Effects of magnesia and borax on the compressive strength and microstructure of phosphoric acid-activated metakaolin geopolymer

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 BO 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 BO bonds.