On the development of MK/BFS alkali-activated materials as repair mortars: Performance under free and restrained shrinkage tests

Abstract

Abstract This paper assessed the variables affecting the shrinkage and cracking of blended metakaolin/blast furnace slag (MK/BFS) alkali-activated mortars (AAM) designed for concrete repair. Twenty-one initial MK/BFS formulations with fixed workability were subjected to compressive strength and static modulus of elasticity, after which five formulations were selected for further tests. They were evaluated as to free shrinkage and restrained shrinkage on a concrete substrate and restrained shrinkage with stress concentrating devices. Mortars were also analysed in a concrete substrate with 0.25% addition of polypropylene fibres. The formulation with the lowest crack density was further studied in other conditions (prevention of water evaporation, thermal curing, higher aggregate/binder ratio, the addition of 0.25 and 0.75% PP fibres). In general, the results show that MK/BFS AAM may present satisfactory mechanical performance (high compressive strength, modulus of elasticity and bond strength) and free shrinkage results. However, this may bring misleading conclusions. Severe cracking is observed when the movement is restricted, irrespective of the formulation. Lower BFS content (20%), higher aggregate/binder ratio, fibres and prevention of moist evaporation help the reduction but not suppress the cracks, which were observed in AAM and also in reference PC mortar.