Influence of the early-age length change of alkali-activated slag mortars on the corrosion of embedded steel

Alkali-activated slag (AAS) materials are one of the most promising alternatives to ordinary Portland cement (PC), as the AAS curing process does not require thermal activation, unlike the activation of other wastes. In addition, AAS exhibit lower porosity than PC, but experience shrinkage problems that can negatively affect their in-service implementation and durability. Shrinkage can directly impact the mechanical properties of AAS as well as the corrosion protection of steel reinforced structures in environments with chlorides, and be a factor affecting durability. The length change during the curing of these mortars can generate high stresses that are released through the formation of microcracks or cracks in their structure. Cracks can act as preferential diffusion paths for aggressive chloride ions and favor the corrosion of the reinforcement. The aim of the present work is to study the reduction in shrinkage that can be achieved for AAS using five different activators: NaOH 4 M, waterglass (WG) with two different SiO₂/Na₂O molar ratios (MR) and Na₂CO₃ solution without and with 10% MgO additions. The results reveal that AAS activated with Na₂CO₃ shows very reduced microcracking. The addition of expansive MgO completely eliminates microcracking but makes the mortar more porous. In the latter case, the pits become much smaller and potentially less dangerous than the ones appearing in the other studied mortars.