Acid Activation in Low-Carbon Binders: A Systematic Literature Review

Abstract

Geopolymers have emerged as an alternative binding material to Ordinary Portland Cement (OPC). Recently, there has been an increase in studies exploring the synthesis of these materials using acid activation rather than traditional alkaline activation. This approach offers benefits such as good strength at an early age, better thermal properties, and a chemical activator that emits less carbon to be produced. In addition, it provides resistance to efflorescence and leaching, which are common challenges associated with alkali-activated products. This work analyzed the scientific advances in acid activation in synthesizing an alternative binder to OPC. To this end, a systematic review of the last five years of scientific literature was carried out using the Systematic Review for Engineering and Experiments (SREE) method. The results show a notable increase in research focused on acid activation over the last few years. The acid activators were always phosphate solutions, mainly phosphoric acid. Metakaolin was the most tested precursor, followed by fly ash, and volcanic ash. The research requires improvements in the methodological quality, providing data on molar ratios (Al/P, Si/Al, and Si/P), Liquid/Solid mass ratio, activator solution molarity, and curing process, in addition to statistical treatment and comparison of results. There exists a paucity of diversity in the examined precursors, activators, and additives. Future research developments need to clarify the behavior of mechanical resistance over time, better curing process, water resistance, durability, and the role of iron, magnesium, and calcium silicates and/or oxides. The paper identifies the main research gaps in the area and functions as a database, guiding researchers in selecting raw materials, dosing methodology, and curing processes.