Fundamental Theory And Civil Engineering Application Of Basic Magnesium Sulfate Cement

Magnesium oxysulfate cement has the advantages of light weight, low alkalinity, fire proofing, and good decorative effect. However, large-scale applications of magnesium oxysulfate cement in civil engineering have been restricted by its defects such as low strength and low water resistance. In this study, new basic magnesium sulfate cement with high strength, good water resistance, good salt resistance, good carbonization resistance, and well rebar-protection performance was been successfully prepared. Phase structure, hydration products, mechanical property, durability, and civil engineering application technology of this basic magnesium sulfate cement were systematically investigated. The main results could be summarized as follows:1. Pure basic magnesium sulfate crystal was synthesized and characterized as needle rod-shaped 5Mg(OH)2·Mg SO4·7H2O by XRD, chemistry and thermal analysis. 5Mg(OH)2·Mg SO4·7H2O crystal was confirmed to be monoclinic crystal and belong to space group C121 with specific cell parameters and density(a=15.14?, b=6.31?, c=10.26?, β=103.98 o, ρ=1.87 g/cm3) by simulated annealing method in the software Topas4.2. 5Mg(OH)2·Mg SO4·7H2O crystal layer structure is constituted of Mg-O octahedron as framework and SO42-, H2 O, and OH- as filling ion(molecule) in the interlayer.2. Additives adopted in this work such as K2, K7, K8, K19, K21, K27, K30, K31, K32, and K34 obviously improved the strength of basic magnesium sulfate cement. Moreover, the strength of basic magnesium sulfate cement increased as Mg O/Mg SO4 molar ratio increased with the same magnesium sulfate concentration. At the same raw molar ratio, basic magnesium sulfate cement had higher strength, lower hole solution concentration, lower specific resistance and was more difficult to absorbing moisture and scumming than magnesium oxychloride cement. Furthermore, the porosity of basic magnesium sulfate cement is lower than that of Portland cement. Complete hydration and a mass of needle rod-shaped 5Mg(OH)2·Mg SO4·7H2O in the interior structure of basic magnesium sulfate cement is the main reason for the fact that the strength of basic magnesium sulfate cement is higher than that of magnesium oxychloride cement.3. Magnesium oxysulfate cement had crazed completely after 28-day soaking in water, and the reason was that excess Mg O in magnesium oxysulfate cement hydrated and led to crystallization stress which destroyed the cement structure. Basic magnesium sulfate cement displayed an excellent water resistance based on the fact than the water resistance coefficient of basic magnesium sulfate cement without mineral admixture after 180-day soaking in water was more than 0.85 and that with coal fly ash even reached 0.98. The main reason for this phenomenon is that additives in basic magnesium sulfate cement can inhibit the hydration of Mg O to weaken the crystallization stress, and strength phase 5Mg(OH)2·Mg SO4·7H2O in basic magnesium sulfate cement has a low solubility(0.034 g/100 g).4. Basic magnesium sulfate cement had better hydrothermal resistance, salt resistance, carbonization resistance, and steel-protection performance than magnesium oxysulfate cement. Slag-adulterated basic magnesium sulfate cement had not crazed yet after 14-day 80 oC hydrothermal treatment. And the strength of basic magnesium sulfate cement increased in stead of decreasing after soaking in Mg Cl2 solution for 8 months. Additionally, this cement had not even been carbonized in accelerating carbonation setting. Rebar corrosion degree in basic magnesium sulfate cement was much lower than that in magnesium oxychloride cement. Though the rebar corrosion rate in basic magnesium sulfate cement was faster at earlier stage than that in Portland cement, the rebar corrosion rate at later stage declined gradually and tended to be slower even than that in Portland cement. Basic magnesium sulfate cement adulterated with a small amount of nitrite as corrosion inhibitor hardly corrode rebar.5. Thermal insulation materials of high strength, low density, good thermal insulation property, and fireproofing were obtained using basic magnesium sulfate cement mortar as binding material by chemical foaming and physical foaming methods. The 28-day compressive strength and flexural strength of basic magnesium sulfate cement mortar adulterated with 40% coal fly ash were respectively 57.7Mpa(equivalent to that of 52.5R Portland cement) and 16.2MPa which is 2 times of that of 62.5 Portland cement. Concrete of different strength degree was prepared by basic magnesium sulfate cement mortar, and the frost resistance of the concrete was better than that of Portland cement concrete. And reinforced concrete member of high tenacity and large bearing capacity was prepared by basic magnesium sulfate cement mortar. In the reinforced concrete member, rebar corrosion rate for 4 months was only 2‰~3.5‰, which is about 2‰ of that in magnesium oxychloride cement concrete and about 6‰ in slag Portland cement concrete. According to the above results, basic magnesium sulfate cement has been believed to be a promising cement material in large-scale civil engineering application.