An investigation of the carbonation of alkaline activated cement made from blast furnace slag generated by charcoal

Abstract

In Brazil, 20% of annual steel production is from charcoal, with growth prospects due to the increase in the use of charcoal as a substitute for coke. The slags from coal blast furnaces (coke) have a large percentage of CaO to remove the present sulfur while those produced in charcoal blast furnaces have a low CaO-SiO2 ratio. Alkali-activated slag (AAS) is a type of cement obtained mostly from GBFS, which has good mechanical properties for engineering and low environmental impact in its production process. This process is based on the activation of the slag by an alkaline solution, such as hydroxides or silicates. However, because the product of hydration consists essentially of CASH, carbonation in AAS has been investigated. Also, studies dedicated to AAS from charcoal (low CaO/SiO2) are not common. Therefore, the goal of this paper was to study the behavior of GBFS-based AAS from charcoal under carbonated conditions by accelerated tests and compare it with that of Portland cement. The results showed that AAS presented higher carbonation depths (127.1 mm) than Portland cement (PC) (77.1 mm) and a sharp decrease in compressive strength from 37 to 28 MPa, which was related to intense shrinkage, decalcification of CSH and formation of vaterite. Although the slag from charcoal has more Al2O3 (13%) and MgO (6%), it was unable to form aluminosilicate or hydrotalcite, which could reduce the susceptibility of AAS carbonation.