Drying shrinkage of alkali-activated cements: effect of humidity and curing temperature

Alkali-activated concrete (AAC) is a recycled and low-CO₂ alternative to ordinary portland cement (OPC) concrete. One challenge with AACs is their propensity to shrinkage and cracking. In this work, drying shrinkage of four different structural-grade AACs was studied, namely: an activated class F fly ash, an activated slag, and two activated fly ash/slag blends. All four binders and a control OPC binder had the same (liquid/solid)_vol and initial porosity. Drying shrinkage and mass loss of paste prisms were monitored as a function of time, relative humidity (RH), and the preceding moist-curing temperature. The results show that all AACs moist cured at 23 °C (73 °F) showed high shrinkage in comparison with the OPC binder and the shrinkage increased with higher proportions of fly ash. Steam curing at 60 °C (140 °F) substantially reduced the shrinkage of fly ash-rich mixtures, but it was less effective for slag-rich mixtures. Slag mixtures showed a significant time dependent response (creep), where the pastes continued to shrink after their mass loss had reached equilibrium.