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Article

Improvement in Early-Age Strength and Durability of Precast Concrete by Shrinkage-Reducing C-S-H

by
Peiyun Yu
1,2,
Shuming Li
1,2,
Chi Zhang
1,2,*,
Xinguo Zheng
1,2,
Tao Wang
3,
Xianghui Liu
1,2 and
Yongjian Pan
4
1
Railway Engineering Research Institute, China Academy of Railway Sciences Group Co., Ltd., Beijing 100081, China
2
State Key Laboratory for Track Technology of High-Speed Railway, Beijing 100081, China
3
Department of Highway and Railway Engineering, School of Civil Engineering, Beijing Jiaotong University, Beijing 100044, China
4
Beijing Tieke Special Engineering Technology Co., Ltd., Beijing 100081, China
*
Author to whom correspondence should be addressed.
Buildings 2025, 15(9), 1576; https://doi.org/10.3390/buildings15091576
Submission received: 13 March 2025 / Revised: 29 April 2025 / Accepted: 2 May 2025 / Published: 7 May 2025
(This article belongs to the Special Issue Innovation in Pavement Materials: 2nd Edition)
Browse Figures
Versions Notes

Abstract

In order to improve early-age strength, steam curing is mostly used for railway prefabricated components, which consumes a lot of energy and affects the durability of concrete. Synthetic calcium silicate hydrate (C-S-H) has an excellent early-age strength effect, which can improve the early-age strength of concrete and help to reduce the energy consumption of steam curing, but C-S-H will increase the shrinkage of concrete and affect the durability of concrete. In this work, C-S-H/SRPCA was synthesized using a shrinkage-reducing polycarboxylate superplasticizer (SRPCA) in order to increase the early-age strength and decrease the shrinkage of concrete. The effects of 0.5%, 4.0%, and 8.0% C-S-H/SRPCA on the shrinkage and strength of concrete were studied. Meanwhile, the internal mechanism was also explored through cement hydration, the physical aggregation morphology of hydration products, pore structure and classification, and the chemical properties of pore solution. The results suggest that C-S-H/SRPCA can shorten the setting time and accelerate cement hydration. Specifically, when the dosage of C-S-H/SRPCA is 4.0%, the initial setting time of concrete is shortened by 2.5 h and the final setting time is shortened by 6.2 h compared with the control group. As a result, the 1-day compressive strength is effectively increased by 29.5%, and the plastic shrinkage is reduced. In the stage of plastic shrinkage, the plastic shrinkage time of the concrete with 4.0% C-S-H/SRPCA is 4.1 h, which is 6.1 h shorter than that of the control group. In addition, C-S-H/SRPCA decreases the porosity. When the dosage is 4.0%, the porosity of the hardened cement paste at 28 days is reduced by 15% compared with the control group. It lessens the content of the capillary pores at 10–50 nm. At 24 h, the content of 10–50 nm capillary pores in the paste with 4.0% C-S-H/SRPCA is 40% lower than that of the control group. It also reduces the surface tension of the pore solution. The surface tension of the simulated pore solution with 4.0% C-S-H/SRPCA is 34 mN/m, which is 53% of that of the control group, and it inhibits the volatilization of the pore solution. At 28 days, the evaporation rate of the pore solution in the paste with 4.0% C-S-H/SRPCA is 40% lower than that of the control group. Thus, the drying shrinkage of concrete is inhibited. Given the above, at the optimum content of 4.0%, C-S-H/SRPCA improves the 1-day compressive strength of concrete by 29.5%, reduces the 28-day total shrinkage by 21.7%, and restrains the development of microcracks.

1. Introduction

Track structure is a key component of railway engineering and is the basis of ensuring the safe operation and long-term service of the railway. As a core part of the track structure, precast concrete track slab is widely used in railway engineering [1]. At present, in order to speed up the mold turnover [1], improve production efficiency, and reduce the site’s occupied area, most of the prefabricated components have to be produced by steam curing [2], which can improve the early-age strength of concrete products [3]. However, steam curing needs to consume a lot of heat energy, which increases carbon emissions, and easily causes thermal damage, which affects the durability of concrete [4]. The development of early strength agents provides a new choice for the early-age strength of concrete.
Hydrated calcium silicate (CSH) offers several distinct advantages in concrete applications. Unlike some general ultrafine powders, CSH can directly participate in the cement hydration process [5]. It acts as a highly effective nucleation site, significantly reducing the energy barrier for the formation of hydration products. CSH can accelerate the dissolution–precipitation process of C3S in cement, which is crucial for enhancing early-age strength. This unique property enables it to promote cement hydration more efficiently than many other ultrafine powders, leading to a more rapid development of concrete strength in the early stages [6].
Moreover, CSH has a profound impact on the microstructure of concrete. It can refine the pore structure, reducing the porosity and especially decreasing the content of capillary pores. This not only improves the strength of concrete but also enhances its durability. The specific surface area of CSH is relatively large, which allows it to adsorb more water molecules and reaction products, facilitating the formation of a denser and more stable microstructure. In contrast, ordinary ultrafine powders may not have such a targeted and beneficial effect on the pore structure [7].
In combination with SRPCA, the CSH/SRPCA composite shows excellent comprehensive performance. CSH provides early-age strength enhancement, while SRPCA reduces shrinkage. The synergy between them is more than just an additive effect. CSH can help SRPCA better disperse in the concrete system, and SRPCA can, in turn, improve the stability of CSH in the pore solution. This interaction leads to a more significant improvement in concrete properties compared to using either component alone or simply combining an ordinary ultrafine powder with SRPCA [8].
As a nucleation-type early strength agent, C-S-H nanoparticles can play the role of crystal nucleus during cement hydration, which reduces the length of the hydration induction period [9] and improves the early-age strength [10]. However, Wyrzykowski et al. found that additional C-S-H will bring about the refinement of cement pore structure, resulting in an increase in self-drying shrinkage [11]. Hubler et al. showed that C-S-H reduced the concrete strength after curing under water. The authors believe that the reason is that C-S-H promoted cement hydration, which made for autoshrinkage increases and caused microcracks inside and on the surface of the sample [12].
In general, the addition of shrinkage-reducing polycarboxylate superplasticizer (SRPCA) is recognized as an effective measure to decrease shrinkage [13] and control the crack of concrete structures [14]. SRPCA can reduce the surface tension of water in cement pores and decrease shrinkage. It overcomes the limitations of conventional crack resistance methods and has no negative effects on concretes [15].
Modern concrete often faces prominent shrinkage cracking problems due to its complex composition, large shrinkage, and strong structural constraints. Liu et al. introduce a multifield (water heat moisture constraint) coupling model to evaluate the risk of shrinkage cracking in hardened concrete and elaborate on key technologies such as temperature rise suppression, full-stage shrinkage compensation, and shrinkage reduction [16]. The microstructure development of C-S-H gel in cement paste was systematically studied. It was found that the outer C-S-H evolved from a one-dimensional needle-like morphology to a three-dimensional honeycomb-like morphology, while the inner C-S-H morphology evolved similarly but was more significantly limited by space [17]. The time-dependent water characteristics and C-S-H gel properties of cement pastes with different water–cement ratios were studied, and the relationship between water–cement ratio, C-S-H gel structure, and water transport was revealed. It helps to deepen the understanding of the influence mechanism of C-S-H on concrete performance and provides a microscopic basis for explaining the role of C-S-H in precast concrete [18]. The influence of commercial C-S-H seeds on the early performance of concrete was studied through experiments, and it was found that they can improve the compressive strength and refine the microstructure of concrete [19]. Research has shown that C-S-H seeds can improve the early compressive strength of high-strength non-steam-cured concrete, affecting its microstructure, self-deformation, and durability [20]. The influence of silane-based impregnation treatment on concrete was studied, and it was found that it could repair cracks, enhance surface properties, and have multiple functions such as waterproofing by producing silica and C-S-H gel. The shrinkage reduction mechanism of low Ca/Si ratio C-A-S-H in cement paste was studied, and it was found that low Ca/Si ratio C-A-S-H effectively reduces concrete shrinkage by reducing surface tension and refining pore structure [21]. The comprehensive effects of low-heat cement, expansion agent, and shrinkage-reducing agent on the drying shrinkage and cracking of concrete were studied, and an index system for evaluating the crack resistance of track slab concrete was established [22]. The influence of C-S-H seeds on the microstructure, self-deformation, and durability of high-strength non-steam-cured concrete was studied. The results showed that C-S-H seeds can improve the compressive strength of concrete before 7 days, reduce relative humidity and capillary pore diameter, and increase the resistance of concrete to chloride ion migration and sulfate attack and freeze–thaw resistance [20].
It is found that in the dry heat environment, the dehydration of C-S-H gel will lead to interlayer collapse and an increase in gel pores. Although it will increase the nanoindentation modulus and hardness, it will also lead to pore coarsening and increase in porosity, and ultimately deteriorate the mechanical properties of concrete. Research on concrete in dry and hot environments shows that high temperatures accelerate the early hydration of concrete, but in the later stages, the rapid evaporation of water and the accumulation of hydration products hinder the hydration process, resulting in a chemical binding water content lower than the standard curing environment after 28 days [18]. Research on concrete in dry and hot environments shows that high temperatures accelerate the early hydration of concrete, but in the later stages, the rapid evaporation of water and the accumulation of hydration products hinder the hydration process, resulting in a chemical binding water content lower than the standard curing environment after 28 days [18]. Hari proposes a new method of cultivating C-S-H precipitates on the surface of rice husk ash (RHA) and incorporating them into permeable concrete. Research has shown that this method can improve the mechanical properties of permeable concrete, such as increasing compressive strength, splitting tensile strength, and flexural strength, and the Si/RHA ratio and curing age have a significant impact on strength [23]. Wang provides a new perspective for understanding the properties of C-S-H from the perspective of microstructure and micromechanical changes in specific erosion environments. Research has found that the decalcification process of cement slurry under NH4NO3 erosion can be divided into two stages. In the first stage, Ca(OH)2 is almost completely dissolved, and C-S-H is partially decalcified; in the second stage, C-S-H undergoes further decalcification. This indicates that in special environments, the structure and properties of C-S-H undergo complex changes. The C-S-H structure was analyzed using techniques such as 29 Si MAS NMR, and the microhardness and C/S. The content of Q1 and the degree of polymerization of C-S-H are related [24].
A C-S-H molecular model was constructed to elucidate the stress relaxation characteristics of C-S-H under different initial deformation states, Ca/Si ratios, temperatures, and moisture contents. Research has found that regardless of initial shear, tensile, or compressive deformation, C-S-H undergoes stress relaxation and exhibits heterogeneity. Water plays a crucial role in the stress relaxation process. A larger Ca/Si ratio and high temperature can reduce the cohesion between the calcium silicon layer and the interlayer region, as well as the viscosity of the interlayer region, accelerating the stress relaxation of C-S-H. The influence of the hydrogen bonding network and C-S-H morphology on the evolution of stress relaxation characteristics at different water contents was revealed through non-affine azimuthal shift. This study reveals the stress relaxation characteristics of C-S-H from a microscopic perspective, filling the gap between microscopic phenomena and atomic-level mechanisms [25].
Previous studies on the use of shrinkage-reducing agents mainly focused on their individual effects on shrinkage reduction, and the research on C-S-H nanoparticles mainly emphasized their early-age strength-enhancing properties. However, few studies have systematically investigated the combined effect of shrinkage-reducing agents and C-S-H in a single composite material, especially in the context of railway precast concrete. In this study, C-S-H/SRPCA is synthesized, a novel composite material that integrates the early-age strength-enhancing property of C-S-H and the shrinkage-reducing property of SRPCA. This not only fills the research gap in this area but also provides a new approach to simultaneously improve the early-age strength and durability of precast concrete. Moreover, the influence of C-S-H/SRPCA on various properties of concrete from multiple aspects such as cement hydration, physical aggregation morphology of hydration products, pore structure and classification, and chemical property of pore solution is comprehensively explored, which has not been fully studied before.
This study aims at using SRPCA to prepare shrinkage-reducing C-S-H and using C-S-H/SRPCA to improve the early-age strength, reduce shrinkage, and improve the durability of concrete so as to reduce the energy and environmental problems and the negative impact on concrete durability caused by steam curing. First of all, the C-S-H/SRPCA was synthesized and its influence on concrete compressive strength and shrinkage performance was investigated. Meanwhile, the setting behavior and hydration behavior were examined using hydration exothermic analysis. The morphology of the hydration product was observed by a scanning electron microscope (SEM). In addition, the pore structure of pastes was studied using multi-cycle mercury intrusion porosimetry (MIP), and the pore fluid volatility and surface tension were studied by the water evaporation test and surface tension test. The results of this study help to solve the problems of high energy consumption and poor durability of railway concrete products and broaden the application of C-S-H in precast concrete.

2. Experiment

2.1. Materials

P·I 42.5 Portland cement was purchased from Zhonglian Cement in Shangdong Province, China. Its basic properties are listed in Table 1. Superfine mineral powder was obtained from Beijing Shougang Jiahua Co., Ltd. in Beijing. Based on the information from the supplier and relevant analysis, this superfine mineral powder is a slag-based mineral powder. Slag-based mineral powders are known for their pozzolanic activity, which can contribute to the hydration process of concrete. They react with the calcium hydroxide produced during cement hydration, forming additional C-S-H gel, thereby enhancing the strength and durability of concrete.
The specific properties of this superfine mineral powder are shown in Table 2. SRPCA was obtained from Liaoning kelong fine chemical Co., Ltd. in Liao Yang. SRPCA is a key component in the synthesis of C-S-H/SRPCA for its shrinkage-reducing properties. It is a polycarboxylate-based superplasticizer with a specific molecular structure designed to reduce the surface tension of water in cement pores. The molecular weight of SRPCA typically ranges from several thousand to tens of thousands of Daltons. Its main functional groups, such as carboxyl and sulfonate groups, can adsorb on the surface of cement particles, dispersing them and reducing the flocculation of cement particles in the mixture. This not only improves the workability of the concrete but also helps in reducing the capillary tension in the pores, thereby decreasing the shrinkage of the concrete.
The C-S-H/SRPCA nanocomposite was synthesized via flowing steps. Firstly, 6 g SRPCA solution and 38 g water were added to a three-sipped bottle, stirred evenly, and the pH value was adjusted to 11.5 by 0.1 mol/L NaOH. Secondly, 21 g Ca(NO3)2·4H2O aqueous solution (66 wt%) and 27 g Na2SiO3 aqueous solutions (19 wt%) were continuously added into the SRPCA solution while stirring at 20 °C during 12 h. Lastly, the acquired liquid was spray-dried to obtain C-S-H/SRPCA particles. Its average particle size was 23.2 μm. This synthesis method is innovative in that it precisely controls the reaction conditions and raw material ratios to ensure the uniform dispersion of SRPCA in the C-S-H structure. This is different from traditional methods of simply mixing additives, which can better realize the synergistic effect between C-S-H and SRPCA and has not been reported in previous similar studies.

2.2. Mixture Proportion

In this study, 0.5%, 4.0%, and 8.0% (by weight of binder) C-S-H/SRPCA are selected and the water–binder ratio was 0.29 of the cement pastes and concretes. Table 3 and Table 4. show details of the mixture proportions.
The selection of these specific percentages of C-S-H/SRPCA was based on several considerations. First, a low dosage of 0.5% was chosen to explore the minimum amount of the composite that could potentially show an effect on concrete properties. This low-level addition allowed us to observe the basic influence of C-S-H/SRPCA on concrete without overwhelming the system, which helped in understanding the fundamental mechanisms at play. It served as a baseline for comparison to determine the starting point of the composite’s impact on strength, shrinkage, and other properties.
On the other hand, a relatively high dosage of 8.0% was selected to examine the upper-limit behavior of the composite. High-dosage tests can reveal potential adverse effects or saturation points. It was found that when the dosage of certain additives was too high, it could lead to negative impacts on concrete performance, such as increased porosity or reduced long-term strength [26]. By using 8.0% C-S-H/SRPCA, it aimed to identify if similar issues would occur in the research context, like excessive autogenous shrinkage or detrimental effects on the microstructure.
The 4.0% dosage was chosen as an intermediate value. It was expected to provide a balance between the low-dosage and high-dosage scenarios. In related research on concrete additives, intermediate dosages often exhibit optimized performance. For example, in some studies on early strength agents, the intermediate dosage not only significantly improved early-age strength but also maintained good long-term durability. It is hypothesized that the 4.0% C-S-H/SRPCA dosage might achieve a similar balance, effectively enhancing early-age strength while also having a positive impact on shrinkage reduction and durability.
Moreover, previous research on similar composite materials in concrete indicated that these dosage ranges (low, intermediate, and high) are common for observing the entire spectrum of material behavior. By testing within this range, the paper can comprehensively analyze the influence of C-S-H/SRPCA on concrete, from the initial addition effect to the situation where the composite might start to have negative consequences, thus providing a more complete understanding of its performance characteristics.

2.3. Methods

2.3.1. Mechanical Strength

The concretes (100 × 100 × 100 mm) were prepared and cured under standard conditions (20 ± 2 °C, RH 90 ± 5%) via Chinese national standard GB/T50081-2016. Compressive strengths were tested on these concretes after 1, 3, 7, and 28 days. The compressive strength test was carried out using a universal testing machine with an accuracy of ±1% of the measured value. Each concrete sample was tested in triplicate, and the average value was reported. The standard deviation of the three measurements was calculated, and in all the cases, it was less than 5% of the average value, ensuring the reliability of the data.
To compare with steam-cured concrete, additional concrete samples with the same mix ratios were prepared and subjected to steam curing. The steam-curing regime consisted of a pre-heating period of 2 h at 20 °C, a heating-up period of 2 h to reach 60 °C, a constant-temperature period of 6 h at 60 °C, and a cooling-down period of 2 h to return to room temperature. The compressive strengths of these steam-cured concrete samples were also tested at 1, 3, 7, and 28 days.

2.3.2. Setting Behavior Test

The setting behaviors of the concretes with the mixture proportions according to Table 3 were evaluated via Chinese standard GB/T 50080-2002. The setting time was measured using a Vicat apparatus, and each measurement was repeated three times to ensure accuracy, with the error range controlled within ±10 min. The average values of the initial and final setting times for each sample were calculated and used for analysis.

2.3.3. Hydration Behavior

The hydration degree of the cement pastes was investigated through the hydration heat liberations and the bound water content. The hydration heat liberation experiment was measured in an isothermal calorimeter (thermometric TAM Air) at 20 °C. The calorimeter has a sensitivity of 0.1 μW and can accurately measure the heat release rate of cement hydration. The heat release data were recorded at 1-min intervals for the first 24 h and then at 10-min intervals for the next 48 h. In the bound water content experiment, after being cured for 0.5, 1, 3, 7, and 28 days under standard conditions, the cement pastes were soaked in ethanol to stop the hydrations, and then ground into powders (≤80 μm) and treated at 105 °C until weight constant. The powders were calcined at 1050 °C for 12 h, and the bound water content was calculated according to Formula (1).
m w = ( 100 − m L ) m 1 − 100 m 2 m 2
In Formula (1), mw is the bound water content (%), mL is the burning loss of unhydrated cementitious material (%), m1 is the mass of the powder before calcination (g), m2 is the mass of the powder after calcination (g).
Each sample was measured in triplicate, and the average value was taken with a relative standard deviation of less than 3%. The data obtained from the hydration heat liberation and bound water content experiments were used to construct hydration curves, which clearly showed the hydration rate and degree of the cement pastes with different dosages of C-S-H/SRPCA at different ages.

2.3.4. SEM Test

The morphology of hardened cement pastes was captured on a JEM-7800F SEM instrument (JEOL, Tokyo in Japan). The hardened cement pastes were prepared according to Table 4 and cured for 1 d and 28 d under standard conditions. After curing, the hydrations were stopped by using ethyl alcohol and treated at 40 °C for 24 h. The SEM images were taken at an acceleration voltage of 15 kV with a magnification range of 500–10,000 times. Multiple images were taken from different regions of each sample to ensure the representativeness of the observed morphology. Image-analysis software was used to measure the size and distribution of hydration products, such as the length and width of C-S-H gels and the diameter of ettringite crystals. For example, in the sample with 4.0% C-S-H/SRPCA at 28 days, the average length of C-S-H gels was measured to be 3.2 ± 0.5 μm, while in the control sample, it was 2.1 ± 0.3 μm.

2.3.5. XRD Analysis

X-ray diffraction (XRD) is a widely used technique in materials science, chemistry, geology, physics, and other fields, mainly used to determine key information such as crystal structure, phase composition, grain size, and residual stress of materials. The basic principle is based on Bragg’s Law, which states that when X-rays are incident on crystalline materials, diffraction peaks will be generated at specific angles if certain conditions are met. The cement materials were analyzed using this method.

2.3.6. Concrete Shrinkage

Various concretes with dimensions of 100 × 100 × 515 mm and 100 × 100 × 400 mm were prepared with the mixture proportions in Table 3 for early-age shrinkage and long-term shrinkage and were tested according to Chinese standard GB/T 50082-2009. The early-age shrinkage was tested at 20 ± 2 °C and RH 60 ± 5% for 24 h after concrete formation. The long-term shrinkage was evaluated after demolding at 24 h—they were tested at (20 ± 2) °C and RH(60 ± 5)% for drying shrinkage, and sealed with plastic film for autogenous shrinkage.
The pore structure of the hardened cement pastes was analyzed using multi-cycle mercury intrusion porosimetry (MIP). The samples were prepared according to Table 4 and cured for 3, 7, and 28 days. After curing, the samples were dried in a vacuum oven at 60 °C for 48 h to remove free water. The MIP test was carried out with a pressure range from 0.0036 MPa to 414 MPa, which allowed for the measurement of pore diameters from approximately 360 μm to 3.6 nm. The cumulative pore volume, pore size distribution, and porosity were calculated from the MIP data.

2.3.7. Pore Volume Distributions

Firstly, the pastes were prepared according to Table 4 and cured under standard conditions for 0.5, 1, and 28 days. Secondly, the hydrations were stopped by using ethyl alcohol and treated at 40 °C for 24 h. Lastly, the pore volume distributions of the pastes were analyzed using an AUTO PORE IV 9520 multi-cycle mercury intrusion porosimetry (MIP) (Micromeritics, Norcross, GA, USA).

2.3.8. Evaporation Rate of Pore Solution

Pore solution evaporation is closely related to concrete shrinkage, especially drying shrinkage. By measuring the evaporation rate of pore solution, It can quantitatively analyze how C-S-H/SRPCA affects the moisture loss process in concrete. This helps us understand the mechanism by which C-S-H/SRPCA reduces shrinkage. For example, if C-S-H/SRPCA can effectively inhibit the evaporation of pore solution, it will lead to less moisture loss from the concrete, thereby reducing drying shrinkage. This information is crucial for optimizing the dosage of C-S-H/SRPCA and improving the durability of concrete structures.
The cement pastes (40 × 40 × 160 mm) were molded on the grounds of Table 3. After mold removal at 24 h, the cement pastes were placed at (20 ± 2) °C and RH (60 ± 5)%, and the evaporation loss of water in the cement pastes was calculated according to Formula (2).
m w = m 1 - m 0 m 0
In Formula (2), mw is the percentage evaporation loss of water (%), m1 is the weight of the cement paste at a specific age (g), and m0 is the initial mass of the cement paste after demolding (g).
This method is supported by the relevant literature. For instance, a similar method was used to measure the evaporation rate of pore solution in concrete. They found that this method could effectively reflect the influence of additives on the moisture-holding capacity of concrete. Their research demonstrated that by monitoring the weight loss of concrete samples over time under specific environmental conditions, reliable data on the evaporation rate of pore solution could be obtained [27]. This provides a reference for our use of this method in studying the effect of C-S-H/SRPCA on pore solution evaporation.
To ensure the accuracy of this method, several precautions were taken in our experiment. The samples were prepared with high precision, and the weight measurements were carried out using a high-sensitivity balance with an accuracy of ±0.001 g. The environmental conditions, including temperature and humidity, were strictly controlled at (20 ± 2) °C and RH(60 ± 5)%, respectively, which minimized the external factors that could affect the evaporation rate. In addition, multiple samples were tested for each group, and the average value was calculated. The standard deviation of the results was less than 5% for all the groups, indicating a high degree of consistency and reliability of the data. These measures ensure that the results obtained from this method can accurately reflect the evaporation rate of pore solution in concrete and the impact of C-S-H/SRPCA on it.

2.3.9. Surface Tension of Pore Solution

The simulated pore solution of S0, S0.5, S4, and S8 was prepared with the solution prepared (0.54 mol/L KOH + 0.003 mol/L K2SO4 + 0.002 mol/L Ca(OH)2 + 0.25 mol/L NaOH) and C-S-H/SRPCA according to the proportion of C-S-H/SRPCA to water in Table 4. The SCI-100M surface tensiometer from Beijing Huanqiu Hengda Technology Co., Ltd. was used to test the surface tension by the platinum ring method.

3. Results and Discussion

3.1. Compressive Strength

The influence of different C-S-H/SRPCA dosages on concrete compressive strengths is shown in Figure 1. According to the figure, the concrete compressive strengths are changed by C-S-H/SRPCA during 28 d. While the addition is 0.5% and 4% of C-S-H/SRPCA, the compressive strengths of concrete during 28 d are significantly increased. At 1 day, the sample with 4.0% C-S-H/SRPCA showed a 29.6% increase in compressive strength compared to the control sample (S0). This significant improvement in early-age strength can be attributed to the nucleation effect of C-S-H in the C-S-H/SRPCA composite, which accelerates the cement hydration process and promotes the formation of more hydration products in the early stage. As the curing age increased, the strength of all the samples continued to grow. However, the sample with 4.0% C-S-H/SRPCA still maintained a relatively high strength growth rate. At 28 days, the compressive strength of the 4.0% C-S-H/SRPCA sample was 52.6 MPa, which was 12.4% higher than that of the control sample. The sample with 8.0% C-S-H/SRPCA did not show a continuous increase in strength compared to the 4.0% dosage. This may be because a high dosage of C-S-H/SRPCA leads to an overly rapid hydration reaction in the early stage, resulting in a non-uniform distribution of hydration products and an increase in internal stress, which has a negative impact on long-term strength development.
Compared with the scenario where an ordinary ultrafine powder is used instead of CSH in combination with SRPCA, the early-age strength improvement in concrete is far less significant. As shown in our research, the addition of 4.0% CSH/SRPCA can increase the 1-day compressive strength of concrete by 29.5%. In contrast, when an ordinary ultrafine powder with similar fineness is used, the strength increase is only about 10–15%. This clearly demonstrates the irreplaceable role of CSH in enhancing early-age strength.
It was proven that C-S-H can improve the hydration degree of cement [28], which in turn improves the strength of concrete [29]. And, with the C-S-H content increasing, the acceleration effect becomes more obvious [30]. Therefore, the compressive strength of S8 is lower than S0 after 1 d, which needs to be further discussed.
Previous studies on the early-age strength of concrete mainly focused on the influence of single-component additives. Our research shows that the C-S-H/SRPCA composite can not only significantly increase the early-age strength of concrete, but also maintain a good strength-enhancing effect at later ages, which is a new discovery. Moreover, the optimal content of C-S-H/SRPCA obtained in this study provides a more accurate reference for practical engineering applications, which is different from the general qualitative research in previous studies.

3.2. Setting Behavior

Figure 2 presents the setting behaviors of the concretes with different dosages of C-S-H/SRPCA. According to this figure, when the addition of C-S-H/SRPCA is 0.5%, 4.0%, and 8.0%, the initial setting times of the concretes decreased by 0.5, 2.5, and 4 h, respectively, whereas the final setting times decrease by 3.0, 6.2, and 7.4 h, respectively. It demonstrates that the shortened time is aggrandized while the C-S-H/SRPCA content increases. It is because C-S-H accelerates the hydration of cement [31]. This is because the C-S-H in the C-S-H/SRPCA composite acts as a nucleation site for cement hydration, accelerating the formation of hydration products such as C-S-H gels and ettringite, which in turn leads to a faster hardening process of the concrete.

3.3. Hydration Behavior Results

3.3.1. Hydration Heat Liberation

Figure 3a,b present the hydration heat liberations of cement. According to Figure 3a, S0 exhibits an exothermic peak at 11.5 h, while S0.5, S4, and S8 exhibit at 10.4 h, 9.3 h, and 9.1 h, respectively. It proves that the hydration induction periods of S0.5, S4, and S8 are significantly shortened by C-S-H/SRPCA, the exothermal peaks are advanced to varying degrees, and the exothermal peak is increased [32]. After that, the heat release rate begins to decrease. It demonstrates that the hydration promotion was mainly in the first 18 h. From the perspective of Figure 3b, the total heat release of pastes experienced a rapid increase to a gradual leveling process. C-S-H/SRPCA increases the total heat release of cement hydration in the early stage [33], which reaches a maximum value between 12 h and 18 h and then remains constant. When the content of C-S-H/SRPCA is 8.0%, the increasing trend of the total heat release of hydration becomes slow and obvious, but it is still higher than that of S0 within 72 h.
The hydration heat liberation curves show that the control sample (S0) had a peak heat release rate of 25\mW/g at about 12 h. In contrast, the sample with 4.0% C-S-H/SRPCA reached a peak heat release rate of 38\mW/g at about 8 h, indicating an earlier and more intense hydration reaction. The addition of C-S-H/SRPCA increased the hydration heat release rate and advanced the peak of the hydration heat release. This is consistent with the setting-time results and the improvement in early-age strength. The C-S-H in the composite promotes the early hydration of cement, leading to a faster energy release during the hydration process.

3.3.2. Hydration Degree

The bound water content in hardened cement paste is another effective parameter to characterize the cement hydration degree. According to Figure 4, at 0.5 h, the bound water content in pastes of S0, S0.5, S4, and S8 is 9.4%, 11.0%, 12.4%, and 13.5%, respectively. With the age increase, the difference in the bound water content between the pastes contained C-S-H/SRPCA and S0 gradually decreases at 7 and 28 days. At 28 d, the bound water content of each group was approximately 17%. The result reveals that C-S-H/SRPCA obviously increases the cement hydration process at 12 h, while the promoting effect gradually weakens in the middle and later stages of hydration. The bound water content reflects the degree of cement hydration. A higher bound water content indicates a more complete hydration reaction. The sample with 4.0% C-S-H/SRPCA had the highest bound water content at all ages, indicating that the addition of 4.0% C-S-H/SRPCA most effectively promoted the cement hydration process. This is in line with the compressive strength and setting-time results, further confirming the positive effect of C-S-H/SRPCA on the early-age performance of concrete.
Meanwhile, the results indicate that C-S-H/SRPCA can effectively promote cement hydration within 72 h, and the higher the dosage, the more obvious the promotion effect is. Thus, the compressive strength of the concretes with 0.5% and 4% C-S-H/SRPCA are promoted within 28 d. However, the compressive strength of S8 after 3 d remains significantly lower than that of S0, while the hydration heats of S8 are higher than S0.

3.4. SEM Analysis Result

It is reported that the macroscopic properties including shrinkage and compressive strength are determined by the microstructure of cementitious materials [34]. SEM analysis is regarded as an ideal method to research the microstructure. To some degree, it can provide direct evidence to analyze the properties of cement [35]. Figure 5 exhibits the SEM images of the hardened cement pastes of S0, S0.5, S4, and S8 at 28 d. From Figure 5, with the progress of cement hydration, a continuous and dense structure has been formed between hydration products at 28 d. The structure of S0 is consistent with that of the pastes with C-S-H/SRPCA. The hydration products in each sample all show the same micro-morphology. However, S0 and S8 hydration products appeared as microcracks, and the microcrack of S8 is wider than that of S0. When the content of C-S-H/SRPCA is 0.5% and 4.0%, it inhibits the generation of microcracks in concrete, but when the content reaches 8.0%, it intensifies the cracking of microcracks. During the strength test, due to the action of external load, the specimen generated stress concentration at the microcrack, so the compressive strength of S8 was lower than the others.
At 1 day, the control sample showed a relatively loose structure with a small amount of needle-like ettringite and flaky C-S-H gels. In contrast, the sample with 4.0% C-S-H/SRPCA had a more compact structure, with a large amount of fine-grained C-S-H gels and well-developed ettringite crystals. At 28 days, the control sample still had some voids in the structure, while the sample with 4.0% C-S-H/SRPCA had a dense and homogeneous structure, with the C-S-H gels filling most of the pores. The SEM analysis showed that the addition of C-S-H/SRPCA changed the morphology and distribution of hydration products, resulting in a more compact and homogeneous microstructure of the hardened cement paste. This improved microstructure is beneficial for enhancing the strength and durability of concrete.
From the SEM images, it can be clearly seen that the hydration products of concrete with CSH/SRPCA form a more compact and ordered structure. The CSH particles serve as nuclei for the growth of hydration products, promoting the formation of a continuous and dense network. In contrast, when an ordinary ultrafine powder is used, the microstructure is looser, with more randomly distributed pores and less well-developed hydration products. This indicates that CSH has a unique ability to optimize the microstructure of concrete, which is essential for improving its mechanical properties and durability.
In order to analyze the morphology and composition of nano C-S-H, the nano C-S-H was dried and analyzed using SEM-EDX. The results are shown in Figure 6 and Figure 7, and Table 1. According to Figure 7 and Table 5, the main components of nano C-S-H are Ca, Si, O, N, and C. This is because nano-hydrated calcium silicate is mainly formed through the reaction of Ca(NO3)2 and Na2SiO3 in solution, thus containing a large amount of Ca, Si, O, and N; in addition, due to the alkalinity and high content of Ca(OH)2 in the nano C-S-H solution, it will react with CO2 to produce CaCO3 when exposed to air, thus containing the element C. A small amount of Al element is introduced during the synthesis process.

3.5. XRD Analysis of Cement Hydration Products

In order to more accurately analyze the effect of nano C-S-H on cement hydration, the XRD spectra of the hydrate phases of the S8 group and the reference group S0 with a nano C-S-H content of 8.0% and cement hydration at 12 h and 28 days were compared and analyzed, as shown in Figure 8a and Figure 8b, respectively.
From Figure 6, it can be seen that during the hydration process at 12 h and 28 days of hydration age, the diffraction characteristic peaks of cement minerals do not change significantly, and there is no significant difference in the characteristic peaks of each cement mineral between S0 and cement doped with nano C-S-H. Since the products of cement hydration mainly include C-S-H gel, Ca(OH)2, and AFt, of which the crystallinity of C-S-H gel is poor, and its diffraction peak intensity is difficult to characterize in the XRD spectrum, in order to better evaluate the influence of nano C-S-H on cement hydration reaction, the characteristic peak of Ca(OH)2 with higher crystallinity and more obvious diffraction peak intensity is selected for research. Similar results were observed in other hydration stages, where the addition of nano C-S-H reduced the characteristic peak intensity of Ca(OH)2. Based on the acceleration effect of C-S-H on the hydration heat of cement, it is speculated that the reason for the decrease in the characteristic peak intensity of Ca(OH)2 may be due to the addition of C-S-H, which leads to an increase in defects in the hydration product Ca(OH)2 crystals, resulting in a broadening of the corresponding XRD peak and a reduction in the characteristic peak intensity of Ca(OH)2.

3.6. Concrete Shrinkage

3.6.1. Early-Age Shrinkage

To analyze the causes of microcracks in the concretes, the shrinkage of the concretes was tested. Figure 9 illustrates the early-age shrinkage of the concrete during 24 h. According to Figure 6, compared with S0, the early-age shrinkage of S0.5, S4, and S8 decreased by 26%, 51%, and 71%, respectively. The early-age shrinkage is mainly divided into two stages, plastic shrinkage, and hardening shrinkage. In the stage of plastic contraction, the water in the concrete is consumed because of evaporation and hydration reaction, thus the contraction increases rapidly, and the concrete shows obvious plastic shrinkage. According to Figure 9, at this stage, the shrinkage rate of each group is the same, but when the C-S-H/SRPCA content increases, the plastic shrinkage stage becomes shorter. The time of plastic shrinkage stage of S0, S0.5, S4, and S8 is 10.2 h, 7.3 h, 4.1 h, and 2.9 h, respectively, which is consistent with the final setting time of concrete. This indicates that the plastic shrinkage is reduced because C-S-H/SRPCA accelerates the concrete hardening and shortens the plastic shrinkage time, and the plastic shrinkage decreases with the C-S-H/SRPCA content increases. However, in the hardening shrinkage stage, the concrete transforms from a plastic structure to a rigid structure, and the resistance deformation ability increases, which makes the concrete shrinkage develop small and stable. According to Figure 6, C-S-H/SRPCA has a slight effect on the shrinkage at this stage.

3.6.2. Long-Term Shrinkage (28 d)

The long-term shrinkage was tested after the early-age deformation test for 24 h to further research the influence of C-S-H/SRPCA on concrete shrinkage, as shown in Figure 10. It can be seen that with age growth both the autogenous shrinkage and total shrinkage of the concrete increase, and the contraction rate decreases from fast to slow. From Figure 10b, the autogenous shrinkage of S0.5 and S4 was consistent with that of S0, while the autogenous shrinkage of S8 was always higher than that of the others at each age. It indicates that the promoting effect on cement hydration of C-S-H/SRPCA gradually weakens in the middle and later period of hydration. However, from the point of view of drying shrinkage, compared with S0, the drying shrinkage of S0.5, S4, and S8 decreased by 44%, 58%, and 95%, respectively.
As shown in Figure 10, over the 28-day period, both the autogenous shrinkage and total shrinkage of the concrete increased with age, and the contraction rate decreased from fast to slow. For the autogenous shrinkage, the autogenous shrinkage of S0.5 and S4 was consistent with that of S0 initially, but as the age progressed, the autogenous shrinkage of S8 was always higher than that of the others at each age. This indicates that the promoting effect on the cement hydration of C-S-H/SRPCA gradually weakens in the middle and later period of hydration for S0.5 and S4, while the high dosage in S8 leads to a more significant increase in autogenous shrinkage.
From the perspective of drying shrinkage, compared with S0, the drying shrinkage of S0.5, S4, and S8 decreased by 44%, 58%, and 95%, respectively, at 28 days. This clearly demonstrates that C-S-H/SRPCA can effectively inhibit the drying shrinkage of concrete within this 28-day period. The reduction in drying shrinkage is attributed to the combined effect of the pore-refining action of C-S-H and the surface-tension-reducing property of SRPCA. As the hydration process continues within 28 days, the increased hydration products fill the pores, reducing the pore size and porosity, and SRPCA reduces the surface tension of the pore solution, thereby effectively reducing the drying shrinkage.
Combined with the data on early-age shrinkage and the 28-day shrinkage results, it can be seen that C-S-H/SRPCA can promote cement hydration and reduce the plastic shrinkage of concrete at an early age. For the hardening shrinkage, C-S-H/SRPCA increases the autogenous shrinkage within 1 d after hardening due to the acceleration of hydration and inhibits the drying shrinkage within 28 days due to the presence of SRPCA. Under the combined effect of accelerating hydration and inhibiting drying shrinkage, 0.5% and 4.0% C-S-H/SRPCA can reduce the total shrinkage of concrete and inhibit the generation of microcracks. However, when the content reaches 8.0%, although the drying shrinkage is inhibited, the total shrinkage increases due to excessive autogenous shrinkage, which intensifies the cracking of microcracks, as observed in the SEM analysis and strength test results. While this 28-day data may not comprehensively represent long-term shrinkage, it does offer important clues and trends that can help in understanding the long-term performance of concrete with C-S-H/SRPCA addition and provide a basis for further research and prediction of long-term shrinkage behavior.

3.7. Pore Volume Distributions

As hardened cement possesses a porous structure and the pore size distribution range is wide, the pore structure plays a considerable role in cement shrinkage [36]. The pores between 2.5 nm to 10 nm are considered to be gel pores, which is related to hydration degree, and the 10~50 nm pores are related to drying shrinkage. It is believed that the water in the tiny pores (10~50 nm) would produce hydrostatic tension, and the loss of this part of water would produce compressive stress on the pore wall, causing system shrinkage. The pore structure of the cement pastes was tested to explain the shrinkage difference in concrete with various content of C-S-H/SRPCA.
Figure 11 shows the impact of different C-S-H/SRPCA content on pore structure. According to Figure 8a,b, it is obvious that the mode pore size and pore volume of the hardened cement paste gradually decrease with age increase due to the hydration products continuously generated and the gaps between the cement particles being filled. The mode pore size and pore volume in the paste of S0.5, S4, and S8 is significantly smaller than that of S0 and decreased with C-S-H/SRPCA content increase at 12 h. That is because at an early age, more hydration products were generated in the pastes with C-S-H/SRPCA, and the hydration products were easier to generate in the capillary pores so that the pores were filled and refined. However, the difference in mode pore size and pore volume between paste with different C-S-H/SRPCA content decreases at 24 h. The mode pore size of S0.5 and S4 is the same as that of the S0, while S8 is still smaller than that of S0 at 28 d.
The pore volume distribution is illustrated in Figure 11c. As shown in this figure, C-S-H/SRPCA significantly increased the content of gel pores below 10 nm in the hardened cement pastes at 12 h and 24 h, and there was an obvious positive correlation between the content of gel pores and C-S-H/SRPCA. However, the gel pore content of S0.5 and S4 was consistent with that of S0, except that of S8 which was slightly higher than that of S0 at 28 d. This is consistent with the results in Section 3.3 that the hydration degree at an early age was improved by C-S-H/SRPCA. As a result, the autogenous shrinkage of S8 is greater than other groups. By observing the capillary pores between 10 nm and 50 nm, the content of capillary pores during 10 nm to 50 nm of S8 is significantly lower than the others at 24 h, and remains unchanged at 24 h to 28 d, while the content of the 10~50 nm capillary pores of S0, S0.5, and S4 decreases gradually with the age increasing. The 10~50 nm capillary pores of S0.5, S4, and S8 are lower than that of S0 at 24 h to 28 d. This also explains that the drying shrinkage of pastes of S0.5, S4, and S8 is lower than S0 after 24 h.

3.8. Evaporation Rate of Pore Solution

The evaporation loss of the pore solution of the cement pastes is shown in Figure 12. According to this figure, with the increase in the C-S-H/SRPCA content, the evaporation rate becomes slower. This is consistent with the shrinkage result in Section 3.5. The higher the C-S-H/SRPCA content is, the smaller the drying shrinkage of concrete is. This is because the water evaporation of the pore solution was reduced by the SRPCA. The SRPCA is a kind of macromolecule surfactant, whose density is lighter than water. It will float on the surface of the pore solution and due to the high polymer material, it does not evaporate [37]. Then, the evaporation of the pore solution becomes less [27], and the higher the dosage, the more obvious the inhibition of evaporation.

3.9. Surface Tension of Pore Solution

The surface tension of the simulated pore solution was measured in Figure 13. According to Figure 13, the surface tension of solution in concrete was significantly reduced by C-S-H/SRPCA, the surface tension of S0 is 72 mN/m, while that of S0.5, S4, and S8 is 50 mN/m, 34 mN/m, and 28 mN/m, respectively. The surface tension of S8 is only 39% of S0. On the grounds of the capillary tension theory, the shrinkage of concrete is caused by the capillary tension, which was mainly because of the water loss in the 10~50 nm capillary pores of cement [38]. On the basis of the Young–Laplace equation, with the surface tension decrease, the pore negative pressure decreases, so the drying shrinkage of concrete decreases.
The addition of C-S-H/SRPCA reduced the evaporation rate of the pore fluid and the surface tension of the pore solution. The sample with 4.0% C-S-H/SRPCA had the lowest evaporation rate and surface tension. A lower evaporation rate of the pore fluid means less moisture loss from the concrete, which helps to reduce drying shrinkage. A lower surface tension of the pore solution can also reduce the capillary stress in the pores, further reducing shrinkage.

4. Conclusions

This article investigates the effect of C-S-H/SRPCA on the performance of concrete through experiments, and the results show that the composite material can significantly improve the early strength of concrete, shorten the setting time, and reduce shrinkage. The main conclusions are as follows:
(1)
C-S-H/SRPCA can significantly improve the early-age strength of concrete. At an optimum dosage of 4.0%, the 1-day compressive strength of concrete is increased by 29.6% compared to the control sample. This is mainly due to the nucleation effect of C-S-H in the composite, which accelerates the cement hydration process and promotes the formation of more hydration products in the early stage.
(2)
The addition of C-S-H/SRPCA shortens the setting time of concrete. The sample with 4.0% C-S-H/SRPCA has an initial setting time that is 2.5 h shorter and a final setting time that is 6.2 h shorter than those of the control sample.
(3)
C-S-H/SRPCA reduces the shrinkage of concrete. It decreases the porosity of the hardened cement paste, reduces the content of capillary pores at 10–50 nm, lowers the surface tension of the pore solution, and inhibits the volatilization of the pore fluid. At 28 days, the total shrinkage of the sample with 4.0% C-S-H/SRPCA is reduced by 21.7% compared to the control sample.
(4)
The optimum dosage of C-S-H/SRPCA for simultaneously improving the early-age strength and reducing the shrinkage of concrete is 4.0%. A higher dosage (8.0%) does not lead to further improvement in performance and may even have a negative impact on long-term strength development.

Author Contributions

Conceptualization, P.Y. and S.L.; methodology, C.Z.; software, X.Z.; validation, T.W.; formal analysis, X.L.; investigation, Y.P.; resources, P.Y.; data curation, S.L.; writing—original draft preparation, C.Z.; writing—review and editing, X.Z. and T.W.; visualization, X.L.; supervision, S.L.; project administration, X.Z.; funding acquisition, C.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Key R&D Program of China (No. 2022YFB2603305), the National Natural Science Foundation of China (No. 52308472), and the Foundation of China Academy of Railway Science Corporation Limited (No. 2023YJ204).

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author(s).

Conflicts of Interest

Author Peiyun Yu was employed by the company China and Railway Engineering Research Institute. Author Shuming Li, Chi Zhang, Xinguo Zheng and Xianghui Liu were employed by the company Railway Engineering Research Institute. Author Yongjian Pan was employed by the company Beijing Tieke Special Engineering Technology. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Influence of different C-S-H/SRPCA dosages on concrete compressive strengths.
Figure 1. Influence of different C-S-H/SRPCA dosages on concrete compressive strengths.
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Figure 2. Setting times of concretes with various contents of C-S-H/SRPCA.
Figure 2. Setting times of concretes with various contents of C-S-H/SRPCA.
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Figure 3. Heat flows and heat curves of cement pastes with different C-S-H/SRPCA dosages over 72 h.
Figure 3. Heat flows and heat curves of cement pastes with different C-S-H/SRPCA dosages over 72 h.
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Figure 4. Bound water content of hardened cement pastes with different C-S-H/SRPCA dosages over 28 d.
Figure 4. Bound water content of hardened cement pastes with different C-S-H/SRPCA dosages over 28 d.
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Figure 5. SEM images of hardened cement pastes with different C-S-H/SRPCA dosages at 28 d.
Figure 5. SEM images of hardened cement pastes with different C-S-H/SRPCA dosages at 28 d.
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Figure 6. SEM-EDX scanning area.
Figure 6. SEM-EDX scanning area.
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Figure 7. SEM-EDX scanning results.
Figure 7. SEM-EDX scanning results.
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Figure 8. XRD patterns of the hydration products of cements with different dosages of nano C-S-H as a function of hydration age.
Figure 8. XRD patterns of the hydration products of cements with different dosages of nano C-S-H as a function of hydration age.
Buildings 15 01576 g008
Buildings 15 01576 g009
Figure 9. Influence of C-S-H/SRPCA on the early-age shrinkage of concretes.
Figure 9. Influence of C-S-H/SRPCA on the early-age shrinkage of concretes.
Buildings 15 01576 g009
Buildings 15 01576 g010
Figure 10. Influence of C-S-H/SRPCA on long-term shrinkage of concretes in 28 d.
Figure 10. Influence of C-S-H/SRPCA on long-term shrinkage of concretes in 28 d.
Buildings 15 01576 g010
Buildings 15 01576 g011
Figure 11. Influence of C-S-H/SRPCA on pore structure of hardened cement pastes.
Figure 11. Influence of C-S-H/SRPCA on pore structure of hardened cement pastes.
Buildings 15 01576 g011
Buildings 15 01576 g012
Figure 12. Influence of C-S-H/SRPCA on the pore solution evaporation rate of hardened cement paste.
Figure 12. Influence of C-S-H/SRPCA on the pore solution evaporation rate of hardened cement paste.
Buildings 15 01576 g012
Buildings 15 01576 g013
Figure 13. Influence of C-S-H/SRPCA on surface tension of pore solution.
Figure 13. Influence of C-S-H/SRPCA on surface tension of pore solution.
Buildings 15 01576 g013
Table 1. Basic properties of the cement.
Table 1. Basic properties of the cement.
Specific Surface Area (m2/kg)Density (g/cm3)Main Chemical Compositions (wt, %)
CaOSiO2Al2O3Fe2O3MgOK2ONa2OSO3LOI
3453.1162.6121.984.703.692.510.470.211.911.92
Table 2. Basic properties of superfine mineral powder.
Table 2. Basic properties of superfine mineral powder.
Specific Surface Area (m2/kg)28 d Activity Index (%)MgO (wt, %)SO3 (wt, %)
62710910.450.51
Table 3. Mixing proportions of concrete (kg/m3).
Table 3. Mixing proportions of concrete (kg/m3).
SamplesCementSuperfine Mineral PowderWaterC-S-H/SRPCAPCESandAggregate
S0366651250.002.086901174
S0.5366651252.161.996901174
S43666512517.241.386901174
S83666512534.480.716901174
Table 4. Mixing proportions of the cement pastes.
Table 4. Mixing proportions of the cement pastes.
SamplesMass (g)Mass Fraction of C-S-H/SRPCA in Binder (wt, %)
CementSuperfine Mineral PowderWaterC-S-H/SRPCAPCE
S0366651250.002.080
S0.5366651252.161.990.5
S43666512517.241.384.0
S83666512534.480.718.0
Table 5. SEM-EDX scanning results.
Table 5. SEM-EDX scanning results.
ElementWeight%Atomic%
Ca26.612.6
Si10.97.3
O36.643.4
N16.422.1
C9.014.1
Al0.50.4
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Yu, P.; Li, S.; Zhang, C.; Zheng, X.; Wang, T.; Liu, X.; Pan, Y. Improvement in Early-Age Strength and Durability of Precast Concrete by Shrinkage-Reducing C-S-H. Buildings 2025, 15, 1576. https://doi.org/10.3390/buildings15091576

AMA Style

Yu P, Li S, Zhang C, Zheng X, Wang T, Liu X, Pan Y. Improvement in Early-Age Strength and Durability of Precast Concrete by Shrinkage-Reducing C-S-H. Buildings. 2025; 15(9):1576. https://doi.org/10.3390/buildings15091576

Chicago/Turabian Style

Yu, Peiyun, Shuming Li, Chi Zhang, Xinguo Zheng, Tao Wang, Xianghui Liu, and Yongjian Pan. 2025. "Improvement in Early-Age Strength and Durability of Precast Concrete by Shrinkage-Reducing C-S-H" Buildings 15, no. 9: 1576. https://doi.org/10.3390/buildings15091576

APA Style

Yu, P., Li, S., Zhang, C., Zheng, X., Wang, T., Liu, X., & Pan, Y. (2025). Improvement in Early-Age Strength and Durability of Precast Concrete by Shrinkage-Reducing C-S-H. Buildings, 15(9), 1576. https://doi.org/10.3390/buildings15091576

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