Usage of grinding sludge in the own production of cast stone

Large amounts of grinding and concrete sludge accumulate in the course of the production of cast stones, while high costs are involved in the disposal of the same. The objective of a research cooperation of TH Köln - University of Applied Sciences and Metten Stein+Design was the investigation of the reuse potentials of grinding sludge in the production of paving blocks as well as the possibility for a substitution of the cement contents in the core mix concrete. The investigation program comprised the characterization of grinding sludge samples taken from the ongoing production of cast stones and an assessment of the influence on the concrete-specific properties such as workability and strength, when used in the core mix concrete of concrete pavers. The laboratory tests conducted revealed that a substitution of up 5 vol.% of cement by grinding sludge is possible without significant impact on the mechanical properties or the workability of the concrete. However, the findings suggest that the grinding sludge primarily assumes the function of a filler in the concrete and thus categorizing as strength developing is only possible to a lesser degree. An implementation on an industrial scale would require an application of the grinding sludge in a fresh and moist condition, complemented by continuous monitoring of the water content for adapting the mixing water accordingly. Hence, an amount of up to 10 kg of grinding sludge per m³ of fresh concrete can roughly be reused for usual core concrete mix designs, what is corresponding to a saving of an equal amount of cement. With respect to the environmental footprint, there are positive effects in terms of the resource efficiency and the carbon emissions. The findings constitute a basis for further investigations, in particular, concerning the durability of the paving blocks manufactured using grinding sludge.