Approaches to enhance the carbonation resistance of fly ash and slag based alkali-activated mortar- experimental evaluations

Abstract

Alkali-activated composites using by-products such as fly ash and slag have been widely studied to make greener construction materials. However, the carbonation of alkali-activated composites has been reported to induce cracking and corrosion of steel reinforcements in structural elements. This shortcoming has greatly limited the structural applications of alkali-activated composites due to durability concerns. This study aims at exploring possible approaches to enhance the carbonation resistance of fly ash and slag based alkali-activated mortar (AAM) by adding different additives, including calcined hydrotalcite (C-HT), calcium silicate (CS), gypsum and silica fume (SF), in the mix proportions. Various blend ratios of additives were considered in the series of laboratory investigations. After 28 d of curing, AAM specimens were subjected to accelerated carbonation condition with 1% CO2 concentration at 25 °C up to additional 56 d. The carbonation resistance was evaluated by examining the carbonation depths, followed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and nitrogen (N2) adsorption/desorption tests for in-depth investigations. In addition, the workability of the fresh mortar and the compressive strengths at different ages were also considered for comprehensive evaluation of the feasibility in structural applications. According to the test results, it is found that the addition of C-HT is the most effective measure among the approaches considered in the present study in terms of improving the carbonation resistance and compressive strength, while assuring the workability of the mortar.