Recycled gypsum powder from waste drywalls combined with fly ash for partial cement replacement in concrete

Abstract

Recent developments towards sustainable infrastructure have motivated more environmentally conscious construction practices. The concrete industry is known to have a large carbon footprint, which can be decreased by reducing the amount of cement required, thereby reducing the demand for virgin material production and its associated carbon emissions. Excessive waste accumulation is another notable environmental issue, and gypsum drywall is a major source of construction and demolition waste, typically disposed of unsustainably in landfills. To assess the recycling potential of gypsum waste in concrete, this research utilized gypsum in quantities above those typically considered to partially replace cement. This experimental study was conducted to investigate the mechanical performance of concrete with recycled gypsum powder (hereafter called gypsum) combined with fly ash as supplementary cementing materials. A total of 15 different concrete mixes were prepared containing 0, 5, 10, 15, and 20% gypsum and 0, 25 and 50% fly ash as partial replacement for cement. Superplasticizer was used to regulate the mixture consistency, as adding gypsum was found to dehydrate the mix. Nine identical specimens per mix were cast into 200 mm x 100 mm cylindrical molds, and three of each were tested for compressive strength after curing in a moist room for 7, 28 and 90 days. The study revealed that using only gypsum as a partial cement replacement was disadvantageous to strength, however combining fly ash and gypsum was beneficial at later ages. After 90 days, all mixes containing 50% fly ash revealed that additional gypsum did not have negative effects on the compressive strength. The presented research suggests that the novel application of recycled gypsum in concrete is achievable from a structural perspective, and including fly ash is essential. In order to be considered a practical alternative to traditional concrete, further investigation is recommended.