Investigation of the structure of heated Portland cement paste by using various techniques

Portland cement paste, a porous material with a multi-scale structure, consists of unhydrated clinker, hydration products and pores. At elevated temperatures, the structure of Portland cement paste changes, which might affect the strength and permeability of cement paste. This paper investigates the structure of heated Portland cement paste by means of scanning election microscope, mercury intrusion porosimetry and nitrogen adsorption, respectively. The structures of cement paste at different scales, i.e., nano-, micro- and meso-scale, determined by various methods are compared and discussed. At microscale (0.1–100 μm), the capillary pore volume in heated cement paste increases slightly at temperatures from 105 °C to 400 °C, and increases dramatically at temperatures beyond 500 °C. The cracking in heated cement paste is caused by the dehydration-induced volume change of hydration products. At nanoscale (2–100 nm), the structure of C–S–H mainly remains constant at temperatures from 105 °C to 400 °C, and is partly transformed from gel structure to crystalline particles at temperatures beyond 500 °C. The size of crystalline particles increases with the increase of temperature. At atomic scale (<2 nm), the tobermorite-like C–S–H starts to be transformed into belite (β-C2S) at 500 °C.