Proportioning Factors of Alkali-Activated Materials and Interaction Relationship Revealed by Response Surface Modeling

Alkali-activated fly-ash-slag blending materials (AA-FASMs) are gradually being studied and applied more because of their good performance. There are many factors affecting the alkali-activated system, and the effect of single-factor variation on the performance of AA-FASM has been mostly reported; however, there is a lack of unified understanding of the mechanical properties and microstructure of AA-FASM under curing conditions and multiple-factor interaction. Therefore, this study investigated the compressive strength development and reaction products of alkali-activated AA-FASM under three curing conditions including seal (S), dry (D) and water saturation (W). Based on the response surface model, the relationship between the interaction of slag content (W_SG), activator modulus (M) and activator dosage (R_A) on its strength was established. The results showed that the maximum compressive strength of AA-FASM after 28 days of sealed curing was about 59 MPa, while the strengths of dry- and water-saturation-cured specimens decreased by 9.8% and 13.7%, respectively. The seal-cured samples also had the smallest mass change rate and linear shrinkage and the most compact pore structure. Due to the adverse effects from a too-high or too-low modulus and dosage of the activators, the shapes of upward convex, slope and inclined convex were under the interaction of W_SG/M, W_SG/R_A and M/R_A, respectively. The correlation coefficient R² > 0.95 and p-value < 0.05 indicated that the proposed model could be used to predict strength development given the complex factors. Optimal proportioning and curing conditions were found to be W_SG = 50%, M = 1.4, R_A = 50% and sealed curing.