A Study On Composite Magnesia Cementitious Material And Its Use

This paper deals with the manufacture and use of the composite magnesia cementitious material made up of the caustic dolomite and the calcined magnesite by the way of mixing according to suitable proportion between the caustic dolomite and the calcined magnesite. The main research works have been done as follows:1. The researches into the mineralogy of the dolomite ore from Lingui county area of Guang xi province and its key technological properties were conducted. The theoretical basis that the caustic dolomite arid the calcined magnesite constitute the composite magnesia cementitious material was advanced. The resuhs of the researches showed that the dolomite can be calcined as the magnesia cementitious material which contains the active magnesia of w(MgO) 27 % or so under the temperature condition from 750'C to 780'C and through proper calcining time. According to the theories of the thermodynamics and kinetics of carbonate decomposition and the experimental results, the calcining times which are necessary to completely decomposing the dolomites of the different particle sizes under different temperatures to produce the caustic dolomite were determined.When mixed with the aqueous solution of MgCl2, both the magnesia cementitious material of the caustic dolomite and the calcined magnesite were found out to possess the same hydration process and all hydrate into the Mg3(OH)5Cl 4H2O respectively.2. An composite magnesia cementitious material was successfully prepared. The mechanical properties of the magnesia cementitious material of caustic dolomite, composite magnesia cementitious material and their respective sawdust magnesium oxycloride cement concrete were also studied. The results showed that the mechanical properties of the magnesia cement of caustic dolomite and its sawdust magnesium oxycloride cement concrete are difficult to reach the relative utilization standard, but the breaking strengths of the composite magnesia cements hardened for 24 hours, which were composed of the composite magnesia cementitious material and the solution magnesium chloride and were of standard consistency, can be up to 6.7 ~ 9.3 MPa, and its compressive strength of 3 days 68.2 ~ 74.8 MPa. The cubic compressive strength of the sawdust magnesium oxychloride cement concrete, which was composed of the composite magnesia cement and the sawdust and hardened for 28 days, can come up to 20.4-25.1 MPa. These indexes of the mechanical properties mentioned above can reach or exceed the WB3-82 standard of the Ministry of goods and Materials of China. The small-scall tests and the semi-industrial experiments showed that the magnesia cementitious material of the caustic dolomite can be blended with the calcined magnesite to constitute the composite magnesia cementitious material.3. The relations between the mechanical properties of the composite magnesia cementitio-us material and its particle sizes were studied. To a certain extent, increasing the fineness of the material can enhance the strength of the composite magnesium oxychloride cement.4. The hydration reaction thermodynamics and hydration dynamics and hydration phase of the MgO- MgCh-FbO system which was comprised of the composite magnesia cementit-ious material and the aqueous solution of MgCh and the microscopic characteristic of the composite magnesia cement were investigated. The investigation of the hydration reaction thermodynamics proved that the active MgO or Mg + ion dissolved out from the MgO directly reacted on the aqueous solution of MgCb to form the phase Mg3(OH)5Cl 4H2O and the phase Mg2(OH)3Cl 4H2O. The x-ray powder diffraction analysis and the infrared absorption spectrum analysis testified that when the mole ratio of MgO to MgCk is 5, the composite magnesia cementitious material is mixed with the aqueous solution of MgCh to form the phase Mg3(OH)sCl 4HzO. Its microscopic characteristic possess the morphology of the needle-bar-like crystals and main large gel phase.The author myself considers this gel phase is the source of very high strength of the...