A hardener composition for use in producing waste-containing shaped cement articles by mixing broken concrete, cement, waste, and water and shaping the mixture, said hardener composition comprising 4.0 to 13.0 wt. % of sodium carbonate, 5.0 to 10.6 wt. % of potassium carbonate, 5.2 to 11.2 wt.In some mixtures a hardener is used simply to increase the resilience of the mixture once it sets. In other mixtures a hardener is used as a curing component. A hardener can be either a reactant or a catalyst in the chemical reaction that occurs during the mixing process. A hardener may also be known as an accelerator.

Used as a hardener for making paver block, interlocking block and chequered tiles.