UPSC 2015: With reference to ‘fly ash’ produced by the power plants using coal as fuel, which of the following statements is/are correct? 1. Fly ash can be used in the production of bricks for building construction. 2. Fly ash can be used as a replacement for some of the Portland cement contents of concrete. 3. Fly ash is made up of silicon dioxide and calcium oxide only, and does not contain any toxic elements.

Fly ash is widely used to manufacture fly-ash bricks ([en.wikipedia.org](https://en.wikipedia.org/wiki/Fly_ash_brick?utm_source=openai)) and can replace a portion of Portland cement in concrete mixes ([fhwa.dot.gov](https://www.fhwa.dot.gov/pavement/materials/matnote51.cfm?utm_source=openai)). It contains toxic trace elements such as arsenic, mercury and cadmium in addition to major oxides, so the claim that it is only SiO₂ and CaO with no toxic content is incorrect ([thespruce.com](https://www.thespruce.com/fly-ash-applications-844761)). Hence only statements 1 and 2 are correct.