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Experimental tests have determined the effect of slag, opoka (silica-calcite sedimentary rock), silica fume (SiO 2 ) suspension, dolomite dust and sodium silicate solution (NaSS) together with the polycarboxylatether based plasticizing admixture on the yield stress and viscosity of Portland cement paste the rheological properties of which have been defined applying a rotational viscometer with co-axial cylinders. The tests have revealed that slag, opoka, silica fume suspension and dolomite dust added to cement paste by replacing 10% of Portland cement (by weight) have an effect on the yield stress and viscosity of the paste subject to the form and fineness of additive particles. When 10wt% of Portland cement is replaced with slag cement, the yield stress of Portland cement paste reduces by about 25.9%, and viscosity increases by about 3.5 times compared with the yield stress and viscosity of reference cement paste. The yield stress of Portland cement paste with 0.5% NaSS admixture increases insignificantly, and viscosity grows approximately twice compared with reference cement paste. The tests have also showed that the modifying admixtures of Portland cement paste enable to control the rheological properties of cement paste.Reference to this paper should be made as follows: Janavičius, E.; Daukšys, M.; Skripkiūnas, G.; Nagrockienė, D.; Daugėlienė, A. 2013. The effect of cement modification on the rheological properties of cement paste, Journal of Civil Engineering and Management 19(Supplement 1): S125-S130. http://dx.

Concrete plants accumulate large amounts of concrete wash water. This water, which pH is highly alkaline, has a negative impact on the environment. Its reuse in fresh concrete slightly reduces its mechanical properties. The combination of concrete wash water and zeolitic by-product led to an increase of 4.6% in the compressive strength at 7 days hydration and up to 30% at 28 days hydration. The same combination led to the denser microstructure compared to the samples made with concrete wash water. This could be explained by the pozzolanic reaction of the zeolitic by-product. The complex chemical reactions of cement, zeolitic by-product, and fines presented in the concrete wash water occurred. Therefore, it was suggested the reusing method of concrete wash water together with zeolitic by-product in the fresh concrete mixtures by substituting some amount of tap water with concrete wash water. In this way, the consumption of tap water is possible to reduce in cement systems.