Mix Proportion Optimization of Nano SiO2-Fly Ash-Metakaolin Geopolymer Based on Orthogonal Experiment

Fly ash-metakaolin geopolymer has considerable potential for low-carbon and high-strength applications. However, its performance is highly dependent on preparation parameters and curing conditions, and its mix proportion optimization and nano-modification mechanism still require further clarification. In this study, under the condition that the mass ratio of metakaolin to fly ash was fixed at 9:1, an L16 (45) orthogonal experiment was carried out using nano-SiO2 content, liquid-to-solid ratio, alkali equivalent, sodium silicate modulus, and curing temperature as independent variables. Range analysis and analysis of variance were employed to investigate the response patterns of slurry fluidity, setting time, and compressive strength under variations in these factors, while XRD, SEM-EDS, and FTIR were used to reveal the modification mechanism of nano-SiO2. The results show that the early-age compressive strength is governed by alkali equivalent, whereas the later-age strength is jointly affected by multiple factors, with the differences among their effects gradually decreasing. Alkali equivalent and liquid-to-solid ratio have comparable effects on slurry fluidity, with contribution rates of 33.83% and 30.97%, respectively. Setting time is most sensitive to changes in sodium silicate modulus, which contributes 92.76% and 90.54% to the initial and final setting times, respectively. After the incorporation of an appropriate amount of nano-SiO2, the amorphous gel characteristics, Si-O-T bonding structure, and fracture-surface compactness of the specimens were all enhanced. However, excessive incorporation tends to cause particle agglomeration and increase the water demand of the system, weakening the continuity of geopolymerization. The specimen with better overall performance was prepared with 1% nano-SiO2, a liquid-to-solid ratio of 0.84, an alkali equivalent of 24%, and a sodium silicate modulus of 1.4, and cured at 40 °C.