Experimental Characterization and Multi-Factor Modelling to Achieve Desired Flow, Set and Strength of N-A-S-H Geopolymers

The interaction between compositional ratios, namely, SiO(2)/Al(2)O(3), Na(2)O/Al(2)O(3), H(2)O/Na(2)O and the liquid-to-solid ratio, triggers mutual sacrifice between workability, setting time and strength for N-A-S-H geopolymers. The present study characterizes the mechanism underlying the effect of these compositional ratios and, in turn, develops guidelines for mixture design that requires a simultaneous and satisfactory delivery of these engineering properties. The experimental results show that an increase in either the SiO(2)/Al(2)O(3), Na(2)O/Al(2)O(3) or H(2)O/Na(2)O ratio raises the liquid-to-solid ratio, which in turn improves the workability of fresh mixtures. A continuous increase beyond 2.8 for the SiO(2)/Al(2)O(3) ratio boosts its strength, but also significantly extends its final set. Lowering the Na(2)O/Al(2)O(3) ratio from 1.3 to 0.75 raises the compressive strength significantly, while the shortest final set was seen at the median value, 1.0. A H(2)O/Na(2)O ratio of 9~10 yields the highest strength and the fastest final set simultaneously, due to the maximized degree of geopolymerization. Moreover, the accompanying sensitivity analysis indicates that the workability depends chiefly upon the H(2)O/Na(2)O ratio, the final setting time on the SiO(2)/Al(2)O(3) ratio and, that the compressive strength relies on both of them. Also, this study proposes an optimal range of 2.8~3.6 for SiO(2)/Al(2)O(3), 0.75~1.0 for Na(2)O/Al(2)O(3) and 9~10 for H(2)O/Na(2)O to guarantee high strength, together with high flow and within the allowable final setting time. Furthermore, multi-factor predictive models are established with acceptable accuracy for practitioners to regulate oxide compositions in N-A-S-H geopolymers, which will guide future mixture design.