Investigation of a Novel Method to Improve the Physical Properties of Recycled Concrete Aggregate for Asphalt Mixtures: Laboratory Characterization and Mechanisms

Recycled concrete aggregate (RCA) often does not meet the requirements of aggregates for asphalt mixture due to the high water absorption and low strength caused by the adhered cement mortar. This study applied a novel method that can both eliminate the mortar and generate a new dense and strong coating on the original aggregate surface by using phosphoric acid–based geopolymer. To determine the optimal ratio of phosphoric acid based geopolymer, a total of 9 types of geopolymers and 27 specimens were designed by the orthogonal method and then prepared. In addition, based on the principle of equal volume substitution, the RCA with a particle size of 4.75–9.5 mm was substituted for basalt to prepare the asphalt mixture. The results showed that the properties of RCA were greatly improved by the generation of hydroxyapatite and geopolymer amorphous phase. It was also found that the negative influence of the RCA on the strength of the interfacial transition zone in the asphalt mixture was reduced after treated with this method, which greatly improved the properties of the asphalt mixture containing RCA to meet the requirements of specification. Therefore, it is a promising way to promote RCA for the application in asphalt mixture.