Recycling of construction and demolition waste generated by building infrastructure for the production of glassy materials

1 Recycling of construction and demolition waste generated by building infrastructure for the production of glassy materials A. Domínguez, M.I. Domínguez*, S. Ivanova*, M. A. Centeno, J.A. Odriozola Departamento de Química Inorgánica e Instituto de Ciencia de Materiales de Sevilla. Avda. Américo Vespucio, 49, 41092 Sevilla (Spain) corresponding author: [email protected], [email protected] Abstract The use of waste materials generated by construction and demolition industry to yield valuable glassy materials, i.e. enamel for glazed ceramic tiles and cellular glasses is presented in this study. Both types of materials are produced by one-step treatment at moderate temperatures after simple waste chemical composition adjust. The enamels are manufactured directly from the initial waste powder by melting, while the expanded materials result from mixing of the vitreous material obtained after waste vitrification with an adequate foaming agent and posterior thermal treatment. Through the manuscript the feasibility of one step production of second generation profit materials is discussed in order to help achieving sustainable development and environmental protection. Keywords: construction and demolition waste, expanded materials, enamel, glazed tiles 2 Introduction In the last century the construction industry, either building or assembling, reached very high activity indexes. Although a sign of wellness, and considered as one of the key industries for economical worldwide growth, this industry is accompanied with an extraordinary waste generation. Thereby, construction and demolition waste (CDW) is considered as the heaviest and bulkiest waste in the European Union (EU), representing about 25% - 30% of all generated waste. The CDW issued principally from activities such as construction or demolition of buildings and civil infrastructure, road planning and maintenance varies in composition and contains different materials, like concrete, bricks, gypsum, wood, glass, metals, plastics, solvents, asbestos or excavated soil, many of which can be recycled [1]. Referring to the Age of sustainable development, the waste generation problems must be addressed, especially in the construction industry, where valuable raw materials can result. In this sense, CDWs are considered as priority treatment for European Union. In addition, CDWs present high potential for recycling and reuse, since some of its components possess high resource value. For example, aggregates derived from CDW are frequently reused in roads, drainage or other construction projects [2,3]. Nowadays the technology for separation and recovery of CDW is well established, being readily accessible and in general inexpensive. However and despite its potential, the percentage of recycling and material recovery of CDW varies greatly (from 10% to 90%) across the EU [1]. In this context, it is worth to mention the European zero waste program, which estimates that the waste management could reduce the material’s input needs by 17– 24% in 2030, representing an overall saving potential of €630 billion/year for the 3 European industry. The later will lead to the satisfaction of 10% to 40% of the raw materials demand, while contributing to achieve also the EU target to reduce greenhouse gas emissions by 40% [4]. Waste reduction, reuse and recycling are very important elements in the waste management resulting in natural resources conservation, valuable landfill space reduction, raw materials and energy needs diminishing, air and water pollution control and, not at last place, new jobs creation potential [5]. There are, however, some constraints on reusing waste materials; they must fulfill some engineering requirements in terms of physical and compositional properties and they should not contain excessive amounts of harmful components which might cause problems in use [6]. The properties of CDWs vary considerably depending on its origin and composition. It is convenient to distinguish between materials originated from construction and demolition of buildings and those from pavement. The former offers composition in which a wide variety of wastes are included, presenting sometimes even dangerous components, which can contaminate other recyclable items and are time and money spending. The vitrification is reported as the safest approach for non flammable hazardous waste treatment [7]. It is a process able to convert directly the waste materials without separation into homogeneous stable glasses for subsequent application [8-10]. However, vitrification is reported as high-energy consuming treatment, producing glasses with low transparency. The use of this treatment appears more expensive than waste deposition in landfills or waste inmobilization into cementitious matrix. Thus, a possible cost reduction of this technology involves conversion of the produced glassy materials into directly marketable products such as cellular glasses and enamels for glazed ceramic tiles production [11]. 4 The cellular glasses are light rigid materials, with low thermal conductivity, good mechanical properties and acoustic insulation capacity [12]. Being fire and waterproof materials, the cellular glasses are increasingly considered in civil engineering as insulating or lightweight filling materials [13-15]. Cellular glasses are formed in two steps: i) viscous flow sintering of fine glass powders and ii) subsequent foaming of the pyroplastic mass with specific additives under heating at 850–1000 °C [14]. The foaming process occurs upon releasing different gases (CO, CO2, SOx) generated from the thermal decomposition of the additives (typically carbonates [16] or sulfates). However, the gas release may represent an environmental problem. An alternative to the above-mentioned foaming process is cellular glass production by means of redox reactions between additives and glass components. However if carbon-containing species (C, SiC) are used as additives greenhouse effect gases will be produced, meanwhile the use of iron(III) oxide or aluminium nitride [14, 17], producing oxygen or nitrogen during redox reaction, are examples for environmentally friendly process. The enamel is a glassy substance chemically formulated to adhere on the surface of preformed tile and subsequently fused into the body when fired. It is essentially composed by silica (glass forming element), alumina (for stability) and additives (to help melting). Various minerals, oxides and chemical compounds could be also employed for color. The CDWs due to their composition are good candidates for being recycled as raw materials for the production of the above-mentioned materials. In this context, the present work is devoted to the feasibility of glassy materials directly obtained from construction waste as marketable products with a special emphasis made on material’s final state and their possible future quick utilization without any further treatments. 5 1. Materials and methods Three groups of CDWs are used as raw materials, i.e. bricks, glazed tiles and concrete wastes. Additionally, some commercial products are employed either to adjust chemical compositions (MgO, SiO2 and NaOH) or as foaming agents (AlN and CaCN2). The chemical composition of the samples is determined by X-Ray microfluorescence spectrometry (XRMF) in an EDAX Eagle III spectrophotometer using Rh source of radiation. X-ray diffraction (XRD) analysis is performed on X‘Pert Pro PANalytical diffractometer using Cu-K radiation ( = 1,5404 Å), working at 40 mA and 45 kV and equipped with position sensitive detector. The diffractograms are recorded over 2 - range 5–800 using 0,050 step size and 80 s step time. Thermogravimetric analysis is carried out on Seiko Exstar 6000 thermobalance, up to 1200ºC in air, using a heating rate of 10ºC·min-1. The material’s density is measured by the He pycnometry method on Pentapycnometer 5200e Quantachrome Instrument. The tribological properties are tested on Microtest® pin-on-disk apparatus, providing continuous measurement of the material’s friction coefficient. The mechanical strength analysis is carried out on Microstest EM1/FR under compression strength. 6 2. Results and discussion All waste materials are milled prior any characterization or use. The chemical compositions obtained by XRMF are shown in table 1. For bricks and glazed tiles silicon is the main element according to the preponderance of silica and silicate phases, observed in the diffractograms (figures 1, brick, and 2, glazed tile). Similarly, the concrete sample is rich in silicon and calcium, in agreement with the presence of silica, silicates and especially calcium carbonates as main crystalline phases (figure 3). Table 1. Chemical composition of the used CDW (oxides wt%) Waste % weight SiO2 CaO Al2O3 Fe2O3 K2O Na2O MgO Others oxides Brick 69,7 5,9 14,2 4,7 3,7 1,6 -- 0,13 (V, Mn) Glazed tile 68,7 7,9 15,4 4,9 3,0 -- -- 0,01 (Mn) Concrete 20,4 67,0 1,5 2,7 0,7 -- 7,4 0,4 (Ti) Figure 1. Diffractogram of brick sample 10 20 30 40 50 60 70 80 # ## # # # # # #-·* * * "" "^&** * * - · Intensity (a.u.) 2 * & ·* ·* *** * ·& * SiO2 & Fe2O3 · NaAl3Si3O8 - (Na,K)(Si3Al)O8 ^ CaCO3 " Na5FeO4 # (K,H3O)Al2Si3AlO10(OH)2 ^ # 7 10 20 30 40 50 60 70 80 * * ^&& &^ &* * * SiO2 & Fe2O3 + (Ca,Na)(Al,Si)2Si2O8 ^ Ca2Mg(Si2O7) 2 Intensity (a.u.) * + ** + + + + * & + * +* *** * +& ^ Figure 2. Diffractogram of glazed tile sample 10 20 30 40 50 60 70 80 ··# ## # # # # # # ## # # # # ^ ^ ^ ^ ^ ^ ^ ^ ^ ^^ ^ ^ ^ ^ ^ * * * * * * * * * SiO2 ^ CaCO3 # CaMg(CO3)2 · NaAlSi3O8 Intensity (a.u.) 2 Figure 3. Diffractogram of concrete sample In the case of the brick sample, seven characteristic phases are identified by XRD (figure 1): iron (III) oxide, Fe2O3 (JCPDS#01-073-0603); silica, SiO2 (JCPDS#01-0781253), two feldspars NaAlSi3O8 (JCPDS#00-009-0466) and (Na,K)(Si3Al)O8 8 (JCPDS#00-009-0478), illite (JCPDS#00-026-0911), calcium carbonate, CaCO3 (JCPDS#01-086-2334) and sodium iron oxide, Na5FeO4 (JCPDS#00-036-0874). For the glazed tile sample, the aluminosilicate occurrence is confirmed by the presence of (Ca,Na)(AlSi)2Si2O8 (JCPDS#00-020-0528); together with crystalline calcium magnesium silicate, Ca2MgSi2O7 (JCPDS#01-076-0841); iron (III) oxide Fe2O3 (JCPDS#01-073-0603) and silica, SiO2 (JCPDS#01-078-1253) (figure 2). Manganese traces determined by elemental analysis were not identified as crystalline phase. The components of the concrete sample are identified by XRD as calcium carbonate, CaCO3 (JCPDS#01-086-2334) and calcium-magnesium carbonate, CaMg(CO3)2 (JCPDS#00-036-0426), silica, SiO2 (JCPDS#01-078-1253) and sodium aluminosilicate, NaAlSi3O8 (JCPDS#00-009-0466) (figure 3). Usually the main problem of reusing CDW wastes is their high aluminum content, which indicates necessity of high temperatures for vitrification. Jordan et al. [18] reported the elaboration of soda-lime type glass at temperatures around 1100ºC from natural zeolite with comparable aluminum content to that observed for our brick and glazed tile samples. Based on this study, and on the fact that the compositions of the waste materials, either bricks and or glazed tiles, are very similar between them, the later is selected for the preparation of two samples (Z1 and Z2) with compositions reported by Jordan et al. [18]. To re-adjust the glassy precursor composition various additives are employed: MgO, SiO2 and NaOH. The relative proportions of waste material and additives and the corresponding chemical compositions of the glassy precursor are listed in table 2 and table 3 respectively. 9 Table 2. Mixtures compositions, wt.% of wastes and commercially available additives Component (wt%) Brick Concrete Glazed tile MgO SiO2 NaOH Z1 75,59 0,75 22,68 0,98 Z2 89,10 0,89 8,91 1,15 Z3 50,33 11,18 30,01 8,48 Table 3. Chemical composition of the prepared powder mixture (oxides wt%) Sample Na2O Al2O3 SiO2 K2O CaO V2O5 MnO Fe2O3 TiO2 MgO Natural zeolite [18] 2,00 11,70 69,00 1,31 4,20 0 0 2,10 0,14 0,41 Z1 0,76 11,67 74,79 2,3 5,99 0 0,01 3,73 0 0,75 Z2 0,89 13,75 70,29 2,72 7,07 0 0,01 4,39 0 0,88 Z3 7,5 7,48 68,72 1,97 10,70 0,05 0,01 2,7 0,03 0,83 The vitrification temperature is fixed to 1200 ºC, with a heating rate of with 10ºC.min-1 and 4h duration. The sample is then cooled down to room temperature under the inertia of the oven without special precaution considered. Figure 4 presents the diffractograms corresponding to the samples Z1 and Z2 before and after thermal treatment. Signals due to the presence of crystalline phases are detected together with the characteristic halo of all amorphous materials. 16 On the other hand, calcium cyanamide (CaCN2) is a product commonly used in agriculture, with the advantage of lower cost compared to aluminum nitride, but with disadvantage of generating CO during the reaction: 2 M2O3 + 3 CaCN2  4 M + 3 CaO + 3 N2 (g) + 3 CO (g) The presence of iron oxide in the CDW (table 3) used in the preparation of the glass enables such redox reactions during the melting process. Therefore, expansion of the molten material is possible. Both, aluminum nitride and calcium cyanamide yield to positive results as foaming agents. The porous materials are prepared by joint milling of the Z3 glass powder and foaming agent. Then the obtained mixture is thermally treated at 1100ºC during 85 min, with a heating rate of 5 ºCmin-1, being those conditions a result of optimization after various tests. When samples equal in initial weight and percentages of foaming agent are compared, the use of AlN results in greater expansions and more porous materials than the use of CaCN2 (figure 9). Figure 9. Comparison between 5% CaCN2 and 5%AlN expanded materials. With the aim of more easily comparison of the obtained foams, its apparent density is estimated. With this purpose, the foams are cut in parallelepipeds (figure 10) and by 5% CaCN2 5% AlN 17 measuring their dimensions and weight the corresponding volume and apparent density could be calculated (table 4). Figure 10. Examples of obtained foams Table 4. Dimensions, volume, weight and apparent density of the selected foams Blowing agent (weight %) Weight (g) Length (cm) Width (cm) Height (cm) Volume (cm3) Apparent density (g.cm-3) Porosity, % 4% AlN 1,167 3,42 1,67 1,65 9,49 0,12 94.3 4% CaCN2 1,233 2,24 1,87 1,28 5,39 0,23 89.3 2% AlN 0,807 2,67 2,00 0,90 4,82 0,17 92.0 1% AlN 0,805 2,33 2,19 0,98 5,04 0,16 92.8 Taking into account the real ( r) and the apparent density ( a) (Table 5) the porosity ( %) could be estimated according to the following relationship: % = (1a/r)*100. Porosity at around 90 % is obtained (Table 4). Within the series prepared with the same foaming agent (AlN), similar porosity is detected when lower charges of AlN are used (1 and 2%). However, when the AlN percentage is increased up to 4%, more expansion of the glass is reached, resulting in 95% of porosity. On the other hand, the same percentage of different expansion agents produces very dissimilar apparent densities. Foam prepared with 4% CaCN2 presents an apparent density of 0,23 g.cm-3, nearly the double of that obtained with 4% of AlN (0,12 g.cm-3). Moreover, the average pore size obtained with calcium cyanamide is smaller than the formed with aluminum nitride (figure 10). 4% AlN 2% AlN 1% AlN 4% CaCN2 18 In addition to the bulk density, the real density of the materials is determined in helium pycnometer (Table 5). As the starting glass is the same in all cases, the same density is expected for all foams. Excepting the samples 5%AlN and 5%CaCN2, all the others present very similar values of density with an average of 2.16 g.cm-3. Although, the final material composition could vary with the concentration and nature of the foaming agents and could influence the density, the more plausible explication of the values below 2 g.cm-3 are the presence of very small pores fraction whose volume was not occupied by helium and, therefore, the total volume of the sample is overestimated and its density underestimated. Table 5. Real density obtained for some of the prepared foams. Blowing agent (weight %) Density (g.cm-3) 5% AlN 1,83 4% AlN 2,12 2% AlN 2,13 1% AlN 2,22 5% CaCN2 1,76 4% CaCN2 2,15 X-ray diffraction analysis of the products shows the formation of two silicates: (Mg0.992Fe0.008)(Ca0.999Fe0.029)(Si2O6), JCPDS#01-083-008 and Ca2.87Fe0.13(SiO3)3, JCPDS#01-083-2198 for all samples (Figures 11 and 12) which implies partial crystallization of the starting glass during the thermal treatment to achieve the expansion of the material. With the aim to check if the crystallization is related to the presence of foaming agent, the same heat treatment of the bare glass sample without expansion agents is carried out and also results in crystallization of some silicates. This indicates that the crystallization is only a thermal effect of the initial glass material. 19 10 15 20 25 30 35 40 45 50 55 60 65 ^ * ^ ^ ^ ^ *^^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ * * * * * * * * * * * * * Angle (2 ) Intensity (a.u.) * ^ * (Mg0.992Fe0.008)(Ca0.999Fe0.029)(Si2O6) ^ Ca2.87Fe0.13(SiO3)3 Figure 11. Diffractogram of sample 4%AlN 10 20 30 40 50 60 70 80 Intensity (a.u.) Angle (2 ) no expansion agent 1% AlN 2% AlN 4% AlN 5% AlN Figure 12. Comparison of the diffractograms of the samples with and without foaming agents The resistance of the cellular glasses under compression strength is also studied. A sample obtained without foaming agent is compared to samples foamed with 4% AlN and 4% CaCN2 respectively. As expected, the resistance of the materials is a function of their porosity. The sample prepared without foaming agent presents initial resistance to crack formation of 500 N/m2 then quick propagation of the later leading to material breaking at 4750 N/m2. Within the series with foaming agent, the smaller the pores size 20 and distribution the higher the mechanical resistance. The high apparent density measured for the sample prepared with cyanamide, and smaller porosity (89.3%), corresponds to higher mechanical strength, being the later slightly inferior to that of the material prepared without foaming agent (3600 vs. 4750 N/m2). On the other hand the increased porosity (94%) observed for the sample obtained with 4 wt.% AlN shows an instantaneous crack formation and maximum resistance to applied load of 450 N/m2. 4. Conclusions This study shows that the generated CDW could be recycled to generate products able to be applied in the same industry, thus reducing the waste generation and the problems related to the its treatment. A sustainable recycling could be applied in this industry. Simple procedures, such as milling, composition adjusting and vitrification could be applied to convert the waste into primary materials. The glazed tile materials resulting are produced from CDW with the same qualities as the commercial units. The direct fusion process is chosen as the more appropriate and cost saving method for such production. In a similar manner, a method for production of cellular glasses with possible application as thermal and acoustic insulators is proposed. The production path consists in glass formation from CDW and its expansion with the use of foaming agent, being the properties of the final material easily controlled by agent’s quantity and composition. 21 5. 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