Yong Ge

Thermal conductivity plays a significant role in controlling thermal cracking of cement-based materials. In this study, the thermal conductivity of cement paste at an early age was measured by the hot plate method. The test results showed that the thermal conductivity of cement paste decreased with the increase of water/cement ratio and curing age. Meanwhile, a multiphase model for the thermal conductivity of cement paste was proposed and used to study the influence of saturation and curing temperature on the thermal conductivity of cement paste. To determine the parameters involved in this model, the thermal conductivity of each phase in cement paste was calculated by the molecular dynamic simulation method, and the hydration of cement was simulated by the Virtual Cement and Concrete Testing Laboratory. The inversion results showed that the relative error between experimental and simulation results lay between 1.1% and 6.5%. The thermal conductivity of paste in the saturated condition was 14.9–32.3% higher than that in the dry state. With the curing temperature increasing from 10 °C to 60 °C, the thermal conductivity of cement paste decreased by 3.9–4.9% depending on the water/cement ratio.

The efficiency and stability of air entrainment in concrete are sometimes found to be weaker at higher elevation. This phenomenon was attributed to the low atmospheric pressure by many researchers, however, the level of influence of atmospheric pressure on concrete air content dramatically varied among different studies. In order to clarify the effect of low atmospheric pressure on air entrainment in cement-based materials, an on-site experimental study was conducted with a rigorous control of irrelevant variables. The study focused on the air-entraining efficiency in cement paste, mortar, and concrete prepared in both low and standard atmospheric pressures. The air bubble stability in fresh mortar and air void characteristics of hardened mortar in different atmospheric pressures were also included. In the study, little effect of low atmospheric pressure on the air-entraining efficiency and air bubble stability in mortar with studied air-entraining agents (AEAs) was found. The air void characteristics were found to be similar between mortar with SJ-2 or 303R type AEAs prepared in different atmospheric pressures. Concrete with either SJ-2 or 303R type AEA prepared in low atmospheric pressure presented a satisfactory air content. These conclusions indicate that it is not necessary to worry excessively about the potentially adverse effect of atmospheric pressure on the frost resistance of concrete if a suitable AEA is applied. Additionally, a supplementary mortar study found that the low temperature of raw materials stored at high elevation would significantly weaken the air entrainment, reminding that potential causes in addition to low atmospheric pressure should also be taken seriously.

Tribology, as a classical physical science, plays a key role in the understanding of long-term performance of pavements, especially that of hydraulic cement concrete pavement. Recent studies have indicated that graphene oxide (GO) can be used as a promising nano-sized additive to improve the mechanical properties and durability performances of cement composites. However, the effects of admixing GO on the tribological properties of ordinary portland cement (OPC) concrete and related mechanisms are still unclear. In this study, we tested the compressive strength and scratch properties of OPC mixtures to investigate their possible correlations with the abrasion resistance. Results reveal that the addition of GO (up to 0.1% by weight of cement) can increase the compressive strength at the macro level and scratch depth at the micro level, both of which could be related to the abrasion resistance of OPC concrete. XRD and SEM/EDS analyses were also conducted to help explain the beneficial effects of GO in OPC concrete. The mechanisms underlying the improved performance are mainly through accelerated hydration of cement and nano-reinforcements induced by the admixed GO. GO can provide nucleation sites for hydration, which facilitates the consumption of cement clinker and production of hydration products. This work helps achieve a mechanistic understanding of the tribology of concrete containing GO, at both macro and micro levels.