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Study of reuse from recycled aggregate processing
M.I. Sánchez de Rojas1*, E. Asensio1, M. Frías1, C. Medina2 1
Department Cements and Materials Recycling, “Eduardo Torroja” Institute for Construction Science, National Research Council (CSIC), 28033 - Madrid, Spain
2
Department of Construction, School of Engineering, University of Extremadura, UEX-CSIC Partnering Unit, 10071 - Cáceres, Spain
Abstract
Construction and demolition waste (C&DW) is a source of alternative raw materials. The promising research findings on fired clay-based construction and demolition waste as active cement additions herald its practical application in several construction sub-industries. Nonetheless, new dimensions and applications have yet to be studied. One would be the feasibility of valorising other C&DW products to ensure the integral use of this type of waste, 500 million tonnes of which are generated in Europe yearly. This study consequently explores C&DW materials, such as the “fines” generated in recycled aggregate processing, that have been put to no specific use to date. Given that this refuse is presently not regarded as apt for use in concrete, research on the development of new cements in which it could be included is described hereunder.
Keywords: C&DW, cement, pozzolanic activity, supplementary cementitious materials.
Introduction
Construction and demolition waste (C&DW) plays a particularly significant role in the present waste recycling scenario, as it constitutes one of the main sources of rubble in the European Union. Its composition is heterogeneous, varying with origin and subsequent processing.
Research conducted with fired clay industry C&DW has shown that this type of waste exhibits pozzolanicity. The economic, environmental and technical benefits of pozzolans have led to their widespread use in the global cement industry. Including active additions in portland cement improves the performance of the resulting matrices. Ordinary burnt natural pozzolan-bearing cement (CEM II/A&B-Q) is one of the types listed in European standard EN 197-1, which defines such pozzolans as thermally activated volcanic, clay, schist or sedimentary materials. Metakaolin, the most common of such products, is obtained by burning kaolinite (Frías et al., 2000). Initially inert clay minerals exhibit significant pozzolanicity when burnt at temperatures of 600 ºC-1000 ºC and ground to the same fineness as cement. The loss of chemically combined water during calcination destroys the crystalline network of the clay constituents, rendering their components amorphous or vitreous. Pozzolanic behaviour is largely governed by the resulting thermodynamic instability (Hea et al., 1995). Prior research (Sánchez de Rojas et al., 2006, 2014) has shown that fired clay roof tile and brick industry rejects exhibit pozzolanicity. C&DW consisting wholly or partially of fired clay materials is another source of such pozzolans. Such waste has also been observed to perform well as an addition (Asensio et al., 2016; Medina et al., 2013) and the procedures for its use have been patented (ES2512065).
This article establishes the differences and similarities among fired clay plant rejects, fired clay-bearing C&DW and the ‘fines’ generated during recycled concrete aggregate crushing and cleaning from the standpoint of their use as alternative pozzolans.
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1. Materials
The materials studied included:
- roof tile (T) and brick (B) rejects from fired clay plants
- construction and demolition waste (C&DW) from different recycle plants containing over 20 % fired clay material
- ‘fines’ (F) generated during the preparation of concrete aggregate in C&DW recycle plants.
2. Methodology
Chemical composition was determined on a Bruker S8 Tiger X-ray fluorescence spectrometer and mineralogy on a Bruker AXS D8 X-ray diffractometer. Electronic microscope studies were conducted on a Hitachi S-4800 SEM fitted with a Bruker EDX energy dispersive spectrometer.
Pozzolanicity was assessed with an accelerated chemical test that consisted in monitoring the reaction of the materials in a lime-saturated solution at 40±1 °C (Sánchez de Rojas et al., 2006). The lime fixed by the sample (%) was found as the difference in concentration between the initial saturated solution and the solution in contact with the sample at each pre-established test time.
Results and Discussion
The chemical composition of the materials is given in Table 1. The composition of the fired clay roof tile and brick rejects resembled that of other pozzolans. Their silica, alumina and iron oxide contents, which together accounted for nearly 75 % of the tiles and around 67 % of the brick, explained their high acidity. Both materials had low SO3 contents. Table 1 also lists the mean values for 20 samples, all with a fired clay content of over 20 %, taken from different C&DW management plants. These materials were also highly acidic, with 59 % SiO2 + Al2O3 + Fe2O3 and a CaO content of 18 %. The mean SO3 concentration was 1 % and no greater than 3 % in any of the samples tested. The low sulfate content ruled out the presence of significant amounts of gypsum. Acidic oxides also predominated in the 'fines', which exhibited a CaO content on the same order as the roof tiles and brick.
The mineralogical composition of all the waste studied was very similar, with quartz (Q), illite (I), calcite (C), dolomite (D), hematite (H), orthoclase (O) and anorthite (An) as the main crystalline components. No signal indicative of gypsum (Figure 1) was observed in any of the samples.
Table 1. Chemical composition: majority elements
Component F CDW T B SiO2 57,71 45,84 53,28 45,18 Al2O3 10,35 9,83 16,57 17,06 Fe2O3 1,86 3,63 5,29 5,05 CaO 12,74 18,20 12,41 15,01 Na2O 1,19 0,60 0,58 0,51 K2O 3,45 2,13 3,10 2,63 MgO 1,44 3,59 3,65 3,15 TiO2 0,26 0,26 0,58 0,48 ZrO2 0,04 0,02 0,02 0,02
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SO3 1,57 1,19 0,79 0,04
Cl- 0,02 0,02 0,02 0,02
PPC 9,18 14,55 3,44 10,82
SiO2+Al2O3+Fe2O3 69,92 59,30 75,14 67,29
Total 99,81 99,86 99,73 99,97
Since acidic components induce pozzolanicity, their presence would initially suggest the existence of that property. That was confirmed by the chemical pozzolanicity test conducted on the samples crushed and milled to a cement-like Blaine fineness, with a specific surface ranging from 3500 cm2/g to 4000 cm2/g. The finest material was obtained with brick, a finding that impacted (particularly early age) pozzolanicity, as discussed below.
Fig. 1. Mineralogical composition
Further to the test findings shown in Figure 2, fired clay waste exhibited acceptable pozzolanicity, for up to 40 % of the lime was fixed after 1 day by the brick (the finest material), over 20 % by the fired clay material-bearing C&DW and 10 % by the roof tile. Around 70 % of the lime was fixed by all three materials after 7 days. The curve subsequently tended to flatten, with around 80 % fixation after 90 days. Similar behaviour was recorded for the ‘fines’, although the lime fixation values were lower, with only 70 % of the total fixed after 90 days.
Fig. 2. Pozzolanic activity test
Fired clay rubble, be it in the form of roof tile or brick rejects, fired clay C&DW or fines, exhibited behaviour comparable to that of metakaolin, the burnt clay most commonly used as a pozzolanic addition in cements. The pozzolanic reaction taking place when the fines were in
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contact with the lime-saturated solution was monitored under a scanning electron microscope. The 90 day findings showed that reaction products containing Ca(OH)2 had formed.
The C-S-H contained small proportion of Al and other elements (Figure 3). The presence of these pozzolanic reaction products confirmed the ‘fines’ to be reactive and hence apt for use as an alternative pozzolanic material.
Figure 3. SEM micrograph and EDX microanalysis Conclusions
The conclusions drawn from this study are as follows.
1. The chemical and mineralogical compositions of fired clay brick and roof tile rejects, fired clay-bearing C&DW and the ‘fines’ generated during concrete aggregate crushing and cleaning were essentially similar.
2. Further to the XRD findings, the increase in the CaO content in some of the materials studied, due primarily to calcite formations, was unrelated to the presence of gypsum, which may jeopardise cement manufacture.
3. The pozzolanicity exhibited by fired clay brick and roof tile rejects confirmed earlier research according to which the firing temperature used in their manufacture suffices to activate their clay component, inducing pozzolanic activity.
4. Whilst the ‘fines’ exhibited exhibited lower pozzolanicity than observed in metakaolin, fired clay C&DW or roof tile or brick rejects, they tested to values suggestive of their possible use in cement.
Acknowledgement
This research was funded by several Spanish ministries under project references BIA 2013-48876-C3-1-R, BIA 2013-48876-C3-2-R and BIA2016-76643-C3-1-R, as well as by the SOSMAT University of Extremadura- (Spanish) National Research Council Associated Research Unit.
References
-Asensio, E., Medina, C., Frías, M., Sánchez de Rojas, M.I. (2016) “Characterization of ceramic – based construction and demolition waste: use as pozzolan in cements”. Journal of the American Ceramic Society, 99 (12), 4121-4127.
- Frías, M.; Sánchez de Rojas, M.I.; Cabrera J. (2000): "Effect that the pozzolanic reaction of metakaolin has on the heat evolution in metakaolin-cement". Cement and Concrete Research, 30, 209-216.
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-Hea Ch, Osbzckb B, Makovicky, E (1995), “Pozzolanic reactions of six principal clay minerals: activation, reactivity assessments and technological effects”, Cement and Concrete Research, 25, 8, 1691-1702.
-Medina, C., Banfill, P.F.G., Sánchez de Rojas, MI, Frías, M.: Rheological behaviour of cements bended with containing ceramic wastes. 7th International Confeence on Self-Compacting Concrete and 1st RILEM International Conference on Rheology and precessing of Construction Materials. Paris, september 2013.
- EN 197-1:2011. Standards. Cement. Part 1: Composition, specifications and conformity criteria for common cements
- Patent ES2512065: Ceramic residue useful for making cements, obtaining process and cements comprising it. M.I. Sánchez de Rojas, M Frías, E. Asensio, C. Medina
-Sánchez de Rojas, M.I., Marín, F., Rivera, J., Frías, M. (2006), Morphology and properties in blended cements with ceramic waste materials recycled as pozzolanic addition, Journal of the American Ceramic Society, 89, 12, 3701-3705.
-Sánchez de Rojas, M.I., Frías, M., Rodríguez, O., Rivera, J. (2014), Durability of blended cement pastes containing ceramic waste as a pozzolanic addition, Journal of the American Ceramic Society, 97,5, 1543-1551.
