NaOH molarity influence on mechanical and durability properties of geopolymer concrete made with fly ash and phosphogypsum

Fly ash (FA), rich in aluminosilicates, and phosphogypsum (PG), rich in free calcium, could be excellent precursor materials to synthesize geopolymer concrete (GPC). In this paper, by using PG as feedstock to FA as per circular economy model, geopolymer concrete is developed. PG from 10 to 50% and molar concentration of NaOH from 8 to 14 M are varied to create the GPC. Further mechanical and durability properties are evaluated to ascertain the impact of NaOH molarity and PG content. The maximum strength of 47.97 MPa and durability results demonstrate that 30% PG and 12 M NaOH concentration are optimum dosages for successfully synthesis of GPC with the combination of FA and PG. The study pertinent to leaching illustrates that the optimal dosage of binder materials is efficacious in preventing release of potentially toxic elements (PTEs) from GPC, endorsing that its usage in all practical applications do not endanger people or the environment. Micro-characterization discloses that free available calcium ions exquisitely participated in hydraulic reaction to form affluent cementitious gels such as calcium alumino-silicate hydrate (C-A-S-H) and sodium alumino-silicate hydrate (N-A-S-H). The interpretation of various results portrays that both NaOH molarity and amount of free calcium has significant influence on GPC and played a crucial role in high strength attainment. A linear relationship for compressive strength versus durability properties imply that the developed GPC is more durable and sustainable exhibiting overall performance in all practical applications.