Alkali activated cement made from blast furnace slag generated by charcoal: Resistance to attack by sodium and magnesium sulfates

Abstract

In Brazil, 20% of annual steel production where the furnaces are fueled using charcoal, with growth prospects 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, while those produced in charcoal blast furnaces have a low CaO-SiO2 ratio. This study investigated the behavior of alkali activated slag (AAS) made from GBFS generated by charcoal under attack by sulfates. Acceleration of the attack was achieved using miniature paste prisms and cylinder specimens. Sulfate-resistant blended cement samples (PC) were used for comparison. The results showed that AAS presented good performance in sodium sulfate solution, superior to that of the sulfate-resistant blended cement, such that no expansion, loss of compressive strength or degradation of C-S-H were observed. However, when AAS was subjected to magnesium sulfate solution, decalcification of C-S-H, formation of gypsum and alteration of physical, chemical and mechanical properties were observed. Also, the decreasing in compressive strength was much more intense for AAS. The tests using miniature past specimens were efficient at accelerating the sulfate attack, but the intense shrinkage of AAS indicated the need to monitor the mechanical performance.