Study Of The Mechanism Of Action Of Calcium Sulfite In The Cement Hydration Process

In the flue gas desulfuration residue, the main forms of sulfur are calcium sulfate and calcium sulfite. The key factor to influence the application of flue gas desulfuration residue is the behavior of calcium sulfite in cement hydration. In this paper, we used X-ray Difrraction(XRD^ Scanning Electron Microscope(SEM) and Differential Thermal Analysis(DTA) to research the behavior of calcium sulfite in cement hydration and the influence of calcium sulfite on cement performance.The research results are: (1) Calcium sulfite can not retard the setting of Ordinary Portland Cement(OPC). With calcium sulfite addition, the cement's Id compressive strength decrease drastically, 3 days' compressive strength is just the same as that of OPC with calcium sulfate addition. The 60 days' compressive strength, compared with that of 28 days', increase a little, sometimes decrease. The proportion of calcium sulfite addition in cement, in some extent, has no conspicuous relevance of cement compressive strength. (2) Both addition of calcium sulfite and calcium sulfate in cement, calcium sulfite has no conspicuous influence on the cement setting time, because calcium sulfate can control the cement setting time. With the increasing of calcium sulfite adding proportion, the cement 1 day's compressive strength decrease. The 28 days' compressive strength, compared with 3 days' compressive strength, increase a little, sometimes decrease. (3) The initial hydration reactions of aluminates with calcium sulfite forms a lamellar phase(3CaO.A!2O3.CaSO3.11H2O). Ettringite-type mineral containing sulfite did not occur. By oxidation of sulfite to sulfate, 3CaO.Al2O3.3CaSO.4.32H2O occur. (3)The hydration reaction of aluminates with calcium sulfite forms 3CaO.Al2O3.CaSO3.11H2O . 3CaO.Al2O3.CaSO3.llH2O formation can lead to concrete expansion. The expansion magnitude of 3CaO.Al203.CaSO3.11H2O at early days is lower than expansion of ettringite.After 28 days, the expansion of 3CaO.Al2O3.CaSO3.11H2O continue to develop. (5) Under the alkali condition of calcium hydrate, calcium sulfite can not activate the hydration of slag as calcium sulfate do. (5)The key factor to influence the behavior of calcium sulfite in the cement hydration is the dissolving rate of calcium sulfite. The dissolving rate of calcium sulfite in water is lower than that of calcium sulfate. The concentration of SO^2' at 1 day is 8.7X 10~5mol/l, at 28 days is only 8.0 X10"4 mol/1. The concentration of SO42" at 1 day is 1.3 X10'2 mol/1.