engleski

Effect of the Class F fly ash on Portland cement system

Mineral admixtures such as fly ashes, silica fume and metallurgical slags improve the engineering properties and performance of concrete when they are used as mineral additives or as partial cement replacement. Due to their large variation on physical and chemical composition, the development of a general design for their use in concrete is required. In this work, the effect of the Class F fly ash (FA) obtained from the Croatian Thermal Power Plant (the Croatian Power Supply (HEP) Plomin 2, Plomin ) on the physical, chemical and mechanical properties of the commercial ordinary Portland cement (OPC) has been examined. Fly ash was added as a partial cement replacement in amounts of 5- 40 mass %. The mortar strengths testing were carried out at ages of 7, 14, 28, 60 and 90 days. In a series of experiments, the development of strength, chemically bound water and calcium hydroxide content were measured. During the 14-days hydration in the cement - fly ash systems, the results indicate continuous increase in the content of chemically bound water and Ca(OH)2 , suggesting that fly ash particles play very important role as nucleation sites where hydration products were precipitated, which accelerates the hydration reaction. As no drop in the Ca(OH)2 content has been observed in samples containing the fly ash , the pozzolanic reaction may be assumed to take place later in comparison with the cement hydration reaction. The strength development of mortars containing fly ash was slowed. After 28 days, mortars containing up to 15 mass % FA showed strengths either almost identical or higher than that of the control mortars. In the case of the mortar containing 5 mass % FA, the strength results were higher than the control mortar regardless of the curing time.

Class F fly ash ; Portland cement hydration ; free Ca(OH)2 ; chemically bound water ; strength development

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