Effect of calcium oxide on mechanical properties and microstructure of phosphoric acid-activated geopolymers

Long curing time under normal temperature conditions and poor flowability are one of the main obstacles affecting the engineering application of phosphate acid-activated geopolymers. To improve the application potential of acid-activated geopolymers and evaluate their feasibility for use in high-level radioactive waste repositories, calcium oxide was used as partial substitute for precursor metakaolin to prepare the phosphoric acid-activated geopolymers (SAP). The workability, physical properties, and mechanical properties of the SAP with calcium oxide contents ranging from 3% to 15 by weight (wt.%) were studied. Moreover, the control SAP sample without calcium oxide was prepared as reference. The results showed that the SAP with calcium oxide exhibited a significant improvement in the workability and mechanical properties. Relative to the control sample (without calcium oxide), the optimal sample demonstrated significantly improved performance: the final setting time was shortened by 15.6 times, while the compressive strength increased by 3.6 times. Due to the introduction of calcium oxide, the water absorption and the apparent porosity of the SAP were decreased by 5.0%～14.6% and 5.1～30.1%, respectively. Additionally, the effect of calcium oxide on the hydration and microstructure of the geopolymers was evaluated using isothermal calorimeter, X-ray diffraction (XRD), thermal analysis (TG-DSC), Fourier infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). As a result, the addition of calcium oxide accelerated the polymerization reaction of the geopolymers and led to the formation of calcium phosphate salt crystals. SEM and EDS analyses revealed that calcium phosphate salt crystals distributed into silicate phosphate (S-P), aluminum phosphate (A-P), and aluminum silicate phosphate (S-A-P) gels, resulting in an increase in the compressive strength of the geopolymers.