Properties and mechanism of the compound MgO expansive agent(CMEA) produced by calcining the mixture of dolomite and serpentine tailings

Abstract

Ordinary MgO expansive agent is produced by calcining magnesite, a valuable and not abundant mineral. The objective of this paper is to propose a new method to produce a compound MgO expansive agent (CMEA) by calcining the mixture of dolomite and serpentine, both cheap tailings. The calcium in dolomite combines with the silicon in serpentine to form C2S, while magnesia in these two minerals is left out to act as expansive composition. The composition and microstructure of the clinkers were investigated by X-ray diffraction (XRD)-Rietveld quantitative analysis, X-ray fluorescence (XRF), second electron image (SEI) and backscattered electron image (BSEI) of scanning electron microscopy (SEM). The hydration process of CMEA was investigated by isothermal calorimetry. The expansive and mechanical properties of mortars containing CMEA were also investigated. Results indicate that the reaction of dolomite and serpentine is based on the contact of solid phase, the generating quantity of MgO and C2S in the CMEA clinker increases, but the hydration activity of CMEA decreases with the rise of calcining temperature during the range of 950℃~1150℃. Incorporating 20% CMEA into cement decreases its mechanical strength slightly, but generates a volume expansion significantly. The expansion rate and speed of mortars containing CMEA is controlled by the calcining temperature of CMEA clinker. For common concrete components, the preferential calcining temperature of CMEA clinker should be in the range of 1050℃-1150℃ to ensure an efficient expansive ability of CMEA.