The assessment of clinker and cement regenerated from completely recyclable concrete

As the construction sector uses 50% of the earth’s raw material and produces 50% of its waste, the development of more durable and sustainable building products is crucial. Nowadays, Construction and Demolition Waste (CDW) is already used as recycled aggregates in low-value concrete applications, since it is mostly inert material. On the other hand, the general trend today for the cement industries is the use of alternative raw materials for the production of cement clinker. From this and the above mentioned need for high-value recycling of CDW, the concept of completely recyclable concrete (CRC) has been developed. After demolition of a CRC-construction, the material cycle is closed as the concrete rubble is given a second life as raw material for cement production, without need for adjustments. Therefore, the concrete mixture is designed to be chemically equivalent to raw material for cement production by adequately incorporating limestone aggregates, different types of cement and industrial by-products. For this study, completely recyclable concrete was designed and produced. Within the design process, the chemical composition of the produced CRC was evaluated with the lime saturation factor, the silica modulus, the alumina modulus and the hydraulic modulus. In addition, the potential mineralogical composition was calculated according to the formulas of Bogue. Then, clinker was regenerated by burning ground CRC in a laboratory furnace by raising the temperature at a constant rate (15 °C/min) to 1350, 1400 and 1450 °C and maintaining it for 30 minutes. After burning, the clinker was immediately air-cooled by removing it from the furnace. The quality of the produced clinker and the influence of the burning temperature thereupon were investigated by determining the free lime content, and by microscopic and X-ray diffraction analysis. Based on these results, the ideal burning temperature was selected to produce cement. This cement was produced by grinding the clinker with calcium sulphate anhydrite. The hydration heat of cement pastes was measured in isothermal conditions and mortars were produced for compressive strength test