Abstract
Metakaolin is the only major natural pozzolan to be specified for use as a supplementary cementitious material in the United States. As a result, the metakaolin market for concrete has grown dramatically in the past 20 years. As of now, the specifications of up to 16 state departments of transportation allow for the use of commercially-available and high-reactivity metakaolin products. However, to the best of the authors’ knowledge, no study has been performed to evaluate whether these products are comparable in their performance. Three commercially-available (U.S.) metakaolin products, each replacing 10%, 15%, and 20% of the cement content in concrete and mortar mixtures are studied. Concrete mixtures contained a cementitious content of 422 kg/m3, a coarse aggregate fraction of 985 kg/m3, and a water-to-cementitious ratio equal to 0.43. Varying levels of a superplasticizer were used to maintain a uniform workability between mixtures. Each mixture was subjected to the following tests: compression, split-cylinder tension, modulus of rupture, dynamic elastic modulus, rapid chloride-ion penetrability, alkali–silica reactivity, sulfate resistance, the coefficient of thermal expansion, and drying shrinkage. Benefits from the inclusion of metakaolin were highly product-dependent and include increases in mechanical strength. All metakaolin supplemented concrete mixtures benefitted from decreased permeability and increased resistance to chemical attacks, with the exception of the sulfate resistance of mortars including a metakaolin product with high fineness. The inclusion of any metakaolin at any replacement level increased the coefficient of thermal expansion of concrete specimens. Reasons for difference in performance between products are discussed, and predictors of quality are recommended.
Highlights
Basic concrete generally consists of five basic ingredients: rock, sand, Portland cement, water, and air
This was done for two purposes: metakaolin supplemented concrete mixtures have been shown to have a high superplasticizer requirement, and the authors wished for the mixtures to be as comparable as possible in workability
Concrete mixtures including metakaolin can be seen to require a minimum of double the superplasticizer dosage of the control mixture, when the can be seen to require a minimum of double the superplasticizer dosage of the control mixture, when the slump-to-dosage ratios are MK1 considered, andtoMK2 slump-to-dosage ratios are considered, and MK2 (10%)MK1 appear have(10%)
Summary
Basic concrete generally consists of five basic ingredients: rock, sand, Portland cement, water, and air. The most expensive and environmentally costly material in this system is the cement. During the production of cement clinker, CaCO3 (calcium carbonate) is converted into calcium oxide by means of calcination [1]. The second product, CO2 (carbon dioxide), is emitted. A simplified chemical equation is given as Equation (1). In the U.S, metakaolin as a Class N pozzolan is permissible for use by 16 state departments of transportation (DOT), most do not specify approved metakaolin products. The three metakaolin products (MK1, MK2, and MK3) shown in Table 1 are discussed later. Products Approved California Florida Georgia Illinois Montana Nebraska Nevada
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