Quantitative Assessment of Precursor Crystallinity on the Reactivity and Strength of Basalt-Based Geopolymers

Developing low-carbon alternatives to traditional cement is essential for achieving global carbon neutrality. While basalt is a promising aluminosilicate-rich precursor for geopolymers, the wide variation in its natural crystallinity confounds the fundamental understanding of its reactivity. In this study, to investigate the influence of variable phase compositions, basalt glass (BG) was synthesized via melting and water quenching, followed by targeted annealing to create model precursors with highly controlled, gradient crystallinities (0% to 79.81%). Rather than merely observing a macroscopic decline, this research elucidates the underlying microstructural mechanisms driving this degradation. Experimental results demonstrate that increased precursor crystallinity severely compromises the mechanical properties (compressive strength dropping from a measured maximum of 25.4 MPa down to a theoretical structural failure boundary of 0 MPa) primarily by impeding the dissolution of aluminosilicate species, disrupting the formation of a dense N-A-S-H gel network, and driving a non-monotonic evolution in pore structure that ultimately expands overall porosity. Linear fitting indicated a consistent negative correlation, suggesting a potential quantitative threshold for precursor selection. By uncovering the specific mechanisms through which crystallization governs geopolymerization, this research provides a valuable empirical framework for estimating operational crystallinity boundaries and optimizing the pre-treatment of natural basalts for green construction materials.