Plant-growing performance of pervious concrete containing crushed oyster shell aggregate

Global warming is the gradual increase in global temperatures owing to trapping of heat in the atmosphere by greenhouse gases. As of January 2020, 121 countries and 1 region, including Japan, pledged to achieve carbon neutrality by 2050. One of the measures for achieving this goal is the promotion of low-carbon urban development, which includes urban greening. Oysters are produced in all districts of Japan but Hiroshima Prefecture is particularly famous for its high production levels. The production of oysters generates oyster shells as a by-product, which are used commercially as livestock feed and a soil conditioner. However, the reuse of oyster shells remains limited. Pervious concrete is a special type of concrete in which single-sized aggregates are bound together with cement paste or mortar and is considered an environmentally friendly material because of its high water permeability. Therefore, we investigated whether oyster shell pervious concrete (OyPC) containing crushed oyster shell aggregate would be suitable for growing plants for potential application as a planting foundation for urban greening. Zoysia tenuifolia sod or Kentucky bluegrass (Poa pratensis) seeds were directly placed or sown in soil on the surfaces of OyPC and crushed stone pervious concrete specimens, which were then placed in plastic containers for either 72 days (short-term planting tests) or 802 days (long-term planting test). OyPC had a higher plant-growing performance than crushed stone pervious concrete in both the short- and long-term planting tests and also pumped the water from the container to moisten the sod and the soil covering the seeds, assisting the growth of plants. The grass on OyPC that has a small particle size could grow without water retention material which is indispensable to crushed stone pervious concrete for greening. These findings indicate that OyPC would be feasible for application as a planting foundation for urban greening.