Advanced phase change materials (PCMs) with tunable phase transition behaviors are increasingly meaningful to the regulation and control of temperature in changeable application environments. This study presents a graphene oxide-modified hydrate salt/poly(acrylamide-co-acrylic acid) copolymer hydrogel (GO-EHS/PAAAM) form-stable phase change composite wherein the phase transition behaviors of the hydrate salt PCM could be tailored through the special interaction of GO and PAAAM with the hydrate salt. The DSC results described that the peak shape during the endothermic process was converted from the single peak of a pure hydrate salt into two interconnected peaks from the GO-EHS/PAAAM composite, of which the mechanistic analysis was validated via SEM, FT-IR and Raman spectroscopy. The thermal conductivity of GO-EHS/PAAAM with 2 wt% of GO increased by 54% when compared with EHS/PAAAM. Moreover, the GO-EHS/PAAAM composite remained chemically and thermally stable after 300 thermal cycles. The obtained results suggested that GO-EHS/PAAAM composites were a promising route for achieving adjustable phase transition behavior targets of the hydrate salt PCMs used for smart thermal management.

As infrastructure construction advances toward the cold and highland environment, concrete preparation technology and service performance in the cold climate is receiving much attention. The freezing of pore water inside concrete leads to significant changes in its mechanical properties at negative temperatures. Therefore, to ensure the safety of civil engineering structures in cold regions, it is necessary to fully understand the change law and enhancement mechanism of the mechanical properties of concrete and be able to predict mechanical properties at a negative temperature. Since the 1970s, scholars have studied concrete’s negative temperature mechanical properties in different water-content states. This paper presents a comprehensive review of the changes in mechanical properties of concrete at low and ultra-low temperatures and further elucidates the evolution of its compressive strength, tensile strength, flexural strength, elastic modulus, and stress-strain relationship at low temperatures. It was found that the main factors affecting the mechanical properties of concrete at low temperatures were temperature and moisture content. The strength of concrete increases significantly with the decrease in temperature and the increase in moisture content. To better understand and predict the mechanical properties of concrete at low temperatures, the best model was suggested by analyzing the prediction models of different researchers and considering the dispersion of the data. Further, based on the G. Wiedemann pore model, the changes in the internal structure of concrete at low temperatures are described in detail, and the mechanism of its mechanical property enhancement is analyzed.

The essential role of the air void size distribution in air-entrained cementitious materials is widely accepted. However, how the air-entraining behavior is affected by features such as the molecular structure of air-entraining agents (AEAs), the type of solid particles, or the chemical environment of the pore solution in fresh mortars is still not well understood. Besides, methods to assess the interaction between AEAs and cement particles are limited. Thus, in this study, the air-entraining behaviors of three kinds of surfactant (cationic, anionic, and nonionic) were examined. The general working mechanisms of these surfactants were studied by zeta potential and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy. Results indicate that the cationic surfactant entrains improper coarse air voids due to the strong electrical interaction between air bubbles formed by the cationic surfactant and negatively charged cement particles. The anionic surfactant interacts with the positively charged part of cement particles, and thus entrains finer air voids. The interaction between the nonionic surfactant and cement particles is very weak; as a result, the nonionic surfactant entrains the finest and homogeneous air voids.

Fly ash (FA) has been an important ingredient for engineered cementitious composite (ECC) with excellent tensile strain capacity and multiple cracking. Unfortunately, the frost resistance of ECC with high‐volume FA has always been a problem. This paper discusses the influence of silica fume (SF) and ground‐granulated blast‐furnace slag (GGBS) on the frost resistance of ECC with high volume of FA. Four ECC mixtures, ECC (50% FA), ECC (70% FA), ECC (30% FA + 40% SL), and ECC (65% FA + 5% SF), are evaluated by freezing‐thawing cycles up to 200 cycles in tap water and sodium chloride solution. The result shows the relative dynamic elastic modulus and mass loss of ECC in sodium chloride solution by freeze‐thaw cycles are larger than those in tap water by freeze‐thaw cycles. Moreover, the relative dynamic elastic modulus and mass loss of ECC by freeze‐thaw cycles increase with FA content increasing. However, the ECC (30% FA + 40% SL) shows a lower relative dynamic elastic modulus and mass loss, but its deflection upon four‐point bending test is relatively smaller before and after freeze‐thaw cycles. By contrast, the ECC (65% FA + 5% SF) exhibits a significant deflection increase with higher first cracking load, and the toughness increases sharply after freeze‐thaw cycles, meaning ECC has good toughness property.