## 1 Introduction  
[PERSON] and [PERSON] [1] added a super absorbent polymer (SAP) into concrete for internal curing (IC) in 2001. Since then, the role of SAP in the internal curing (IC) of high performance concrete has gained wide recognition. SAP can significantly delay the decrease in the internal moisture content of concrete-based materials and can inhibit self-shrinkage [2, 3]. Therefore, it can be used in the internal maintenance of concrete. However, SAP can also release water and create pores during the process of hydration. The created pores have a negative effect on the strength development of concrete [4], especially on the early mechanical properties. Super absorbent polymer (SAP) is a polymer that can be obtained by the polymerization of monomers containing a multitude of hydrophilic groups (e.g., carboxyl (-COOH), hydroxyl (-OH), and amide groups (-CONH2) [5, 6]. It has a three-dimensional network structure, which is formed by a low degree of cross-linking. It also has excellent salt and alkali resistance and excellent water absorption-release properties [7]. By releasing the pre-absorbed water during hydration, SAP added to concrete enables the hydration of cement, keeps the internal humidity of concrete and promotes further hydration of cement [8]. Compared with other materials for internal curing, SAP has two unique properties: it can not only keep the internal humidity of concrete and inhibit shrinkage [9], but can also increase the resistance to frost erosion and improve the durability of concrete by the increased porosity and air content inside the concrete with the water released from it [10; 11]; moreover, the SAP water absorption multiplier can be designed in different situations so as to adapt to the shrinkage under different conditions introduced by different water-to-cement ratio concrete materials [12]. Autogenous shrinkage and drying shrinkage are the direct consequences of the decrease in internal relative humidity (IRH) due to autogenous shrinkage and external drying [13]. SAP can be used to inhibit concrete autogenous and drying shrinkage. [PERSON] et al. [8] studied the effect of different amounts of SAP on the shrinkage and cracking resistance of concrete, by the concrete restraint ring test, and the results showed that the shrinkage value of concrete decreased and concrete cracking time increased with the increase in SAP, indicating a positive effect of SAP on shrinkage restraint. [PERSON] et al. [14] studied the effect of the particle sizes of SAP internal curing materials on the performances of self-compacting concrete. It was considered that both the relative humidity inside the cement and the early autogenous shrinkage of self-compacting concrete (SCC) will decrease with the increase in the introduced amount of internal curing water. Incorporation of highly absorbent polymer SAP can effectively regulate the decrease in IRH of cementite due to self-shrinkage, thus reducing capillary stress and the self-shrinkage of concrete. [PERSON] et al. [15] pointed out that SAP alone delayed the hydration of cement and increased the percentage of macropores above 100 nm, and introducing lime-type expansion agent (KEA) into SAP mortar could promote the formation of micropores and increase the compressive strength slightly. After regression analysis, a relationship was found between the microscopic volume of pores above 50 nm and the macroscopic compressive strength. The compressive strength decreases gradually with the increase in the percentage of pore sizes above 50 nm. Super absorbent polymer (SAP) with high water absorption can be used in internal curing of concrete with a low water-to-cement ratio [16; 17]. It can release water and create pores as the cement hydrates [18]. The pores can adversely affect the strength of concrete. [PERSON] et al. [19] stated that both the particle size and powder SAP have significant influences on the performances of cementitious materials.