Drying and carbonation shrinkage of cement paste containing alkalis

In this work, shrinkage performance of ordinary portland cement (OPC) paste containing various alkali salts was characterized at two drying conditions, namely: nitrogen gas and air. The results show that incorporation of alkalis dramatically increases shrinkage magnitude, but reduces shrinkage kinetics of OPC, regardless of source and type of alkalis (e.g. Na⁺ or K⁺). The amount of alkalis bound in the solid hydrated phases, rather than the free alkalis remaining in the pore solution, is crucial in controlling the shrinkage performance of OPC. It is suggested that the alkali enrichment in OPC increases the visco-elastic/visco-plastic compliance (reduce creep modulus) of its solid skeleton under drying-induced internal stresses. This phenomenon is likely to be attributed to the alkalis binding in calcium–silicate–hydrate (C–S–H), which promotes the packing of C–S–H nanoparticles. Carbonation results in shrinkage (i.e. carbonation shrinkage) in plain OPC, but expansion in OPC with alkali enrichment. The overall volume change of OPC due to carbonation may be a result of competition between dissolution-induced shrinkage and crystallization-induced expansion.