. This indicates that HPMC has fully exerted its main functions and its ether bond structure completely disappears; the intensity of the -OH absorption peak increases with the curing age.

3.6.4. XRD Analysis

From Figure 14, it can be seen that the hydration product of HBSAC is mainly AFt, and there are also trace amounts of Ca(OH)2. According to Figure 14a, after 3 days of hydration, there are still C3A6 S _ that has not participated in hydration and C2S that has already reacted slowly. After the addition of VAE, the diffraction peak of ettringite showed a certain enhancement, and the higher the addition amount, the higher the peak. This phenomenon is more evident in the 28 d XRD pattern, indicating that VAE promotes the generation of AFt to some extent. According to Figure 14c,d, it can be seen that the diffraction peak intensity of AFt after hydration of HBSAC with HPMC for 3 days shows a trend of first increasing and then decreasing with the increase in content; 0.2% HPMC has a certain inhibitory effect on the formation of ettringite. After 28 days of hydration, the AFt diffraction peak of pure cement changed significantly, and the peak intensity was higher than that of HPMC-HBSAC, indicating that HPMC has an inhibitory effect on the formation of AFt in the later stage of HBSAC. But with the increase in HPMC dosage, the intensity of the C2S diffraction peak decreases, while the intensity of the Ca(OH)2 diffraction peak increases, indicating that HPMC promotes the hydration reaction of C2S. Figure 15, Figure 16 and Figure 17 indicate the XRD quantitative analysis of HBSAC repair mortar samples. In HBSAC mortar paste, the AFt content was 44.6% at 3 d and 61.8% at 28 d, respectively. At 0.1% HPMC, these values increased to 55.1% and 57.5%, whereas at 0.2% HPMC, they decreased to 45.4% and 44.6%. This indicates that 0.2% HPMC significantly inhibits the formation of AFt in the later hydration stage. In contrast, as the VAE dosage increased from 0% to 10%, the AFt content at 3 d and 28 d rose to 56.5% and 67.7%, demonstrating that 10% VAE promotes the generation of AFt.

3.6.5. SEM Microscopic Morphology

As can be seen from Figure 18a,b, the presence of VAE results in a tight arrangement of HBSAC hydration products, while AFt mostly appears as short and thin needle-like structures with a large number and presents a petal-like morphology that spreads from the middle to the periphery. After 28 days of hydration, more flocculent gel appeared around AFt. As shown in Figure 18c,d, the morphology of AFt in HBSAC doped with HPMC is thicker and has a distinct rod-shaped structure compared to VAE-doped HBSAC. At the same time, as the content and curing age increase, more spherical structures appear at the end of AFt and pile up tightly, which may be the result of the interweaving of HPMC molecules and ettringite, increasing the density of the internal structure. The density of the bonding interface is an important factor affecting the bonding strength and durability of the repair material.
As shown in Figure 19, after forming pure HBSAC on the old mortar block, the crack width at the interface is relatively large, and the new mortar around the interface also has defects. After the addition of VAE, the crack width at the interface is significantly reduced, and the red box is a magnified local image. There is a clear polymer film connecting the new and old materials, indicating that the presence of polymers not only enhances the bonding strength but also reduces crack development. In contrast, the addition of HPMC further reduced the crack width, and the mortar around the crack did not show any self-cracks, indicating that HPMC can effectively resist the shrinkage cracking of cement mortar. This may be because that, different from ordinary Portland cement (OPC), high-belite sulfoaluminate cement (HBSAC) produces a large amount of ettringite during hydration and brings moderate micro-expansion, which can effectively compensate for the volume shrinkage of repaired mortar in the hardening stage. The two polymers, VAE and HPMC, adopted in this study can improve the overall water retention capacity of repaired mortar and prevent drying shrinkage caused by rapid water loss [44]. Meanwhile, the polymers form continuous films inside the hardened matrix to disperse internal shrinkage stress and reduce the possibility of shrinkage cracks in mortar.
Combined with the above analysis on workability, mechanical properties and microstructures, the optimal dosage of VAE is determined to be 5.0%. At this dosage, the fluidity, water retention, bonding strength and microstructure of repaired mortar are significantly improved, with a relatively controllable strength loss. Meanwhile, 0.1% is selected as the optimal dosage of HPMC. It can effectively prolong the setting time and greatly enhance water retention, while exerting minor adverse effects on flexural and compressive strength.

4. Conclusions

In this study, HBSAC is used as a cementitious material to prepare high-performance repair mortar. The effects of VAE and HPMC on the workability, mechanical properties, hydration characteristics, and microstructure of HBSAC repair mortar were systematically studied. The conclusions drawn are as follows:
  • The addition of VAE and HPMC can prolong the setting time of HBSAC and OPC pastes, and the retarding effect on HBSAC is more prominent. Both types of polymers can enhance the flowability of the mortar, lubricate the paste–aggregate interface, and significantly improve the water retention rate of the mortar. Among them, HPMC modification has a better effect, with a water retention rate of over 90% achieved at a content of 0.1%.
  • Low content of VAE can slightly improve the flexural strength of HBSAC mortar, while high content significantly reduces both flexural and compressive strength. The mechanical strength of HPMC-modified HBSAC mortar fluctuates with the content. The bond strength of the two polymers to the mortar was significantly improved, with the 5% VAE group and 0.1% HPMC group increasing by 56.7% and 15.1% respectively at 28 d. The polymer induces a smaller reduction in the compressive strength of HBSAC mortar, but a larger improvement in the bond strength.
  • The hydration heat release of HBSAC has two characteristic heat release peaks: cement dissolution and formation of AFt. The polymers can delay the AFt heat release peak and reduce the peak rate, while increasing the total hydration heat release within 48 h, making the hydration heat release process smoother and more persistent. The XRD results indicate that VAE is beneficial for the generation of AFt and weakens the diffraction peaks of other minerals, while HPMC inhibits the generation of AFt and enhances the diffraction peak intensity of Ca(OH)2.
  • FTIR analysis shows that the characteristic peaks of polymer functional groups decay after hydration reaction, and VAE can enhance the infrared characteristic peak of AFt, while HPMC has a weaker effect on the characteristic peak of AFt. VAE has a spherical microstructure, which can refine AFt needle-like crystals and form a network-like polymer film. HPMC has a band-like structure, which can generate a large number of stacked spherical structures after hydration, optimizing the morphology of hydration products and improving the density of the paste.
  • The HBSAC-based repair mortar modified with 5.0% VAE or 0.1% HPMC features rapid hardening and high early strength, as well as excellent workability, bonding performance and mechanical properties. It can be widely used for repairing damaged concrete building components, emergency maintenance of municipal roads and partial restoration of old structures. The mortar can meet the engineering requirements for rapid construction service, processing a good practical value and promotion prospects in the field of repair material.

Author Contributions

L.W.: writing—original draft. Y.W.: methodology, software. C.G.: software, investigation. Y.G.: conceptualization. G.Y.: formal analysis, supervision. Q.L.: writing—review and editing, funding acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the [Natural Science Foundation of Shandong Province] grant number [No.ZR2023ME044], [National Natural Science Foundation of China] grant number [No.52478262]. And The APC was funded by [National Natural Science Foundation of China] grant number [No.52478262].

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding authors.

Acknowledgments

The authors thank the Natural Science Foundation of Shandong Province (Grant No.ZR2023ME044) and the National Natural Science Foundation of China (Grant No.52478262).

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. SEM micro-morphology of different polymers (a): VAE; (b): HPMC.
Figure 1. SEM micro-morphology of different polymers (a): VAE; (b): HPMC.
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Figure 2. The detailed experimental procedure of HBSAC repair mortar.
Figure 2. The detailed experimental procedure of HBSAC repair mortar.
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Figure 3. The initial and final setting time of pastes with different polymers: (a) VAE; (b) HPMC; (c) OPC.
Figure 3. The initial and final setting time of pastes with different polymers: (a) VAE; (b) HPMC; (c) OPC.
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Figure 4. Fluidity of HBSAC and OPC mortar: (a) VAE series; (b) HPMC series.
Figure 4. Fluidity of HBSAC and OPC mortar: (a) VAE series; (b) HPMC series.
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Figure 5. Water retention ratio of HBSAC and OPC mortar: (a) VAE series; (b) HPMC series.
Figure 5. Water retention ratio of HBSAC and OPC mortar: (a) VAE series; (b) HPMC series.
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Figure 6. Flexural strengths of HBSAC repair mortar: (a) VAE series; (b) HPMC series.
Figure 6. Flexural strengths of HBSAC repair mortar: (a) VAE series; (b) HPMC series.
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Figure 7. Compressive strength of HBSAC repair mortar: (a) VAE series; (b) HPMC series.
Figure 7. Compressive strength of HBSAC repair mortar: (a) VAE series; (b) HPMC series.
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Figure 8. Bond strength of HBSAC repair mortar: (a) VAE series; (b) HPMC series.
Figure 8. Bond strength of HBSAC repair mortar: (a) VAE series; (b) HPMC series.
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Figure 9. Hydration heat curves of HBSAC repair mortar with different polymers: (a) Heat flow; (b) cumulative heat.
Figure 9. Hydration heat curves of HBSAC repair mortar with different polymers: (a) Heat flow; (b) cumulative heat.
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Figure 10. TG/DTG curves of HBSAC repair mortar: (a) HBSAC paste-3d; (b) HBSAC paste-28d; (c) HBSAC-5%VAE-3d; (d) HBSAC-5%VAE-28d; (e) HBSAC-10%VAE-3d; (f) HBSAC-10%VAE-28d.
Figure 10. TG/DTG curves of HBSAC repair mortar: (a) HBSAC paste-3d; (b) HBSAC paste-28d; (c) HBSAC-5%VAE-3d; (d) HBSAC-5%VAE-28d; (e) HBSAC-10%VAE-3d; (f) HBSAC-10%VAE-28d.
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Figure 11. TG/DTG curves of HBSAC repair mortar: (a) HBSAC-0.1%HPMC-3d; (b) HBSAC-0.1%HPMC-28d; (c) HBSAC-0.2%HPMC-3d; (d) HBSAC-0.2%HPMC-28d.
Figure 11. TG/DTG curves of HBSAC repair mortar: (a) HBSAC-0.1%HPMC-3d; (b) HBSAC-0.1%HPMC-28d; (c) HBSAC-0.2%HPMC-3d; (d) HBSAC-0.2%HPMC-28d.
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Figure 12. FTIR curves of HBSAC repair mortar: (a) HBSAC-VAE-3d; (b) HBSAC-VAE-28d.
Figure 12. FTIR curves of HBSAC repair mortar: (a) HBSAC-VAE-3d; (b) HBSAC-VAE-28d.
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Figure 13. FTIR curves of HBSAC repair mortar: (a) HBSAC-HPMC-3d; (b) HBSAC-HPMC-28d.
Figure 13. FTIR curves of HBSAC repair mortar: (a) HBSAC-HPMC-3d; (b) HBSAC-HPMC-28d.
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Figure 14. XRD patterns of HBSAC repair mortar: (a) HBSAC-VAE-3d; (b) HBSAC-VAE-28d; (c) HBSAC-HPMC-3d; (d) HBSAC-HPMC-28d.
Figure 14. XRD patterns of HBSAC repair mortar: (a) HBSAC-VAE-3d; (b) HBSAC-VAE-28d; (c) HBSAC-HPMC-3d; (d) HBSAC-HPMC-28d.
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Figure 15. XRD quantitative analysis of HBSAC repair mortar without polymer: (a) 3 d; (b) 28 d.
Figure 15. XRD quantitative analysis of HBSAC repair mortar without polymer: (a) 3 d; (b) 28 d.
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Figure 16. XRD quantitative analysis of HBSAC repair mortar: (a) 0.2% HPMC-3d; (b) 0.2% HPMC-28d; (c) 0.1% HPMC-3d; (d) 0.1% HPMC-28d.
Figure 16. XRD quantitative analysis of HBSAC repair mortar: (a) 0.2% HPMC-3d; (b) 0.2% HPMC-28d; (c) 0.1% HPMC-3d; (d) 0.1% HPMC-28d.
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Figure 17. XRD quantitative analysis of HBSAC repair mortar: (a) 10%VAE-3d; (b) 10%VAE-28d; (c) 5%VAE-3d; (d) 5%VAE-28d.
Figure 17. XRD quantitative analysis of HBSAC repair mortar: (a) 10%VAE-3d; (b) 10%VAE-28d; (c) 5%VAE-3d; (d) 5%VAE-28d.
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Figure 18. SEM morphology of HBSAC paste with different polymers at curing age of 3 d and 28 d.
Figure 18. SEM morphology of HBSAC paste with different polymers at curing age of 3 d and 28 d.
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Figure 19. SEM microscopic morphology of HBSAC paste and substrate bonding surface: (a) HBSAC; (b) VAE-HBSAC; (c) HPMC-HBSAC.
Figure 19. SEM microscopic morphology of HBSAC paste and substrate bonding surface: (a) HBSAC; (b) VAE-HBSAC; (c) HPMC-HBSAC.
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Table 1. The physical performance parameters of HBSAC and OPC.
Table 1. The physical performance parameters of HBSAC and OPC.
CementSpecific Surface Area/m2·kg−1Initial Set/minFinal Set/minFlexural Strength/MPaCompressive Strength/MPaStability
3 d28 d3 d28 d
HBSAC50118.524.56.77.834.643.2Qualified
OPC3851522754.67.325.747.2Qualified
Table 2. The XRF chemical composition of OPC and HBSAC (%).
Table 2. The XRF chemical composition of OPC and HBSAC (%).
CementCaOSiO2Al2O3Fe3O4MgOSO3NaOK2OTiO2L.O.I
OPC61.6518.586.565.872.921.880.840.650.560.49
HBSAC50.513.815.31.52.114.20.580.480.620.92
Table 3. The main performance of VAE powder.
Table 3. The main performance of VAE powder.
ColorSolid Content/%pH ValueAsh Content/%Film-Forming Temperature/°C
White powder≥986–810 ± 20–5
Table 4. The main performance of HPMC powder.
Table 4. The main performance of HPMC powder.
ColorFineness/%pH ValueViscosity/Pa·sApparent Density/kg·m−3
Pale yellow powder≥905–7.5300–6000.5
Table 5. Mix proportion design of HBSAC repair mortar.
Table 5. Mix proportion design of HBSAC repair mortar.
Serial NumberVAE/C
/%		HPMC/C
/%		Cement
(kg/m3)		Sand
(kg/m3)		Binder/Sand RatioWater/Binder RatioWater Reducer
(kg/m3)
HBSAC-00045013501:30.56.75
HBSAC-P12.5045013501:30.56.75
HBSAC-P25.0045013501:30.56.75
HBSAC-P37.5045013501:30.56.75
HBSAC-P410.0045013501:30.56.75
HBSAC-P500.0545013501:30.56.75
HBSAC-P600.1045013501:30.56.75
HBSAC-P700.1545013501:30.56.75
HBSAC-P800.2045013501:30.56.75
OPC-00045013501:30.56.75
OPC-P15.0045013501:30.56.75
OPC-P200.1045013501:30.56.75
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MDPI and ACS Style

Wang, L.; Wu, Y.; Guo, C.; Guo, Y.; Yue, G.; Li, Q. Study on Properties and Hydration Mechanism of Polymer-Modified High-Belite Sulfoaluminate Cement Repair Mortar. Buildings 2026, 16, 2352. https://doi.org/10.3390/buildings16122352

AMA Style

Wang L, Wu Y, Guo C, Guo Y, Yue G, Li Q. Study on Properties and Hydration Mechanism of Polymer-Modified High-Belite Sulfoaluminate Cement Repair Mortar. Buildings. 2026; 16(12):2352. https://doi.org/10.3390/buildings16122352

Chicago/Turabian Style

Wang, Liang, Yaning Wu, Chao Guo, Yuanxin Guo, Gongbing Yue, and Qiuyi Li. 2026. "Study on Properties and Hydration Mechanism of Polymer-Modified High-Belite Sulfoaluminate Cement Repair Mortar" Buildings 16, no. 12: 2352. https://doi.org/10.3390/buildings16122352

APA Style

Wang, L., Wu, Y., Guo, C., Guo, Y., Yue, G., & Li, Q. (2026). Study on Properties and Hydration Mechanism of Polymer-Modified High-Belite Sulfoaluminate Cement Repair Mortar. Buildings, 16(12), 2352. https://doi.org/10.3390/buildings16122352

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