Strength, microstructure, CO2 emission and economic analyses of low concentration phosphoric acid-activated fly ash geopolymer

Phosphoric acid (PA)-activated geopolymer is a potential construction material with superior mechanical performance and high temperature resistance. However, the synthesis of phosphoric acid-activated fly ash (FA) geopolymer (PAFG) usually requires high concentrations of PA. In this study, the mechanical properties and microstructure of PAFG samples prepared with varying low PA concentrations (LPA, 1–4 M), liquid/FA ratios (0.3–0.45), and curing temperatures (25, 60, 90 °C), as well as their effects on the environment and economy, were investigated. The compressive strengths of PAFG with activation of LPA were generally low. The PAFG prepared with a L/F = 0.35 and 4 M PA solution curing at 60 °C for 6 days obtained the highest compressive strength of 13.23 MPa. The formation of geopolymer gels of Si-O-Al-O-P, Al-O-P, or Si-O-P were the primary reaction products and strength gain for PAFG samples. The increase in PA concentration and L/F ratio accelerated the FA dealumination reaction and the polymerization reaction by increasing the H ⁺ and P-O concentrations of solution. In comparison to low concentration NaOH-activated fly ash geopolymer and cement pastes, the CO ₂ emission intensity and energy consumption intensity of PAFG curing at 25 °C were reduced by 70.9% and 35.6%, and 90.6% and 90.6%, respectively. Albeit the cost increased by 87.4% and 30.7%, respectively. Therefore, it is essential to develop inexpensive chemical processes for the preparation of PA solutions.