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An investigation on the recycling of hydrated cement from concrete demolition waste
Cement & Concrete Composites 61 (2015) 29–35
Contents lists available at ScienceDirect
Cement & Concrete Composites journal homepage: www.elsevier.com/locate/cemconcomp
An investigation on the recycling of hydrated cement from concrete demolition waste D. Gastaldi a,⇑, F. Canonico a, L. Capelli a, L. Buzzi a, E. Boccaleri b,⇑, S. Irico b a b
Buzzi Unicem S.p.A., Via Luigi Buzzi 6, 15033 Casale Monferrato, Alessandria, Italy Science and Technological Innovation Department, University of Piemonte Orientale, Viale T. Michel 11, 15121 Alessandria, Italy
a r t i c l e
i n f o
Article history: Received 30 May 2014 Received in revised form 1 April 2015 Accepted 11 April 2015 Available online 24 April 2015 Keywords: Hydrated cement waste Recycled aggregate Concrete demolition waste Fine fraction CO2 reduction
a b s t r a c t Construction and demolition waste (CDW) recycling is generally limited to the use of the coarser fraction as aggregate for new concrete. The recovery of fine aggregates requires a cleaning by removing the hydrated cement waste (HCW). In this paper, the possibility to use HCW extracted from CDW as alternative component for the production of new clinker is explored. A pure HCW sample was prepared and used in partial replacement of natural materials in raw admixtures for new clinker production. At a replacement degree of 30%, a new Portland clinker containing almost 50% of C3S could be produced with a huge spare in the release of CO2 (about 1/3 less). At higher HCW dosage a non-Portland clinker containing almost 80% of C2S has been obtained: its use as supplementary cementing material in blended cements revealed satisfying long term performances. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Recycling is the issue for sustainable development. Main streams in the construction industry are constituted by concrete, brickworks, pieces of plaster and gypsum board characterized by extremely wide chemical composition, color, mechanical properties. In Europe about of 180 million tons of concrete demolition waste (CDW) are produced every year, corresponding annually to 500 kg for each citizen [1]: this amount represents around 31% of all the waste produced in the European Union [2]. For long time concrete and brickworks waste have only been used as a filling material or disposed to landfill. Nevertheless, in the late 20th century concrete recycling gained more and more importance, due to the increasing attention toward environmental protection and to the progressively reducing landfill capacity [3]. Current concrete recycling consists of crushing waste concrete and use it again as aggregate for new concrete [4], according to specifications which are based on local regulations in different countries. The production of recycled concrete aggregates (RCA) is a well established practice in Belgium, Denmark and the Nederlands, where recycling rates raise 80% [5], while it is less common in Southern Europe. ⇑ Corresponding authors. Tel.: +39 0161 809740 (D. Gastaldi), +39 0131 360264 (E. Boccaleri). E-mail addresses: [email protected] (D. Gastaldi), enrico.boccaleri@ uniupo.it (E. Boccaleri). http://dx.doi.org/10.1016/j.cemconcomp.2015.04.010 0958-9465/Ó 2015 Elsevier Ltd. All rights reserved.
The quality of RCA is generally lower than that of natural aggregates, due to presence of residual mortar [6]: for this reason, when dealing with concrete recycling, a differentiation between coarse (nominal size >5 mm) and fine aggregates (maximum size <5 mm) is generally done. Coarse recycled concrete aggregates (CRCA) are commonly used in partial replacement of natural aggregates in concrete [7–9], however the concrete mix design has to be adjusted in order to correct the worsening of final properties such as workability and durability, especially in respect to alkali–silica reaction, corrosion (due to chloride content) and freeze thaw resistance [10]. On the contrary, fine recycled concrete aggregates (FRCA) are less useful as aggregates in concrete as they can be highly detrimental for what concern strength, workability and durability [11–15]. The problem with FCRA is mainly associated to its high content of fine material, i.e. smaller than 75 lm. Shui et al. [16] investigated the finest fraction of FCRA by sieving out the fraction smaller than 75 lm and separating sand from hydrated cement after thermal treatment at 500 °C: they demonstrated that dehydrated cement can be successfully rehydrated after thermal treatment, but the obtained hydration products have low performances than the original ones, probably because of a reduced packing density and crystallization degree of the new hydrates. For this reason, at present, recycling industry has a very limited interest in fine concrete waste, even if they account at least for about the 30% of the entire building material waste. Recently, few researchers evaluated the application of recycled aggregate in the precast industrial production obtaining satisfying
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perspectives, even in presence of fine aggregates: in a wide research programme, Soutos et al. [17–19] described the preparation of building blocks, paving blocks and building pavement flags which achieved mechanical properties similar to those produced with newly quarried aggregates; Ledesma et al. [20] successfully produced masonry mortar replacing natural sand by FRCA at a replacement ratio up to 40%; Florea et al. [21] observed that thermally activated recycled fine fraction in mortars behaves similarly to fly ashes; Ahmari et al. [22] proposed a ‘‘complete recycling’’ of waste concrete (with no separation between coarse and fine fraction) in the preparation of a geopolymeric binder in combination with fly ashes. In a very recent study, Schoon et al. [23] evaluated the efficiency of three installations to maximize the separation between the fine aggregates and the attached residual mortar and the possibility to use the latter as alternative raw material in cold clinker powders: they found that, depending on the different meal produced, the recovered material they obtained could only be used in replacement amount lower than 15%. Hydrated cement waste (HCW) obtained as by-product from an efficient separation of FRCA would be of great interest as recycled material in the cement industry. Firstly, it is an inorganic material whose chemical composition is, after drying, the same as raw clinker meal and its use in replacement of natural quarried minerals would reduce the consumption of non-renewable material. Moreover, its CO2 content is limited depending on its environmental carbonation degree. It is well known that Portland cement manufactory process is responsible for the emission of CO2: estimating that the emission of carbon dioxide is around 0.87 ton for every ton of cement produced, cement industry accounts globally for 5% of annual manmade CO2 emissions [24]. Therefore, the possibility to use a raw recycled material with a reduced CO2 content is a further advantage from the environmental point of view. Hereafter, the use of HCW in the production cycle of Portland cement is investigated.
2. Materials and method A pure hydrated cement sample (HCW) was prepared by hydrating grinded Portland clinker supplied by Buzzi Unicem S.p.A. (water-to-cement ratio = 0.4) and curing the paste under vacuum in a sealed plastic case up to 28 days; the paste was then crushed in a laboratory jaw crusher and grinded in a laboratory vibrating mill until particle size was <90 lm. Chemical analyses were performed by dispersive X-ray fluorescence (XRF), using a Panalytical Axios spectrometer on fused bead. The specimen were prepared with a Breithländer autofluxer mixing 0.9 g of calcined sample with Li-tethraborate in a 1:10 dilution. The data treatment has been performed with the IQ+ semi-quantitative software, which allows to obtain the element content expressed as a percentage in weight of the corresponding metal oxide. The chemical composition of the resulting HCW is shown in Table 1, where also the chemical composition of the other mineral phases (natural limestone and schist) used in the following is summarized. Three different powders were prepared, whose composition is described in Table 2: An ordinary Portland powder (OPp), prepared as reference sample from natural materials. A first experimental powder based on cement waste (CWp-A) prepared by replacing 30% in weight of OPp by HCW, thus maintaining the same limestone/schist ratio. A second experimental powder based on cement waste (CWp-B) prepared with a higher amount of HCW, 55% in weight: in this case a reduction in limestone/schist ratio was
Table 1 Chemical composition of HCW and other raw powder components expressed as % in weight of oxide (average error <±2%); LOI = loss on ignition.
SiO2 Al2O3 Fe2O3 CaO MgO SO3 Na2O K2O LOI
HCW
Limestone
Schist
14.7 3.9 1.7 51.1 1.1 0.8 0.2 0.7 25.3
2.3 0.6 0.2 53.0 1.2 0.1 0.1 0.1 42.4
54.1 9.4 5.4 14.8 2.4 – – – 13.9
Table 2 Composition of the raw powder formulation.
Limestone (%) Schist (%) HPC (%)
OPp
CWp-A
CWp-B
76 24 –
53 17 30
25 20 55
necessary to compensate the CaO surplus; the optimal composition was arranged on the basis of the Bouge calculation in order to maximize the calcium silicate phases (C3S + C2S) in the final clinker. The three powders were treated on small scale in a Carbolite tubular furnace using a 50 15 10 mm platinum vessel. 25 g of raw powder were prepared for each formulation by manually mixing the powders (d95 < 90 lm) with the minimum amount of water necessary for obtaining the slurry for manual granulation (for quantification purposes the amount of water was the same for the three powders). 30 min of firing time at 1500 °C were enough, according to previous experience, to obtain a fully burnt clinker. A rapid cooling was realized through a compressed air flow. The obtained clinker samples (respectively labeled OPc, CWc-A and CWc-B) were finally grinded in a vibrating mill and characterized by X-ray diffraction. For a larger scale production, 750 g of the powder were prepared, hand granulated with water and dried in a oven at 110 °C; the dried grains were then burned in a muffle furnace in a refractory vessel for 30 min. A compressed air flow was used for the cooling. The mineralogical investigations were performed by X-ray diffraction (XRD) analyses, using a Bruker AXS D4 Endeavor diffractometer working in Bragg–Brentano geometry, equipped with a ceramic X-ray tube KFF (Cu Ka radiation) and a ‘‘Linx Eye’’ energy dispersive detector. Powdered samples were manually placed on a special zero-background sample holder. Mineral phase identification was performed through the EVA software. Refinement for semi-quantitative mineralogical analyses, when possible, was conducted by Rietveld method using the Topas 2.0 package; both software packages were commercially supplied by Bruker AXS. High temperature investigations were performed by TG/DCS using a Mettler Toledo TGA/DSC-1 analyser. For characterization purposes, 100 ll alumina crucible were used and the samples heated up to 950 °C at 20 °C/min. The high temperature behavior of the different powders was investigated by burning small amount of slurry in 30 ll Pt crucible in air flow (80 ml/min); in this case the burning procedure consisted of a first heating ramp at 20 °C/min up to 1500 °C followed by a 10 min high temperature isothermal treatment. The experimental clinkers were grinded in a laboratory mill for 30 min (d95 < 90 lm). Compressive strength was determined according to the UNI EN 196-1 on reference mortars (40 40 40 mm cubic specimen, tested for mechanical strength from 2 days up to 6 months).
D. Gastaldi et al. / Cement & Concrete Composites 61 (2015) 29–35
3. Results and discussion 3.1. Characterization of HCW The chemical composition of the HCW is summarized in Table 1. Loss on ignition is mainly due to the water content. The sample is constituted for more than half of CaO (51.1%), SiO2 accounts for 14.7% and Al2O3 and Fe2O3 are detected in amount of 3.9% and 1.7% respectively. The thermogravimetric analyses (TG/DSC) of the HCW and the corresponding X-ray Diffraction Pattern (XRPD) are shown in Figs. 1 and 2 respectively. Fig. 1 shows that a first continuous weight loss starts already at 50–70 °C and continues up to 400 °C: this is due to the gradual water evaporation from the calcium silicate and aluminate hydrates, accounting for about 15– 20% of the sample. A second, well defined weight loss takes place between 440 and 540 °C, corresponding to the dehydration of Ca(OH)2 (Portlandite). Calcium hydroxide is the main hydrated phase observed in Fig. 2, as far as calcium silicate hydrates are amorphous and not detectable by means of X-ray analysis; for the same reason, i.e. a high content of amorphous phases, a quantitative evaluation of the mineralogical composition based on the XRD pattern is not possible. The other detectable phases are residual anhydrous silicates and aluminates and calcium/magnesium carbonate (calcite/dolomite).
3.2. Experimental clinkers: production and characterization The high temperature behavior of the three powders prepared as described in Section 2 (Table 2) was investigated by TG/DSC analyses and is shown in Fig. 3. The reference OPp shows the weight loss due to dehydration of the sample (granulation water – below 150 °C) and a huge weight decrease between 600 and 900 °C caused by the limestone decarbonation reaction. CWp-A and CWp-B have more complex patterns: the dehydration step is more consistent due to the contribution of the loss of water molecules from hydrated phases of HCW, resulting in a slow and continuous weight decrease from environmental temperature up to 300– 400 °C, while between 400 and 500 °C the weight loss due to the dehydration of calcium hydroxide happens. In these formulations the weight loss associated to limestone decarbonation is clearly much more reduced with respect to the reference sample (reduced limestone content – see Table 2). All the described weight losses are associated with endo-thermal bands in the heat flow pattern (Fig. 3b): clinkerization reactions take place above 1100 °C, without any variation in weight. The XRD patterns of the three clinkers obtained are shown in Fig. 4; the semi-quantitative compositions of the same, obtained by Rietveld refinement on the same patterns, are summarized in Table 3. The patterns of the OPc reference and of the CWc-A sample are completely overlapping and the semi-quantitative composition (Table 3) reveals that in the latter a slightly higher amount of C3S is found against a reduced content of C2S and almost no free lime: these observation attest the fact that HCW can be conveniently reused in replacement of a certain share of raw powder for the production of new clinker. The CWc-B sample obtained after burning the CWp-B sample consists mainly of C2S-b (the Rietveld refinement indicates an amount around 80%): as a matter of fact, the lower CaO/SiO2 ratio in this powder than in the previous one is unfavourable to the formation of C3S. Moreover, it is worth to report that during cooling, the b ? a polymorphic transition took place and could be appreciated even by naked eye: the volume change happening during the transition cause the rupture of the clinker grains and
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a rapid partial pulverization. A faster cooling could probably avoid that the transition takes place and would even increase the amount of C2S-b.
3.3. CWc in cement production Even if the absence of C3S does not allow to consider CWc-B as a conventional clinker, its possible application in a cement mixture should not be overlooked. C2S-b, already originally present in Portland clinker, can be indeed a valuable hydraulic phase and its origin and mineralogy makes it fully compatible with the mineral phases of the ordinary Portland clinker. For this purposes, the CWc-B was used for the preparation of two blended cements: two dosages (20% and 40% respectively) of the produced clinker were added to a grinded industrial Portland reference clinker and to gypsum, both supplied by Buzzi Unicem. The two final binders were: – CWc-B20 containing CWc-B, Portland clinker and gypsum 20:75:5; – CWc-B40 containing CWc-B, Portland clinker and gypsum 40:55:5. A third reference cement was manually prepared in laboratory without the addition of CWc-B: – OPC-I containing Portland clinker and gypsum 95:5. In Fig. 5, the results of the compressive strength test up to 6 months are shown. For comparison purpose, mechanical performances of commercial Portland cements supplied by Buzzi Unicem (a limestone cement Type II and a pozzolanic cement type IV) are also reported. The reference OPC-I sample represents the target, having the higher strength value at all aging time, while the commercial cement Type II should be considered as the lower limit, being limestone a filler without hydraulic features. As expected, the commercial cement Type IV shows slower initial performances, but after 28 days it starts recovering strength. Comparing the performances of CWc-B20 with the reference cements, the following observation arise: – between 2 and 7 days, the mechanical performances of sample CWc-B20 are in between those of cement Type II and Type IV; – after 28 days the limestone cement Type II sets on a fixed value, around 43 MPa, while for the other samples mechanical strength continuously increases with time, as a demonstration of late hydraulic activity; – between 1 and 6 months the sample CWc-B20 performs better than both the reference commercial cements. Similarly, also the performances of the sample CWc-B40 improve with time: it shows compressive strength worse than all the other formulation up to 28 days, while after 3 months it overpasses the limestone cement Type II, even if its clinker content is smaller. Overall, the results demonstrates that the sample containing 40% of CWc-B has behavior similar to those of a pozzolanic cement. It is worth to notice that the difference between the formulation containing 20% and 40% of CWc-B significantly decreases with time: for example, after 1 month compressive strength are 43 and 33 MPa respectively for the binder containing 20% and 40% of CWc-B, while after 6 months they are 54 and 50 MPa. This is a further confirmation of the late but huge hydraulic activity of the CWc-B.
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Fig. 1. TG (above) and DSC (below) patterns of the HCW sample.
Fig. 2. XRD pattern of the HCW sample (P: Portlandite; Q: Quartz; C: Calcite; D: Dolomite; S: Clinker silicates; A: Clinker aluminate).
3.4. CWc production and use: CO2 reduction Compared to the production of Ordinary Portland clinker from natural quarried materials, the use of cement waste in the formulation of the clinker meals leads to different advantages from the environmental point of view. The main feature is, of course, the possibility to reduce environmental CO2 emissions due to decarbonation of the powder: the analysis of TG patterns of the clinker produced without and with HCW (Fig. 3a) allows to estimate the amount of CO2 released during the decarbonation phase from the corresponding weight loss at 800–900 °C. OPc and CWc-A shows 29% and 20% weight loss respectively: this means that, when 30% of HCW is used, a clinker with the same mineralogical composition can be produced releasing almost 1/3 less CO2 from the powder during burning. Some further consideration can be done when the use of CWc is limited to that of supplementary cementing material, as in the two binder formulation prepared above (CWc-B20 and CWc-B40). The release of CO2 observed during the decarbonation phase in the
burning of CWc-B, evaluated through the analysis of the TG pattern (Fig. 3a) is around 10% in weight; subsequently, considering the composition of the two binders (see Section 3.3), the overall CO2 released can be estimated as follows:
%CO2½CWcB20 ¼ %CO2½CWcB 0:20 þ %CO2½OPc 0:75 ffi 24% %CO2½CWcB40 ¼ %CO2½CWcB 0:40 þ %CO2½OPc 0:55 ffi 20% In the reference binder produced without the addition of CWc-B the release of CO2 could is:
%CO2½OPCI ¼ %CO2½OPc 0:95 ffi 28% As expected, if ordinary Portland clinker is replaced by the recovered CWc-B, the released CO2 decreases: at a replace extent of 40%, the CO2 released during the cement production1 is more than 1/4 less than without replacement. 1 Only the CO2 released during the decarbonation phase of meal burning is here considered.
D. Gastaldi et al. / Cement & Concrete Composites 61 (2015) 29–35
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Fig. 3. TG (a) and DSC (b) pattern of the three powders described in Table 2.
Fig. 4. X-ray diffraction patterns of the clinker produced after burning the three powders described in Table 2 (only the main diffraction peaks are assigned: b = belite C2S-b; A = alite C3S).
Of course, any supplementary cementing material would show a consistent reduction in the amount of CO2 released, decreasing as far as the replacement amount increases: the advantage of using CWc instead other SCMs consists in the high environmental value
of recovering a waste material against the consumption of new quarried material, in line with the sustainable development initiatives proposed as a contribution from cement industry to Climate Change Initiative [25].
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Table 3 Rietveld semi-quantitative mineralogical composition of the experimental clinker produced after burning the powder described in Table 2 (residual weighted profile lower than 10%; estimated average error < ±2%).
C3S (%) C2S-b (%) C2S-c (%) C3A/C4AF (%) MgO (%) CaO (%)
OPc (reference)
CWc-A
CWc-B
47.5 31.6 – 16.1 3.3 1.5
49.7 28.7 – 16.7 3.1 0.6
– 78.4 10.8 10.4 0.6 –
was therefore tested. Two blended cement were prepared by adding respectively 20% and 40% of CWc-B to Portland clinker in presence of 5% gypsum and submitted to compressive strength test from 2 days up to 6 months. The evaluation of their mechanical performances revealed that the two blend cements are characterized by a significant late hydraulicity and that long term strength are comparable to the ones shown by a pozzolanic cement tested as reference material, even at higher replacement degree. On the whole, the following consideration can be drawn: – the hydrated cement extracted from CDW is not a waste but a resource, as far as its composition makes it appealing as raw component to be used in the chain of cement production in partial substitution of natural quarried materials; – depending on the chemical composition of the waste, and in particular on the CaO/SiO2 ratio, the amount of HCW that can be used in a raw powder for Portland clinker production is limited; – if a higher amount of HCW is used, a non-Portland clinker with valuable properties as supplementary cementing material can be produced; – in both cases (low or high amount of HCW) a huge reduction of CO2 emission associated to the clinker/cement production is observed; – beyond the CO2 reduction, another environmental advantage in using recycled hydrated cement lies in the fact that the consumption of natural resources is reduced.
Fig. 5. Comparison among the compressive strengths of an ordinary Portland cement (OPC-I, prepared as described in Section 3.2), two commercial cement (Type II and Type IV) and the two blend cements containing CWc-B (respectively 20% and 40% in replacement of Portland clinker).
4. Conclusions In the field of concrete recycling, the possible recovery of hydrated cement extracted from CDW has been explored. Hydrated cement consists of amorphous calcium silicate and calcium aluminate hydrates, calcium hydroxide and minor amount of calcium/magnesium carbonate; its chemical composition makes it appealing for the re-use in the chain of cement production as far as the elemental ratios reflect the chemical composition of cement clinker. To investigate this application, a pure sample of hydrated cement waste (HCW) was prepared and used as raw material in combination with limestone and schist for the production of new clinker. Two powder formulation were investigated. (I) In the first case, 30% of HCW was used in replacement of a reference meal for Portland clinker production: a clinker (CWc-A) with a mineralogical composition very close to that of the reference clinker could be produced with a spare in the emission of CO2 from decarbonation of about 1/3. (II) In the second case (CWp-B), a powder based on a higher amount of HCW was formulated – 55% – and the amount of limestone and schist were regulated as to maximize the amount of calcium silicates (C3S + C2S-b) in the final product: the clinker produced by burning this meal (CWc-B) resulted rich in C2S-b. The mineralogical composition, and in particular, the absence of C3S (that can be easily explained considering stoichiometrical balance reasons) makes this second material unsuitable to be used alone. An alternative use as supplementary cementing material
On the basis of the encouraging results obtained, similar experiences will be performed on real waste of hydrated cement extracted from CDW; a pilot scale production is also planned in order to produce enough material for durability investigation.
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[21] Florea MVA, Ning Z, Browers HJH. Activation of liberated concrete fines and their application in mortars. Construct Build Mater 2014;50:1–12. [22] Ahmari S, Ren X, Toufigh V, Zhang L. Production of geopolymeric binder from blended waste concrete powder and fly ash. Construct Build Mater 2012;35:718–29. [23] Schoon J, De Buysser K, Van Driessche I, De Belie N. Fines extracted from recycled concrete as alternative raw material for Portland cement clinker production. Cement Concr Compos 2015;58:70–80. [24] Wray P. Straight talk with Karen Scrivener on cements, CO2 and sustainable development. Am Ceram Soc Bull 2012;91:47–50. [25] Damtoft JS, Lukasik J, Herfort D, Sorrentino D, Gartner EM. Sustainable development and climate change initiatives. Cem Concr Res 2008;38:115–27.
Contents lists available at ScienceDirect
Cement & Concrete Composites journal homepage: www.elsevier.com/locate/cemconcomp
An investigation on the recycling of hydrated cement from concrete demolition waste D. Gastaldi a,⇑, F. Canonico a, L. Capelli a, L. Buzzi a, E. Boccaleri b,⇑, S. Irico b a b
Buzzi Unicem S.p.A., Via Luigi Buzzi 6, 15033 Casale Monferrato, Alessandria, Italy Science and Technological Innovation Department, University of Piemonte Orientale, Viale T. Michel 11, 15121 Alessandria, Italy
a r t i c l e
i n f o
Article history: Received 30 May 2014 Received in revised form 1 April 2015 Accepted 11 April 2015 Available online 24 April 2015 Keywords: Hydrated cement waste Recycled aggregate Concrete demolition waste Fine fraction CO2 reduction
a b s t r a c t Construction and demolition waste (CDW) recycling is generally limited to the use of the coarser fraction as aggregate for new concrete. The recovery of fine aggregates requires a cleaning by removing the hydrated cement waste (HCW). In this paper, the possibility to use HCW extracted from CDW as alternative component for the production of new clinker is explored. A pure HCW sample was prepared and used in partial replacement of natural materials in raw admixtures for new clinker production. At a replacement degree of 30%, a new Portland clinker containing almost 50% of C3S could be produced with a huge spare in the release of CO2 (about 1/3 less). At higher HCW dosage a non-Portland clinker containing almost 80% of C2S has been obtained: its use as supplementary cementing material in blended cements revealed satisfying long term performances. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Recycling is the issue for sustainable development. Main streams in the construction industry are constituted by concrete, brickworks, pieces of plaster and gypsum board characterized by extremely wide chemical composition, color, mechanical properties. In Europe about of 180 million tons of concrete demolition waste (CDW) are produced every year, corresponding annually to 500 kg for each citizen [1]: this amount represents around 31% of all the waste produced in the European Union [2]. For long time concrete and brickworks waste have only been used as a filling material or disposed to landfill. Nevertheless, in the late 20th century concrete recycling gained more and more importance, due to the increasing attention toward environmental protection and to the progressively reducing landfill capacity [3]. Current concrete recycling consists of crushing waste concrete and use it again as aggregate for new concrete [4], according to specifications which are based on local regulations in different countries. The production of recycled concrete aggregates (RCA) is a well established practice in Belgium, Denmark and the Nederlands, where recycling rates raise 80% [5], while it is less common in Southern Europe. ⇑ Corresponding authors. Tel.: +39 0161 809740 (D. Gastaldi), +39 0131 360264 (E. Boccaleri). E-mail addresses: [email protected] (D. Gastaldi), enrico.boccaleri@ uniupo.it (E. Boccaleri). http://dx.doi.org/10.1016/j.cemconcomp.2015.04.010 0958-9465/Ó 2015 Elsevier Ltd. All rights reserved.
The quality of RCA is generally lower than that of natural aggregates, due to presence of residual mortar [6]: for this reason, when dealing with concrete recycling, a differentiation between coarse (nominal size >5 mm) and fine aggregates (maximum size <5 mm) is generally done. Coarse recycled concrete aggregates (CRCA) are commonly used in partial replacement of natural aggregates in concrete [7–9], however the concrete mix design has to be adjusted in order to correct the worsening of final properties such as workability and durability, especially in respect to alkali–silica reaction, corrosion (due to chloride content) and freeze thaw resistance [10]. On the contrary, fine recycled concrete aggregates (FRCA) are less useful as aggregates in concrete as they can be highly detrimental for what concern strength, workability and durability [11–15]. The problem with FCRA is mainly associated to its high content of fine material, i.e. smaller than 75 lm. Shui et al. [16] investigated the finest fraction of FCRA by sieving out the fraction smaller than 75 lm and separating sand from hydrated cement after thermal treatment at 500 °C: they demonstrated that dehydrated cement can be successfully rehydrated after thermal treatment, but the obtained hydration products have low performances than the original ones, probably because of a reduced packing density and crystallization degree of the new hydrates. For this reason, at present, recycling industry has a very limited interest in fine concrete waste, even if they account at least for about the 30% of the entire building material waste. Recently, few researchers evaluated the application of recycled aggregate in the precast industrial production obtaining satisfying
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perspectives, even in presence of fine aggregates: in a wide research programme, Soutos et al. [17–19] described the preparation of building blocks, paving blocks and building pavement flags which achieved mechanical properties similar to those produced with newly quarried aggregates; Ledesma et al. [20] successfully produced masonry mortar replacing natural sand by FRCA at a replacement ratio up to 40%; Florea et al. [21] observed that thermally activated recycled fine fraction in mortars behaves similarly to fly ashes; Ahmari et al. [22] proposed a ‘‘complete recycling’’ of waste concrete (with no separation between coarse and fine fraction) in the preparation of a geopolymeric binder in combination with fly ashes. In a very recent study, Schoon et al. [23] evaluated the efficiency of three installations to maximize the separation between the fine aggregates and the attached residual mortar and the possibility to use the latter as alternative raw material in cold clinker powders: they found that, depending on the different meal produced, the recovered material they obtained could only be used in replacement amount lower than 15%. Hydrated cement waste (HCW) obtained as by-product from an efficient separation of FRCA would be of great interest as recycled material in the cement industry. Firstly, it is an inorganic material whose chemical composition is, after drying, the same as raw clinker meal and its use in replacement of natural quarried minerals would reduce the consumption of non-renewable material. Moreover, its CO2 content is limited depending on its environmental carbonation degree. It is well known that Portland cement manufactory process is responsible for the emission of CO2: estimating that the emission of carbon dioxide is around 0.87 ton for every ton of cement produced, cement industry accounts globally for 5% of annual manmade CO2 emissions [24]. Therefore, the possibility to use a raw recycled material with a reduced CO2 content is a further advantage from the environmental point of view. Hereafter, the use of HCW in the production cycle of Portland cement is investigated.
2. Materials and method A pure hydrated cement sample (HCW) was prepared by hydrating grinded Portland clinker supplied by Buzzi Unicem S.p.A. (water-to-cement ratio = 0.4) and curing the paste under vacuum in a sealed plastic case up to 28 days; the paste was then crushed in a laboratory jaw crusher and grinded in a laboratory vibrating mill until particle size was <90 lm. Chemical analyses were performed by dispersive X-ray fluorescence (XRF), using a Panalytical Axios spectrometer on fused bead. The specimen were prepared with a Breithländer autofluxer mixing 0.9 g of calcined sample with Li-tethraborate in a 1:10 dilution. The data treatment has been performed with the IQ+ semi-quantitative software, which allows to obtain the element content expressed as a percentage in weight of the corresponding metal oxide. The chemical composition of the resulting HCW is shown in Table 1, where also the chemical composition of the other mineral phases (natural limestone and schist) used in the following is summarized. Three different powders were prepared, whose composition is described in Table 2: An ordinary Portland powder (OPp), prepared as reference sample from natural materials. A first experimental powder based on cement waste (CWp-A) prepared by replacing 30% in weight of OPp by HCW, thus maintaining the same limestone/schist ratio. A second experimental powder based on cement waste (CWp-B) prepared with a higher amount of HCW, 55% in weight: in this case a reduction in limestone/schist ratio was
Table 1 Chemical composition of HCW and other raw powder components expressed as % in weight of oxide (average error <±2%); LOI = loss on ignition.
SiO2 Al2O3 Fe2O3 CaO MgO SO3 Na2O K2O LOI
HCW
Limestone
Schist
14.7 3.9 1.7 51.1 1.1 0.8 0.2 0.7 25.3
2.3 0.6 0.2 53.0 1.2 0.1 0.1 0.1 42.4
54.1 9.4 5.4 14.8 2.4 – – – 13.9
Table 2 Composition of the raw powder formulation.
Limestone (%) Schist (%) HPC (%)
OPp
CWp-A
CWp-B
76 24 –
53 17 30
25 20 55
necessary to compensate the CaO surplus; the optimal composition was arranged on the basis of the Bouge calculation in order to maximize the calcium silicate phases (C3S + C2S) in the final clinker. The three powders were treated on small scale in a Carbolite tubular furnace using a 50 15 10 mm platinum vessel. 25 g of raw powder were prepared for each formulation by manually mixing the powders (d95 < 90 lm) with the minimum amount of water necessary for obtaining the slurry for manual granulation (for quantification purposes the amount of water was the same for the three powders). 30 min of firing time at 1500 °C were enough, according to previous experience, to obtain a fully burnt clinker. A rapid cooling was realized through a compressed air flow. The obtained clinker samples (respectively labeled OPc, CWc-A and CWc-B) were finally grinded in a vibrating mill and characterized by X-ray diffraction. For a larger scale production, 750 g of the powder were prepared, hand granulated with water and dried in a oven at 110 °C; the dried grains were then burned in a muffle furnace in a refractory vessel for 30 min. A compressed air flow was used for the cooling. The mineralogical investigations were performed by X-ray diffraction (XRD) analyses, using a Bruker AXS D4 Endeavor diffractometer working in Bragg–Brentano geometry, equipped with a ceramic X-ray tube KFF (Cu Ka radiation) and a ‘‘Linx Eye’’ energy dispersive detector. Powdered samples were manually placed on a special zero-background sample holder. Mineral phase identification was performed through the EVA software. Refinement for semi-quantitative mineralogical analyses, when possible, was conducted by Rietveld method using the Topas 2.0 package; both software packages were commercially supplied by Bruker AXS. High temperature investigations were performed by TG/DCS using a Mettler Toledo TGA/DSC-1 analyser. For characterization purposes, 100 ll alumina crucible were used and the samples heated up to 950 °C at 20 °C/min. The high temperature behavior of the different powders was investigated by burning small amount of slurry in 30 ll Pt crucible in air flow (80 ml/min); in this case the burning procedure consisted of a first heating ramp at 20 °C/min up to 1500 °C followed by a 10 min high temperature isothermal treatment. The experimental clinkers were grinded in a laboratory mill for 30 min (d95 < 90 lm). Compressive strength was determined according to the UNI EN 196-1 on reference mortars (40 40 40 mm cubic specimen, tested for mechanical strength from 2 days up to 6 months).
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3. Results and discussion 3.1. Characterization of HCW The chemical composition of the HCW is summarized in Table 1. Loss on ignition is mainly due to the water content. The sample is constituted for more than half of CaO (51.1%), SiO2 accounts for 14.7% and Al2O3 and Fe2O3 are detected in amount of 3.9% and 1.7% respectively. The thermogravimetric analyses (TG/DSC) of the HCW and the corresponding X-ray Diffraction Pattern (XRPD) are shown in Figs. 1 and 2 respectively. Fig. 1 shows that a first continuous weight loss starts already at 50–70 °C and continues up to 400 °C: this is due to the gradual water evaporation from the calcium silicate and aluminate hydrates, accounting for about 15– 20% of the sample. A second, well defined weight loss takes place between 440 and 540 °C, corresponding to the dehydration of Ca(OH)2 (Portlandite). Calcium hydroxide is the main hydrated phase observed in Fig. 2, as far as calcium silicate hydrates are amorphous and not detectable by means of X-ray analysis; for the same reason, i.e. a high content of amorphous phases, a quantitative evaluation of the mineralogical composition based on the XRD pattern is not possible. The other detectable phases are residual anhydrous silicates and aluminates and calcium/magnesium carbonate (calcite/dolomite).
3.2. Experimental clinkers: production and characterization The high temperature behavior of the three powders prepared as described in Section 2 (Table 2) was investigated by TG/DSC analyses and is shown in Fig. 3. The reference OPp shows the weight loss due to dehydration of the sample (granulation water – below 150 °C) and a huge weight decrease between 600 and 900 °C caused by the limestone decarbonation reaction. CWp-A and CWp-B have more complex patterns: the dehydration step is more consistent due to the contribution of the loss of water molecules from hydrated phases of HCW, resulting in a slow and continuous weight decrease from environmental temperature up to 300– 400 °C, while between 400 and 500 °C the weight loss due to the dehydration of calcium hydroxide happens. In these formulations the weight loss associated to limestone decarbonation is clearly much more reduced with respect to the reference sample (reduced limestone content – see Table 2). All the described weight losses are associated with endo-thermal bands in the heat flow pattern (Fig. 3b): clinkerization reactions take place above 1100 °C, without any variation in weight. The XRD patterns of the three clinkers obtained are shown in Fig. 4; the semi-quantitative compositions of the same, obtained by Rietveld refinement on the same patterns, are summarized in Table 3. The patterns of the OPc reference and of the CWc-A sample are completely overlapping and the semi-quantitative composition (Table 3) reveals that in the latter a slightly higher amount of C3S is found against a reduced content of C2S and almost no free lime: these observation attest the fact that HCW can be conveniently reused in replacement of a certain share of raw powder for the production of new clinker. The CWc-B sample obtained after burning the CWp-B sample consists mainly of C2S-b (the Rietveld refinement indicates an amount around 80%): as a matter of fact, the lower CaO/SiO2 ratio in this powder than in the previous one is unfavourable to the formation of C3S. Moreover, it is worth to report that during cooling, the b ? a polymorphic transition took place and could be appreciated even by naked eye: the volume change happening during the transition cause the rupture of the clinker grains and
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a rapid partial pulverization. A faster cooling could probably avoid that the transition takes place and would even increase the amount of C2S-b.
3.3. CWc in cement production Even if the absence of C3S does not allow to consider CWc-B as a conventional clinker, its possible application in a cement mixture should not be overlooked. C2S-b, already originally present in Portland clinker, can be indeed a valuable hydraulic phase and its origin and mineralogy makes it fully compatible with the mineral phases of the ordinary Portland clinker. For this purposes, the CWc-B was used for the preparation of two blended cements: two dosages (20% and 40% respectively) of the produced clinker were added to a grinded industrial Portland reference clinker and to gypsum, both supplied by Buzzi Unicem. The two final binders were: – CWc-B20 containing CWc-B, Portland clinker and gypsum 20:75:5; – CWc-B40 containing CWc-B, Portland clinker and gypsum 40:55:5. A third reference cement was manually prepared in laboratory without the addition of CWc-B: – OPC-I containing Portland clinker and gypsum 95:5. In Fig. 5, the results of the compressive strength test up to 6 months are shown. For comparison purpose, mechanical performances of commercial Portland cements supplied by Buzzi Unicem (a limestone cement Type II and a pozzolanic cement type IV) are also reported. The reference OPC-I sample represents the target, having the higher strength value at all aging time, while the commercial cement Type II should be considered as the lower limit, being limestone a filler without hydraulic features. As expected, the commercial cement Type IV shows slower initial performances, but after 28 days it starts recovering strength. Comparing the performances of CWc-B20 with the reference cements, the following observation arise: – between 2 and 7 days, the mechanical performances of sample CWc-B20 are in between those of cement Type II and Type IV; – after 28 days the limestone cement Type II sets on a fixed value, around 43 MPa, while for the other samples mechanical strength continuously increases with time, as a demonstration of late hydraulic activity; – between 1 and 6 months the sample CWc-B20 performs better than both the reference commercial cements. Similarly, also the performances of the sample CWc-B40 improve with time: it shows compressive strength worse than all the other formulation up to 28 days, while after 3 months it overpasses the limestone cement Type II, even if its clinker content is smaller. Overall, the results demonstrates that the sample containing 40% of CWc-B has behavior similar to those of a pozzolanic cement. It is worth to notice that the difference between the formulation containing 20% and 40% of CWc-B significantly decreases with time: for example, after 1 month compressive strength are 43 and 33 MPa respectively for the binder containing 20% and 40% of CWc-B, while after 6 months they are 54 and 50 MPa. This is a further confirmation of the late but huge hydraulic activity of the CWc-B.
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Fig. 1. TG (above) and DSC (below) patterns of the HCW sample.
Fig. 2. XRD pattern of the HCW sample (P: Portlandite; Q: Quartz; C: Calcite; D: Dolomite; S: Clinker silicates; A: Clinker aluminate).
3.4. CWc production and use: CO2 reduction Compared to the production of Ordinary Portland clinker from natural quarried materials, the use of cement waste in the formulation of the clinker meals leads to different advantages from the environmental point of view. The main feature is, of course, the possibility to reduce environmental CO2 emissions due to decarbonation of the powder: the analysis of TG patterns of the clinker produced without and with HCW (Fig. 3a) allows to estimate the amount of CO2 released during the decarbonation phase from the corresponding weight loss at 800–900 °C. OPc and CWc-A shows 29% and 20% weight loss respectively: this means that, when 30% of HCW is used, a clinker with the same mineralogical composition can be produced releasing almost 1/3 less CO2 from the powder during burning. Some further consideration can be done when the use of CWc is limited to that of supplementary cementing material, as in the two binder formulation prepared above (CWc-B20 and CWc-B40). The release of CO2 observed during the decarbonation phase in the
burning of CWc-B, evaluated through the analysis of the TG pattern (Fig. 3a) is around 10% in weight; subsequently, considering the composition of the two binders (see Section 3.3), the overall CO2 released can be estimated as follows:
%CO2½CWcB20 ¼ %CO2½CWcB 0:20 þ %CO2½OPc 0:75 ffi 24% %CO2½CWcB40 ¼ %CO2½CWcB 0:40 þ %CO2½OPc 0:55 ffi 20% In the reference binder produced without the addition of CWc-B the release of CO2 could is:
%CO2½OPCI ¼ %CO2½OPc 0:95 ffi 28% As expected, if ordinary Portland clinker is replaced by the recovered CWc-B, the released CO2 decreases: at a replace extent of 40%, the CO2 released during the cement production1 is more than 1/4 less than without replacement. 1 Only the CO2 released during the decarbonation phase of meal burning is here considered.
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Fig. 3. TG (a) and DSC (b) pattern of the three powders described in Table 2.
Fig. 4. X-ray diffraction patterns of the clinker produced after burning the three powders described in Table 2 (only the main diffraction peaks are assigned: b = belite C2S-b; A = alite C3S).
Of course, any supplementary cementing material would show a consistent reduction in the amount of CO2 released, decreasing as far as the replacement amount increases: the advantage of using CWc instead other SCMs consists in the high environmental value
of recovering a waste material against the consumption of new quarried material, in line with the sustainable development initiatives proposed as a contribution from cement industry to Climate Change Initiative [25].
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Table 3 Rietveld semi-quantitative mineralogical composition of the experimental clinker produced after burning the powder described in Table 2 (residual weighted profile lower than 10%; estimated average error < ±2%).
C3S (%) C2S-b (%) C2S-c (%) C3A/C4AF (%) MgO (%) CaO (%)
OPc (reference)
CWc-A
CWc-B
47.5 31.6 – 16.1 3.3 1.5
49.7 28.7 – 16.7 3.1 0.6
– 78.4 10.8 10.4 0.6 –
was therefore tested. Two blended cement were prepared by adding respectively 20% and 40% of CWc-B to Portland clinker in presence of 5% gypsum and submitted to compressive strength test from 2 days up to 6 months. The evaluation of their mechanical performances revealed that the two blend cements are characterized by a significant late hydraulicity and that long term strength are comparable to the ones shown by a pozzolanic cement tested as reference material, even at higher replacement degree. On the whole, the following consideration can be drawn: – the hydrated cement extracted from CDW is not a waste but a resource, as far as its composition makes it appealing as raw component to be used in the chain of cement production in partial substitution of natural quarried materials; – depending on the chemical composition of the waste, and in particular on the CaO/SiO2 ratio, the amount of HCW that can be used in a raw powder for Portland clinker production is limited; – if a higher amount of HCW is used, a non-Portland clinker with valuable properties as supplementary cementing material can be produced; – in both cases (low or high amount of HCW) a huge reduction of CO2 emission associated to the clinker/cement production is observed; – beyond the CO2 reduction, another environmental advantage in using recycled hydrated cement lies in the fact that the consumption of natural resources is reduced.
Fig. 5. Comparison among the compressive strengths of an ordinary Portland cement (OPC-I, prepared as described in Section 3.2), two commercial cement (Type II and Type IV) and the two blend cements containing CWc-B (respectively 20% and 40% in replacement of Portland clinker).
4. Conclusions In the field of concrete recycling, the possible recovery of hydrated cement extracted from CDW has been explored. Hydrated cement consists of amorphous calcium silicate and calcium aluminate hydrates, calcium hydroxide and minor amount of calcium/magnesium carbonate; its chemical composition makes it appealing for the re-use in the chain of cement production as far as the elemental ratios reflect the chemical composition of cement clinker. To investigate this application, a pure sample of hydrated cement waste (HCW) was prepared and used as raw material in combination with limestone and schist for the production of new clinker. Two powder formulation were investigated. (I) In the first case, 30% of HCW was used in replacement of a reference meal for Portland clinker production: a clinker (CWc-A) with a mineralogical composition very close to that of the reference clinker could be produced with a spare in the emission of CO2 from decarbonation of about 1/3. (II) In the second case (CWp-B), a powder based on a higher amount of HCW was formulated – 55% – and the amount of limestone and schist were regulated as to maximize the amount of calcium silicates (C3S + C2S-b) in the final product: the clinker produced by burning this meal (CWc-B) resulted rich in C2S-b. The mineralogical composition, and in particular, the absence of C3S (that can be easily explained considering stoichiometrical balance reasons) makes this second material unsuitable to be used alone. An alternative use as supplementary cementing material
On the basis of the encouraging results obtained, similar experiences will be performed on real waste of hydrated cement extracted from CDW; a pilot scale production is also planned in order to produce enough material for durability investigation.
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