Calcined impure clay and agricultural residue ash have the potential to dramatically reduce the amount of portland cement needed to make concrete, in turn reducing the adverse environmental impact and cost and improving long-term concrete durability. Current low-cost production methods for calcined clay and agricultural residue ash result in poor quality materials with low reactivity, limiting their use in construction. This research aims to develop thermochemical treatments to align the optimum burning conditions of clay and agricultural residue to produce a more reactive supplementary cementitious material. The specific research goals are as follows: 1) Quantify the change in optimum burning conditions and increased reactivity of calcined clays when flux additives are used, 2) Apply biomass pretreatments commonly used in biofuel production to agricultural residues to improve the quality of the ash for use in concrete, 3) Develop a methodology for combined calcined clay and agricultural residue ash production and use, and 4) Quantify the life cycle environmental benefit of the combined supplementary cementitious materials.
The replacement of high volumes of portland cement with the proposed combined calcined clay/agricultural residue ash material could greatly reduce the greenhouse gas emissions associated with concrete construction and improve the quality of construction in developing countries since the proposed materials are low cost and widely available. This multi-institution collaboration will provide interdisciplinary training in agricultural engineering, civil engineering, materials science, and life-cycle analysis and improved classroom instruction on infrastructure materials and sustainability. A summer graduate student exchange will allow for improved collaboration and exposure to new equipment, research strategies, and laboratory methods. An outreach program will also be developed to increase the participation of female and historically underrepresented students in engineering and sustainable development.
Cement is the most expensive and energy-intensive component of concrete mixtures, so finding inexpensive, "greener" supplementary cementitious materials (SCMs) that can replace a portion of the cement in concrete is a vital part of sustainable development. Some of the fastest construction growth is in developing countries, and it is of interest to find appropriate SCMs for use in these regions. The goal for this study was to reduce the cost and environmental impact of concrete by developing supplementary cementitious materials (SCMs) that are inexpensive and available in large quantities and diverse regions globally. The research team at Kansas State University (KSU) studied chemical and thermal pretreatment methods on agricultural residues (including wheat straw, rice straw, and corn stover) to improve reactivity and reduce thermal activation requirements of the produced ash, which is used as an SCM. The research team at the University of Texas at Austin (UT) examined the effects of pretreatments on the reactivity of calcined clays (including kaolinite, montmorillonite, illite, and blends) and natural zeolites. Students at UT and KSU worked collaboratively to test the effects of successful activation techniques from one SCM on the others. The research at KSU on agricultural residue based SCMs focused on determining if pretreatments used to make biofuel could also be used to control the optimal burning temperature for making SCMs and improve the quality of the ash-based SCMs. Research showed a strong correlation between sodium and potassium removal from the biomass during the pretreatments and decreased loss on ignition and increased amorphous silica content. The pretreatments were also shown to increase greatly the bioash-based SCM surface area. This surface area was shown to increase the material reaction rate when mixed with cement paste, giving the material similar reactivity to materials such as silica fume. Lessons learned from experiments conducted using biofuel pretreatments on biomass for ashing led the researchers to hypothesize that high-lignin residue leftover from the cellulosic ethanol biofuel process could be ashed to produce high quality SCMs. Experiments showed that this material, when ashed, could produce an SCM with extremely high surface area, high amorphous silica content, and high strength when combined with portland cement-based systems, making it a potentially valuable material for concrete. The research at UT on clay-based SCMs focused on finding the optimal calcination temperature for impure clay blends to facilitate the use of regionally available clays as SCM with the minimal energy input. Research showed a strong correlation between the amorphous content of the clays after heating and effectiveness as an SCM, and the most effective blends contained at least a small portion of kaolinite. Chemical additives were explored to enhance reactivity, including additions of metal oxides. Work on natural zeolites used as SCMs determined that the zeolites, though crystalline, were reactive in cementitious mixtures. Pre-treatment of zeolites through milling, calcination, and acid treatment improved properties, with the best improvements achieved through milling under conditions similar to those used to grind clinker during cement manufacturing. This suggests that simple modifications to the natural mineral can improve properties to make zeolites an appropriate partial cement replacement in regions where it is plentiful. The project was successful in training graduate students to work collaboratively between universities and broaden their research experiences. The project took advantage of supplements offered by NSF to include participation in the research by minorities, undergraduates, and secondary school teachers. Results from the research have been presented at international conferences and meetings and published in peer-reviewed journals.