Monitoring the carbonation-induced microcracking in alkali-activated slag (AAS) by nonlinear resonant acoustic spectroscopy (NRAS)

This article presents the potential of nonlinear resonant acoustic spectroscopy (NRAS) for noninvasive monitoring of the carbonation progress in alkali-activated slag (AAS) mortars. In the search for sustainable concrete, AAS has emerged as a potential substitute for ordinary portland cement binder. However, carbonation is reported to be an important durability concern for AAS due to the absence of portlandite. In this study, the correspondence between the physical properties and microstructural evolution of sodium silicate-activated slag (SS-AS) and sodium hydroxide-activated slag (SH-AS) mortars were studied over the full course of accelerated carbonation. The measured properties include the following: compressive strength, carbonation depth, porosity, pore size distribution, and phase assemblage. In addition, NRAS was used to track the changes in materials stiffness (linear resonance frequency) and hysteretic nonlinearity (amplitude dependency of resonance frequency). Scanning electron microscopy (SEM) images and porosimetry results showed the formation of microcracks and increased micrometer porosity in a carbonated AAS binder caused by calcium aluminum silicate hydrate decalcification; the cracking was more severe in the SS-AS than in the SH-AS. The NRAS results revealed a close correspondence between the observed microscopic changes in the samples and measured macroscopic test parameters, indicating the potential of acoustic techniques for monitoring the advancement of carbonation fronts in AAS mortars.