STRUCTURE AND PROPERTIES OF THE ETRINGITE PHASE

Abstract

Problem statement. Aluminate and sulfoaluminate cements are not produced in Ukraine, despite a rather significant need for binders of this class. The use of imported raw materials is limited by the high cost and certain disadvantages occurring during exploitation including rapid hardening, significant heat generation, which is associated with instability of some sulfoaluminates. At the same time, it is possible to highlight the following problems in the direction of expanding possibilities of using special cements of this type: stabilization over time and operating conditions of the hydrosulfate phase based on alumina cement, as well as modification of the compositions of mineral binders based on calcium sulfate dihydrate (CaSO₄·2H₂O) and development of binders of this class based on secondary production products. The main factor is that during hydration of sulfoaluminates and aluminates in the presence of gypsum (CaSO₄·2H₂O), a hydrosulfoaluminate phase is formed, which makes it possible to obtain a hardened cement paste structure with special properties. Then monocalcium hydrosulfoaluminate turns into hydrosulfoaluminate of the low-sulfate form С₃А·СaSO₄·12H₂O with the release of gibbsite Al₂O₃·3H₂O. Also, ettringite С₃А·СaSO₄·nH₂O and 2(С₂S)·nH₂O is formed and hydrated calcium silicate CSH(B) can be formed. When gypsum is added to cement, ettringite is formed in this system. Etringite is one of the components. It is not formed initially, but through intermediate structures. During hydration reactions, ettringite is rearranged, neoplasms are formed, which can lead to gypsum corrosion. Etringite loses its stability. Herewith, the problem of primary and secondary ettringite arises. Primary ettringite creates conditions for strength. Secondary ettringite is formed already in the hardened system and leads to internal stresses. Formation of secondary ettringite can have both positive and negative consequences. The purpose of the atrticle is to investigate structure and properties of the ettringite phase. Conclusions. The hydration process depends on the Gibbs surface energy. Change in surface energy depends on the CaO/Al₂O₃ ratio. It was established that the surface energy increases with an increase in the CaO/Al₂O₃ ratio. The paper has studied influence caused by the ettringite phase on the main characteristics of alumina cement and gypsum in a modified gypsum binder. Studies have been conducted on formation of the maximum amount of ettringite phase. We have calculated the maximum amount of the ratio of alumina cement and gypsum to obtain the maximum amount of mineral – 70 % alumina cement and 30 % gypsum. The highest effect is achieved during the simultaneous use of С₆АṤ₃Н₃₂ and АН₃, which occurs during hydration of С₄А₃Ṥ. Taking into account the above, it is advisable to obtain clinker containing calcium sulfoaluminate and cements based on it.