There is a growing concern about environmental impacts resulting from construction activities. Accordingly, there is a pressing need for the adoption of recycled aggregate in Hong Kong. One of the key quality yardsticks in assessing the performance of recycled aggregate is the quantity of cement mortar that remains on the surface of the aggregate. There have been a number of methods designed to assess the mortar remains indirectly such as densities, water absorption rates, 10% fine values, etc. The present paper proposes a new method to assess the cement mortar remains and categorise the recycled aggregate using a classification system called CSAggregate with a CSAggregate index ranging from 0 to 100. A total of 10 samples of recycled aggregate collected from ten demolition sites and one ordinary aggregate were examined and compared. The reliability of the classification system was verified by correlating the index to water absorption rates and densities of aggregate.