Influence of low-temperature curing on the mechanical strength, hydration process, and microstructure of alkali-activated fly ash and ground granulated blast furnace slag mortar

Abstract

Alkali‑activated concrete, a green building material with excellent mechanical properties and durability, is the most promising building material for use in the continuously increasing number of infrastructure construction projects in cold regions. However, at present, there are few studies evaluating the performance of alkali‑activated concrete cured in a low-temperature environment. In the present work, 50% fly ash (FA) and 50% ground granulated blast furnace slag (GGBFS) were used as composite cementitious materials to prepare four groups of alkali‑activated mortar samples, which were cured at −5 °C, 0 °C, 5 °C and 20 °C. The effects of low curing temperature (−5 °C, 0 °C, and 5 °C) on the mechanical properties, hydration process, hydration products, pore characteristics and microstructure of alkali‑activated FA and GGBFS (AAFG) mortar were investigated with uniaxial compressive strength tests, resistivity measurements, X-ray diffraction (XRD), scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP). The results indicated that low-temperature curing reduced the compressive strength of the alkali‑activated mortar. The resistivity curves of the four groups of samples well reflected the influence of different curing temperatures on the early hydration process of the alkali‑activated binder. A low curing temperature delayed the hydration process and showed an obvious “hysteresis effect”. The lower the curing temperature was, the more significant the effect. The hydration reaction was not stopped when the sample was cured at −5 °C. Low curing temperatures did not change the type of hydration products but changed their amount. Low-temperature curing resulted in the formation of harmful pores.