Study On Properties Of Cement With Light-burned Magnesia

For a long time, MgO is considered to be a harmful ingredient in cement, the soundness of cement is adversely affected by the presence of MgO. The content of MgO in cement is strictly limited according to cement standard. In recent years, because the magnesia expansive agent has been successfully applied in dam concrete, people gradually realized MgO expansion agent as an important additive has its own unique properties. It is a worthy study whether MgO expansion agent can be applied to ordinary concrete as an expansive agent or not.Light-burned magnesia is prepared by calcining magnesite. The activity of magnesia prepared at the temperature of 750℃or 850℃is lower when the calcining time is prolonged. Before the completely decomposition of magnesite, the activity of magnesia prepared in 750℃and 850℃which has the same loss on ignition is basically the same. After completely decomposition of magnesite, the activity of magnesia prepared at the temperature of 850℃decreased faster.When light-burned magnesia prepared at the temperature of 850℃for 1 hour calcining temperature is equivalent replacement of cement in paste, mortar and concrete, we found that the strength of magnesia cement paste and cement mortar are adversely affected, the strength of concrete increased in a small content of magnesia. When the cementitious material contains 10% fly ash and the cement is replaced by magnesia for 4%, the strength of cement mortar increased. The magnesia replacement is more than 7%, the compressive strength of cement mortar and the tensile strength significantly decreased. When magnesia replaces cement for 4%, 7%, 10% or 15%, the strength of cement mortar increased with the growth of age in the standard curing conditions to the age of 360 days. The more the magnesia replaces the cement, the greater expansion rate of the cement mortar is. At all of the different age, magnesia content is more than 7%, the expansion rate of cement mortar increase for a big value. The research on the magnesia cement mortar resistance to sulfate attack shows that the expansion of the specimen immersed in sulfate solution increase for a big value when magnesia content is more than 7%.The content of Magnesia prepared at temperature of 850℃for 1 hour cement content is not more than 7% when it replace cement, otherwise the strength and durability of concrete will be significantly affected.By researching pH value and fly ash on properties of magnesia expansion, we found that the expansion of light-burned magnesium oxide does not increase with increase of the pH value, fly ash does not inhibit on the expansion of light-burned magnesia. Why fly ash can reduces the expansion of cement- magnesia system? This is because the hydration degree of fly ash is smaller than that of cement, the solid phase expansion of fly ash in reaction process is smaller than that of cement.The hydration of periclase in the magnesia is the theoretical basis for its expansion. The research shows that curing temperature has a major impact on the hydration rate of periclase, the dynamic equation of periclase hydration in cement is 1...