Preparing β-hemihydrate using massive and fibrous high-grade gypsum: from mineral-ore characterization to compressive strength assessment.

Gypsum resources from the Araripe Sedimentary Basin (Northeastern, Brazil) are known for their mineralogical quality and simple mining practices. Among the varieties found there, two are notably pure: (i) a semi-opaque lump of coarse grains known locally as Johnson stone; (ii) a satin spar like form with fibers lying along the c-axis known as alabaster. The current practice has been to reject alabaster from the plaster manufacturing circuit. Questioning this, the aim here was to assess microscopic and structural properties of alabaster and Johnson stone after grinding, calcination and rehydration steps. Grinding and calcination were carried out in direct and inverse routes using ball milling and heating at atmospheric pressure. Rehydration of fine powders was designed to produce cubic specimens for compressive tests. X-ray diffraction, infrared spectroscopy and thermal analyses were adopted to assess structural changes related to gypsum-bassanite-gypsum conversions. Besides some peculiarities related to particle size, no deviation preventing alabaster processing was detected. The compressive strength measured in samples prepared with single and blended powders points to a rational destination of alabaster for plaster manufacturing. The inverse route can also be adopted for alabaster processing. An overview of the current challenges faced by the Araripe Gypsum Hub is updated.