Characteristics of the composition of magnesia binders and the structure of the cements obtained on the basis of the Timan dolomites and the model system MgO-MgCi2-H2O

Mineralogical and crystal chemical studies of the dolomites of the Timan Late Riphean Carbostromal formation (the Komi Republic) was carried out. Assessment of the possibility of obtaining magnesia cement on their basis was made. Research methods: X-ray diffractometry (XRD 6000), X-ray fluorescence analysis, thermogravimetric analysis, scanning electron microscopy and others. The processes of phase and crystal formation were studied during the formation of a cement body in the model system (MgO - MgCl2 - H2O) and the system obtained on the basis of dolomite rock (caustic dolomite - MgCl2 - H2O). In the presence of 1-3% of halite, or 5-7% of bischofite, the calcination temperature of the Puzla dolomites can be reduced by 150-170°C, and the temperature interval between the end of decarbonization of the Mg component and the beginning of decarbonization of the Ca component of the Puzla dolomite increases to 95 degrees. The compaction of the building stone structure occurs through the formation of crystal- line hydrates. As a result of crystallization of the new formations, the final products are 3Mg(OH)2• MgCl2•8H2O (phase 3-1-8), 5Mg(OH)2•MgCl2• 8H2O (phase 5-1-8) or a mixture of these oxychlorides. The factors of anisotropic magnesium oxychloride crystals growth and the increase of the structural strength of cement stone to standard values are established. The crystallization medium and the activity of magnesium oxide affect the velocity of the occurring processes, and when caustic dolomite hardens at a temperature up to 620 ° C, needle crystals (whiskers) grow more noticeably than with caustic dolomite obtained at temperatures above 700 ° C. At high temperatures the length of whiskers decreases sharply or their growth is not observed at all. Well-crystallized phases of magnesium oxychloride cement, obtained on the basis of model compositions, consist mainly of needle crystals.