**1. Introduction**

The inevitable ingress of moisture with harmful ions into concrete could reduce the pH value of pore solution. Temperature is a key factor in the rate of moisture transport in concrete because it can change pore pressures to cause concrete spalling at a critical degree [1–3]. The rate of moisture transport in concrete directly affects the time and degree of concrete deterioration. Therefore, understanding the moisture transport process in concrete is essential to design durable concrete structures [4–6]. The main factors determining the rate of moisture transport in concrete include temperature, relative humidity (RH), microstructure and porosity of concrete.

The isothermal absorption and desorption of moisture vapor in a cement-based material reflect the ability of its pore structure to absorb and desorb moisture [7]. Therefore, the moisture absorption and desorption processes are typically characterized using isothermal absorption–desorption curves [8]. Concrete under an environment with different RH levels will finally attain an equilibrium state at a constant temperature when the pore structure reaches a particular moisture saturation level [9]. The Young-Laplace equation describes the relationship between the capillary pressure and aperture, while the Kelvin equation expresses gas–liquid equilibrium relationship between the curvature of liquid surface and vapor pressure. These two equations can be used to transform the isothermal absorption– desorption curve so as to represent the relationship between the capillary pressure and saturation [10,11]. Trabelsi et al. developed an isothermal absorption–desorption curve to describe the moisture desorption using statistical and finite element methods [12]. Neithalath et al. [13] calculated the intrinsic permeability of concrete based on the porosity, specific surface area and tortuosity. Baroghel-Bouny et al. [14] obtained isothermal desorption curves for concrete and determined a theoretical relationship between the relative permeability coefficient and saturation based on findings in the literature [15]. The diffusion

**Citation:** Zhang, Q.; Kang, Z.; Ling, Y.; Chen, H.; Li, K. Influence of Temperature on the Moisture Transport in Concrete. *Crystals* **2021**, *11*, 8. https://dx.doi.org/10.3390/cryst 11010008

Received: 9 December 2020 Accepted: 21 December 2020 Published: 23 December 2020

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coefficient of moisture in concrete was determined, and then the moisture transfer curves under the drying process were obtained.

Zhou and Li [16] studied the concrete permeability using a three-phase composite random aggregate concrete simulation model based on the finite element method. Wang and Ueda [17] discretely divided concrete at the mesoscale to characterize moisture transport and investigated the influence of the interfacial transition zone on the capillary absorption of the concrete. Li et al. [18] established a three-dimensional mesoscale model to evaluate concrete permeability.

In previous numerical studies on moisture transport, the parameters that determine the influences of temperature and RH in the driving force of capillary pressure, have not been derived. In this paper, isothermal adsorption and desorption experiments were carried out at three temperatures (20 ◦C, 35 ◦C, 50 ◦C) to obtain the adsorption–desorption curves. The influences of different temperatures and RHs on the moisture transport process of concrete were analyzed. Further, a model in function of RH and saturations of adsorption and desorption process was regressed based on experimental data. Using the Kelvin equation, the relationship between capillary pressure and saturation was evaluated. A moisture transport model in respect of capillary pressure and saturation of concrete was established and verified with the experiment results. In addition, the moisture transport behavior of concrete at different temperatures and RHs was simulated.

### **2. Materials and Methods**
