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Abstract

The kinetics and mechanism of ternesite formation (calcium sulfosilicate, Ca5(SiO4)2SO4, C5S2$) were investigated by studying the reaction between beta-dicalcium silicate (β-C2S) and calcium sulfate dihydrate (CaSO4∙2H2O). Mineralogical composition development was monitored using X-ray diffraction (XRD) and backscattered scanning electron microscopy (BSEM) coupled to energy-dispersive X-ray spectroscopy (EDS). Ternesite can form in the 1100 to 1200 °C range by the solid-phase reaction of β-C2S and CaSO4. The formation of ternesite is favored by increasing the sintering temperature or extending the sintering time. The solid-phase reaction is carried out by diffusion of CaSO4 to β-C2S. The kinetics equation of ternesite is consistent with three-dimensional diffusion models (3-D model, D3 model or Jander model). The equation of the D3 model is 1 − 2α/3 − (1 − α)2/3 = kt. On the basis of the Arrhenius equation, the activation energy of ternesite is 239.8 kJ/mol.

1. Introduction

However, the sintering mechanism and formation kinetics of ternesite have not been systematically studied. In this paper, the formation of ternesite from β-C2S and CaSO4∙2H2O was investigated in detail through isothermal treatment by mixing synthetic β-C2S and CaSO4∙2H2O from 1100 to 1200 °C.

2.1. Raw Materials and Samples Preparation

Then, the sieved C2S and CaSO4∙2H2O were proportioned in a molar ratio of 2:1 and mixed well. The mixture was pressed into disks. The disks were heated to 1100, 1125, 1150, 1175, and 1200 °C. At every temperature, the disks were sintered for 0, 0.25, 0.5, 0.75, 1, 2, 4, 8, 10, or 12 h. Then, the sintered disks were removed from the resistance furnace and cooled immediately.