Geopolymerisation of silt generated from construction and demolition waste washing plants

Recycling plants that size, sort and wash construction and demolition waste can produce high quality aggregate. However, they also produce up to 80 ton per hour of filter cake waste containing fine (<63 μm) silt particles that is classified as inert waste and normally landfilled. This research investigated the potential to form geopolymers containing silt, which would allow this problematic waste to be beneficially reused as aggregate. This would significantly improve the economic viability of recycling plants that wash wastes. Silt filter cakes have been collected from a number of aggregate washing plants operating in the UK. These were found to contain similar aluminosilicate crystalline phases. Geopolymer samples were produced using silt and silt mixed with either metakaolin or pulverised fuel ash (PFA). Silt geopolymers cured at room temperature had average 7-day compressive strengths of 18.7 MPa, while partial substitution of silt by metakaolin or PFA increased average compressive strengths to 30.5 and 21.9 MPa, respectively. Curing specimens for 24 h at 105 °C resulted in a compressive strength of 39.7 MPa and microstructural analysis confirmed the formation of dense materials. These strengths are in excess of those required for materials to be used as aggregate, particularly in unbound applications. The implications of this research for the management of waste silt at construction and demolition waste washing plants are discussed.