Recycling of construction and demolition waste in cement

by University of Lorraine

Treatments of construction and demolition waste (CDW) produce aggregates with sizes ranging from few micrometers to several centimeters. Because of the high water absorption and impurities in the finest fraction (grain size below 2 mm), recycled sands can hardly be used as secondary raw materials for concrete production. Therefore, in the framework of the SeRaMCo project, University of Lorraine and the cement manufacturer Vicat use these fine recycled aggregates as raw material for the production of cement. The aim of the project is to (i) produce industrial cement containing recycled sands and (ii) develop a methodology to optimize these CDW valorization in cement manufacture. The use of these materials in cement plants offers a new application for these recycled products and avoids consumption of natural raw materials (limestone, marls and clays) traditionally used for cement production.

By-products or waste (wood, slags, foundry sands, etc.) are commonly used in cement manufacture as combustible or raw materials but their addition is complex. Indeed, these components can affect the chemistry of the raw meal and consequently the cement quality (percentage of the different cement phases, crystals size, etc.). These cement properties influence directly the performance of the concrete used for building construction, in terms of strength, dimensional stability, durability, and so on. Therefore, valorization of recycled fines in cement plant requires a comprehensive study of the recycled aggregates chemistry as well as the reactivity and durability of the cement produced. Preliminary laboratory tests were made using two recycled aggregates provided by the project partner Tradecowall. Two types of aggregates were produced: the first one using “concrete” waste and the second one using “mixed” waste (concrete, bricks, soil, etc). In laboratory, they were dried, crushed and their chemical composition was determined. Based on the results (percentage of calcium oxide, aluminum oxide, etc.), the proportion of recycled sand in the raw meal was calculated in order to comply with specific chemical criteria (lime saturation factor, percentage of liquid phase, silica and alumina modulus, etc.). Mixes of marl and limestone with 22% of recycled “concrete” aggregates or 15% of recycled “mix” aggregates were prepared for burning in a laboratory oven at 1450°C (figure 1). The lower incorporation of the “mixed” aggregate in the raw meal is linked to the higher silica and lower calcium percentage in this type of CDW.

Figure 1. Sample quenching after burning at 1450°C

Figure 2. Free lime titration - change of color at the equivalent point

The effect of the recycled aggregates on the burnability of the raw meal is assessed by the measurement of the free lime content formed during burning. The values are then compared with a “reference” clinker prepared without recycled aggregates. A low free lime content indicates a good reactivity between the raw materials. Free lime proportion is measured by chemical titration (figure 2) with ethylenediaminetetraacetic acid (EDTA) after extraction with ethylene glycol. The results after burning at 1200°C, 1300°C, 1400°C and 1450°C indicate that the addition of these secondary raw materials don’t modify significantly the quantity of free lime formed during the cement phase formation. The study of the cement composition and reactivity is now under progress.