CDW fines as mortar aggregate

Abstract

The increasing global demand for construction materials has raised concerns about the sustainability of natural sand, a key component in concrete production. In the Netherlands and across Europe, the long term availability of sand is uncertain, and the environmental and social impacts of sand extraction are becoming more apparent. One promising alternative is the use of recycled aggregates, particularly construction and demolition waste (CDW) fines, as a replacement for natural fine aggregates in mortar and concrete. This master thesis investigates the potential of Dutch CDW fines to address this challenge, with a focus on their characterisation, performance, and impact on microstructure.

The study begins with a comprehensive characterisation of CDW fines provided by Miner-
alz/Renewi, assessing their chemical, physical, and mineralogical properties using techniques such thermogravimetric analysis (TGA), X-ray fluorescence (XRF), X-ray diffraction (XRD), and
polarised light microscopy. These analyses revealed that CDW fines are highly heterogeneous but largely consistent in composition over time. Quartz is the dominant phase at 72.5% according to qXRD, with smaller amounts of calcite, gypsum, and amorphous material also being present. The presence of contaminants such as glass and metals was confirmed and quantified using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS).

Performance tests on mortar samples incorporating varying percentages of CDW fines demonstrated a notable reduction in compressive and flexural strength of about 25% when 100% of natural aggregates are replaced with CDW fines. The compressive strength of 24.7 Mpa (100% CDW fines) still falls into the highest defined strength class in the European standard for masonry mortar. A decrease in strength while other parameters are kept constant is consistent with previous studies. The setting time does drastically increase with the replacement percentage of CDW fines, by as much as 100%. The workability on the other hand remains the same, contrary to what other is reported in other literature. All this would make CDW fines viable for plenty of applications. Before knowing more about the durability aspects though it is recommended to start with non-structural ones like pavement tiles.

Microstructural analyses using SEM indicated no significant changes in hydration products
but highlighted the presence of impurities, microcracks, and damaged and low quality interfacial transition zones (ITZs). The impurities such as brick, various metals and attached old cement paste contribute to the observed strength performance differences. The low amount of old cement paste (4-6%) observed with EDS also makes the low water absorption, high density, and stable workability more plausible. Furthermore, calorimetry testing on mortar samples shows significant differences in the hydration process over time. One difference is that the reaction of C3A is slowed down by the presence of gypsum in the CDW fines. This is in line with the previously mentioned increase in setting time.

This research provides insights into the properties and behaviour of CDW fines and extends the foundation for their improved utilisation in sustainable construction practices. By leveraging this abundant waste stream, the construction industry can make meaningful steps toward reducing its environmental footprint and conserving natural resources.