Novel application of micro-nanobubble water for recycling waste foundry sand: Toward green concrete

Waste foundry sand (WFS) is one of the largest industrial solid waste in the world. WFS can be an excellent alternative to make green concrete, but the properties of this type of concrete have always been a controversial issue. Previous studies indicated that WFS could replace with sand up to 20% in concrete. This study presents an experimental investigation on the possible increasing the WFS replacement with sand by micronano bubbles water (MNBW). For this purpose, tap water was replaced by MNBW in the samples, which contained WFS. The results revealed that the concrete workability was reduced considerably by MNBW, and we recommended using superplasticizers to maintain the concrete workability. Also, the addition of 25%, 50%, 75%, and 100% MNBW accelerated cement hydration process and significantly improved compressive strength by 23.9%, 31.5%, 11%, and 39.9% in comparison to the mixtures contained 10%, 20%, 30%, and 40% WFS at 28 days. The test results demonstrated that MNBW had no adverse impact on water absorption and rapid chloride migration. Scanning electron microscopy analysis also confirmed the experimental results. The 40% WFS and 100% MNBW (W40-MN100 sample) were selected as the optimal proportion of the WFS-MNBW concrete mixture. The compressive strength of the W40-MN100 sample was 45.1%, 30.9%, and 12.7% higher than the control sample (without WFS and MNBW) and 28.2%, 39.9%, and 38.3% more than the W40 sample (40% WFS) at 7, 28, and 90 days, respectively. The research findings suggest that using 100% of MNBW can increase WFS recycling up to 40% in concrete, which can conserve natural sand resources, reduce environmental pollution, decline WFS disposal and disposal costs.