Influence of defoamer on mechanical performance and pore structure of steam-cured mortar containing polycarboxylate superplasticizer

To counteract the excessive air entrainment of polycarboxylate superplasticizers (PCEs) and diminish the damage caused by the expansion of air bubbles during steam curing, defoamers were used in this study to solve the problem of high porosity and coarse pore size of the steam-cured mortars containing PCEs. The effects of mineral oil defoamer and polyol defoamer on fluidity, heat of hydration, compressive strength and pore structure were investigated at different dosages (0.03–0.09 %), and the performance differences between the two defoamers were compared in fresh and hardened mortar. The results showed that the defoamers reduced the fluidity and fluidity loss of cement and had a retarding effect on cement hydration. Besides, the defoamers increased the compressive strength due to the improvement of the pore sizes and spacing coefficients. The differences in the properties and action mechanisms of the two defoamers resulted in different effects on the mortar, especially on compressive strength, and the differences were more significant in the 28-day strength. Overall, the mineral oil defoamer was better adapted to steam curing with an optimal dosage of 0.05 %.