1. Introduction
The phenomenon of microcrack initiation and insufficient hydration are significantly generated in the cement concrete structure of highway engineering that is exposed to the atmosphere and are most likely caused by the frequent evaporation of water and water consumption during the hydration process, thus increasing the risk of deterioration of the concrete structure during the service period. Thus, reasonable curing measures should be taken to inhibit the generation of humidity shrinkage cracks effectively [
1]. At present, the material composition design of cement concrete tends to reach higher performance than before, which makes it difficult for curing water to infiltrate the material adequately due to its respectively dense structure. This will cause the frequent initiation of humidity shrinkage cracks, which may increase the risk of deterioration during the structure service period.
Cement concrete internal curing technology is one of the most promising technologies of shrinkage and crack resistance that can inhibit the humidity shrinkage cracks by releasing water via the internal curing agent that is incorporated into the cement paste [
2]. Nowadays, superabsorbent polymers (SAP) are one of the best internal curing agents. The pre-absorbent SAP particles can release water in a timely fashion, while the humidity of the cement paste decreases and the ion concentration of the pore solution increases, which can effectively reduce the shrinkage strain, promote the hydration of cementitious materials and improve the mechanical properties and durability of cement concrete [
3,
4].
In effect, the development of a curing effect, microstructure properties and the macro-performance of the internal curing cement concrete mainly depends on the water absorption and release performance of the SAP. However, the water absorption of SAP always changes dynamically with the ion concentration and pH of the cement paste in the early stage of mixing [
5]. The calculation error of internal curing water was often caused by the inaccuracy of the SAP water absorption test in previous studies. Too much or too little absorption and the absorbed rate will adversely affect the construction workability, mechanical properties and durability of cement concrete. Therefore, in order to control the water absorption and water release performance of SAP precisely and gain the most benefit from the internal curing effect of SAP in cement concrete, it is necessary to research the ‘water absorption-release’ mechanism of SAP in cement paste.
Nevertheless, most of the existing studies focus on the improvement of the macro-properties of cement concrete by SAP, including the mechanical property, shrinkage resistance, crack resistance and durability. Sun et al. [
6] found that SAP can reduce the compressive strength of concrete under water-curing conditions, while SAP can improve the later compressive strength of concrete under air-curing conditions. Liu [
7] and Kalinowski [
8] further analyzed the effect of SAP on the compressive strength of concrete. They believe that in the early stage, the addition of SAP would form large pores and reduce the compressive strength of concrete. With the increase of age, due to the hydration products formed by the internal curing water provided by SAP to compensate for the pores, the microstructure near the SAP was densified and the mechanical properties were compensated.
In the study of water absorption and the release performance of SAP, Schröfl [
9] and Kang [
10] studied the water absorption kinetics of SAP and proposed that SAP absorption in a solution is mainly controlled by osmotic pressure, which could be regarded as a diffusion process and can be expressed by Fick’s second law. Bi [
11] studied the short-term SAP absorption characteristics in cement paste and clarified the SAP absorption characteristics with the change of the dynamic chemical environment of water paste. However, the studies above only focused on the self-absorption of SAP. In addition, Yang [
12] and Lee [
13] found that the SAP absorption capacity was affected by the total ion concentration and concentration of [Ca
2+] in the process of cement hydration. Girum [
14] and Yun [
15] proposed that the water absorption-release capacity of different SAP in cement paste depends on its crosslinking density and the density of anionic functional groups. Tan et al. [
16] explored the SAP swelling characteristics in different solutions.
Li et al. [
17] studied the water absorption and release behavior of SAP by using the squeezing pore solution and autogenous shrinkage test. Zhang et al. [
18] have shown that the SAP water absorbed rate in a cement centrifugate increased rapidly, then gradually decreased and finally stabilized. Snoeck et al. [
19] used nuclear magnetic resonance (NMR) to study the water release effect of SAP cementitious materials during hydration, and proposed water release kinetics that effectively alleviate autogenous shrinkage. Mehdi et al. [
20] observed the water migration of cement-based materials during hydration by neutron radiography.
In conclusion, there is a certain amount of research on the water absorption-release performance of SAP in pure water and cement paste, but it mainly focuses on the simple study of water absorption performance. The existing research results have not yet involved the systematic study of the water absorption-release characteristics of SAP in fresh cement slurry, and related dynamic absorption-release mechanism research has not yet been carried out. Therefore, there is a lack of more scientific and reasonable methods to determine the amount of internal curing water, which causes the construction workability, mechanical properties and durability of the designed internal curing concrete to be unstable.
In this paper, the influence of environmental factors, such as the water–binder ratio, fly ash content, ionic valence, temperature and SAP particle size on the absorption performance of SAP was studied. Based on the swelling kinetics equation and image analysis technology, the absorption kinetics of SAP in cement paste were further studied. Moreover, the influence mechanism of temperature, humidity and capillary pressure on the water release behavior of SAP in cement paste was explored. Finally, the water absorption-release mechanism of SAP in fresh cement paste was proposed.