Surface quality of steam-cured concrete with defoamer-modified mould release agents

Steam curing is widely used for precast concrete, but mixtures containing polycarboxylate ether superplasticiser often develop surface bugholes and coarser pores near the mould-contact region. This study examined whether a water-based mould release agent (MRA) modified with a defoamer based on mineral oil (ODA) or a polyol-based defoamer (ADA) could improve the surface quality of steam-cured concrete. Concrete compressive strength, demoulded surface appearance and rebound-based near-surface uniformity were assessed. Steam-cured mortar was used to compare image-based pore characteristics in the mould-contact region and the interior. The image-based pore-structure response was more evident in the mould-contact near-surface region than in the interior. Compared with the blank group, ADA-6 (0.6% defoamer by mass of the MRA) increased the demoulded and 28-day compressive strengths by 21.84% and 10.13%, respectively, whereas ADA-9 (0.9% defoamer) gave the greatest near-surface pore refinement, reducing the mean pore diameter from 0.67 mm to 0.26 mm and the porosity from 6.58% to 1.63%. ODA was more dosage-sensitive, and complete replacement of the MRA reduced the 28-day strength by 15.70%. The results indicate that defoamer-modified water-based MRAs can improve demoulded surface quality, but strength and surface pore refinement should be optimised separately.