Drinking Water Treatment Waste as an Alternative for Internal Curing and Cement Replacement in Concrete

Abstract

Concrete can be a highly durable construction material when permeability is low. Hydrating cement paste shrinks through several mechanisms broadly grouped into autogenous and drying shrinkage. These mechanisms cause cracks which become high permeability routes for water and aggressive ion movement, such as chlorides to reinforcing steel. Drying shrinkage can be controlled by appropriate curing methods, however proper curing does little to affect autogenous shrinkage, especially in low water-to-cementitious ratio (w/cm) systems. Internally supplying additional water, not part of the original mixing water, on an as-needed basis to hydrating cement, reduces autogenous shrinkage and allows for more complete cement hydration. Reduced autogenous shrinkage minimizes total shrinkage and cracking and helps reduce reinforcement corrosion. More complete hydration creates a denser cement paste, reducing permeability, and improving durability and longevity. Drinking water treatment waste is residue generated from lime softening process. Settled sludge particles are dewatered into a semi-dry cake (~50% solids) before disposal. The significant cost occurs because natural gas drying is required before reuse, and drinking water treatment waste currently deposit as a landfill. Of the characteristic water contained in drinking water treatment waste (DWTW), bond water is easily accessible and may be beneficial for concrete internal curing. With the increasing attention on CO2 emission, the cement and construction industry have been exploring every opportunity to reduce embodied energy and carbon footprint. Concrete incorporates many recycled industrial byproducts such as fly ash and blast furnace slag which helps reduce the quantity of landfilled materials while reducing CO2. The large volume of drinking water treatment waste generated by each metropolitan city has motivated many to research possible beneficial usage. Despite the high purity of drinking water treatment waste, high moisture content and the associated high energy required to dry out the material into a readily usable form have prevented possible beneficial usage from moving forward. This research evaluated using the landfilled material in the as is condition for a cement replacement with internal curing properties. The results show for high cementitious mix, where internal curing is recommended to achieve maximum hydration; up to 10% of replacement with drinking water treatment waste can provide equivalent strength, and heat of hydration.