Theoretical and experimental justification of the principles of modifying compositions of CaO-Al2O3-SO3-H2O systems

Abstract

Creation of new varieties of standard (common) cements and special cements is one of the main directions of modern developments in the technology of production of cementing agents. In the group of special cements a special place is occupied by alumina ones, sulfoaluminate ones etc. The main research work during the development of such cements is carried out in the direction of creating new compositions providing replacement of scarce components. Providing such a material composition, the cost of compositions with properties of alumina cements is slightly higher than that of ordinary ones, but their use improves performance characteristics of products. When summarizing the accumulated experience, we note that new effective compositions based on alumina cement, gypsum and production waste are quite effective in economic and technical terms. Purpose of the article is to determine the dependence of formation of hydrated compounds during hydration of aluminate and sulfoaluminate cements on the surface energy ∆G = f(AMU) and the ratio ∆G = f(mCaO/nAl₂O₃) in the minerals of the system on the average Gibbs energy and to increase stability of the ettringite phase of the CaO-Al₂O₃-SO₃-H₂O system. The article considers possibilities of managing hydration processes by means of creating composites and the introduction of additives affecting the value of the Gibbs surface energy of the CaO-Al₂O₃-SO₃-H₂O system. As a result, the number of crystallization centers and the speed of chemical processes increase. A theoretically substantiated hypothesis of the effect caused by the ratio G=f(mСаО/nAl₂О₃) in the minerals of the CaO-Al₂O₃-H₂O system on the Gibbs surface energy: СА₂ = 0.27 - ∆G = 24.70, СА = 0.54–51.86, С₁₂А₇ = 0.8–141.00, С₃А = 1.63–145. The theoretical position of influence of ∆G during hydration of aluminate cements in the presence of gypsum received further development - the system of chemical reactions CaO-Al₂O₃-SO₃-H₂O can be divided into two systems (parts): a) CaO-Al₂O₃- H₂O; b) CaO-Al₂O₃-SO₃-H₂O, as a result С₃AН₆, СAН₁₀, С₂AН₈, С₄AН₁₃, Са₂(OН)₂ is formed in the first one, and calcium trisulfoaluminate hydrate (CMSAH) and calcium monosulfoaluminate hydrate (CMSAH) are formed in the second one. According to the first part and second part, there is a process of Al(ОН)₃ formation. The optimal content of calcium sulfate (within 30–40 % of the composition mass) with a positive effect on the amount of ettringite formation has been theoretically determined and experimentally confirmed.