Investigation of the Influence of Recycled Concrete Powder (RCP) on the Setting Behavior of Cement When Used as Supplementary Cementitious Material (SCM)

Abstract

The building materials industry makes a major contribution to greenhouse gases emitted each year, particularly by the cement clinker production. Therefore, the aim should be to maintain an increased part of building material from demolition sites in the material cycle. The use of the fine material (<2 mm) from demolition waste in concrete has so far proved to be problematic due to the increased water demand deterioration of durability properties. One approach is the use of recycled concrete powder (RCP) as supplementary cementitious material (SCM). Demolition material used in this study has been obtained from discarded railroad sleepers and pre-crushed as sand (<2 mm). The recycled sand was subjected to a mechanical and thermal activation process before use. It was ground to a particle size < 63 μm and then fired at different temperatures (750 °C and 800 °C). The aim was to convert parts of the hydrated C-S–H structure back into reactive silicate phases through thermal treatment process. They can contribute again to the hydration process when used SCM. The ground and thermally treated SCM was examined for its physical and chemical properties. Subsequently, 10 to 40 wt.-% of Portland cement was replaced by the SCM in the binder mixture. In a first step, the different water demand of the binders was documented. Ultrasonic methods were used to investigate the stiffening and setting behavior of the binders. The key factor here was the proportion of chemically bound water in the binder mixtures.