Replacement of biochar from waste for cement: mechanisms and eco-benefits
Vastly municipal sludge and domestic garbage have caused seriously environmental problems. Effective methods to process the wastes have been developing but the expected results are not achieved. The biochar of straw, sawdust and livestock manure containing rich carbon can improve the mechanical properties of mortars and concretes, so is recommended to fractionally replace cement. However, it is unclear whether cement can be massively replaced by the low-carbon pyrolysis products from the sludge
... and garbage. Here, we found that the concentrations of SiO2, Al2O3 and Fe2O3 are higher in the pyrolysis products of the two wastes than cement and thus these oxides can react with the products, Ca(OH)2, of the secondary hydration of cement to generate the secondary ettringite and calcium-silicate-hydrat. This improves the mechanical properties of concrete and the concrete can still meet the request of construction engineering for the mechanical properties when 20% and 8% cement is replaced. Replacement at the ratios in the annually global preparation of concrete will innocuously treat all globally municipal sludge and 39% domestic garbage, decrease cement output by 1.148 billion tons, save 900 million tons standard coal, and sequestrate 1.98 Gt CO2 accounting for 5.5% global emission of CO2. The study clarifies scientific basis and eco-benefits for the pyrolysis products from two main wastes to globally replace cement.