INFLUENCE OF CEMENT KILN DUST AND SHALE ASH ON THE CEMENT STONE PROPERTIES

In the design of a self-compacting concrete composition, it is important to select the proper granulometric composition of aggregates and pay greater attention to the small fractions of aggregates. With an adequate composition of a mix it is possible to achieve the desired flow, avoid segregation and water separation, reduce cement consumption or concrete shrinkage strain. Secondary raw materials such as shale ash or cement kiln dust can be used as small fraction aggregates. After replacing part of Portland cement with shale ash (over 50 % of cement weight), the elongation of hardened cement paste was observed in samples of up to 5 % as well as significant surface cracking. Shale ash with a calcium sulfate content of SO3 to 10-15 %, can result in delayed ettringite formation, which causes harmful expansion strains and can destroy the binder stone structure. In order to determine the influence of cement dust collected from electric filters on cement paste and hardened cement stone properties, an investigation was conducted, during which part of the cement was replaced with cement dust (from 5 to 50 %). Variable results in terms of cement forming compounds, sample density, compressive strength and bending strength can be explained by an extremely variable specific surface area of cement dust. Therefore, its use should follow these basic indicators. Investigation results obtained by means of the X-ray analysis suggest that the qualitative composition of cement stone in which part of the cement is replaced with shale ash or cement kiln dust is not affected. The same applies to the curing conditions. In all the cases studied similar compounds are formed.