A critical review of the advancements in acid-activated metakaolin geopolymers

Alternative to ordinary Portland cement (OPC), geopolymer binders emit low CO2 during manufacturing. Recently, acid-activated geopolymers were found to transform the mechanical performance of the binder. The review uses scientometric analysis to summarize the advancements in phosphoric acid-based metakaolin geopolymer (MGPacid). Web of Science (WOS) was used to retrieve relevant literature data. Science mapping and bibliometric viewpoint by CiteSpace presents: (1) the growth of the literature with the highest citations, (2) keyword analysis, (3) the contributing journals, (4) the most influential researchers, and (5) countries' citation impact targeting the relative importance of the studied direction. Findings reflect the improvement in properties of geopolymer, including denser microstructure, mechanical strength, and evolution of reaction mechanism. Reaction kinetics of MGPacid were assessed under different factors of calcination temperature, type of precursor, acid molarities, molar mass ratios, and liquid/solid ratios exposed to various curing conditions. For assessment appraisal, the keyword occurrences (frequency analysis) were followed by the impact of influencing factors on the mechanical performance of the geopolymer. MGPacid was primarily used as a construction material, but areas of durability and its adoption in other applications need further investigation. MGPacid shows higher strength compared to alkali-activated geopolymer (MGPalkali). Various studies have conducted CO2 adsorption of MGPalkali after developing composites having activated carbon and zeolites. MGPalkali geopolymers showed better chemical stability than OPC under an acidic CO2 storage environment. As per authors' knowledge, the application of MGPacid in CO2 sequestration application is not made yet, particularly for enhanced oil and gas recovery (EOR and EGR) applications. Our findings suggest exploring alternative low-cost acid activators for geopolymers to develop an anti-CO2 corrosive binder to mitigate the well integrity issues of CO2 sequestration.