Acid-resistance of one-part geopolymers: Sodium aluminate and carbonate as alternative activators to conventional sodium metasilicate and hydroxide

In this study, the durability of one-part geopolymers (OPGs) made from high calcium fly ash is assessed by investigating their resistance to acid attacks. The predominant use of less environmentally sustainable sodium metasilicate (Na2SiO3) and sodium hydroxide (NaOH) in OPG, along with the limited understanding about the influence of solid alkali activators on OPG's acid resistance propelled the investigation and comparison of acid resistance of OPGs activated using both conventional activators and potential alternatives like sodium aluminate (NaAlO2) and sodium carbonate (Na2CO3). The OPGs developed were exposed to sulphuric acid (H2SO4) solution for 28 days. The MH sample (activated with Na2SiO3 and NaOH) was vulnerable to acid attack, with a 66% compressive strength drop after 5% H2SO4 solution exposure, due to high sorptivity. In contrast, the MA sample (activated with Na2SiO3 and NaAlO2) demonstrated excellent acid resistance, with only a 32% strength reduction, attributed to its Al-rich hydrated gel. The MC sample (activated with Na2SiO3 and Na2CO3) exhibited a 41% strength reduction, where the formation of calcite reduced extensive ion exchange, gradually mitigated deterioration. The residual compressive strength of MA and MC samples increased by 7% and 9% from 7 to 28 days of immersion in 5% H2SO4 solution, respectively. The NaAlO2 and Na2CO3 are potential activators for OPG, serving as alternative construction materials to OPC and traditional geopolymers in acidic environments.