https://doi.org/10.14311/APP.2022.33.0591 Acta Polytechnica CTU Proceedings 33:591–596, 2022 © 2022 The Author(s). Licensed under a CC-BY 4.0 licence Published by the Czech Technical University in Prague USE OF RECYCLED CONCRETE FINES IN CEMENT AND AS AGGREGATE Simone Stuerwald∗, Ronny Meglin, Susanne Kytzia, Sabrina Gilg Open University, Walton Hall, Milton Keynes, Keynes MK7 6AA, UK ∗ corresponding author: simone.stuerwald@ost.com Abstract. The research project focused on investigating and optimizing the processing and use of recycled crushed sand 0/4 from concrete demolition waste, as an alternative raw material in the cement and concrete industry. Crushed sand is produced during the processing of concrete demolition waste. The goal was to identify the optimum way of using the processed material along the entire process chain so that greenhouse gas emissions, waste volumes are reduced, and natural resources are conserved. Different samples of laboratory and real crushed concrete fines were collected and examined in relation to various possible applications in accordance with the applicable standards. Results highlight, that crushed concrete fines can be used in various applications in the concrete value-chain. However, for an optimal usage, additional processing is needed. Keywords: Cement, concrete, construction and demolition waste, LCA, recycling. 1. Introduction More than 10 years ago, a swiss standard was in- troduced that regulated the use of recycled concrete demolition waste as an aggregate for structural con- crete [1]. Nowadays the use of such aggregates is very common in Europe and Switzerland [2–4]. In the coming decades, the amount of construction waste in Switzerland will increase - especially concrete de- molition waste from structures built in the 1950s to 1970s [5]. However, most of the processed construc- tion waste is "down-cycled" into materials with sub- ordinate technical requirements such as lean concrete or as road subbase material [6]. For this reason, vari- ous studies have recently been carried out to optimize the use of concrete demolition waste [7–9]. However, most studies focus on the complete concrete demo- lition waste. We focus on the crushed concrete fines because it has a significant impact on the possible use of the demolition material due to its less favourable properties [10]. We think, that the usage can be op- timized, especially as it is assumed that recycling of concrete fine from demolition could further reduce CO2 emissions in the concrete value chain [11–13]. We aim to optimize the usage of concrete demoli- tion waste, especially the crushed concrete fines as a by-product. The economic efficiency of concrete re- cycling will be significantly improved if this crushed sand can be used profitably in cement and concrete production. We evaluate three economically viable uses of recycled crushed sand 0/4 according to sus- tainability criteria such as climate protection, re- source conservation and waste avoidance. Possible applications would be (i) use as a low-CO2 substi- tute in cement kiln, (ii) use as an additive in cement mill or concrete production, or (iii) use as an aggre- gate in concrete production (Figure 1). The aim is to identify the optimum place of use, the quality of the crushed sand required and the requirements for processing methods. In this way, crushed sand is to be optimally returned to the material cycle, natural raw materials are conserved, and CO2 is saved. 2. Methodology We estimate the potential for the use of crushed con- crete fines by using various test methods and calcu- lations to determine the properties of real and lab- oratory specimens. Three samples of crushed sands (BS-L) were produced in the laboratory with differ- ent cement paste contents. Another three samples (BS-R) were taken from recycling plants and a sam- ple of sand from natural resources was used as a ref- erence. Based on chemical analyses of the samples, the use of ground crushed fines in clinker production is evaluated in simulations and the possible degree of substitution as a raw material substitute is esti- mated. Subsequently, mortar samples produced with the ground crushed fines as an admixture are tested according to SN EN 196. The results are then com- pared with the current standards and allow predic- tions on the possible use of the crushed sand as an additive and the possible amount to be added. In a further step, crushed fines samples are used as a substitute for primary sand in the production of con- crete. The concretes produced are then subjected to extensive testing according to SN EN 206. A compar- ison with the standards will allow us to estimate the maximum degree of substitution of the primary sand. Finally, the results will be evaluated and discussed. 591 https://doi.org/10.14311/APP.2022.33.0591 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en S. Stuerwald, R. Meglin, S. Kytzia, S. Gilg Acta Polytechnica CTU Proceedings Figure 1. In the process chain, raw materials are used at various processes: in the cement kiln (green), in the cement mill/concrete production (blue) and in concrete production (red). Figure 2. Thin section to determine the content of cement paste. 3. Results 3.1. Substitution of Raw Material in Cement Production Recycled crushed fines contains sand and hardened cement paste and can be used as an alternative raw material in cement production [6, 10]. In particular, the cement paste portion contains the raw materials that can be reactivated in the cement kiln. Crushed fines samples (BS-L) from the laboratory were specif- ically made with 40%, 55% and 70% cement paste. The cement paste content of real crushed fines (BS- R) was determined on prepared thin sections by po- larized light microscopy (Figure 2). It was observed that the content of cement paste in the real crushed fines samples was 14% - 15%, which is lower than the expected typical content of about 30% (Figure 3). It is assumed that parts of the ce- ment paste were lost in the demolition and separation process. Material analysis by X-ray fluorescence spec- troscopy (XRF) was performed on each of the real crushed fines samples (BS-R). The CaO content was between 24 % and 29 % and the LSF (Limestone Sat- uration Factor) was between 16.9 and 27.8. This re- flects the low cement paste content, which is decisive for the maximum substitution of natural raw mate- rial. Calculations showed that only small amounts of the raw material can be substituted. To raise the sub- stitution rate, the cement paste of crushed sand must be separated completely. Hardened cement paste by itself could replace more than 90% of the natural raw materials (Figure 4). 592 vol. 33/2022 Use of Recycled Concrete Fines in Cement and as Aggregate Figure 3. Samples of crushed sand and their content of hardened cement paste. Figure 4. Maximum substitution rate depending on Lime-Saturation-Factor (LSF). 3.2. SCMs in cement mill or concrete production To evaluate the possible use of crushed sand as an admixture, selected samples of crushed sand (BS-R- 1, BS-L-40, BS-L-55 and BS-L-70) were milled and mixed in a ratio of 40:60 % with CEM I 52.5 R. These cements were then used to prepare cement paste for further testing of strength (according to EN 196-1), setting time and room stability (both according to EN 196-3). Results of CEM II/B-LL were used as reference. The results of the tests (Table 1) indicate, that a lower compressive strength compared to a CEM II/B- LL must be expected. Since the strengths of cements with crushed fines do not differ significantly, it can be assumed that the crushed fine is inert. The results of the setting time do not show a clear picture. While the cement with BS-R-1 crushed fine sets significantly faster than a CEM II/B-LL, the cement with BS-L-40 shows a somewhat slower setting time. The room sta- bility test does not indicate any significant differences between the cements tested. The processing and preparation of the crushed sand as an additive is more complex than for the other two types of use since high demands are made on purity and homogeneity of the material. 3.2.1. Substitution of Sand in Concrete Production Using the crushed fines as a substitute for natural sand is a good method to reduce the consumption of primary resources. However, sand is often segre- gated from demolition material because the fresh and hardened concrete properties may deteriorate. First, the particle size distribution (Figure 5) and the properties of the crushed sands (Table 2) were determined. In a test series on concrete mixtures, it was inves- tigated whether samples of crushed sand are suitable as a substitute for sand and which concrete proper- ties are derived. In each mixture, the entire aggregate fraction 0/4 was replaced by recycled crushed sand. 593 S. Stuerwald, R. Meglin, S. Kytzia, S. Gilg Acta Polytechnica CTU Proceedings CEM BS-L-70 CEM BS-L-55 CEM BS-L-40 CEM BS-R-1 CEM II/B-LL Water [%] 30.1 30 30 28 28 Compressive strength [N/mm2] 2 d 21.3 21.6 20.9 20.8 25 7 d 30.8 34.9 32.4 33.3 37 28 d 36.9 41.4 39.4 39.2 47 Flexural strength [N/mm2] 2 d 4.8 4.5 4.4 4.9 N/A 7 d 5.7 5.9 6 6.3 N/A 28 d 6.2 6.3 5.9 6.3 N/A Setting time [min] begin 157 187 216 127 200 end 204 231 256 207 240 Volume stability [mm] 0.5 1 1 1.5 1 Table 1. Results of cement paste tests compared to CEM II B/LL. Figure 5. Particle size distribution of crushed sands. A mixture with natural sand was used as a reference. The different water absorption of the crushed sand samples was considered, so that a constant effective w/c ratio of 0.6 should be achieved. The compressive strength of concrete with recy- cled crushed sand was only reduced by about 10-15%. However, this reduction may also be related to differ- ences in the effective water content and need not be related to the mechanical properties of the crushed sand. Crushed sand with high content of cement paste (BS-L-70) show a young’s modulus that is up to 17 % lower, while a mixture with low content of cement paste shows a young’s modulus that is about 6 % lower compared to the reference. Natural sand can be completely replaced by recy- cled crushed sand. If the cement paste is separated during processing, the remaining sand can be used to produce concrete, which is technically equivalent to concrete made from primary material. 4. Discussion The results show that the crushed fines can, with certain restrictions, be used for the investigated ap- plication possibilities in the value chain of concrete production. The results of the mortar tests (section 3.2.) indicate that a complete substitution of lime- stone powder is possible without any significant loss of quality. The use as a substitute for natural aggre- gates is also possible according to the results in 3.3. To do this, however, further concrete technology mea- sures must be taken to prevent any significant loss of quality. The use as a raw material substitute in ce- ment production (3.1.), however, does not seem to be possible without further processing steps. This is due to the low cement content found in the real samples, which leads to a low lime saturation factor and thus prevent a high degree of substitution. An aspect not presented here in detail is the CO2- uptake of the concrete waste. The cement paste ab- sorbs CO2 from the air through natural carbonation 594 vol. 33/2022 Use of Recycled Concrete Fines in Cement and as Aggregate BS-R-1 BS-L-40 BS-L-70 ratio of cement paste [%] 14 40 70 dry density [kg/m3] 2332 2175 1940 water absorption WA24 [%] 6.2 9.7 12.4 Table 2. Properties of the crushed sands. reference BS-L-70 BS-L-40 BS-R-1 cement [kg/m3] 300 300 300 300 water [kg/m3] 180 180 180 180 w/z (eff.) [−] 0.6 0.6 0.6 0.6 aggregate [kg/m3] 1887 1605 1678 1738 density [kg/m3] 2367 2085 2158 2218 air content [Vol. %] 1.5% 1.5% 1.5% 1.5% Table 3. Concrete mix and properties of fresh concrete mixtures. reference BS-L-70 BS-L-40 BS-R-1 Density concrete [kg/m3] 2400 2310 2300 2320 Strength 28d [N/mm2] 40.5 36.0 37.0 34.5 E-modulus [N/mm2] 38ă419 32ă883 31ă867 35ă976 Table 4. Properties of hardened concrete mixtures. and binds this in the formation of calcium carbonate (CaCO3). This effect has already been investigated by various studies, although there is still no generally valid assumption about the exact maximum possible degree of carbonation [14–16]. Carbonation occurs particularly when the cement stone is ground and therefore has a very large surface. The CO2 uptake is particularly useful in the production of clinker, as the CO2 emissions of the rotary kiln can be kept in the "cycle" by means of forced carbonation. In this way, we could achieve a certain degree of CO2 neutrality in clinker production. Based on these results, we would like to suggest the following approaches: • The greatest possible separation and recovery of the cement paste when processing concrete demo- lition. When developing processing methods, the aim should be to separate the cement stone as far as possible and to hold it back as a separate frac- tion. This gives you (i) a high-quality and climate- neutral secondary raw material for cement produc- tion and (ii) a high-quality RC aggregate for con- crete production, which enables the further opti- mization of recipes to produce RC concretes. • As far as possible carbonation of the cement paste in the entire process chain of the preparation of the cement paste in the concrete demolition (i.e. the preparation and storage in the various pro- cess stages). We assume that concrete demolition in Switzerland has a significant potential for re- binding CO2, since it is mostly concrete that was produced with Portland cement and only absorbed limited amounts of CO2 during use. In the develop- ment of processing methods, the aim should be to increase the carbonation of the cement paste in the concrete demolition. This is achieved above all by increasing the surface area and designing the stor- age conditions in a targeted manner. This allows CO2 to be extracted from the air and stored in the building material. This corresponds to the "carbon capture and storage" approach that is much dis- cussed today, with which one would like to reduce net CO2 emissions by 2050. 5. Conclusions The demolition of concrete is not yet considered as a problem and the potential of its recycling is not yet sufficiently recognised. This will change in the coming years or decades, because: (i) The amount of concrete demolition will increase once the concrete structures from the second half of the last century have reached the end of life. Therefore, the pro- portion of mixed demolition in mineral construction waste will decrease in the future and the proportion of concrete demolition will increase accordingly. (ii) The possible uses of RC concretes in structural en- gineering will increase and with it the demands on the building material. Today, qualitative deficiencies in the concrete granulate are compensated by adjust- ments in the concrete mix design (more cement, more admixtures). This leads to higher costs and envi- ronmental pollution. In the future, attempts will be made to improve the quality of concrete demolition. (iii) The pressure on the cement industry to make its contribution to climate protection is increasing (e.g. 595 S. Stuerwald, R. Meglin, S. Kytzia, S. Gilg Acta Polytechnica CTU Proceedings through higher CO2 fees and measures taken by pub- lic clients). Many representatives of this industry see a promising approach to solving this problem by re- binding CO2 in concrete. For this, the greatest po- tential lies in the rebinding capacity in the crushed sand of the concrete demolition. For these reasons, efforts to develop new processes for the separation and processing of C&DW will in- crease significantly in the coming years and further investigations for the usage of C&DW will be nec- essary. 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