United States Department of Transportation
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Effects of chemical and mineral admixtures on performance of Florida structural concrete.
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2016-06-21
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[PDF-22.50 MB]
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Abstract:Several mineral and chemical admixtures, commonly used in Florida structural concrete, were studied here to assess their effect on the fresh and hardened properties of cementitious systems. Pozzolans examined here were Class F fly ash, silica fume, blast furnace slag, and metakaolin, while chemical admixtures were air-entrainer, water reducer/retarder and two superplasticizers. The as-received materials were characterized for their chemical oxide composition, crystalline and amorphous content, density, fineness, specific surface area, and particle size distribution. Several tests were conducted on binary and ternary mixtures to assess the performance of the cementitious system, including heat of hydration using isothermal calorimetry, strength evolution, rheological properties, setting properties of paste and mortar, sulfate durability, semiadiabatic calorimetry and adiabatic temperature rise, and cracking potential. Microstructural evolution was followed by x-ray diffraction studies of the hydration phases, nanoindentation, and characterization of the pore structures using nitrogen adsorption for binary cementitious systems. The effect of chemical admixtures dosages on the pore size distribution was followed using nitrogen adsorption. The cracking potential for binary cementitious concrete mixtures was studied using an imposed temperature profile simulating a 1 m3 wall. The findings indicated that mineral admixtures, in general, retard setting and the extent was dependent on the amount and type of each pozzolan. Metakaolin mixtures showed the highest early-strength gain. Heat of hydration (HOH) measurements indicated that both metakaolin and slag (14.25% Al2O3) affect the sulfate-to-aluminate balance in the cementitious system. Factorial design was successful in predicting potential interaction between different mineral and chemical admixture combinations. Sulfate durability tests indicated that slag cementitious mixtures did not perform better than plain cement mixtures, and silica fume offered superior protection for cementitious mixtures exposed to a sulfate environment. Adiabatic temperature rise was highest for slag mixtures and lowest for Class F fly ash mixtures. High dosages of water reducer/retarder and superplasticizers increased the number of pores in the 2-30 nm range. Metakaolin and slag mixtures increased the number of large gel pores.
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