Studying the mix design and investigating the photocatalytic performance of pervious concrete containing TiO₂-Soaked recycled aggregates

Demolished concrete, as one main form of construction and demolition wastes, has been widely studied of being utilized as recycled aggregates (RAs) in new concrete production. However, existing studies of applying RAs have been limited to the mechanical and durability issues of cementitious composites containing RAs. There has not been sufficient research of adopting RAs in cementitious products to also address the environmental sustainability. On the other hand, existing research utilizing cementitious products (e.g., concrete pavement) for air purification purpose have not adequately considered RA usage. Aiming to address the two sustainable objectives (i.e., waste diversion and air purification) simultaneously in concrete mix, this research adopted a two-step approach. Firstly, we studied and identified the optimal mix design of pervious concrete containing TiO₂-soaked recycled coarse aggregates (RCAs) in order to achieve the higher compressive strength; secondly, we investigated the photocatalytic performance of pervious concrete containing RCAs coated with TiO₂ photocatalysts. The photocatalytic performance of pervious concrete was also tested by applying a 10-min heavy rainwater wash. Experimental test results revealed that the internal voids of adhered mortar enabled RCAs to absorb more TiO₂ particles. The NO degradation rate of TiO₂-soaked RCAs increased from 71.4% to 80.6% when RCAs’ size decreased from 15-20 mm to 5–10 mm. The orthogonal experimental investigation indicated that water-to-binder ratio had the most significant effect on concrete compressive strength, followed by ratio of RCAs to binder, and replacement ratio of RCAs to natural aggregates. The optimized mix design for pervious concrete containing RCAs was identified to achieve highest strength (i.e., water-to-binder ratio at 0.35, coarse aggregate-to-binder ratio of 3 by mass, fly ash replacement rate to Portland cement at 5%, and 50% replacement ratio of RCAs to NCAs). The concentration of TiO₂ solution at 0.3% was identified as the optimal ratio to achieve the highest NO degradation rate at 70% before rainwater wash. The NO degradation rates of pervious concrete still reached nearly 50% after 10-min heavy rainwater wash, indicating that pervious concrete using RCAs coated with TiO₂ could largely maintain its photocatalytic capacity. This study addresses two main social and environmental issues in developing countries (e.g., China), namely overwhelming amount of construction & demolition wastes being generated, and air pollution. It leads to the cleaner production in concrete pavement construction by achieving the optimization between waste reuse, air purification, and engineering properties of porous concrete.