## 2 Materials and Methods  
### Materials  
The materials used include OPC, DE, and RS. OPC is used as a binder, while DE is used to partially replace cement and serves as an internal curing agent. RS is used as aggregate. The DE is industrial-grade diatomic produced by Henan Boxu Environmental Protection, and its main component is silica, with a SiO\({}_{2}\) content of 71.56% and a loss on ignition of 0.303%. Meanwhile, the OPC used is produced by Tianshan Cement Factory in Urumqi City, with a loss on ignition of 1.75%. DE, made of industrial-grade diatomic from Henan Boxu Environmental Protection, is crushed and sieved to ensure it passes through a #200 sieve, with a loss on ignition of 0.303%. The microscopic morphology of DE was captured by scanning electron microscopy, as shown in Figure 1. The chemical composition of OPC and DE, determined using X-ray fluorescence testing, is presented in Table 1. Tap water from Urumqi City is used for mixing, and polycarboxylate water-reducing agent is added to adjust the fluidity of the cement mortar.  
The water absorption and release performance of internal curing agents are crucial indicators for determining their efficiency. Water absorption determines the amount of internal curing water available for practical use, and a higher water absorption rate leads to more internal curing water being introduced. This, in turn, provides more water during hydration and evaporation, resulting in better shrinkage reduction in the slurry. The water release rate has a significant impact on the internal curing effect. If an internal curing agent has a high water absorption rate but a low water release rate, introducing it into concrete with a low water release rate or no water release would not only waste water resources and increase the self-weight of the concrete but also fail to achieve a good internal curing effect. Therefore, it is necessary to measure the water absorption and release characteristics  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Chemical Analysis** & **OPC (\%)** & **DE (\%)** \\ \hline SiO\({}_{2}\) & 21.34 & 71.56 \\ Al\({}_{2}\)O\({}_{3}\) & 4.86 & 14.87 \\ Fe\({}_{2}\)O\({}_{3}\) & 3.56 & 1.57 \\ CaO & 63.92 & 4.16 \\ MgO & 1.78 & 4.95 \\ Na\({}_{2}\)O & 0.56 & 1.59 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Chemical composition of OPC and DE.  
Figure 1: Microstructural image of DE.  
of diatomite. Since DE is in powder form, with pores on its particle surface, its water absorption rate, measured using the tea bag method [31], is 230% of its mass, as shown in Figure 2a. By simulating the internal solution environment of cement mortar using potassium nitrate and a supersaturated solution of sodium bromide, and controlling the relative humidity at 94.5% and 50.5%, respectively, the water release performance of DE under different relative humidity conditions was measured, as shown in Figure 2b.  
As shown in Figure 2b, under a relatively low humidity of 50.5%, the water release rate (the percentage of water loss to total water absorption) of DE tends to be stable at a 4-day age, releasing approximately 95% of water. Under a relatively high humidity of 94.5%, the water release rate of DE tends to be stable at a 16-day age, releasing approximately 89% of water. The excellent water absorption and release properties of DE lay a foundation for its application as an internal curing agent.