Pore structure controlling the carbonation of a hardened cement matrix blended with mineral admixture

Abstract

The pore size that controls the rate of carbonation of hardened cement matrix is identified. The rate of carbonation of mortar with and without mineral admixture (pulverized fuel ash (PFA) and ground granulated blastfurnace slag (GGBS)) is shown to depend on the total volume of pores having a radius >450 angstroms present in the hardened matrix prior to accelerated carbonation. The greater the volume of pores above the critical radius in the hardened matrix prior to accelerated carbonation, the faster is the rate of carbonation, and vice versa. The above relationship also holds for plain cement concrete and concrete blended with PFA and GGBS, as both concrete and mortar of similar type (water/binder (w/b) ratio, PFA or GGBS content) follow an identical carbonation trend. The results of the present study also demonstrate that the rate of carbonation of hardened cement matrix, apart from depending on the total volume of pores having a radius >450 angstroms, is also controlled by the parameters (cementitious content, type and proportion of mineral admixture in the mix) that control its alkalinity. The present study also emphasizes the need to precure the hardened cement matrix with and without PFA and GGBS in water for a minimum period of 7 days to minimize the pore volume above the critical radius (>450 angstroms), and thereby to optimize the resistance of the matrix to carbonation. The total volume of pores having a radius >450 angstroms is also shown to be linearly related to the 7 day compressive strengths of mortar blended with and without PFA and GGBS.