Behavior of Fresh Mortar in a Vacuum and Microstructure of Mortar Hardened in a Vacuum

The effects of vacuum and microgravity on the hydration of cement are primary concerns regarding casting concrete both on the moon and in space. Previous studies by others have focused on the behavior and mechanical properties of concrete cast in a vacuum without addressing the microstructural characteristics of the paste. More recently, an experiment was carried out onboard a space shuttle to investigate the effect of microgravity on cement hydration. In the present study, mortars hardened in a vacuum for 4 to 12 hours and control mortars not exposed to a vacuum were studied by light microscopy and scanning electron microscopy in order to examine differences in microstructural and microchemical characteristics. The principal findings, that rapid loss of water from the fresh mortar leads to frothy texture, poor cement hydration and paste development, and low compressive strength, appears to preclude casting water-rich mortar or concrete in a vacuum.