New fired bricks based on municipal solid waste incinerator bottom ash

Abstract

The main objective of this work was to study the sintering process and technological properties of new fired bricks based on high amount of post-treated municipal solid waste incinerator bottom ash and refractory clay. In addition, the effect of the minor addition of flux (Na₂CO₃) or reinforce (corundum) was also highlighted.

Several methods were used to study the effect of compositions variations on the sintering process, structure and the mechanical characteristics of the test briquettes. Differential thermal analysis (TG/DTA) and dilatometry techniques were applied to study the thermal behaviour while scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy and high-temperature X-ray diffraction were used to elucidate the structure and the phase composition. The mechanical characteristics were estimated by micro-indentation, strength and various physical tests (porosity, linear shrinkage and water absorption, etc).

The results highlight the possibility to use very high amount of municipal solid waste incinerator bottom ashes in the production of new fired bricks with good performances at all levels.

It is also shown that the addition of additives managed the final properties, affecting the crystal phase formation, porosity and greatly the strength of the samples.

Keywords

Bricks bottom ash sintering process physical-mechanical properties microstructure analysis

Introduction

Since the construction industry still uses a large amount of clay bricks in buildings, several studies have been made to incorporate coal fly ash, bottom ash (BA) and other residues in the production of fired bricks (Sena da Fonseca B et al., 2015; Aineto et al., 2006; Little et al., 2008; Lingling et al., 2005; Sokolar and Vodova, 2011; Demir, 2006) to contribute to conserve traditional raw materials and to avoid waste disposal (Naganathan et al., 2011). In this context, because of limited landfill sites on the one hand and the presence of silicon, calcium, aluminium and iron in BA on the other hand, municipal solid waste incinerator (MSWI) BAs are now used in the ceramic and building sector as opposed to landfill disposal (Naganathan et al., 2011). In 2004, 2.4 million tonnes of municipal solid waste (MSW) were disposed of by incinerators only in Italy (Aloisi et al., 2006; ISPRA, 2008). This situation demands that the construction industry stakeholders investigate new alternative materials and to develop novel methodologies for effective use of wastes produced from MSW incineration.

Therefore, the development of innovative fired bricks based on a huge amount of MSWI BA can be a substantial step towards the decrease of pollution and environmental impact.

Since MSWI ash contains CaO, SiO₂, Fe₂O₃ and Al₂O₃, a number of studies have investigated their feasibility in the production of mortar and concrete (Pan et al., 2008; Huang and Chu, 2003). Mandal and Sinha (2017a) studied the effect of BA finesse on the properties of red mud geopolymer. Bricks with higher strength and density values with increasing temperature, fineness of BA and iron slime content have been obtained Mandal and Sinha (2017b). Moreover, in a previous work the authors showed the feasibility to use a high amount of post-treated BA (60 wt%) as an alternative raw material in clay-based ceramics compositions.

Even though in recent years, the partial substitution of clay in bricks by several types of waste has been investigated focusing on the physical and mechanical properties of bricks, (Lingling et al., 2005; Demir, 2006; Pan et al., 2008; Huang and Chu, 2003; Mandal and Sinha, 2017a; Mandal and Sinha, 2017b; Shabback et al., 2012; Muñoz et al., 2016; Martínez-García et al., 2012; Menezes et al., 2005; He et al., 2012; Monteiro, 2005; Raut et al., 2012; Sena da Fonseca et al., 2011; Pérez-Villarejo et al., 2012; Barbieri et al., 2013), extensive experimental studies related to detailed studies of a densification process, phase transformation and microstructures are very limited. Moreover, no information on the effects of additions of reinforcements (as corundum) or intensive fluxes (as Na₂CO₃) in fired clay bricks based on MSWI has been reported.

Generally, similar additives are used to improve some properties of ceramic batches, such as shaping characteristics, drying behaviour, firing behaviour, etc. (Cultrone et al., 2005). Corundum is a raw material with high physical and mechanical properties, which is used in glazes and ceramic materials to increase their mechanical and physical properties (Asenov et al., 2013). Na₂CO₃ is an additive widely used in the glass industry, since it, as foaming agent, affects the density and porosity and additionally enables bathes to sinter at a lower temperature (Mular et al., 2002).

For this purpose, the article examined the densification process and the related phase transformation of bricks made of clay and secondary raw materials from MSWI BA, as well as the supplementary effect of the addition of small amounts (5 wt%) of corundum (Al₂O₃) and sodium carbonate (Na₂CO₃). First, the study was based on the evaluation of the optimal heat-treatment by optical dilatometer and differential thermal analysis. The elemental and phase composition fraction of fired bricks were determined by energy dispersion spectroscopy (EDS) and X-ray diffraction (XRD). The use of scanning electron microscopy (SEM) allowed the capturing of microstructural features. Furthermore, physical and mechanical at micro- and macroscale properties of these heterogeneous materials were investigated and reported.

In the present research, a pre-treated BA and commercialised as ‘end of waste material’, according to the European Norm 2008/98/CE, was used in the production of new fired bricks. The mechanical pre-treatment process is composed of three steps: (i) aging in open air, (ii) grinding in a mill (obtaining different grain sizes 0–2; 2–8 mm) and (iii) separation of iron and aluminium pieces by means of magnetic and eddy current systems. Nowadays, this inert material is exploited in many sectors of construction material, such as cement (in clinker production), concrete, ceramic product (tiles and bricks) and bituminous conglomerate. Then the incorporation of such BA into the bricks would pose no risk at all, as discussed in other works (Shabback et al., 2012; Dermates and Meng, 2003).

Materials and methods

Materials

The raw material used in this study is an artificial silica-based aggregate, derived from a MSWI BA treatment plant in the North of Italy. Other raw materials, used for the production of bricks, were industrial kaolin (K)-ceramic grade (Balco, Italy), sodium carbonate (Na₂CO₃) and corundum (Al₂O₃) purchased from Sigma–Aldrich Chemical Co.

The results of the chemical analysis (determined by Inductively Coupled Plasma, ICP, Varian Liberty 200) of the raw materials (in oxides wt%, experimental error ±0.05 wt%) are reported in Table 1, while their mineralogical compositions, are reported in Table 2. The data elucidated that kaolin has a typical composition for this type of clay. At the same time, the chemical and physical composition of ash will depend on the compositions of raw MSW, the operational conditions, the incinerator and air pollution control system design (He et al., 2004). The BA used in this study, received from an industrial plant located in the North of Italy, is composed of silica, alumina, calcium oxide, sodium oxide and iron in different amounts. SiO₂ is the most abundant compound that exists in BA, containing up to 45 wt%. After heating at 600 °C and burnout of the residual organic part in BA (at about 4 wt%), a small increase in inorganic oxides content is measured in BA material.

Table 1.

Chemical composition (wt%) of the used raw materials.

Oxides	Kaolin	BA	BA-b
SiO₂	52.5	46.8	48.7
TiO₂	0.5	0.7	0.8
Al₂O₃	33.3	9.8	10.2
Fe₂O₃	0.6	4.3	4.5
CaO	0.2	18.6	19.3
MgO	0.4	2.9	3.0
K₂O	0.9	1.0	1.0
Na₂O	0.1	4.5	4.7
B₂O₃	0	0.6	0.6
MnO	0	0.3	0.3
ZnO	0	0.3	0.3
PbO	0	0.3	0.3
SO₃	0	1.0	1.0
P₂O₃	0	1.2	1.3
CuO	0	0.5	0.5
Others	0	0.2	0.2
L.O.I	11.7	7.1	3.2

BA: bottom ash; L.O.I: loss of ignition.

Table 2.

Mineralogy of kaolin (K) and BA used in the mixtures.

Materials	Crystalline phases (ICDD)
Kaolin	Quartz, SiO₂ [01–085–0457] Kaolinite, Al₂ (Si₂O₅) (OH)₄ [01–078–1996] Illite, (K, H₃O) Al₂Si₃AlO₁₀ (OH)₂ [026–0911]
BA	Quartz, SiO₂ [01–085–0794] Calcite, CaCO₃ [024–0027] Akermanite-Gehlenite, Ca₂(Mg_0.5Al_0.5)(Si_1.5Al_0.5O₇) [01–079–2423] Albite calcian ordered, (Na,Ca) Al (Si, Al)₃O₈ [041–1480] Covellite, CuS [03–065–3928]

Table 3 reports the leaching test results performed on the BA, according to UNI EN 12457-2 (2002). The pH, conductivity and leaching values for all metals and anions were lower than values reported in Decree Ministerial 27/9/2010 (eluate limits for residues to landfill).

Table 3.

Leaching test results of MSWI BA used in this work.

Metals/Anions	BA (mg L⁻¹)	Italian limit D.M 27/9/2010 not hazardous waste (mg L⁻¹)
Cu	0.480	5.0
Zn	0.191	5.0
Ni	0.050	1.0
As	<0.05	0.2
Cd	0.008	0.02
Cr total	<0.001	1.0
Pb	0.530	1.0
Al	90.40	—
Fe	8.653	—
Na	151.00	—
Cl-	200	2500
SO₄ ^2-	375	5000
pH	10.68	5–12
S.C (µS cm⁻¹)	1500	—

Particle size distribution of kaolin in Figure 1 was obtained by laser diffraction using a Malvern Mastersizer (Malvern Instruments, UK). This indicates that the as-received clay has an average particle size (d₅₀) of approximately 7 µm.

The range of apparent particle density on an oven-dried basis of AB was 1.9–2.1 g cm⁻³ and of kaolin was 1.8–2.5 g cm⁻³.

Samples preparation

All the parent ceramic batches, reported in Table 4, were prepared by mixing the raw materials, grounding, milling and sieved below 75 µm. Each batch was prepared by dry-grinding in ball jar for 10 min and humidified with 6 wt% of distilled water, then green samples were uniaxially pressed at 40 MPa. The samples used with a gauge length of 50 mm and cross section of 5 × 4 mm² were used for sintering study (thermal analysis) and in the evaluation of technological properties, such as density and water absorption. Testing cylindrical samples, for mechanical tests, with 18 mm diameter and 36 mm length, were manufactured by compressing at 40 MPa.

Table 4.

Chemical composition (wt%) of the studied ceramic batches.

Oxides	C	C-b	C-b-Al	C-b-Na
SiO₂	54.4	54.2	51.5	52.7
TiO₂	0.7	0.7	0.7	0.7
Al₂O₃	22.5	22.2	26.9	22.1
Fe₂O₃	2.9	3.1	2.7	2.8
CaO	11.4	11.6	10.5	10.7
MgO	2.0	2.0	1.8	1.8
K₂O	1.1	1.0	1.0	1.0
Na₂O	2.8	2.8	2.6	5.9
B₂O₃	0.4	0.4	0.3	0.3
MnO	0.2	0.2	0.2	0.2
ZnO	0.2	0.2	0.2	0.2
PbO	0.2	0.2	0.2	0.2
SO₃	0.6	0.6	0.5	0.5
P₂O₃	0.7	0.8	0.7	0.7
CuO	0.3	0.3	0.3	0.3

The mixing ratio of BA and kaolin of 55:45 was selected to obtain a ceramic batch with a higher content of CaCO₃ with respect to the kaolin. In fact, as reported by several researchers (Peters and Iberg, 1978; Cultrone et al., 1998), carbonates can lead to a reduction in shrinkage, anisotropic behaviour and to an increase in compressive strength.

The first ceramic (labelled C) was obtained by mixing 55 wt% ‘as it is’ BA with 45 wt% kaolin. The second composition (labelled C-b) was prepared using 55 wt% BA, preliminary heat-treated for 2 h at 600 °C (BA–b) (in order to eliminate entirely the residual organic phase) and 45 wt% kaolin. Moreover, in the ceramic batches, labelled C–b–Al and C–b–Na, 5 wt% of BA–b was substituted by fine Al₂O₃ and Na₂CO₃, respectively. The chemical compositions of these four ceramics, determined by X-ray fluorescence analysis (XRF) (ARL ADVANT’XP X-ray fluorescence spectrometer, Thermo Fischer Scientific Inc., Waltham, MA, experimental error ±0.1 wt%) are summarised in Table 4. These samples were dried overnight at 110 °C before the thermal analysis.

Samples characterisation

The sintering process was studied with a horizontal contactless optical dilatometer (Expert System Solutions, Misura HSML ODLT 1400) using a heating rate of 20 °C min⁻¹ in the temperature range of 20 °C–1300 °C. The temperature precision given by the equipment is ± 1 °C (Paganelli, 1997).

The phase transformations were followed by differential thermal analysis (TG/DTA, Netzsch STA 409) using the same conditions. The temperature precision given by the equipment is ±5 °C.

The fired bricks mineralogical compositions were evaluated by a conventional Bragg-Brentano powder diffractometer (X’Pert PRO, Panalytical) with nickel -filtered copper Kα radiation using bracket holder. The spectra were collected over 2θ intervals ranging from 10° to 40°, with a step size 0.02° and time step of 25 s. Moreover, to analyse the formation of crystalline phases as a function of the temperature, a high-temperature X-ray diffractometer with a heating chamber (HTK 16) was used. Powders of mixtures (<38 µm) were heated on a platinum bar from room temperature to 900 °C and 1000 °C, where the XRD scanning was performed. The identification of crystalline phases was made under comparison with data on the JCPDS files.

The microstructure and crystal morphology of the fired samples were observed by SEM (JEOL JSM 6390) coupled with EDS equipment (EDS, INCA 350, Oxford). Before metallisation with gold, chemical etching with 2% hydrofluoric acid for 5 s was carried out.

Linear shrinkage (LS%), water absorption (WA%) and measurement of weight loss of ignition (WLOI%) of the fired samples were performed according to ISO 10545-3 (1995).

The apparent density, ρ_a, skeleton, ρ_s and absolute, ρ_a_s, densities of the sintered samples were determined and the results were used to evaluate total P_T, closed P_C and open P_O porosity:

P_{T} = 100 \times (ρ_{a s} - ρ_{a}) / ρ_{a s}

(1)

P_{C} = 100 \times (ρ_{a s} - ρ_{s}) / ρ_{a s}

(2)

P_{O} = 100 \times (ρ_{s} - ρ_{a}) / ρ_{a s}

(3)

ρ_a was estimated by an Envelope Density Analyser (GeoPyc 1360, Micrometrics) using a dry-flow medium, while ρ_s and ρ_as by gas (Ar) pycnomoter (AccyPy 133, Micrometric) before and after crashing and milling the samples below 26 µm, respectively.

The results are the mean of three measurements. The experimental errors in the evaluations ρ_a, ρ_s and ρ_as, are ±0.03, ±0.01 and ±0.01 g cm⁻³ leading to mistakes of about 2% for total and open porosity and at about 1% for closed porosity, respectively. The experimental errors for LS and WA are ±0.5% and 1%, respectively.

The effect of composition and microstructure on the mechanical characteristics of the investigated new samples was studied using a combination of classical strength tests and micro-indentation tests.

The compressive strength (σ_c) was obtained by standard compression tests on cylindrical samples with a height to diameter ratio of two (h/D = 2). Three samples of each type were tested. The measurements were carried out using an electro-mechanical Instron 3300 (Instron, MA, USA).

Samples’ hardness and Young’s modulus of indentation were measured by depth-sensing micro-indentation (Depth-Sensing Vickers Microindenter, C.S.M. Instruments, Peseux, Switzerland). The continuous stiffness measurement continuously measured the stiffness and allowed the hardness and elastic modulus to be determined as continuous function of penetration depth. A maximum load of 10000 mN was set and it was reached applying a loading rate of 1500 mN min⁻¹, with 10 s holding time at the load necessary to reach the maximum displacement. Fifteen measurements were performed on different regions of three samples of each type and an average value was obtained. The hardness and the Young’s modulus values for each sample were calculated according to the Oliver and Pharr methods, respectively (Oliver and Pharr, 1992).

Results and discussion

Sintering and phase formation

The densification behaviour of samples C and C–b (i.e. the variations of linear shrinkages versus temperature) is reported in Figure 2. Both dilatometric curves present a first linear shrinkage of about 0.5% at about 530 °C, which was related to the kaolinite deoxydrilation. After that, the sintering plot of the C–b sample presented a second linear shrinkage of about 3.5%, which started at ~850 °C and stopped at the beginning of the phase formation range (970 °C–980 °C). At the same time, this sintering step in C sample was related with a lower linear shrinkage of ~2.5%. Moreover, a third densification range, leading to shrinkage of 4.5% was observed in the range 1250 °C–1300 °C. Similar high temperature densification was, practically absents in C–b sample.

This difference in the sintering behaviour could be related to the presence of 4% residual organic phase in sample C. Most probably, during the heat-treatment, the burning of this residual organic phase leads to an increase of porosity and then a densification process happens with the formation of melt trapped inside the porosity.

The sintering process of C–b sample was studied also at 1100 °C, 1120 °C and 1130 °C for 1 h. The plots are shown in Figure 2(b) and highlight that no additional densification or significant expansion of the samples were carried out at these temperatures. These results clearly demonstrate that the heat-treatment for bricks sintering might be limited to the modest temperature of 1000 °C.

The effects of the addition of Na₂CO₃ and Al₂O₃ (i.e. flux and inert reinforcement) in the C–b batch was also investigated by non-isothermal dilatometry and the sintering curves are reported in Figure 3(a). The decomposition of Na₂CO₃ at 700 °C–800 °C leads to some expansion but, after that, the formed Na₂O acts as a very intensive fluxing agent, decreasing the densification temperature with about 100 °C. At the same time, the addition of Al₂O₃ reduces the densification rate, which decreases the firing shrinkage by around 1%. The additional isothermal results (5 min at 1000 °C), which are presented in Figure 3(b), highlight that very short heat-treatment is needed to complete the densification process

The DTA plots of C–b, C–b–Al and C–b–Na are presented in Figure 4. The effects related to kaolinite deoxydrilation are identical for the all the compositions. The crystallisation exo-peaks, Tp, for samples C–b–Al and C–b were detected at 985 °C–980 °C, whereas the melting endo-effect occurred in the range of 1160 °C–1200 °C for both samples. The results demonstrated that the addition of an inert corundum fraction does not influence on the phase formation and melting processes. At the same time, the addition of 5 wt% of Na₂CO₃ leads to simultaneous decreasing of both crystallisation and melting temperatures with about 100 °C.

The comparison of non-isothermal DTA and dilatometry curves show that the low temperature partial densification is related to the neo-formation crystalline phase processes, while the secondary high temperature sintering and/or overfiring coincide with the melting of some neo-formed crystalline phases.

The DTA and dilatometry results also confirm that the low-cost heat-treatment schedule (5–10 min at 1000 °C) can be used for the samples densification.

Fired samples physico-mechanical properties

The physico-mechanical quality of fired briquette specimens was investigated to estimate the firing shrinkage, water absorption, weight loss on ignition of bricks, density and the load bearing capacity.

Water absorption (WA) gives information for open porosity. Low values imply good resistance to the natural environment and acceptable permeability of bricks. From the results in Table 5, it is clear the addition of Na₂CO₃ produces a decrease of brick WA. This can be explained by the difference in the open porosity of the mixtures. The WA for the C–b–Na sample is less with respect to C–b and C–b–Al, because the formation of the Na₂O-enriched phase with low viscosity can reduce the open porosity, which corresponds to an increase of closed porosity.

Table 5.

Technological properties: Compressive strength (σ_c), loss on ignition of bricks (WLOI%), linear shrinkage (LS%), water absorption (WA%), measured densities, hardness (H_IT) and indentation modulus (E_IT) for the fired samples.

Properties	C–b	C–b–Na	C–b–Al
σ_c (MPa)	55±6	33±1	59±6
WLOI (%)	7.3	10.6	6.9
LS (%)	4.0	3.5	3.5
WA (%)	14.5	11.5	13.0
ρ_a (g cm⁻³)	1.84	1.92	1.95
ρ_s (g cm⁻³)	2.67	2.62	2.77
ρ_as (g cm⁻³)	2.70	2.74	2.78
P_O (%)	31	26	30
P_C (%)	1.1	4.4	0.4
P_T (%)	32.0	30.0	30.0
H_IT (GPa)	0.80±0.35	1.41±0.50	1.75±0.24
E_IT (GPa)	16±3	22±5	19±2

Table 5 summarises the values of the weight of loss on ignition (WLOI) for the fired bricks. As reported by several works, the WLOI for a normal clay brick is 15% (Long Lin, 2006; Lissy and Sreeja, 2014). Based on Table 5, all the fired bricks have a lower WLOI. The C–b sample has low WLOI followed by C–b–Al with percentages of loss of 7.3% and 6.9%, respectively. The higher WLOI value, measured for the C–b–Na sample, was mainly owing to the contribution of Na₂CO₃ dissociation and gas evolution during the firing process in the temperature ranging from 700 °C to 800 °C.

The quality of brick could be further measured by examining the linear shrinkage (LS) of samples. Shrinkage in the ceramic process is a significant parameter, since structural change and solidification, implying densification, may create tensions and failures in fired bricks (Mekki et al., 2008). All the LS values obtained for the new fired bricks (Table 5) are lower than 4%. Furthermore, the progressive addition of Al₂O₃ and Na₂CO₃ produces a decrease of the sample’s LS with respect to the C–b sample.

To investigate the load-bearing capacity of these new building materials, including the Al₂O₃ and Na₂CO₃ addition, a uniaxial-compressive loading test was carried out.

The results reported in Table 5 indicate that the incorporation of Na₂CO₃ and Al₂O₃ influence the strength of the bricks. Even if the C–b–Na sample has a higher density and lower open porosity, a decrease of the compressive strength was measured. The decreased compressive strength can arise, as reported in the next paragraph, from the new crystalline phases created, the albite phase to the detriment of anorthite. However, the compressive strength results range from 33 MPa in the C–b–Na sample, to 59 MPa in the C–b–Al sample. Depending on the countries, there are different standards specifying brick grades according to the compressive strength values. For instance, withdrawn European standards specify a minimum strength of 5 MPa for burnt clay bricks (BS 3921, 1985; TS 705, 1985).

All the samples of our study, made with BA–b, have compressive strengths comparable with the standard strength values. In fact, according to ASTM C-67 (1992), the minimum required compressive strength of a paving brick subjected to light traffic is 17.2–20.7 MPa (Loryuenyong et al., 2009).

From another point of view, ASTM C62 (2005), for building and facing bricks, respectively, correlate the compressive strength with the weathering resistance specifying a minimum value of 20.7 MPa, to not be susceptible to degradation. In these terms, all the samples in this study should be durable to severe weathering.

The mechanical behaviour is inherently linked to the microstructure and constituent properties at micro-scale. In order to quantify this link and correlate the effect of open and closed porosity on micromechanical properties, an indentation technique was used (Piatasik and Hasselman, 1964). This testing procedure, based on the traditional Vickers hardness measurement, is based on continuous monitoring of the load and displacement of the hard probe as it is driven and withdrawn from the material at discrete locations of a grid. The obtained load–displacement diagrams, not provided in the manuscript, allow the determination of the material hardness H_IT and indentation modulus E_IT (Krakowiak et al., 2011).

The values reported in Table 5 evidence an increase of indentation hardness (H_IT) and indentation Young modulus (E_IT) after the addition of Na₂CO₃ and Al₂O₃. At a sub-millimetre scale the fired bricks, obtained in this work, can be ascribed to form a porous composite material whose behaviour is driven by matrix and porosity. The increase of average micro-hardness and indentation Young modulus in the C–b–Na sample can be attributed to reduction of open porosity and increase of closed porosity. Moreover, the presence of a harder phase as Al₂O₃ enhances the hardness of the bricks matrix from 0.80 GPa to 1.75 GPa. Then, the average micro-hardness obtained for sintered C–b–Na and C–b–Al samples are more than a doubling of the microhardness compared with the C–b sample.

Phase assemblage and microstructure

The evolution of the phase composition during heating and then the characteristic feature of structural brick materials was studied by XRD analysis of the sample using a heating chamber. Figure 5 reports the spectra for C–b, C–b–Na and C–b–Al samples respectively, gathered during heating.

The spectra of initial ceramic batches correspond to the main crystalline phases of used raw materials (i.e. kaolinite, quartz, calcite and akermanite). The spectra of batches sintered at 900 °C, present alpha quartz [JCPDF file 01-082-0511] as the main crystalline phase and traces of ghelenite (Ca₂Al(AlSi)O₇) [JCPDF file 01-089-5917] deriving from the initial BA–b waste component. At 1000 °C, in agreement with the DTA results, which elucidated an intensive phase formation in the interval 900 °C–1000 °C, formation of anorthite (CaAl₂Si₂O₈) [JCPDF file 00-002-0523] was evidenced. In fact, in a heterogeneous system, such as the used BA (Lam et al., 2010), the alkaline oxides are the most reactive components, resulting in the formation of an intergranular thin liquid phase, involving mass transport phenomena giving rise to the formation of new phases, such as anorthite (Acchar et al., 2013).

The XRD results with the addition of Na₂CO₃ reflect some variations in the crystalline phases developed. As can be seen, the final stages of heating the formation of some albite (NaAlSi₃O₈) [JCPDF file 01-083-1610] is indicated. In fact, the addition of Na₂CO₃ leads to the formation of Na₂O (fluxing oxide) that accelerates the local melting and subsequent crystallisation. On the other hand, the presence of Al₂O₃ does not influence the formation of new crystalline phases and peaks of corundum with identical intensity are observed in all spectra. The increase, with the temperature, of the background signal can be attributed to the presence of an amorphous phase, for example aluminosilicate glasses.

The results of this qualitative analysis are supported by the SEM-EDS results. Figure 6 demonstrates typical images of the main crystal phases in the samples (together with the corresponding EDS spectra). Figure 6(a) and (b) show the main typical crystalline phases, anorthite and albite solid solution. Unreacted glass particles, pyroxene, ghelenite and corundum particles, were also observed in C–b–Al sample (Figure 6(c)).

SEM images of the sample surfaces at low magnification, showing a ceramic body with a degree of sintering for brick production and typical open porosity (Karaman et al., 2001) are presented in Figure 7, respectively. Additionally, the SEM observations of the C–b sample fired at 1120 °C (not reported in the work) confirmed that no drastic structural charges were carried out when temperature is increased by 100 °C–120 °C.

Conclusion

The possibility to manufacture ceramic bricks by re-using of high amount of one inert material, generated from MSWIs, is demonstrated.

It is shown that, during a low-cost treatment (5–10 min at about 1000 °C), some densification and intensive re-crystallisation reactions between the meta-kaolinite and the phases from BA were carried out. It is also demonstrated that the addition of Na₂CO₃ results in the decrease of sintering and re-crystallisation temperatures, resulting in a reduction of open porosity as well as to some variations in the crystalline phases developed. At the same time, no effect in the phase formation was observed with Al₂O₃ addition.

The low firing shrinkage coupled with low WLOI and the improved mechanical properties (all of obtained samples have sufficient compressive strength over 20 MPa) demonstrated the high potential of pre-treated MSW BA to be used as a clay substitute to produce good face solid bricks.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

R Taurino

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