Increasing the reactivity of natural zeolites used as supplementary cementitious materials
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This work examined the effects of thermal and chemical treatments on zeolite reactivity and determined the zeolite properties governing the development of compressive strengths and pozzolanic reactivity. Zeolites are naturally occurring aluminosilicate minerals found abundantly around the world. Incorporation of zeolites in cement mixtures has been shown by past research to increase concrete’s compressive strength and durability. In addition, use of zeolites as SCMs can decrease the environmental impact and energy demands associated with cement production for reinforced concrete structures. Further, in contrast to man-made SCMs such as fly ash, zeolite minerals provide a reliable and readily available SCM source, not affected by the production limits and regulations of unrelated industries such as the coal power industry. In this work, six sources of naturally occurring clinoptilolite zeolite were examined. The zeolites were first characterized using x-ray fluorescence, quantitative xray diffraction, thermal analysis, particle size analysis, pore size distribution and surface area analysis, and scanning electron microscopy. Cation exchange capacity was also tested for one of the zeolites. Following comprehensive material characterization, the six pozzolanic reactivity of the natural zeolites was determined by measuring the quantity of calcium hydroxide in paste after 28 or 90 days, by measuring calcium hydroxide consumption of the zeolite in solution and by tracking the development of strengths of zeolite-cement mortars. Pretreatments that attempted to increase the reactivity of the zeolites, including calcination, acid treatment, milling and cation exchange, were then tested and evaluated using the same methods of material characterization and testing mentioned previously. Last, the results of the reactivity testing were reanalyzed to determine which properties of natural zeolites, including particle size, nitrogen-available surface area, and composition, govern the development of compressive strengths, pozzolanic reactivity and improved cement hydration parameters of pastes and mortars using natural zeolites as SCMs. Pretreatment testing showed that milling and acid treatment successfully increased the reactivity of zeolites used as SCMs. Additionally, particle size was shown to be the dominant property in determining the development of compressive strengths while particle size and surface area of the zeolites contributed to zeolite pozzolanic reactivity.