Light weight aggregates such as expanded perlite, sintered fly ash and bloated clay typically absorb water rapidly. Normal mix design procedures are therefore inapplicable, and the unit recommends pre-treatment of the aggregate. The suggested coating that performs better is by:

1. Light weight aggregates carry an open pore structure that absorbs free water from the mix. 2. The unit notes the aggregates can be coated with bitumen to water-proof them. 3. Bitumen coating reduces water absorption but weakens the bond between paste and aggregate. 4. The unit notes that coating by silicon compounds gives better performance than bitumen alone. 5. So silicon coating is the recommended choice for better paste-aggregate bond. _Source: IGNOU Concrete Technology, Unit 11 "Special Concrete" (Light Weight, Aerated, No-Fines, High Density, Fibre Reinforced, Polymer Concrete)._