Romain Trauchessec, Hichem Krour, Cécile Diliberto, André Lecomte,
University of Lorraine
Laury Barnes-Davin, Breffni Bolze, Arnaud Delhay,
Vicat
Recycled Aggregates (RA) from Construction and Demolition Waste (CDW) contain natural aggregates, hardened cement paste but also masonry, plaster, etc. Part of these recycled aggregates are hardly suitable for concrete production due to their water absorption (cement paste porosity, clays) and reactivity (sulphate or alkalis release, etc.). RA incorporation in Portland cement raw meal is a possible way to use these recycled aggregates and consequently avoid landfill. Several studies [1-3] have already shown the possibility of RA incorporation in cement raw meal. However, complementary investigations require to consider the chemical variability of the RA, the ecologic or economic interest of this recycling, the durability of the cement produced, etc. This article examines the advantages and challenges of the RA incorporation, the RA chemical variability and the clinker synthesis (laboratory and industrial trial).
Advantages and challenges
In the North-Western European (NWE) countries, the interest of recycling Construction and Demolition Waste (CDW) in cement manufacture is supported by the geographic repartition of the cement factories with kiln(s) and the quantity of cement produced. Indeed, the available data (cement producers and associations) show that cement plants are relatively well distributed in the different countries and are often located near important cities (and therefore deconstruction sites). These factories produce more than 50 million tons of Portland clinker per year. Moreover, cement manufacturers are familiar with alternative resources valorization (foundry sands, etc.) currently used to adjust the raw meal composition and substitute natural materials. The main hurdles to use recycled aggregates in cement manufacture are the sustainability of the RA supply chain, the cost of these materials and the regularity of the product characteristics.
Recycled aggregates variability
In order to study the RA variability, chemical analyses (X-Ray Fluorescence, XRF) of recycled aggregates (literature data and complementary experiments) have been
performed. The values show that the composition of recycled (concrete or mixed) aggregates widely vary in terms of calcium oxide (CaO) and silicon oxide (SiO2) contents. In most of the samples, compared to Portland cement, recycled aggregates contain much more silicon oxide but less calcium oxide. Concerning the amount of aluminium and iron oxides (Al2O3 + Fe2O3), the quantity is relatively similar in recycled aggregates and Portland cement. Minor elements (MgO, Na2O, K2O, SO3, etc.) represent around 6% of the recycled aggregates and could limit the incorporate rate. Consequently, Portland cement cannot be produced only from recycled aggregates but can be manufactured by a combination of conventional raw materials (limestone, clay, marl, etc.) and an adjusted fraction of RA.
Clinker manufacture using RA
Laboratory synthesis
Portland clinkers were produced with 5 various RA (concrete or mixed). The incorporation rate (between 11 and 22%) was calculated using the XRF results. These RA percentages cannot be higher due to the silica or alkalis contents. After burning at 1200, 1300, 1400 and 1450°C, the free lime content is measured and the clinker properties (composition and reactivity) are studied.
Industrial trial
In September 2018, recycled aggregates produced by Tradecowall in Belgium (150t) and Agrégats du Centre in France (580t) were used to produce 5137t of raw meal. An incorporation rate higher than 14.2% could not be reached given the high silica amount in the RA. The raw meal also contained limestone and corrective materials (bauxite and iron oxide) but marls were not used. The clinker produced contained free lime rates similar to the standard clinker production.
Conclusion
Replacement of Portland clinker raw materials by RA is promising but challenging because of the variability in the RA composition. The substitution percentage has to be adjusted in function of the cement plant (chemistry of the natural raw material) and the recycled aggregates (type of deconstruction, recycled aggregates treatments, etc.). Other issues can arise due to the silica or alkalis contents. Laboratory and industrial productions of Portland clinker containing near 15% of RA have been successfully achieved. For Créchy cement plant, lower incorporation rates are more realistic (RA supply, marl utilization) and will be chosen for the next trial.
[1] C. Diliberto, A. Lecomte, J-M. Mechling, L. Izoret, A. Smith, Valorization of recycled concrete sands in cement raw meal for cement production. Materials and Structures, (2017) 50:127.
[2] J. Schoon, K. De Buysser, I. Van Driessche, N. De Belies, Fines extracted from recycled concrete as alternative raw material for Portland cement clinker production. Cement and Concrete Composites, 58 (2015) 70–80.
[3] M. Fridrichova, J. Gemrich, Use of recycled concrete constituents as raw materials components for producing Portland and belite cement clinker. Cement International, 4 (2006) 110–116.