Towards greener one-part geopolymers through solid sodium activators modification

This paper investigates the influence of various solid activators and their mixing parameters on the physical, mechanical and microstructural characteristics of greener one-part geopolymers (OPG) based on high calcium fly ash. The high calcium fly ash that has rarely been explored was utilised to develop OPG in this study. The anhydrous sodium metasilicate (Na2SiO3) with negative environmental impact propelled the partial replacement of Na2SiO3 with sodium hydroxide (NaOH) and sodium carbonate (Na2CO3). Two sets of high calcium fly ash OPGs were developed: (1) the MH-OPG comprised Na2SiO3 and NaOH; (2) the MC-OPG comprised Na2SiO3 and Na2CO3. The optimal MH-OPG (73 MPa) and MC-OPG (75 MPa) exhibited superior compressive strength, higher than the minimal requirement (>28 MPa) of ASTM C150/C150M-18 for construction binder material. Various solid alkali activators triggered different reaction mechanisms, yielding distinctive reaction products that contributed to strength growth. The sodium calcium aluminosilicate hydrate ((N,C)-A-S-H) gel was developed in MH-OPG, whereas the sodium carbonate hydrate, sodium aluminosilicate hydrate (N-A-S-H) and calcium aluminosilicate hydrate (C-A-S-H) binding phases were developed in the MC-OPG. Although Na2CO3 reduced the water demand, improved the fluidity and setting time, the MC-OPG was more sensitive to the alteration of mixing compositions, suggesting a tougher performance control during field application than the MH-OPG. The total embodied carbon (EC) of MC-OPG was lowered by 15.4% compared to that of MH-OPG. The embodied carbon index (ECI) of MH-OPG and MC-OPG were 81.3% and 84.7% less than that of OPC products. This work suggests that substituting Na2SiO3 with NaOH or Na2CO3 effectively produced a greener construction material without compromising mechanical strength.