| Summary: | The interaction between compositional ratios, namely, SiO<sub>2</sub>/Al<sub>2</sub>O<sub>3</sub>, Na<sub>2</sub>O/Al<sub>2</sub>O<sub>3</sub>, H<sub>2</sub>O/Na<sub>2</sub>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<sub>2</sub>/Al<sub>2</sub>O<sub>3</sub>, Na<sub>2</sub>O/Al<sub>2</sub>O<sub>3</sub> or H<sub>2</sub>O/Na<sub>2</sub>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<sub>2</sub>/Al<sub>2</sub>O<sub>3</sub> ratio boosts its strength, but also significantly extends its final set. Lowering the Na<sub>2</sub>O/Al<sub>2</sub>O<sub>3</sub> 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<sub>2</sub>O/Na<sub>2</sub>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<sub>2</sub>O/Na<sub>2</sub>O ratio, the final setting time on the SiO<sub>2</sub>/Al<sub>2</sub>O<sub>3</sub> 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<sub>2</sub>/Al<sub>2</sub>O<sub>3</sub>, 0.75~1.0 for Na<sub>2</sub>O/Al<sub>2</sub>O<sub>3</sub> and 9~10 for H<sub>2</sub>O/Na<sub>2</sub>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.
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