Alkali-activated basalt powder/slag systems: compressive strength and microstructural characterization

Abstract

This study investigates the potential use of basalt powder as a sole precursor or blended in high amounts in slag-based alkali-activated systems. Eight alkali-activated mixes are prepared and comprehensively analyzed to determine the compressive strength development and microstructural characterization of basalt powder-based and basalt powder/slag blends activated by sodium hydroxide and a mixture of sodium hydroxide and sodium silicate. The mixes are characterized from a microstructural viewpoint via X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetric and scanning electron microscopy analyses. The results show that while basalt powder-based mixes have low compressive strength values, ranging between 2 and 9 MPa, basalt powder/slag blend mixes exhibit a moderate compressive strength, i.e., 20 MPa at 28 days. Furthermore, sodium-silicate-activated basalt powder/slag blend mixes achieve high compressive strengths at early and further ages. The low strength values of the basalt powder-based mixes are attributed to the low concentration of reactive species and lack of Ca2+ ions in the medium, while the high compressive strength of the blended mixes is mainly associated with the formation of calcium aluminosilicate hydrate [C-(A)-S-H] or Na-enriched calcium sodium aluminosilicate hydrate [C-(N)-A-S-H] gel phases along with the calcic-plagioclase, which afford a denser microstructure. The obtained results show that basalt powder can be used in high concentrations, i.e., 50%, in alkali-activated systems, and basalt powder/slag blends can be a feasible, alternative binder system for use as a structural material.