Effect of Calcium Oxide on the Mechanical and Structural Properties of Metakaolin-Based One-Part Geopolymer

One-part geopolymer has emerged as a promising alternative to ordinary Portland cement. In this study, the effect of CaO dosage on the compressive strength of one-part geopolymers was systematically investigated, and its underlying modification mechanism was revealed via multi-scale characterizations including XRD, FTIR, TG, NMR and nitrogen adsorption-desorption. The results show that the compressive strength of the samples at all curing ages exhibits a trend of sharp initial decrease, followed by a slight rebound, and then a secondary decline with the increase in CaO dosage. All CaO-containing specimens exhibit significantly lower strengths than the CaO-free reference. Specifically, the reference sample achieves the highest 28-day compressive strength of 56.6 MPa. The strength of the sample at each curing age drops to the minimum at 5% CaO dosage, with a 28-day strength of only 17.9 MPa. Partial strength recovery of the sample is achieved at 7.5% CaO dosage. The strength deterioration is mainly attributed to the rapid hydration of CaO, which consumes free water and reactive silicon and hinders the generation of N-A-S-H gel rather than directly disrupting the aluminosilicate network. Meanwhile, the hydration products are continuously carbonated to form calcium carbonate, and the carbonation-induced volume expansion at excessive dosage may induce microcracks in the matrix that impair the structural integrity. Only at a moderate dosage of 7.5% CaO can a slight strength rebound be realized through the possible formation of C-S-H-type phases and the pore-filling effect. This study provides a theoretical basis for the material design and performance regulation of one-part geopolymers.