Investigation of the influence of fine recycled sand on the setting behaviour 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 (< 2mm) from demolition waste in concrete has so far proved to be problematic due to the increased water demand and loss of compressive strength. 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 (< 4 mm). The recycled sand was subjected to a mechanical and thermal activation process before use, then was ground to a particle size <63 μm and then fired at 4 different temperatures (750°C, 800°C, 850°C, 900°C). The aim was to convert parts of the hydrated C-S-H structure back into reactive silicate phases through firing process. They can contribute again to the hydration process when used as supplementary cementitious material. The ground and thermally treated material - called SCM - wasexamined for their physical and chemical properties. Subsequently, 10 and 20 Vol.-% were replaced by the SCM in a binder mixture, respectively. In a first step, the different water demand of the binders was documented. Ultrasonic methods were used to investigate the stiffening and setting behaviour of the binders. The decisive factor here was the proportion of chemically bound water in the binder mixtures. Finally, the mechanical properties of the binders were investigated in mortar tests. Acceptable compressive strengths were achieved compared to the reference mortar (mortar mixture without cement substitution). At first glance, it seems possible to use it as an SCM.