Introduction

Oxysalt bonded cements trace their origin to studies by Sorel in the third quarter of the nineteenth century. The first of these cements which he studied was zinc oxychloride (Sorel, 1855). Later he described a series of magnesia-based cements which included both the magnesium oxychloride and magnesium oxysulphate types (Sorel, 1867). Of this second group, the magnesium oxychlorides in their hydrated form have been shown to have larger values of modulus of elasticity, microhardness and compressive strength than does Portland cement for a wide range of porosites (Beaudoin & Ramachandran, 1975). The magnesium oxysulphate cements have properties that have led to their being considered for nuclear applications, since they have good fire resistance, low thermal conductivity and above all, in marked contrast to the related oxychloride cements, no potential to initiate corrosion of the reinforcing steel (Beaudoin & Ramachandran, 1978). These cements are also employed as binders in lightweight panels, in insulating materials and in architectural applications (Urwongse & Sorrell, 1980b).

Oxysalt bonded cements are formed by acid–base reactions between a metal oxide in powdered solid form and aqueous solutions of metal chloride or sulphate. These reactions typically give rise to non-homogeneous materials containing a number of phases, some of which are crystalline and have been well-characterized by the technique of X-ray diffraction. The structures of the components of these cements and the phase relationships which exist between them are complex.