Natural and accelerated carbonation of concrete containing fly ash and GGBS after different initial curing period

This paper presents a study of the carbonation process of concrete with different initial curing periods consisting of fly ash and ground granulated blast furnace slag (GGBS). Both are exposed in a natural indoor environment for 720 days and at accelerated conditions. The relationship between compressive strength and carbonation depth is analysed. The factors affecting the carbonation of high-volume mineral admixture concrete in a natural environment and accelerated conditions are discussed using the orthogonal method. The factors include: water–binder ratio, content of mineral admixtures, total amount of cementitious materials and fly ash–slag ratio. The influence of different initial curing periods at early ages on carbonation depth is also reported. The results show that there is not always a linear rule between compressive strength and carbonation depth of high-volume mineral admixture concrete. Adequate curing at an early age can decrease the carbonation resistance of concrete significantly. For insufficiently cured concrete, the early stage before 56 days is the main developing stage of carbonation in a natural environment. An evident decrease of carbonation depth can be observed after exposure for 180 days and this will make the carbonated area turn to a red colour again when sprayed with phenolphthalein. In order to guarantee the excellent anti-carbonation ability of concrete in field structure, it is very important to ensure the initial curing time, except for restricting the water–binder ratio and the content of mineral admixtures.