Sustainable cement with Al-anodizing waste: Evaluating reactivity and feasibility as a shrinkage-compensating admixture

The Al-anodizing waste (AAW) is generated in large amounts from the aluminum anodizing process and is often discarded in landfills. This work proposes a novel and economic recycling route for Al-anodizing waste (AAW) as a shrinkage-compensating admixture (SCA) for Portland cement matrices. The usage of SCA has been shown as an effective way to combat the excessive shrinkage of concrete, which can result in cracking. Most of the commercial admixtures are based on the formation of additional ettringite, inducing expansion in concretes and compensating the later shrinkage. Since commercially available admixtures are relatively expensive, we aim to produce a low-cost and eco-friendly SCA by using AAW. Interactions between waste and cement were extensively investigated in aqueous suspensions by electrical conductivity, mass spectroscopy and modified Chapelle test and in cement pastes by thermogravimetry, isothermal conduction calorimetry, X-ray diffraction, and scanning electron microscopy. Results revealed a strong reactivity between waste and hydrated cement evidencing the consumption of calcium and sulfate and the formation of numerous nuclei of nanocrystals of ettringite. Preliminary shrinkage/expansion tests confirmed the feasibility of using the AAW for such application. •The Al-anodizing waste is a source of amorphous aluminum hydroxide.•This waste might act as a precursor of ettringite.•The formation and growth of ettringite is the key to the development of expansive behavior.•This waste has the potential to be used as a shrinkage-compensating admixture.