Improving the Carbonation Resistance of Alkali-Activated Slag Containing Mineral Additions: Investigation under Natural and Accelerated Conditions

Abstract

Alkali-activated slag (AAS) is a type of cement obtained mainly from ground blast furnace slag (GBFS), which has good mechanical properties for engineering and low environmental impacts in its production process, as it is based solely on the activation of the slag by an alkaline solution. However, because the production of hydration consists essentially of C─ S─ H, its resistance to carbonation is low. The present study investigated the improvement in the carbonation resistance of AAS made from GBFS generated by charcoal with the addition of calcium hydroxide, calcium sulfate, and magnesium oxide in its composition. The analyses were conducted under natural and accelerated carbonation conditions in pastes and concretes for 4, 12, 24, and 48 weeks and 4, 8, 12, and 16 weeks, respectively. Carbonation depth, compressive strength, and microstructural analysis by x-ray diffraction were determined during exposure to carbonation. The results showed that, under natural conditions, the AAS containing magnesium oxide presented lower carbonation depths and a smaller decrease in compressive strength. Under accelerated conditions, all AASs were totally carbonated, although the sample containing magnesium oxide showed a higher compressive strength than the other AASs. It was noted that the consumption of this oxide prevented C─ S─ H decalcification. Thus, magnesium oxide could be an alternative for increasing the resistance of AASs to carbonation; however, its efficiency is highly dependent on its physicochemical characteristics.