One-part geopolymer binders with sodium-based activators exposed to acid and sulphate environments

One-part geopolymer (OPG) evolved from traditional geopolymer, intended to substitute the Ordinary Portland cement (OPC) as a construction binder material. OPG is made by mixing of aluminosilicate precursor, solid alkali activator and water. OPG production commonly utilises blends of class F fly ash + ground granulated blast furnace slag (GGBS) as precursors, with Na₂SiO₃ as the solid alkali activator. Addition, limited information regarding the chemical resistance of OPGs, thereby restricting their applications. This research uncovered the influence of solid alkali activators on class C fly ash-based OPGs and their resistance to acid and sulphate attacks. Four OPGs based on were fabricated. The M-OPG (solely sodium metasilicate (Na₂SiO₃)), MH-OPG (Na₂SiO₃ + sodium hydroxide (NaOH)), MA-OPG (Na₂SiO₃ + sodium aluminate (NaAlO₂)) and MC-OPG (Na₂SiO₃ + sodium carbonate (Na₂CO₃)). The optimised mixtures were subjected to 3% and 5% sulphuric (H₂SO₄) solutions, and 5% and 10% magnesium sulphate (MgSO4) solutions. The fluidity, setting time, density, mass, porosity, compressive strength, loss of alkalinity, depth of degradation, leaching behaviour and material characterisation of the OPGs were evaluated. The influence of mix proportions of the OPGs with different solid alkali activators were distinctive. The optimised M-OPG attained the highest compressive strength. For the MH-OPG, the alteration of Na₂SiO₃/NaOH ratio significantly influenced fluidity and setting time but had no impact on compressive strength. The MA-OPG exhibited thixotropic fresh paste. The incorporation of Na₂CO₃ reduced the water demand of the MC-OPG. Optimum mix proportions selected were based on well-balanced fresh properties and compressive strength. The optimum M-OPG, MH-OPG, MA-OPG and MC-OPG exhibited 28-day compressive strength of 83.6, 72.7, 45.1 and 75.1 MPa, respectively. After H₂SO₄ exposure, the compressive strengths of all the OPGs were reduced. Gypsum was precipitated on the out-most layer of the samples, gradually restricting acid migration. After 28 days of exposure to 5% H₂SO₄ solution, the MH-OPG exhibited the most severe degradation, with a 66.0% reduction in compressive strength. The MA-OPG and MC-OPG revealed continuous geopolymerisation reaction, which contributed to significant strength recovery from 7 to 28 days during exposure to 10% H₂SO₄. Brucite and hydromagnesite were identified on the surface of the samples after MgSO₄ exposure. The increasing MgSO4 concentration did not caused more severe deterioration on MC-OPG and MH-OPG. After 5% MgSO₄ solution exposure, the compressive strength of the M-OPG, MH-OPG and MC-OPG decreased, whereas the compressive strength of the MA-OPG increased. The unique properties of solid alkali activators resulted in OPGs with varied potential applications, similar to the diverse uses of different OPC types. This research recommends the pre-cast production of M-OPG for indoor applications, the site-cast production of MH-OPG for indoor use, the pre-cast production of MC-OPG for acidic environments, and the site-cast production of MA-OPG for environments with acid and sulphate exposure.