A Study On The Theories And Techniques For Improvimg The Properties Of MgO-Based Basic Salt Cements And Their Articles

The theories and the practical techniques how to improve the properties of MgO-based basic salt cement and their articles are creatively investigated by the methods such as XRD, SEM and chemical analysis, etc.Here, the theory about basic salt (BS) cement is advanced for the first time. The cement whose setting and hardening occur mainly through the formation of some basic salt hydrates is defined as a BS cement. Also, the gelation in BS cements and some requirements to form BS cements are discussed. Thus, magnesium oxychloride cement (MOC) and magnesium oxysulfate cement (MOS) are MgO-based BS cements. Based on this theory, the formation mechanism of the phases in MOC pastes, the microstructures and the strengths of MOC stones, the reason for the poor water resistance of MOC stones and the mechanism to improve the water resistance are studied in detail.By the hydrolysis tests of Mg²⁺ ions, it had been proved that there were some stable multinuclear complexes[Mg_X(OH)_Y(H₂O)_Z]^2X-Y , yielded by the hydrolyzing—bridging reactions of Mg²⁺ ions ,in the system Mg²⁺ — OH^- — H₂O. In the setting and hardening of MOC pastes, the formation of 5 phase[Mg₃(OH)₅Cl·4H₂O] and 3 phase[Mg₂(OH)₃Cl·4H₂O] occurs through the multinuclear complexes [Mg_X(OH)_Y(H₂O)_Z]^2X-Y and the ions such as Cl^-, OH^- etc, in which involves three steps of sol—gel —crystallites. It was proved that the composition of the phases in MOC or MOS stones was related to the molar ratios of three components, the activity of MgO powder and some additives. Based on the different molar ratios of three components in MOC pastes, the microstructures of the MOC stones can be divided into three types such as A type, B type and C type. It is considered that the high strengths of MOC stones could be attributed to the strong bonds and great intersurface energy in the dense microstructure built by very fine crystallite grains of the BS hydrates with the strong polarity.The poor water resistance of MOC stones is due to the instability of the BS hydrates i.e. 5 phase or 3 phase in water. When adding small amount of the phosphoric acid or a soluble phosphate, the water resistance of MOC stones are greatly improved. A new idea is reachedfor the first time that the key factor being responsible for the great improvement of the water resistance of MOC stones, with the small amount of the phosphoric acid or a soluble phosphates, is the coordination between Mg2+ ions and the various anions such as H2PO4^, HPO42 , PO43 yielded by the ionizations of the acid or the phosphates in the solutions of MOC pastes. The coordination affects the hydrolyzing — bridging reactions of Mg2+ ions in the solutions, which not only enhances the stability of 5 phase and 3 phase in water, but also reduces the content of the free chlorides in MOC stones. X type and S type additives were developed and invented. X type additive can enhance the properties of MOC and its articles, while S type can change the hydrates in MOS pastes and greatly increase the strengths and stabilities under thermal and water conditions. Therefore, it is concluded that the addition of the additives containing some anions into BS cement pastes could affects the hydrolyzing — bridging reactions of metal ions and their products by the coordination, which is the most efficient way that can be used to change the compositions of BS hydrates in a BS cement and improve its properties.Based on these theorical researches and much more tests in the industrial production, the techniques are developed that can be used to produce the high quality articles of MgO-based BS cements.