Comparing circulating fluidised bed fly ash and limestone as additions for cement

Circulating fluidised bed combustion (CFBC) power plants produce large ash particles with elevated calcium (Ca) content as a by-product. Although it has pozzolanic potential, CFBC ash is not commonly accepted as an addition for cement. To enhance the activity of this material, in this experimental study CFBC ash was ground to a fineness equal to that of cement, simulating a joint milling with the clinker. This ground fly ash (GFA) was included in blended cements in ratios of 10, 20 and 40%. These new cements with GFA surpassed 52·5 MPa at 28 d for even substitutions as high as 40%. In comparison with a limestone addition – the most used in Europe – the cements with GFA presented higher compressive strength and better durability, with the only drawback being a slight reduction in workability. This CFBC fly ash could be used as clinker replacement once ground to a similar fineness to that of cement, allowing high reductions in clinker consumption and, consequently, its environmental impact.