Crack Self-Healing in NaOH-Activated Slag-Based Composites Incorporating Calcium Hydroxide

Abstract

Geopolymers and alkali-activated materials (AAM) have emerged as a promising sustainable alternative to ordinary Portland cement (OPC). Although much effort has recently been devoted to a wide range of research on the self-healing of cracks in OPC-based composites, little is known about the self-healing potential of AAMs. Therefore, this study explores the crack self-healing capability of NaOH-activated slag composites using a portfolio of testing methods, including electrical conductivity, mercury intrusion porosimetry, inductively coupled plasma optical emission spectroscopy, scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy, and X-ray microcomputed tomography. Experimental results indicate that alkali-activated slag-based composites incorporating calcium hydroxide achieved higher levels of self-healing than control specimens without calcium hydroxide. X-ray micro-computed tomography coupled with three-dimensional image analysis demonstrated that the observed self-healing was a surface mechanism that only occurred at surface cracks. Calcium carbonate was found to be the main self-healing product in all test specimens. Leaching experimental results indicated that the concentration of Ca2+ ions in the AAM matrix plays a critical role in calcium carbonate precipitation and, thus, in the self-healing potential.