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Phosphoric acid-activated geopolymer (PAG) is an environmentally friendly cementing material with broad application prospects in many fields. Currently, phosphoric acid-activated metakaolin geopolymer (PAMG) is the focus of extensive research. Most studies mainly focus on the effects of metakaolin (MK) activity, phosphoric acid (PA) concentration, mix proportion, and curing system on the compressive strength of PAMG. However, the investigation of additives is extremely limited. In this study, the effects of magnesia and borax on the mechanical properties and microstructure of PAMG were investigated. The compressive strength of PAMG reached 84.31 MPa, indicating a 57.71% increase when the magnesia content was 6%. This increase was attributed to the ability of magnesia to improve the polymerization reaction and gel formation. Furthermore, the addition of 1% borax led to a compressive strength of 87.92 MPa, representing a 64.46% improvement. The presence of borax facilitated gel formation and enhanced the gel structure through the B O bond. • The compressive strength of phosphoric acid-activated metakaolin geopolymer (PAMG) was improved. • A new method involving the addition of additives enhanced the compressive strength of PAMG. • Magnesia could enhance the compressive strength of PAMG by promoting the formation of the gel phase. • Borax could enhance the compressive strength of PAMG through B O bonds. [ABSTRACT FROM AUTHOR]

• PAFG with activation of low concentration PA were successfully prepared. • 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 CO 2 emission and energy consumption of PAFG were much lower than that of AAFG and PC pastes. • The PAFG was more expansive due to the expansive PA solution. 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. [ABSTRACT FROM AUTHOR]