The influence of water absorption and porosity on the deterioration of cement paste and concrete exposed to elevated temperatures, as in a fire event

This study investigates why CaO rehydration causes total strength loss of OPC paste compared to only 65% loss of OPC concrete after 800 °C exposure. Subsequent water sorptivity tests revealed paste reacted instantaneously with water, completely disintegrating within minutes. This was attributed to an accelerated rehydration of CaO into CaOH2. This was not observed in concrete. Sorptivity tests using acetone eliminated the disintegration due to CaO reaction with water. Porosity tests indicated a higher porosity and coarser capillary pore size distribution (PSD) in paste compared to paste in concrete. This has an influence on the rate of water absorption. The rate of water absorption determines the CaO rehydration rate and ultimately the growth rate of CaOH2 crystals and type of CaOH2 crystals formed. Different types of CaOH2 crystals cause different levels of deterioration, not always leading to total disintegration. Therefore, the rate of water absorption is the determining factor controlling the extent of deterioration caused by CaO rehydration.