Mechanical and durability performance of alkali-activated slag cement concretes with carbonate and silicate activators

In the race for decarbonization, the construction industry is increasingly turning its interest to alkali-activated cements (AAC) as an alternative to Portland cement (PC). This paper studies sodium carbonate (Na2CO3) as a potentially environmentally-friendlier and cheaper activator of ground granulated blastfurnace slag (GGBS) in comparison to Na2SiO3 or combined Na2SiO3-Na2CO3 activators. The laboratory study started with an investigation of suitable mixing procedures, after which mechanical and durability testing was performed under four curing conditions. Ambient temperature with high-moisture curing gave better strength gain in time, whereas thermal curing gave the highest early strengths and lowest absorption and porosities but strengths stopped evolving at later times. The strengths of all tested AAC mixes were suitable for structural concrete although those containing Na2SiO3 had the highest strengths at all curing conditions and ages. For the mix with Na2CO3 only (suitable for C30/40 grade concrete vs C50/60 concrete for the mixes containing Na2SiO3) strengths were evolving considerably with curing time despite the lower early strengths. All mixes with AAC performed better in terms of chloride attack (accelerated corrosion test) compared to the PC mix, despite their higher water absorption and, in most cases, porosity. AAC mixes with Na2CO3 only (as activator) were the least affected in terms of strength after exposure to sulphates. Overall, the study gives promise that Na2CO3-activated slag can be a good AAC system as alternative to PC towards an increased sustainability in the construction sector.