Towards circular cement

Abstract

The changing climate and the need for a new purpose for End-of-Life concrete increased the importance and interest of concrete recycling. To decrease the environmental footprint of concrete, in which cement has the highest contribution, it would be of great importance to be able to recycle cement, which would positively affect the environment. The separation efficiency plays an important role in the cement properties. The Smart Crusher is a novel concrete crushing- and separation technology developed to optimize the separation efficiency of the concrete. This study investigates the recycling potential of the cementitious fractions obtained from the Smart Crusher. The aim of this research is to evaluate the reactivity of the cementitious binder retrieved from recycled cement stone to obtain a fully functional binder that can replace primary cement in constructions and structural elements. To gain more insight in the reactivity of cementitious binder retrieved from recycled cement stone, the Smart Crusher cement fractions, which in this study had an unknown origin, are compared with non-hydrated and hydrated reference cements CEM I 52.5 R and CEM III/B 42.5 N. Therefore, the different materials are characterized, using TGA, DSC, MS, XRF and XRD. Analysis of the chemical and mineralogical composition showed the presence of hydration products in the hydrated Smart Crusher, CEM I 52.5 R and CEM III/B 42.5 N samples. Additionally, quartz is found in the cementitious fractions of the Smart Crusher, which indicates the presence of aggregate particles such as sand in these samples. Furthermore, the absence of P2O5 in the cementitious Smart Crusher fractions proved that there is no or a not measurable amount of fly ash present in the cementitious Smart Crusher fractions. Upcycling of the hydrated Smart Crusher, CEM I 52.5 R and CEM III/B 42.5 N materials is done by thermal treatment. To assess the upcycling, again the chemical and mineralogical composition is analysed. Red colouration (500 °C and 800 °C) and melting (1400 °C) of the cementitious Smart Crusher fractions again show the presence of aggregate particles in the material. During thermal treatments of the Smart Crusher, CEM I 52.5 R and CEM III/B 42.5 N materials, the decomposition of hydration products and calcite are monitored, using TGA, DSC and MS. The thermal treatment at 800 °C showed the formation of alite and belite phases in all the samples and was therefore seen as the treatment with the most potential in this research. The only material that shows the formation of alite and belite during other thermal treatments was CEM I. After the thermal treatments the composition and crystalline phases were determined, using XRD and XRF. Insight in the mechanical performance of the different binders was obtained by flexural and compressive strength tests. Mortar prisms were made, containing CEM I and CEM III/B as reference cement and thermally treated CEM I, CEM III/B, 0.0 – 0.063 mm and 0.063 – 0.125 mm cementitious Smart Crusher fractions. The mortar mixtures with the thermally treated binders were observed to be rather dry as a result of an increasing water demand. Compared to new cement, the increase in water demand led to a low degree of compaction, resulting in return to strength properties that were practically unmeasurable. The reference cement showed a lower early strength for CEM III/B compared to CEM I and comparable strength after 28 days due to the higher gain of strength of CEM III/B. Especially the flexural and compressive mechanical tests showed that more research is necessary. To conclude, this study showed that a certain reactivity can be expected in the secondary binders after upcycling, but that the unmeasurable strength properties of the upcycled materials showed the need for more research to make a better estimation of that reactivity.