Co-activation of Blended Blast Furnace Slag and Fly Ash with Sodium Sulfate and Hydroxide

Abstract

The addition of sodium sulfate (Na2SO4) to alkali-activated blast furnace slag (BFS), specifically when sodium hydroxide (NaOH) is used as an activator, allows to reduce the amount of hydroxide necessary to achieve desirable mechanical properties. This kind of activation is suitable for BFS that does not require very high pH to promote its hydration. However, it might fall short in case of hybrid binders such as mix of BFS and fly ash (FA). The objective of this study is to elucidate whether the co-activation with Na2SO4 and NaOH of these blended mixes leads to advantages compared to just NaOH. The effectiveness of such activation on blended systems is evaluated by means of isothermal calorimetry, products assemblage and compressive strength. The FA replacement significantly affects the induction period length, especially when low NaOH concentration is used, leading to poor early age properties. Nevertheless, the obtained results show that, at later ages, it is still possible to obtain mechanical properties suitable for practical application.