Effects of polycarboxylate superplasticisers with various functional groups on the pore structure of cement mortar

The air-entraining capabilities of polycarboxylate superplasticisers (PCEs) with different functional groups were systematically analysed. Using different molecular PCEs, the evolution process from bubbles in fresh cement mortar to pores in hardened cement mortar was investigated, considering their effects on mortar workability and mechanical performance. The results showed that PCEs with various functional groups have different air-entraining capabilities. PCEs with ester groups decreased the surface tension and significantly promoted the air-entraining capability, resulting in a notable increase of air content in fresh cement mortar. PCEs with sulfonic acid groups increased the surface tension and remarkably decreased the air content in the fresh cement mortar. A higher air-entraining capability benefits the dispersing properties of PCEs and therefore the PCEs with ester groups performed better in dispersibility in comparison with the PCEs with sulfonic acid groups. The stability of bubbles entrained in fresh cement mortar leads to differences in the pore structure of hardened cement mortar. The PCEs with ester groups diminished the size of large bubbles, leading to significantly decreased porosity of the hardened mortar. PCEs with sulfonic acid groups in the backbone barely optimised the pore structure, with a large amount of large pores remaining in the hardened mortar. Decreased porosity and pore size hence led to higher compressive strength of the hardened cement mortar.