Effect of steam curing on strength, durability and microstructure of concretes with and without mineral admixtures

This research explored effect of steam curing on strength, durability and microstructure of concretes with and without mineral admixtures of fly ash (FA) and ground granulated blast furnace slag powder (SL). The two types of C55 strength grade concretes (one is pure cement concrete, another one is concrete, replacing part of cement with 12% FA and 18% SL) were prepared and their compressive strength, chloride diffuse on coefficient and carbonation depth under the standard and steam curing conditions were measured and compared. The phase composition and microstructure of the concretes under the two curing conditions were tested by X-ray diffraction (XRD), scanning electron microscope (SEM) and mercury intrusion porosimeter (MIP). The results show that, on the one hand, compared with standard curing, steam curing is beneficial to the early strength, but not conducive to the later strength development and chloride permeability resistance of both two types of concretes. Furthermore, for the steam cured concrete, the 28d carbonation depth of the forming surface is higher than that of the bottom surface. What's more, steam curing can reduce the carbonization resistance of pure cement concrete and improve the carbonization resistance of concrete mixed with FA and SL. On the other hand, composite mineral admixtures of FA and SL reduce the early strength, the carbonization resistance of concrete cured with standard curing, but improve the later strength, the carbonization resistance and chloride permeability resistance of concrete cured with steam curing. FA and SL are thermally activated under steam curing and can react with the calcium hydroxide formed by cement hydration in the early stage, which produces larger amount of secondary hydration products to improve the pore structure and interfacial microstructure of the concrete with FA and SL. Thus, the adverse effects of steam curing on later strength and durability of concrete are significantly restrained by composite mineral admixtures of FA and SL.