Introduction
With the increasing construction of large structures, the use of mass concrete is becoming more and more extensive. When a cement is hydrated, the compositions react with water to acquire stable, low-energy states. During this process, energy is released in the form of heat [
1]. For concrete construction during winter, the ambient temperature may be too low to stimulate the hydration. At this time, hydration heat helps the cement hydration. While for mass concrete structures, large hydration heat may be generated and could be a great threat when the heat cannot be properly released [
2]. If no effective measure is taken, the stress generated by non-uniform temperature distribution easily leads to cracks. These cracks spread rapidly and could exacerbate the severity and extent of other distress which leads to the performance deterioration, shortening of service period, and increase of maintenance cost. Therefore, alleviation of the early hydration heat and autogenous shrinkage of cement-based materials are critical to impede the occurrence of early distress and prolong the lifespan. Some studies revealed certain relationship existed between the hydration heat and the mechanical properties. Kaszyńska [
3] found that exponential relationship existed between the mechanical performance and the amount of hydration heat.
The hydration heat release of cement is usually divided into five periods (Fig. 1). Stage I is the initial period. In this period, after the cement mixed with water, aluminate and sulfate dissolve immediately and release a lot of heat in a few minutes. Stage II is the induction period. When the solubility of aluminate decreases due to sulfate in the solution, the rate of hydration slows down and remains relatively stable. Stage III is the acceleration period. In this period, a part of C3S in cement dissolves and C-S-H forms will release a great deal of heat and reach the peak value of rate of hydration in 4–8 h. The time of setting and hardening of cement can be judged by the acceleration period. Cement can maintain plasticity before the beginning of this stage, and appears thickening and initial setting before reaching the peak value of the rate of hydration. After the peak, it starts to harden. Stage IV is the deceleration period. During this period, the hydration reaction rate begins to decline rapidly. After the deceleration period, the hydration of the cement was very slow and basically completed. Stage V is called the attenuation period.
Autogenous shrinkage refers to the reduction of apparent volume or length of cement-based materials under seal and isothermal conditions [
4]. Nowadays, lots of concrete structures experience early-age cracking. In particular, high-performance concretes and high-strength concretes are more susceptible to cracking caused by autogenous shrinkage due to their low water-to-binder ratio. There is a general agreement about the existence of a relationship between autogenous shrinkage and relative humidity changes in the pores of the hardening cement paste [
4–
6]. It is believed that autogenous shrinkage is caused by the capillary tension when the saturation state of capillary pore changes from saturated to unsaturated [
6]. The capillary tension theory explicates the autogenous shrinkage using pore structure, relative humidity, self-stress, degree of hydration, and interface structure [
6]. During the progressive cement hydration, free water is gradually consumed, and the internal relative humidity reduces. Pores are formed during this process, and the internal tension pressure is generated because of the change of the internal humidity. Autogenous shrinkage is a very complicated process and affected by many factors, including raw materials [
7–
9], supplementary materials [
10,
11], and curing methods [
12,
13], etc. Autogenous shrinkage can be predicted according to the cement compositions. C
3S and C
3A are more susceptible to the hydration and can release lots of heat rapidly, so cement containing more C
3S and C
3A is prone to generate autogenous shrinkage caused by a large amount of water consumption in the early stage [
7]. Different supplementary materials present different effects on the autogenous shrinkage. It was found that silica fume could increase the autogenous shrinkage [
13], while slag and fly ash show an inhabiting effect on autogenous shrinkage [
10,
14].
To counteract the self-desiccation and autogenous shrinkage, internal curing is attracting great concerns and plays promising effect in the prevention of early-age cracking [
12,
15,
16]. Internal curing refers to the process during which some pre-saturated small inclusions distributed within the cement-based materials hold water during mixing and up to setting time and release it during cement hydration [
17]. The so-called ‘inclusions’ are generally light-weight aggregates (LWAs) [
18] and Superabsorbent Polymers (SAPs) [
19,
20]. LWAs absorb lots of water and act as the internal curing material mainly because of the significant pore structure, while SAPs play its role in internal curing because of its strong water absorption ability. The polymeric network in SAPs has the ability to absorb and retain water of up to 500 times of their own mass, and the volume can swell due to osmosis [
19,
21]. Compared to LWAs, when SAP releases water in hardened cement-based materials, voids may be introduced, and thus the mechanical properties and durability may be deteriorated. Because of the introduced extra water, the mixture design is more complicated for SAP cement-based materials. Sun et al. [
19] proposed a design method for SAP cement-based materials based on the criterion of flowability similarity.
Normally, the water in the mixture is gradually consumed in the formation process of hydration products, and the relative humidity in the microstructure decreases. During this process, autogenous shrinkage is caused by the moisture difference between inside and outside of cement-based materials. SAP has a potential to provide the wet inner environment for the concrete, thus to mitigate the autogenous shrinkage. Besides, for the unhydrated cement, the absorbed water helps the hydration process continue [
15]. Research showed that the water release from SAP particles was uniform and instantaneous at an early age, while the depercolation of capillary porosity may substantially inhibit the water transport at later ages [
20]. Because of the water release from SAP particles, SAP could enhance the cement hydration even at a very late stage. Snoeck and de Belie [
22] found that SAP provides superior behaviors in self-healing. The mechanisms of the autogenous healing include two aspects: unhydrated cement is hydrated in cracks; calcium carbonate precipitates on the crack surfaces.
In the research of SAP cement-based materials, it was found that many factors, including SAP content, SAP type, particle size, and water-saturated state of SAP [
23–
27], all play a significant role on the internal curing efficiency. SAP with larger particle size present stronger water absorption ability, and have a better effect on mitigating the autogenous shrinkage [
27,
28]. When the SAP content is very large, or the introduced water by SAP is too much, the total
W/
C will increase, so that SAP will play a negative effect on concrete performance, such as workability, mechanical property and the durability. Based on these considerations, Igarashi and Watanabe [
23] gave the suggestion on
W/
C range and the SAP dosage, in which autogenous shrinkage can be effectively reduced.
The aforementioned studies are mainly about low- and moderate-strength concrete, effects of SAP on high-performance concrete were seldomly explored. Besides, although SAPs help the continuous hydration, there is a lack of knowledge about the time when SAP starts to take effect. Cement concretes’ mechanical properties are closely related to the microstructure, while the research about the correlation between mechanical behaviors and the microstructure evolution is still limited.
Objective and scope
The objective of this research was to fundamentally deepen the understanding of the effect of SAP on some behaviors of cement mortars. To this end, laboratory investigations were systematically and quantitively conducted to explore the effect of SAP on the hydration properties, mechanical behaviors, autogenous shrinkage performance and the microstructure evolution of cement mortars with a low water-cement ratio. Five types of cement mortars were prepared based on different SAP content. Effect of SAP on the hydration heat in different periods was examined. A non-contact test method was employed to examine the autogenous shrinkage behaviors. The starting time of autogenous shrinkage and the 3 d shrinkage value were selected as the parameters to characterize the autogenous shrinkage behaviors. The microstructure at different ages were obtained and further correlated to the mechanical properties of SAP cement-based mortars.
Materials and methods
Materials
A type of low-crosslinking sodium acrylate was selected as the SAP used in this research. SAP could absorb 52±2 mL of normal saline for each gram. Figure 2 presents a Scanning Electron Microscope (SEM) image of SAP particles, whose scale is in 100 mm. In this study, the SAP particles were added in a dry state.
P.O. 42.5 Portland Cement, polycarboxylate superplasticizer, tap water and standard sand were used in the mixtures. The physical and chemical compositions of the cement are presented in Table 1. Particle size distributions of cement, SAP, sand and superplasticizer are shown in Fig. 3. Sand and SAP are coarser than superplasticizer and cement. Size distributions of cement and superplasticizer are very close, while the they are both finer than SAP.
Mix proportion
Table 2 presents the design of mix proportion. Water-cement ratio (
W/
C) of the cement mortar is 0.3, which is typical for high-strength or high-performance concrete [
29]. Sand-cement ratio (
S/
C) is 2. The amount of superplasticizer is 0.3% by mass of the cement. Considering the effect of SAPs on the mechanical and workability behaviors, SAP content used in cement-based materials is generally no more than 1% [
29]. In this study, the amount of SAP is 0%, 0.25%, 0.5%, 0.75%, and 1% by mass of cement, respectively. Because of the significant water absorption ability of SAP, extra water is needed for cement mortars with SAP to achieve the same flowability as the specimens without SAP. Here, the criterion of flowability similarity was employed to determine the extra water content. The so-called ‘criterion of similarity’ is a criterion which determines the extra water absorbed by SAPs, so that the flowabilites of specimens with and without SAPs could be similar. In this paper, the flowability of the cement mortar with and without SAP are both 226 mm, which was the diameter of the cement mortar in the flow test conducted following ASTM C 1437. The extra water content can be seen in Table 2. The detailed information of the extra water content can be obtained in Ref. [
19]. To ensure the uniform distribution of SAPs in cement mortars, cement mortars were prepared in the following procedures: cement, sand, superplasticizer, and SAP were mixed first in one mechanical mixer for 1 min; water was added in the composite and mixing was conducted for another 5 min.
Testing methods
Hydration heat
Generally, 50% of hydration heat is liberated within the first 3 d, and about 70% of heat is released within the first 7 d of hydration [
1]. To explain how SAP affects the hydration heat, the hydration heat was measured using a TAM AIR conduction calorimeter as a function of time on all specimens. The calorimeter is capable of 8 parallel measurements in 8 separate measuring cells with an error less than 2 J/g.
Before the test, the isothermal calorimeter was placed in the condition of 20°C±2°C for more than 4 h. The mass of each sample was about 150 g. During the test, cement mortar was first poured into a glass vial, then sealed and placed into the calorimeter. The whole operation process is controlled within 10 min. The hydration heat of SAP cement mortar was automatically collected by the test system for 10 d. The calorimetry results are normalized by gram of cement in mortars.
Autogenous shrinkage
A non-contact testing method was employed to measure the early shrinkage deformations of cement mortars from casting to 3 d. The deformation characteristics were obtained using several non-contact sensors. Figure 4 shows the setup of the test. The principle of the designed non-contact system is simplified as follows: 1) cement mortar is first cast into the rectangle groove; 2) then the alternating field will be generated under the effect of the electric current field around the sensor; 3) the eddy current field will be generated around the reflection target; 4) the eddy current field can be converted into the length change of the sample.
The autogenous shrinkage test was carried out according to GB/T50082-2009, which could be simplified to the following steps: 1) apply lubricating oil to the inner side of the test mold, then lay the sponge pad on both sides in the test mold, and then put in a plastic film to reduce the friction between the test mold and the specimen; 2) the reflection target is perpendicular to the bottom of the test mode; 3) SAP cement mortar was poured into the test mold in two layers, and after vibration, the specimens were immediately moved to the laboratory with a constant temperature (25°C) and humidity (40%); 4) the sensor is fixed on the test mode and the distance between the probe and the reflection target is kept between the 1.0-1.2 mm to ensure the accuracy of the measurement, and the deformation of the mortar will not exceed the measured range; 5) the autogenous shrinkage of the cement mortar was then tested. The data acquisition frequency is 1 min−1 and the acquisition time is 3 d. The sample size is 100 mm×100 mm×515 mm. During the test, the temperature should be kept at 20°C±1°C. The deformation results were obtained by measuring specimens in triplicate to ensure the more accurate estimation of autogenous shrinkage.
Mechanical properties
After the preparation of the specimens, mechanical tests (compressive and flexural strengths) were performed at 14, 28, and 120 d. The sample sizes are 40 mm×40 mm×40 mm and 40 mm×40 mm×160 mm for the compressive test and the flexural test, respectively. Each test was conducted in triplicate. The strength tests were carried out according to the Chinese Standard GB 17671-1999. Specimens used for the mechanical tests were cured under the standard curing condition. All specimens were cured under the standard curing condition (air temperature 20°C±2°C and relative humidity of 95%) until the mechanical tests were conducted.
Scanning Electron Microscope (SEM)
To study the evolution of microstructure of SAP cement-based materials, the microstructure of cement mortar with 1% SAP at 50 d and 100 d was traced by employing SEM technology. The selection of cement mortars with 1% SAP was due to the fact that the larger the SAP content, the more obvious microstructure change can be observed. In this study, SEM tests were performed using a field-emission electron microscopy system. Before testing, samples were cut and the center section of the cement mortar was coated with gold to make the specimens electrically conductive. Then, photographs of different magnifications were obtained to analyze the morphology evolution.
Results and discussions
Hydration heat
The rate of heat of hydration as influenced by different amount of SAP is shown in Fig. 5. The rate of hydration heat of cement mortar with and without SAP showed little difference before the induction period. The heat release before the end of the induction period generally accounts for only about 5% of the total heat release, which can be neglected relative to the total hydration energy. After the induction period, the reaction between cement and water accelerates and reaches the peak. During the acceleration period, the cement mortar without SAP first reaches the peak of the rate of hydration heat, which was significantly higher than that of cement mortar with SAP. During the deceleration period, at the same time, the more SAP incorporated, the larger the heat evolution rate. This was attributed to the fact that by supplying water to the cement at the later stage of hydration reaction, SAP can extend the hydration reaction time and make the hydration reaction of cement more complete. According to the curve of acceleration period, it can be seen that the addition of SAP can also prolong the plasticity time and increase the thickening time of cement mortar.
Figure 6 presents the peak value of the hydration heat rate and the appearance time. Cement mortar without SAP reached the peak of hydration rate at about 9 h, and the peak rate was about 0.00354 W/g. With the increase of SAP content, the heat peak decreased significantly. Compared to the samples without SAP, the peak reduction was up to 5.1%,5.6%, 7.1%, and 10.3% when SAP dosage was 0.25%, 0.5%, 0.75%, and 1%, respectively. When SAP was added, some alkali ions were absorbed by SAP, and the initial ion concentration in cement mortars was correspondingly reduced, so that the peak heat rate decreased [
30]. Besides, it can be seen that SAP has a significant retarding effect on cement hydration and this retarding effect increased drastically with the increase of SAP content. Compared to the samples without SAP, when the SAP content was 1%, the appearance time of the heat peak was delayed about 2 h.
Figure 7 shows the cumulative heat of cement mortar with different SAP content within 10 d. Four periods can be observed. In period I, which was within the first half hours, the cumulative heat increased sharply. Cumulative heat was very stable in period II, which was from the 0.5 to 6 h. Cumulative heat increased obviously within period III. After 60 h (period IV), cumulative heat went to stable.
Figure 8 is the comparison of cumulative heat of SAP cement mortar at different ages. During the first 18 h, samples with SAP presented less cumulative hydration heat, and samples with 1% SAP showed the smallest heat value. With the increase of hydration time, SAP gradually released water to help hydrate. After 24 h, the cumulative heats of samples with 0.5%, 0.75%, and 1% SAP were all larger than that of the sample without SAP. At the end of 10 d, samples incorporated with SAP showed extremely larger cumulative heat than sample without SAP. It was found that the larger the content of SAP, the larger the cumulative heat of cement mortar at the later stage. Figure 9 shows the heat increment of cement mortars treated by different content of SAP compared to neat specimens. At the end of 10 d, compared to the samples without SAP, the cumulative heat increases were 2.4%, 7%, 12.7%, and 17.5%, respectively, for samples with 0.25%, 0.5%, 0.75, and 1% SAP. The results show that SAP helped the fully hydration of cement particle because of the internal curing, which contributed to the larger cumulative hydration heat.
It should also be mentioned that in Fig. 9, from 6 h to 12 h, compared to the samples without SAP, the cumulative heat decrease changed from -4.19%, -4.09%, -3.67%, and -6.46% to -3.86%, -6.60%, -8.51%, and -13.72% for samples with 0.25%, 0.5%, 0.75%, and 1% SAP, respectively. The first 12 h mainly cover the initial period, induction period and the acceleration period, indicating SAP delayed the heat release from Stage I to Stage III. From 12 h to 18 h, compared to the samples without SAP, the cumulative heat decrease changed from -3.86%, -6.60%, -8.51% and -13.72% to -2.01%, -1.68%, -1.26% and 3.60% for samples with 0.25%, 0.5%, 0.75%, and 1% SAP, respectively. This indicates SAP began to help the cement hydration obviously from the deceleration period (Stage IV).
Autogenous shrinkage
Figure 10 presents the autogenous shrinkage development as a function of age. In Fig. 10, the positive and negative values represent shrinkage and expansion, respectively. It can be observed there was a small expansion for all cement mortars in the first day. The early expansion can be ascribed to the formation and growth of Ca(OH)
2 micro crystal, and also due to the formation of inner C-S-H rims that causes a local increase in solid volume [
31]. Data in the first day are messy and irregular, which may be caused by the experimental error. At the end of the 3rd day, compared to the autogenous shrinkage of cement mortar without SAP, the decrease proportions of cement mortars with SAP content of 0.25% and 0.5% were 36% and 69%, respectively, indicating autogenous shrinkage was effectively alleviated with the presence of SAP and the mitigating effect was obviously enhanced with the increasing dosage. SAP with the content of 0.75% and 1% even helped expand the cement mortars. Nevertheless, it should be mentioned that during 1000 to 3180 min, the autogenous shrinkage of samples with 0.25% SAP was larger than that of samples without SAP, which was caused by some experimental operation.
The main reason why SAP can effectively retard the autogenous shrinkage is that SAP is distributed uniformly in cement-based materials and helps delay the declining process of internal relative humidity by the water release from SAP particles. During the hardening process of cement mortars, due to the continuous loss of moisture in the pores, the surface tension of the pores causes the shrinkage strain of the cement mortar. Meanwhile, the moisture difference between internal water of SAP and pore solution of cement paste is produced. The change of osmotic pressure leads to the continuously release of water out of SAP, which relieves capillary tension and reduces volume shrinkage. It is found that when the content of SAP is from 0.75% to 1%, the specimen of cement mortar even expanded to a certain extent.
The autogenous shrinkage characteristic parameters of SAP cement mortar with different SAP content are listed in Table 3. It can be observed that although the start time of autogenous shrinkage of cement mortar with 0.25% SAP was earlier than that of cement without SAP, SAP generally delayed the occurrence of autogenous shrinkage. It is known that the autogenous shrinkage occurs when the rate of water consumption caused by hydration is larger than the conservation of internal curing water. For SAP cement-based materials, when the relative humidity decreases, the ionic concentration of cement pore solution increases. Under the action of humidity gradient and osmotic pressure, the internal water of SAP released gradually and the capillary water is then replenished. Therefore, SAP can effectively delay the occurrence time of autogenous shrinkage.
Microstructure
Figure 11 shows the microstructure of SAP cement mortar at 50 d. As shown in Fig. 11(a), the smooth dark part is an absorbent SAP particle with a diameter of about 1000 mm. SAP existed in the form of hydrogel after the water absorption. Figure 11(b) shows a clear gap between SAP and the surrounding cement stone. The gap was formed due to: 1) the volume of SAP particle shrunk after the water migration from SAP to the surrounding cement mortar; 2) some new hydration products were produced around the gap because of the release of water from SAP, but the amount of hydration products was not enough to fill the gap, resulting in the separation of SAP from the surrounding cement stone. Because of the volume shrinkage of SAP after the water release, it can be concluded from Fig. 11(a) that some SAP particles did not start to release water due to the tightly bonded interface between SAP and the adjacent calcium silicate hydrates (C-S-H). The hydration products of irregular needle-shaped and block-shaped crystal structures are observed in the Fig. 11(c). Micro-cracks can be also observed clearly. This is because the self-desiccation could not be completely compensated by external water curing, and may lead to micro cracks inside the hardened cement paste. Figure 11(d) reflects the morphology of SAP particles in cement mortar and the common pore morphology in cement mortar. The diameter of SAP particles is 4-5 times larger than that of ordinary pores at 50 d.
Figure 12 shows the microstructure of SAP cement mortar at 100 d. The residual pores of SAP can hardly be distinguished from ordinary pores. As shown in Fig. 12(a), part of SAP was encapsulated by cement stone, and part of SAP was separated from the surrounding cement stone (Fig. 12(b)), which were similar with the micromorphology at 50 d (Figs. 11(a) and 11(b)). The diameter of SAP particle at 100 d is significantly smaller than that of SAP particle at 50 d. What is more, the diameter of a few SAP particles was even reduced to the diameter of its dry state. The results show that when the age reached 50 d, the release of water was still continuing, while at the end of 100 d, some of the SAP particles had completely released water and the releasing process was still ongoing for other SAP particles. With the increase of age, the hydration process continued, resulting in more hydration products. Figure 12(c) shows a large number of needle-like hydration products around SAP particles. The results indicate that the hydration process of cement mortar was prolonged. The diameter of SAP particles was about 100-200 μm, which was 2-3 times larger than that of the common pore in cement. It can be seen that the cement hydration is more complete and the cement stone structure around SAP particles is more compacted.
Strength
Figure 13 presents the compressive strength of cement mortar with different SAP content. From 14 to 28 d, the compressive strength increase was about 14.1%, 10%, 5%, 14.5%, and 13.3% for specimens with 0%, 0.25%, 0.5%, 0.75%, and 1% SAP, respectively. While when the curing period increased from 28 to 120 d, the compressive strength increment was about 2.7%, 5.2%, 12.4%, 5.8%, and 6.3% for specimens with 0%, 0.25%, 0.5%, 0.75%, and 1% SAP, respectively. It can be inferred that the strength development was mainly formed during the first 28 d. It can be observed clearly that the compressive strength decreased with the increase of the SAP dosage. Microstructure analysis shows that lots of pores were introduced, and gaps existed between cement stone and SAP particles which had released water. Besides, micro-cracks were also formed because of the possible self-desiccation effect. Compared to the controlled mix at the same curing age, the pores and gaps contributed to the compressive strength degradation of specimens incorporated with SAP.
Compared to the specimens without SAPs, the compressive strength reductions were 6.9%, 21%, 17%, and 23.4% when the SAP dosage ranged from 0.25% to 1% at 28 d, respectively. While at the age of 120 d, the strength reductions were 4.6%, 13.5%, 14.5%, and 20.7% when SAP content was 0.25%, 0.5%, 0.75%, and 1%, respectively. The strength reductions at 120 d were less obvious than the reductions at 28 d. This is due to the fact that SAP helped the hydration proceeds further during the later stage.
The flexural strengths of all specimens are presented in Fig. 14. Compared to evolution process of the compressive strength, maximum flexural strengths can be obtained at 28 d with the exception of the specimens with SAP 1.0%. Compared to the specimens without SAPs treated, specimens with 0.25%, 0.5%, 0.75%, and 1% SAPs presented the strength reductions of about 12.7%, 12.6%, 23.7%, and 30.1%, respectively. However, at 120 d, the reductions were 20.7%, 8.7%, 12%, 10.9%, and -0.2%, respectively. This indicates that although SAP led to the flexural strength decline compared to the neat specimens, the presence of SAP can effectively inhibit the strength reduction at a later stage. Because of the internal curing effect induced by SAP, unhydrated cement could by continuously hydrated after 28 d, so that the decline trend of the flexural strength was thereby significantly alleviated.
The maximum compressive strengths were obtained at 120 d, while at 28 d for the maximum flexural strengths. This is because the micro-cracks generated during the self-desiccation [
31] play a more significant effect on the flexural strength rather than on the compressive strength. Self-desiccation refers to the reduction of the internal relative humidity because of the various factors during the hardening process. Microstructure shows that from 50 to 120 d, more SAP particles released water, so that more pores were introduced and more gaps between SAP particles and C-S-H were generated. It should be mentioned that the newly formed hydration products are not enough to fill the gap. The microstructure evolution is the main contributor leading to the flexural strength decline from 28 to 120 d, which is consistent with the research results of the existing literature [
29,
32]. It is suggested that SAP with smaller size be used to improve the microstructures and further reduce the negative effect of SAP on the mechanical properties.
Summary and conclusions
The aim of this study was to investigate the effect of SAP on the hydration heat, autogenous shrinkage law, mechanical behaviors and microstructure of cement mortars. A series of laboratory tests were conducted to explore these properties. Based on the results presented, the following conclusions can be drawn.
1) Compared to the samples without SAP, the reduction of the peak heat rate was up to 5.1%, 5.6%, 7.1%, and 10.3% when SAP dosage was 0.25%, 0.5%, 0.75%, and 1%, respectively. SAP significantly delayed the occurrence of the peak value.
2) The cumulative heat of specimen without SAP was larger than those of specimens with SAP at early stage, while SAP significantly promoted the hydration heat liberation. Hydration promotion caused by SAP mainly occurred in deceleration period and attenuation period.
3) At the end of 10 d, compared to the samples without SAP, the cumulative heat increases for samples with 0.25%, 0.5%, 0.75, and 1% SAP were 2.4%, 7%, 12.7%, and 17.5%, respectively.
4) SAP could effectively reduce the autogenous shrinkage and the shrinkage-mitigating effect was obviously enhanced with the increasing dosage. The shrinkage decreases of cement mortar with SAP content of 0.25% and 0.5% were 36% and 69%, respectively. SAP with the content of 0.75% and 1% even helped expand the volume.
5) After the releases of water in the SAP, the additional hydration products could not compensate the hollow space caused by the shrinkage of SAPs. Pores and gaps would be formed around the SAP particles, and the interfacial transition zone between the SAP particles and the surrounding hydration products become weaker.
6) The strength of SAP cement-based materials decreased with the increase of SAP content. The pores and gaps observed from the microstructure evolution contributed to the inferior mechanical behaviors. It is recommended that SAP with smaller sizes be added to reduce the negative effect on early strength in the future study.
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