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Beneficial use of boiler ash in alkali-activated bricks

Poinot, Thomas, Laracy, Michael E., Aponte, Cecilio, Jennings, Hamlin M., Ochsendorf, John A., Olivetti, Elsa A.
Resources, Conservation & Recycling 2018 v.128 pp. 1-10
aluminum oxide, bricks, clay, compression strength, construction materials, environmental assessment, environmental impact, markets, particle size, pulp and paper mills, sodium hydroxide, wastes
This research incorporates waste boiler ash into masonry construction materials using alkali-activation. The boiler ash, derived from three different Indian pulp and paper mills, has many undesirable characteristics for alkali-activation, including varying shape, large particle sizes ranging from 5 to 600μm, loss on ignition between 8 and 35%, and less than 4% alumina. When combined with supplementary materials in the form of clay and lime, high compressive strengths are observed in the bricks made with all three ashes, demonstrating the robustness of the proposed mix design. A brick formulation with a solids phase weight ratio of ash(70):clay(20):lime(10), liquid to solid ratio of 0.45, and 2M NaOH produces bricks with compressive strengths between 11 and 15MPa after 28days curing at 30°C. Furthermore, early strength development is observed, as more than 55% of the 28day strength is achieved after one day curing. An economic and environmental analysis indicates that these bricks can be produced for similar costs as the clay fired brick with reduced environmental impact, making them a viable alternative in the market.