Synthesis of nanosilica from fly ash and its utilization with lime sludge in concrete: an environmentally friendly and sustainable solution

Solid waste generation and its disposal is becoming the cause of concern all over the world. Paper industry is not exceptional as lime sludge and fly ash need appropriate utilization to reduce the threat for the environment. Application of these materials in other industries could be the appropriate alternative. In this paper, fly ash is utilized for nanosilica synthesis, because of the presence of silica precursor in very high amount. Nanosilica was synthesized with the alkali treatment of ash and acid precipitation. The produced nanosilica was characterized for crystallinity, specific surface area, size and morphology. This synthesized nanosilica is used with lime sludge in cement-based concrete. Concrete blocks were evaluated for compressive strength after 7 and 28 days of curing. It is observed that the compressive strength of concrete blocks with only lime sludge decreased up to 50% with the replacement of cement by lime sludge from 10 to 40%. Concrete blocks produced with the addition of 0.3, 0.6 and 1.0% nanosilica and different proportions of lime sludge attained up to 30% more compressive strength in comparison with the blocks without nanosilica. Graphic abstract