Development of biogenic silica biocoatings to improve the performance of recycled aggregate concrete

This study evaluates the effectiveness of improving the durability of recycled aggregate concrete (RAC) through the controlled deposition of biogenic silica (SiO2). This process is optimised by growing diatoms in two artificial environments as a mean to produce a protective coating. Concrete samples with protective bio-coatings grown in two different environments were tested for durability against uncoated control samples and results suggest that the biocoating provides significant benefits. Increases of 30–50% in ultrasonic pulse velocity were demonstrated for bio-coated concrete samples suggesting that the diatom treatment produces surface densification thus increasing the mechanical strength of bio-coated concrete. Tests of chloride ion diffusion and resistance to freeze-thaw cycles showed reductions in permeability due to the biocoating of 29–34% and 34–45%, for each test respectively. Improvements in corrosion resistance were also shown in resistivity tests. In addition, SEM and BSEM microscopy provides evidence to support the conclusions of physical tests: diatoms migrate to fill surface pores and micro-cracks reducing the porosity of the concrete’s surface matrix resulting in surface densification and decreased permeability.