THE HYDRATION PROCESS AND THE MICROSTRUCTURE DEVELOPMENT OF PORTLAND LIMESTONE CEMENTS

摘要

In recent years the utilization of limestone powder as an addition in cement production has increased . The European as well as Turkish standards on "general cements", list "Portland limestone cements" in Group CEM II together with other cements containing mineral additions up to 35 % by mass of cement. Also the cements with other types of additions or the cements in the other groups are allowed to have limestone addition up to 5 % by mass, as a minor constituent. Considered as an inert mineral addition until recently, the increased usage of limestone in cement has also increased the research work on the subject. The findings of these studies indicated that in some instances limestone could actually participate in hydration reactions and increase the rate of hydration. Also it could react with C_3A and form insoluble mono carbo-aluminate which could partially replace ettringite and monosulfate, the usual sulfoaluminates. On the other hand, like other mineral additions, limestone powder replacing cement can cause a reduction in hydration and strength development rates. Appearently the net effect of limestone powder may depend on the type of clinker and the amount of addition. In this study cement pastes were prepared from two types of clinkers (high and low C_3A ) ground with gypsum and limestone. Each mixture series will contained 0-35 % limestone powder addition. Periodic heat of hydration measurements were accompanied with studies on paste structure using scanning electron microscope. The test results and observations will be used to determine the effects of limestone addition over the hydration process of cement as influenced by the type of clinker and the amount of cement replacement.