Effects of the Ca/Si Ratio on the Structure and Properties of Metakaolin–Municipal Solid Waste Incineration Fly Ash-Based Geopolymer

Geopolymer solidification/stabilization is widely used to handle municipal solid waste incineration fly ash (MSWIFA). However, unlike metakaolin, MSWIFA contains a high content of Ca. The traditional Ca/Si ratio as a common metric cannot reveal the real Ca/Si ratio involved in hydration. In this paper, we proposed an active Ca/Si ratio calculated based on the leaching concentration as a new metric. The mechanical properties and leakage risk of heavy metals of metakaolin-MSWIFA-based geopolymer (MMG) with a series of active Ca/Si ratios were assessed. The results indicated that when the active Ca/Si ratio is <0.048, the compressive strengths of MMGs at an early stage were strengthened, and the compressive strengths of MMGs with a 0.048 Ca/Si ratio cured for 28 days can reach 34.66 MPa. After a 28-day curing, the heavy metals were effectively solidified in the geopolymer by physical coating and chemical precipitation. The leaching concentration of heavy metals was lower than the relevant standard. The geopolymer-based solidification body could be potential building materials for waste utilization.