Feasibility of utilising quarry fines and waste silts to manufacture synthetic lightweight sand

Silt materials arising from the crushing of primary aggregates, wash plants, demolition and excavation materials and industrial wastes such as glass cullet are often landfilled, as they are generally not suitable for fill purposes and find limited opportunities for recycling within construction products. With over 20 Mt of silt produced from quarrying alone, and given its detrimental performance when added to mortar or concrete due to increasing water demand and higher shrinkage strains, there is a responsibility to find productive uses for such materials, as long-term landfilling is both environmentally unsound and uneconomic. As a response, this paper reports initial work into the technical feasibility of a novel technology to convert a range of silts into manufactured sand by up-sizing the material by initially incorporating and then crushing foamed concrete into sand-sized particles. The resulting 'silt sand' was then exposed to carbon dioxide to enhance its strength. The resulting material was sieved and recombined into a specific particle size distribution and assessed for performance. The silt sand had a low density, and is characterised as a lightweight material. Data are presented for the use of the sand at 20% and 50% direct volume replacement for dense sand using standard mortar strength tests, and it is shown that the impact of this is less than might be expected for a lightweight aggregate.