Mitigating shrinkage of alkali activated slag with biofilm

As an emerging alternative cementitious binder, alkali activated slag (AAS) is gaining great attention, but considerable shrinkage caused by alkali activation and drying limit its potential applications. Herein, we demonstrate that the addition of an environmentally benign biofilm, cultured from B. subtilis, mitigates both the autogenous and drying shrinkage of AAS. The influences of the biofilm on the hydration kinetics, water absorption and strengths are investigated. Results show the addition of the biofilm increases the hydrophobicity of the pore wall, which in turn decreases the capillary tension. The hydrophobic modification by the biofilm significantly reduces the water loss from the AAS to its direct environment (up to 86% at 35 d exposure). Consequently, both autogenous and drying shrinkage of AAS are dramatically reduced. Moreover, a new mechanism is proposed to explain the mitigation of AAS shrinkage, which takes into account the increase in internal RH and reduction in capillary pressure.