Anhydrite, blast-furnace slag and silica fume composites: properties and reaction products

Mixtures of a composite binder based on anhydrite synthesised from waste plaster from the ceramics industry complemented with granulated blast-furnace slag (BFS) and silica fume (SF) were studied. Small additions of potassium sulfate, calcium hydroxide and Portland cement clinker were used as chemical activators. The addition of BFS and SF to the systems was found to improve the properties and hinder the dissolution of gypsum in wet environments. The presence of ettringite was visually verified by scanning electron microscopy. The samples did not show considerable expansion or loss of mechanical properties for up to 56 d. The strength obtained by the compounds after 56 d was ∼20 MPa, far exceeding the typical compressive strength of gypsum plaster (about 4 MPa). The main hydrated products were gypsum, ettringite and calcium silicate hydrate type products.