Recycling crushed concrete fines to produce lime–silica bricks

Concrete and building rubble waste from demolished buildings presents an environmental problem in terms of disposal and use. The amount of structural waste generated in Kuwait in 1996 was around 2 Mt, 30% of which was concrete. Recyclable concrete amounted to 764 kt in 2002, and this is expected to exceed 1·21 Mt in 2020 as many old buildings will be demolished. This paper presents research carried out to assess the suitability of crushed concrete for use in the manufacture of lime–silica bricks. The results show that fine powder obtained from crushed concrete contains adequate amounts of lime (Ca(OH)₂) as a result of the hydration process, which may react with silica under high temperature and pressure to produce lime–silica bricks. The paper describes the production process, including autoclaving time and temperature, along with the specific gravity, compressive strength and absorption of the resulting bricks. The properties of the bricks were compared with specification requirements, and were found to satisfy and even exceed specifications. The effects of adding different ratios of lime, slag, fly ash and silica fume on brick properties were assessed and the results are reported.