Negative temperature curing is a very harmful factor for geopolymer mortar or concrete, which will decrease the strength and durability. The water in the geopolymer mixture may be frozen into ice, and the water content is a crucial factor. The purpose of this paper is to explore the influence of water content on the properties of alkali-activated binders mortar cured at −5 °C. Fly ash (FA) and ground granulated blast furnace slag (GGBFS) were used as binders. Three groups of experiments with different water content were carried out. The prepared samples were investigated through uniaxial compression strength test, Scanning electron microscopy (SEM), and X-ray diffraction (XRD) for the determination of their compressive strength, microstructural features, phase, and composition. The results indicated that, the compressive strength of samples basically maintained 25.78 MPa–27.10 MPa at an age of 28 days; for 90 days, the values reached 33.4 MPa–34.04 MPa. The results showed that lower water content is beneficial to improving the early strength of mortar at −5 °C curing condition, while it has little impact on long-term strength. These results may provide references for the design and construction of geopolymer concrete in cold regions.

Engineering practices illustrate that the water phase change in soil causes severe damage to roads, canals, airport runways and other buildings. The freezing point is an important indicator to judge whether the soil is frozen or not. Up to now, the influence of salt on the freezing point is still not well described. To resolve this problem, a series of freezing point tests for saline soil were conducted in the laboratory. Based on the relationship between the freezing point and the water activity, a thermodynamic model considering the excess Gibbs energy was proposed for predicting the freezing point of saline soil by inducing the UNIQUAC (universal quasi-chemical) model. The experimental results show that the initial water content has little influence on the freezing point if the initial water content is higher than the critical water content, while the freezing point decreases with the decrease of the water content if the initial water content is lower than the critical water content. Moreover, it is found that the freezing point is related to the energy status of liquid water in saline soils and it decreases with the increase of the salt concentration. Moreover, the freezing point depression of saline soil is mainly caused by the decrease of water activity. Compared with the other two terms, the residual term, accounting for the molecular interactions, has an obvious influence on the water activity. This result is helpful for understanding how salt concentration affects the freezing point of saline soil and provides a reference for engineering construction in saline soil areas.

Electrical resistivity of soil has become one of the most important indicators of compactness and compressive resistance. In previous studies, the temperature of soil tested was always above 0 °C. However, due to the phase change of pore water, the resistivity of freezing soils is different from that of unfrozen soils. Therefore, it is necessary to clarify the influence of temperature on electrical resistivity. To achieve this goal, according to the bundle of the cylindrical capillary model, pores in soil were treated as conducting tubes, and two simple lognormal functions were introduced to describe the distribution of the conducting tubes. Based on the conductivity theory, a new model considering the temperature and the pore size distribution was presented. Results show that only a part of the conducting tubes can be treated as the effective conducting tubes, which are determined by the soil temperature. Furthermore, a small portion of the pores with the smaller size controlled the electrical resistivity of the freezing clay. In other words, the change in the electrical resistivity of freezing clay was caused mainly by the change of unfrozen water content through the decrease in temperature. Based on the experimental results conducted on clay from Northeast China, the validity of the presented model was verified. The comparison results indicate that there was a good agreement between the calculated results and the experimental results. To expand the scope of the model, more laboratory tests are suggested to conduct to verify the applicability of the model to other clays.