Effects of the early curing regime on the properties and pore structure of concrete in an environment with high altitudes and low atmospheric pressures

High altitudes have harsh, complex climates and changing environments. These conditions considerably challenge the service life of concrete structures. The engineering quality of concrete structures at high altitudes can be improved by scientific curing regimes; similarly, carbon emissions can be reduced throughout their life cycle. In this paper, the effect of the early curing environment on the macroscopic properties, microstructure, and hydration products of concrete realistically exposed to harsh environments (10-30 degrees C, RH 20-80 %, 70 kPa) of high altitude was studied. The mechanism of the effect of the early curing environment on the microstructure of the concrete at high altitudes was determined. The concrete performance at high altitudes was investigated regarding the enhancement mechanism of steam curing. At high altitudes, the deterioration of concrete properties was primarily caused by an increase in the percentage of pores sized between 20 and 200 nm and the total porosity. Appropriate steam curing can be employed to enhance concrete hydration in these areas. This process refines the pores in hardened cement paste at the micro-nano scale (20 nm-200 nm and 50 mu m-100 mu m), ultimately reducing total porosity. Additionally, it alleviates the "aging phenomenon" of the C-S-H gel, which is induced by the low atmospheric pressure environment. The comprehensive performance of concrete in high-altitude areas can be significantly improved by steam curing at 35 degrees C. Compared with the uncured and sprinkling curing cases, steam curing at 35 degrees C for 3 days increased the 28day compressive strength of concrete exposed to the natural environment at high altitudes by 42.8 % and 23.5 %, respectively, reduced the 56-day drying shrinkage strain by 38.7 % and 38.5 %, respectively, and could significantly reduce the risk of concrete cracking at high altitudes.