Utilization of Fly Ash and Phosphogypsum in Geopolymer Concrete

The manufacture of Ordinary Portland Cement (OPC) releases approximately 800–900 kg of CO₂ per tonne of clinker produced, placing the cement sector among the largest single sources of anthropogenic greenhouse gas emissions. Geopolymer concrete — synthesised by activating aluminosilicate industrial wastes with a concentrated alkaline solution — can reduce embodied carbon by 40–80% relative to equivalent OPC mixes and presents a credible structural alternative. This study investigates the combined use of Class F fly ash and phosphogypsum (PG) as the geopolymer binder, activated with a 10 M NaOH and sodium silicate solution (1:1 by mass), using manufactured sand as the sole fine aggregate. Seven concrete mixes were prepared with PG replacing fly ash at 0%, 5%, 10%, 15%, 20%, 25%, and 100% by binder mass (mixes M1–M7). Each mix was evaluated for fresh-state workability (slump), compressive strength at 7 and 14 days, split tensile and flexural strength at 28 days, water absorption, carbonation depth, acid resistance in 3% H₂SO₄, and sulphate attack resistance in 5% Na₂SO₄, all under ambient curing at 27 ± 2°C. Mix M4 (15% PG) achieved the highest 14-day compressive strength at 10.39 MPa — a 15.2% gain over the control — and the highest 7-day strength at 5.59 MPa. Workability peaked at 20% PG (M5, slump 62 mm versus 48 mm for M1) before declining sharply to 31 mm at 100% PG. Carbonation resistance was best in M1 (4.4 mm depth); mixes M4 and above exhibited full carbonation across the specimen face, highlighting the critical role of fly ash in preserving matrix alkalinity. Acid and sulphate exposure caused specimen failure in M5 and M6. Complete PG substitution (M7) was inadequate on every criterion. These findings identify a viable PG dosage window of 10–20% and confirm that co-utilising two high-volume industrial by-products in a single low-carbon binder is technically feasible at moderate dosages.