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Journal of Cleaner Production 168 (2017) 452e462

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Transforming wood waste into water-resistant magnesia-phosphate cement particleboard modified by alumina and red mud Lei Wang, Iris K.M. Yu, Daniel C.W. Tsang*, Shuang Li, Jiang-shan Li, Chi Sun Poon, Yan-Shuai Wang, Jian-Guo Dai Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 June 2017 Received in revised form 2 August 2017 Accepted 4 September 2017 Available online 7 September 2017

To transform construction wood waste by magnesia-phosphate cement (MPC) into cement-bonded particleboards, instability of MPC in water that compromises durability of particleboards in moist environment needs to be addressed. This study proposed a novel use of alumina and red mud to improve water resistance of MPC particleboards. Cement hydration chemistry and microstructure characteristics were revealed by quantitative X-ray diffraction analyses and scanning electron microscopy. Addition of alumina or red mud (Mg/Al or Mg/Fe at optimal molar ratio of 10:1) facilitated formation of amorphous MgeAl or MgeAleFe phosphate gel, respectively, which enhanced compressive strength. Alumina improved short-term water resistance, whereas red mud provided better long-term water resistance. Red mud-MPC binder enhanced strength retention (by 22.8%) and reduced water absorption (by 26.4%) of particleboards after 72-h water immersion, probably attributed to stable scaly-like MgeAleFe phosphate gel. This study demonstrated that red mud and wood waste are sustainable materials for producing particleboards. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Hydration chemistry Wood waste Red mud Magnesia-phosphate cement Cement-bonded particleboard Waste recycling

1. Introduction Magnesia phosphate cement (MPC) is a rapid-hardening cementitious material resulting from homogeneous acid-base reaction between dead-burnt magnesia (MgO) and phosphate. It shows superior properties to ordinary Portland cement, such as fast setting and hardening, high early strength, excellent corrosion resistance, low drying shrinkage, low permeability, as well as outstanding compatibility with impurities and contaminants (Fan and Chen, 2014). These advantages have rendered MPC popular in various engineering applications, including fast-repairs and rehabilitation of civil structures (Chau et al., 2011), wastewater treatment, stabilization/solidification of hazardous substances and nuclear wastes (Fu et al., 2016). Our recent study further demonstrated that MPC is an attractive cementitious material to transform the construction

* Corresponding author. E-mail address: [email protected] (D.C.W. Tsang). http://dx.doi.org/10.1016/j.jclepro.2017.09.038 0959-6526/© 2017 Elsevier Ltd. All rights reserved.

wood waste into rapid-shaping cement-bonded particleboard (Wang et al., 2017a). At present, ordinary Portland cement (OPC) is used as the primary binder to produce cement-bonded particleboard production, which requires an unfavourably long moulding time (approximately 24 h) under pressure for maintaining consistent dimensions with suitable porosity (Wang et al., 2016a). Although accelerator addition and CO2 curing can shorten the stiffening time to 6e8 h (Wang et al., 2016b), it still presents a limitation to scaled-up wood waste recycling due to low production rate. In contrast, MPC binder can enable rapid hardening process in the order of minutes (5e8 min) (Wang et al., 2017a). The crystalline and amorphous hydration products of MPC enhanced the bonding with wood particles, which acted as a hygroscopic material to moderate the development of hydration product, namely magnesium potassium phosphate hexahydrate (MgKPO4$6H2O, MKP), within the wood porosity. The MPC binder also showed high tolerance to the wood waste impurities, e.g., water soluble extractives and chemical preservatives, which often interfered with the OPC hydration reaction (Wang et al., 2017b). The synergistic combination of MPC and wood waste produced high-strength particleboard with favourable properties such as

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lightweight and thermal and acoustic insulation. Despite numerous advantages, the application of MPCcontaining products may be questionable in humid environment due to the instability of MPC in water (Lu and Chen, 2016). Previous studies revealed that the MKP crystal and amorphous magnesium phosphate decomposed in aqueous medium (Shi et al., 2014), accounting for the significant reduction of residual strength in the product after long-term water immersion (Zhang et al., 2017a). Moreover, wood waste has a high water absorption capacity that may further debase the durability of MPC particleboards in moist environment. Although the addition of silica fume, water glass, and water repellent could improve the resistance against water (Ding and Li, 2005), the mechanical strength declined significantly as an undesirable trade-off (Li and Chen, 2013). Thus, it is necessary to develop a new approach to improve the water resistance of MPC particleboards. In the MPC system, MgO is the substrate and potassium dihydrogen phosphate (KDP) is the activator. A range of metal oxides have similar properties to MgO, such as calcium oxide (Ambard and Mueninghoff, 2006), zinc oxide (Ma et al., 2014), aluminum oxide (Liu and Chen, 2016), and iron oxide (Mikulci c et al., 2016), which can serve as the alternatives for metalphosphate cement production. In particular, we proposed aluminium oxide (Al2O3) and iron oxide (Fe2O3) with relatively low solubility as high-potential supplements for improving the water resistance of the MPC particleboards. Although their low solubility may hinder the spontaneous cement hydration with phosphate (KH2PO4, KDP), the heat released from exothermic hydration of MgO may enhance the solubility Al2O3 and Fe2O3 (Wagh et al., 2003). Therefore, the combination of MgO and Al2O3/Fe2O3 may improve the homogeneity of product matrix by moderating the setting time that is often too fast in the MPC systems (Mymrin et al., 2017). Partial replacement of MgO by alumina (Al2O3) also presents environmental merits, as it is abundant and does not require energy-intensive sintering process, i.e., a potential low-carbon substitute. The use of Fe2O3- and Al2O3-rich red mud can further promote the sustainability of MPC particleboard production (Kang and Kwon, 2017), because it is an aluminium refinery waste that has been considered as a green supplementary cementitious material (Liu and Poon, 2016a, b). Therefore, we postulated that the incorporation of alumina or red mud could improve the performance of MPC particleboard in humid environment, by retarding the spontaneous MPC hydration process and generating insoluble metal-phosphate gels. In order to develop water-resistant MPC particleboards, this study aimed to: (i) assess the effect of alumina and red mud addition on the physicochemical properties of MPC blocks; (ii) elucidate the dependence of microstructure and mineralogy on the Mg/Al and Mg/Fe ratio; (iii) investigate the relationship between hydration products and water resistance; and (iv) evaluate the compatibility of alumina or red mud modified-MPC with wood waste for producing water-resistant particleboards. 2. Materials and methodology

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possibly attributed to the use of preservative chemicals during the manufacture of wood formwork. The samples were ground into particles in 0.3e2.36 mm and pre-wetted to 55 wt% water content for meeting the saturated surface dry (SSD) condition for MPCbonded particleboard production. Neutral Al2O3 and red mud as an industrial waste were employed to partially substitute MgO for modifying the MPC block and particleboard. The reagent-grade Al2O3 was purchased from Tianjin Chemical Reagent Factory, China. The red mud was obtained from an aluminium refinery in Jiangsu Province, which contained 59.3% Fe2O3 as the major mineral oxide, followed by 16.2% Al2O3, 9.1% SiO2, and 8.2% TiO2 (Table 1). High concentrations of metals/ metalloids were also detected, i.e., As (363 mg kg1), Cu (98.6 mg kg1), Zn (67.4 mg kg1), Pb (49.6 mg kg1), Ni (39.6 mg kg1), and Cr (34.2 mg kg1). Nevertheless, only marginal amounts leached from the red mud according to the toxicity characteristic leaching procedure (TCLP, US EPA 1311, 1992), where the leachability was as follows: 4.5 mg kg1 of Cr, 0.23 mg kg1 of As, and 0.19 mg kg1 of Zn from red mud, which were considered non-hazardous waste. The raw materials of the MPC binder included MgO (calcined at 1400  C, 91.9%; with 3.3% SiO2 and 2.8% CaO) (Table 1) from Liaoning Province, China, and reagent-grade KH2PO4 and borax (Na2B4O7$10H2O, retarding agent) from Tianjin Chemical Reagent Factory, China. Oil-coated polyvinyl alcohol fibre (PVA) with a length of 12 mm and tensile strength of 1400 MPa was obtained from Fujian Province, China. 2.2. Modified MPC blocks and particleboard manufacture For the production of MPC pastes (without wood) for evaluating the binder design, constant values of magnesia-tophosphate (M/P) molar ratio (7), borax-to-MgO ratio (0.13), and water-to-solid ratio (0.15) were adopted from previous findings (Wang et al., 2017a). To examine the effectiveness of modifiers, neutral alumina was added to the MPC paste at a Mg-to-Al (Mg/Al) molar ratio of 12:1, 10:1, 8:1, 6:1, and 4:1 according to the literature (Liu and Chen, 2016), while red mud was dosed at a Mg-to-Fe (Mg/Fe) molar ratio of 40:1, 30:1, 20:1, 10:1, and 5:1 based on our preliminary alumina-modified MPC results. The formulae of the modified MPC blocks are shown in Table 2. The pre-mixed powders (MgO, KDP, borax, and alumina or red mud) were fast mixed with water for 1 min by a planetary stirrer and subsequently poured into steel moulds (20  20  20 mm) for 1-h hardening process. The demoulded blocks were subject to 1-, 3-, 7-, and 28d air curing at 20 ± 1  C under 50 ± 5% humidity before assessment. All the experiments on MPC blocks were quadruplicated for quality assurance and the average values with standard deviations were reported. For manufacturing MPC-wood particleboards, a wood-to-MPC binder ratio of 2:8 wt/wt (i.e., 1:0.57 v/v) was selected based on the preliminary experiments, whereas the optimal Ale and red mud-modified MPC binder formulae were adopted from the test results of MPC blocks. Besides, 2 vol% PVA fibres with respect to wood content was incorporated to improve the mechanical

2.1. Wood waste, red mud, and chemicals The waste wood formwork dumped at construction sites was collected from a wood recycling company in Hong Kong. Inductively coupled plasma-atomic emission spectrometry (ICP-AES, Perkin Elmer Optima 3300DV) revealed a variety of metals/metalloids in the wood waste samples: Mn (1460 mg kg1), Zn (40.4 mg kg1), Cr (39.2 mg kg1), As (10.4 mg kg1), Cu (4.4 mg kg1), and Ni (4.0 mg kg1). Such contamination was

Table 1 Properties of magnesia and red mud. Chemical composition (wt%)

Magnesia Red mud

MgO

CaO

SiO2

Fe2O3

Al2O3

TiO2

P2O5

Na2O

K2O

91.9 0.1

2.8 2.1

3.3 9.1

0 59.3

0.5 16.2

0 8.2

0.2 0.4

0.2 2.8

0.2 0.2

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Table 2 Paste formulations (wt%) for MPC blocks modified by alumina or red mud incorporation.

Mg/Al ¼ 1:0 Mg/Al ¼ 12:1 Mg/Al ¼ 10:1 Mg/Al ¼ 8:1 Mg/Al ¼ 6:1 Mg/Al ¼ 4:1 Mg/Fe ¼ 40:1 Mg/Fe ¼ 30:1 Mg/Fe ¼ 20:1 Mg/Fe ¼ 10:1 Mg/Fe ¼ 5:1

Magnesia

Alumina

Red mud

KDP

Borax

Water

1.000 0.911 0.895 0.872 0.837 0.773 0.949 0.933 0.902 0.822 0.698

0.000 0.089 0.105 0.128 0.163 0.227 0.000 0.000 0.000 0.000 0.000

0.000 0.000 0.000 0.000 0.000 0.000 0.051 0.067 0.098 0.178 0.302

0.443

0.130

0.236

KDP: potassium dihydrogen phosphate.

properties of particleboards, of which the density was maintained at 1.65 g cm3. Sequential addition of materials was required to obtain a homogeneous product (Wang et al., 2017a): wood particles, KDP, and water were mixed for 0.5 min, followed by the addition of PVA fibres for another 0.5-min stirring. The resultant mixture was blended with the dry pre-mixed MgO, borax, and alumina or red mud for 0.5 min. The final mixture was placed in a steel mould (160  160  15 mm) under pressure at 4.0 MPa for 5min hardening. The demoulded particleboards were subsequently cured in an air curing chamber (20 ± 1  C and 50 ± 5% humidity) for 7 d. Triplicated experiments on the MPC particleboards were conducted.

Fig. 1. Compressive strength of Al-modified MPC blocks with varying Mg-to-Al molar ratios.

2.3. Physical properties, water resistance, and spectroscopic/ microscopic/thermal analyses The initial and final setting time of the modified MPC pastes were measured using the Vicat Apparatus (BS EN 480-2, 2006). A multi-function mechanical testing machine (Testometric CXM 50050 KN) was used to assess the unconfined uniaxial compressive strength (BS EN 12390-3, 2009) and flexural strength (ASTM C1185, 2008), at a loading rate of 0.5 and 0.3 mm min1, respectively. The initial and final setting time of modified MPC pastes were measured using the Vicat Apparatus (BS EN 480-2, 2006). The water resistance was evaluated in terms of the change in mechanical strength and water absorption (ASTM D1037, 2012) of the samples (SSD condition) after water immersion. The durations of immersion were 1 and 7 d for the MPC blocks, as well as 1, 6, 24, and 72 h for MPC particleboards. The micro-features and interface chemistry in the MPC products were assessed. Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX, TESCAN VEGA3 XM) was employed to observe the morphology of samples (coated by gold to enhance conductivity), which operated at an accelerating voltage of 15 kV and current of 70e78 mA. Thermogravimetric analysis (TGA, Rigaku Thermo Plus) was performed to study the thermal behaviour, as the temperature rose from 100 to 500  C at a rate of 5  C min1 under argon stripping. The crystallinity was measured by means of quantitative X-ray diffraction analysis (QXRD, Rigaku SmartLab). The amorphous fraction was determined by adding the internal standard (20 wt% CaF2) to the samples. Scanning of samples were conducted over a range of 10 e45 2q with 3 min1 at 45 kV and 200 mA. The whole powder pattern fitting (WPPF) method in the Rigaku's XRD software (PDXL) was adopted to calculate the Rietveld refinement quantitative phase.

Fig. 2. XRD spectra (a) and QXRD analysis (b) of 7-d air cured Al-modified MPC blocks with varying Mg-to-Al molar ratios.

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Fig. 3. SEM image (a) and EDX analysis (b) of MPC blocks (Mg/Al ¼ 1:0); SEM image (c) and EDX analysis (d) of Al-modified MPC blocks (Mg/Al ¼ 10:1); SEM image (e) and EDX analysis (f) of Al-modified MPC blocks (Mg/Al ¼ 4:1).

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3. Results and discussion 3.1. Alumina as a substitute of magnesia

Fig. 4. Compressive strength of red mud-modified MPC blocks with varying Mg-to-Fe molar ratios.

In general, the uniaxial compressive strength of all MPC blocks (without wood) gradually increased with the curing time (Fig. 1). Their strength after 1-d air curing reached approximately 50% of that of 28-d air curing, reflecting the fast-hardening ability of MPC. The partial replacement of MgO by Al2O3 enhanced the compressive strength, which reached the maximum at the Mg/Al molar ratio of 10:1 for all tested curing periods, e.g., up to 57.2 MPa after 28-d curing. The strength enhancement (20%) was on the low side of the reported range (19e36%) in the previous study (Fan and Chen, 2015). Although the strength declined as the Al2O3 dosage increased further (i.e., Mg/Al < 10:1), the performance of Almodified blocks was still superior or comparable to the unmodified MPC block. This implied that a significant portion of MgO can be substituted by the natural Al2O3 without compromising the strength of MPC products. The XRD spectra illustrated that a wide range of crystalline MKP peaks (from 20 to 22 and from 30 to 34 ) existed in the unmodified MPC block (Mg/Al ¼ 1:0) (Fig. 2a). However, the intensity of these MKP peaks significantly diminished in the Al-modified MPC samples (Fig. 2a). This aligned with the TGA results (Fig. S1, Supplementary Information) that indicated smaller peaks of MKP crystals at the temperatures between 70  C and 120  C (Ma et al., 2014). The QXRD results further suggested that the MKP fraction declined by 51.1% and 62.6% in the samples with Mg/Al ratio of 10:1 and 4:1, respectively, relative to the unmodified MPC block (Fig. 2b). However, the amorphous phase fraction was remarkably higher in the Al-modified MPC samples, which might contribute to the outstanding mechanical strength (Fig. 1) by providing additional strength (i.e., overcompensating) for the reduced content of crystalline MKP (Lu and Chen, 2016). The morphologies of Al-modified MPC blocks were observed using SEM, as shown in Fig. 3. The columnar-like crystals with a porous structure were observed in unmodified MPC sample, which had a Mg:K:P:O molar ratio of approximately 1:1:1:6.5 (Fig. 3b), pointing to the combination of well-formed crystalline MKP (MgKPO4$6H2O) and amorphous MgKPO4$nH2O gel, as suggested in a recent study (Fan and Chen, 2015). In comparison, a smooth and bulk material with dense structure was identified in Area 2 of the sample with Mg/Al ¼ 10:1 (Fig. 3c). Such dense structure might refer to the increased amorphous portion as indicated by the QXRD results, accounting for the relatively high compressive strength of the Al-modified MPC blocks (Fig. 1) (Wang et al., 2015a). Area 2 had a Mg:Al:K:P:O molar ratio of 0.72:0.23:1.01:1:4.61 (Fig. 3d), suggesting the formation of Mg0.72Al0.23KPO4. As the elemental compositions were distinctive in different areas, probable composition of the dense structure could be speculated as MgxAlyKPO4$nH2O (Fan and Chen, 2015). In the sample with Mg/Al ¼ 4:1, abundant rock-like structures were observed in Area 3, indicative of the unreacted alumina (Al:O molar ratio of 1.65) that impeded the strength development (Fig. 1). The optimal Mg/Al molar ratio of 10 was selected for the production of Al-modified MPC particleboard in this study. 3.2. Red mud incorporation in MPC paste

Fig. 5. XRD spectra (a) and QXRD analysis (b) of 7-d air cured red mud-modified MPC blocks with varying Mg-to-Fe molar ratios.

The compressive strength of red mud-modified MPC blocks gradually increased with the proportion of red mud until reaching the Mg/Fe molar ratio of 10:1 for all curing ages (Fig. 4), which was similar to the trend of the Al-modified MPC blocks (Fig. 1). Interestingly, red mud incorporation extended the initial setting time from 1.92 to 2.75 min, as well as the final setting time from 3.75 to

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Fig. 6. SEM image (a) and EDX analysis (b) of MPC blocks (Mg/Fe ¼ 1:0); SEM image (c) and EDX analysis (d) of red mud-modified MPC blocks (Mg/Fe ¼ 10:1); SEM image (e) and EDX analysis (f) of red mud-modified MPC blocks (Mg/Fe ¼ 5:1).

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Fig. 7. Compressive strength (a) and strength retention (b) of Al-modified MPC blocks after water immersion; compressive strength (c) and strength retention (d) of red mudmodified MPC blocks after water immersion.

5.67 min for the sample at Mg/Fe ¼ 10:1 (Fig. S2), possibly due to relatively low reactivity of Fe2O3 in red mud (Senff et al., 2011). The use of red mud may reduce the dosage of retardant (borax) and achieve more cost-effective production of MPC products (Nie et al., 2016). The XRD spectra indicated no obvious reduction of crystalline MKP content after red mud replacement at Mg/Fe ¼ 10:1 (Fig. 5a), which differed from the spectra of Al-modified MPC block. The amorphous phase content increased by 17.6% according to the QXRD results (Fig. 5b). The significant amount of hydration products probably contributed to the high compressive strength of the red mud-modified MPC block (58.7 MPa), which was slightly higher than that of the optimal Al-modified one. However, the MKP content significantly dropped by 49.6% at Mg/Fe ¼ 5:1, as corroborated by the TGA results (Fig. S3). The peaks at 21.5, 33.1, and 35.7 in the XRD spectra suggested the presence of unreacted hematite (Fe2O3), owing to the relatively low reactivity of Fe2O3 in red mud (Kim et al., 2017). As a result, the compressive strength at Mg/Fe ¼ 5:1 was much lower than that at 10:1 and the unmodified MPC block (Fig. 4). The SEM images depicted scaly-like irregular structure in the red mud-modified block (Mg/Al ¼ 10:1) (Fig. 6c), contrasting with the smooth bulk structure in the Al-modified sample (Mg/ Fe ¼ 10:1) (Fig. 3c). The rough surface of the irregular structure may be more conducive to a higher mechanical strength than the

smooth structure (Fig. 1 vs 4). The EDX results indicated that such irregular structure comprised Mg, Al, Fe, K, P, and O in molar ratio of 0.3:0.21:0.42:0.88:1:5, corresponding to substance with the formula Mg0.30Al0.21Fe0.42KPO4$H2O. However, we may expect the general formula as MgxAlyFezKPO4$nH2O in view of the observed variation in the elemental ratio from point to point in the Almodified block. Comparatively, abundant granular structure was observed at Mg/Fe ¼ 5:1, which was dominated by Al, Fe, and O according to the EDX results. It was possibly the unreacted red mud due to excessive red mud incorporation. For analysis on the red mud-modified MPC particleboard, the optimal Mg/Fe ratio of 10 was adopted. 3.3. Water resistance of Ale and red mud-modified MPC blocks The compressive strength of unmodified MPC block decreased as the duration of water immersion prolonged, i.e., strength retention was 95.1% and 86.3% after 1- and 7-d immersion, respectively (Fig. 7a and b). The XRD spectra and QXRD results showed that the content of crystalline MKP significantly increased by 162.7% in unmodified MPC blocks after 1-d water immersion (Fig. 8a and d). However, substantial decomposition of the amorphous phosphate gel (Fig. 8d) took place as a side effect, which accounted for the observed strength deterioration (Fig. 7a and b). This was substantiated by the TGA results showing notable loss of

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Fig. 8. XRD spectra and QXRD analysis of modified MPC blocks after water immersion: (a) MPC blocks (Mg/Al ¼ 1:0); (b) Al-modified MPC blocks (Mg/Al ¼ 10:1); (c) Red mudmodified MPC blocks (Mg/Fe ¼ 10:1); (d) QXRD analysis of modified MPC blocks.

KDP at about 240  C (Fig. 9a and b). After 7-d immersion, the crystalline MKP content decreased (Fig. 8d), while the soluble KDP fraction further increased (Fig. 9a and b). In addition, Mg(OH)2 gel transformed from MgO in aqueous environment may cause thickness swelling of the MPC block, reducing the mechanical strength to a greater extent (Wang et al., 2016a). It was noteworthy that the alumina addition significantly improved the water resistance (Fig. 7a and b). The compressive strength of the Al-modified sample (Mg/Al ¼ 10:1) was surprisingly enhanced by 3.3e6.7% after 1-d water immersion, possibly due to the additional hydration reaction. The QXRD results indicated the growth of MKP crystal (Fig. 8b and d), while no precipitation of KDP (decomposition of the amorphous phosphate gel) was observed in the TGA spectra (Fig. 9c and d). This explained the promising water resistance of the amorphous MgxAlyKPO4$nH2O gel in this study (Section 3.1). Despite the strength deterioration after 7d immersion, the performance of the Al-modified MPC blocks remained remarkably higher than that of unmodified MPC (Fig. 7a and b). In contrast, red mud-incorporated MPC blocks exhibited lower strength retention than unmodified MPC samples after 1-d water

immersion (Fig. 7c and d). This might be ascribed to the high water absorption capacity of the unreacted Fe2O3 (Zhang et al., 2017b) and clay minerals (Wang et al., 2015b) in the red mudmodified samples. The QXRD results showed a remarkable reduction of crystalline MKP content (Fig. 8c and d), which was probably transformed to weaker amorphous phosphate compounds as no KDP precipitation or other crystalline compounds were detected. Interestingly, the compressive strength rebounded after 7-d immersion and the residual strength surpassed that of unmodified MPC samples by 5.1e7.3%. The strength retention was comparable to the reported values (85e95%) in the previous study (Shi et al., 2014). Although no obvious change appeared in the XRD results, the TG and DTG curves illustrated a significant increase in the content of hydrates (70e120  C) (Fig. 9e and f), suggesting that further MPC hydration was facilitated by the long-term water curing (Ma et al., 2014). The additional hydration products probably induced the strength rebound after 7-d water immersion (Fig. 7c and d), which could not be directly detected by XRD or TGA analysis and further investigation should be conducted in future studies.

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Fig. 9. TG (a) and DTG (b) analysis of MPC blocks (Mg/Al ¼ 1:0) after water immersion; TG (c) and DTG (d) analysis of Al-modified MPC blocks (Mg/Al ¼ 10:1) after water immersion; TG (e) and DTG (f) analysis of red mud-modified MPC blocks (Mg/Fe ¼ 10:1) after water immersion.

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Fig. 10. Flexural strength, strength retention, and water absorption of modified MPC particleboards of water immersion. (MP: MPC particleboard; AP: Al-modified MPC particleboard; RP: red mud-modified MPC particleboard). (red line at 5.5 MPa: minimum required flexural strength for wet cement-bonded particleboards; blue line at 9 MPa: minimum required flexural strength for dry cement-bonded particleboards). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

3.4. Water resistance of modified MPC particleboards The water resistance of the MPC-wood particleboards was evaluated. The flexural strength of unmodified MPC particleboard gradually decreased from 10.8 to 5.68 MPa after 72-h water immersion (Fig. 10a). Although it fulfilled strength requirement of 5.5 MPa for wet particleboards (ISO 8335, 1987), the strength retention of MPC-wood particleboards (Fig. 10b) was much lower than that of MPC blocks (Fig. 7b). This was attributed to significant swelling of wood upon water absorption, which was negatively correlated with the strength retention (Fig. 10c) in agreement with the previous studies (Wang et al., 2016b). The flexural strength of dry Al-modified MPC particleboard (Mg/Al ¼ 10:1) was lower than the unmodified MPC particleboard, although it complied with the strength requirement of 9 MPa for dry particleboards (ISO 8335, 1987). This contrasted with the results of MPC blocks that the addition of alumina improved the strength (Fig. 1). Such discrepancy was probably associated with the smooth MgeAl phosphate gel as illustrated in the SEM image (Fig. 3c), which was unfavourable for bonding with wood waste particles. Nevertheless, the Al-modified particleboard presented promising water resistance, aligned with the observation for Al-modified blocks (Fig. 10a). After 72-h water

immersion, the flexural strength of Al-particleboard even surpassed the unmodified MPC particleboards. Its strength retention was 22.8% higher and the water absorption was 26.4% lower compared to unmodified MPC particleboard after 72-h water immersion (Fig. 10b and c), implying that additional hydration reaction also took place in the wood particleboard. In contrast, the red mud incorporation enhanced the flexural strength of dry MPC particleboards by 10.8% with respect to the unmodified particleboard (Fig. 10a). This was attributed to the formation of scaly-like irregular MgeAleFe phosphate gel as revealed by the SEM image (Fig. 6c), which enhanced the compatibility with the wood waste particles. More importantly, red mud considerably improved the water resistance of MPC particleboards (Fig. 10a). After 24-h and 72-h water immersion, there were high strength retention of 70.1% and marginal water absorption of 5.8% (Fig. 10b and c). The long-term water resistance was superior in red mud-modified MPC than Al-modified MPC particleboards, which probably originated from the additional formation of amorphous hydrates upon long-term water immersion (as discussed in Section 3.3). The enhanced durability in moist environment may promote the applications of MPC particleboards in outdoor environment.

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4. Conclusions This study proposed an innovative approach to transform the wood waste into water-resistant particleboards using alumina- and red mud-modified MPC. Both alumina and red mud incorporation enhanced the mechanical strength of MPC blocks due to the formation of amorphous MgeAl and MgeAleFe phosphate gel, respectively. The molar ratio of Mg/Al or Mg/Fe at 10:1 was considered as the optimal for strength development. The water resistance of unmodified MPC block was relatively low because of the decomposition of MKP crystal and magnesium phosphate gel in aqueous medium. The alumina addition notably improved the short-term water resistance, whereas red mud addition provided even better long-term water resistance. The red mud-modified MPC particleboards presented excellent compatibility with the wood waste, because of the formation of scaly-like irregular MgeAleFe phosphate gel. Red mud incorporation enhanced strength retention by 22.8% and reduced water absorption by 26.4% after long-term water immersion, which could improve the durability of MPC particleboards and extend their application for outdoor facilities. This study demonstrates that red mud and wood waste are low-cost and sustainable materials for producing highperformance particleboards in 5 min, which can reduce waste disposal and virgin material consumption as well as present a cleaner production approach for the manufacture of wood-based panels. Acknowledgement The authors appreciate the financial support from the Construction Industry Council Research Fund (project account: K-ZJJS) and PolyU's Project of Strategic Importance (project account: 1ZE12) for this study. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jclepro.2017.09.038. References Ambard, A.J., Mueninghoff, L., 2006. Calcium phosphate cement: review of mechanical and biological properties. J. Prosthodont 15, 321e328. ASTM C1185, 2008. Standard Test Methods for Sampling and Testing Non-asbestos Fiber-cement Flat Sheet, Roofing and Siding Shingles, and Clapboards. ASTM Standard C1185e08. http://dx.doi.org/10.1520/C1185-08R12. ASTM D1037, 2012. Standard Test Methods for Evaluating Properties of Wood-base Fiber and Particle Panel Materials. ASTM Standard D1037e06a. http:// dx.doi.org/10.1520/D1037-12. BS EN 12390-3, 2009. Testing Hardened Concrete - Part 3: Compressive Strength of Test Specimens. BS EN Standard 12390-3:2009, ISBN 978-0-580-76658-9. BS EN 480-2, 2006. Admixtures for Concrete, Mortar and Grout-test Methods - Part 2: Determination of Setting Time. BS EN Standard 480-2:2006, ISBN 0-58049250-8. Chau, C.K., Qiao, F., Li, Z., 2011. Microstructure of magnesium potassium phosphate cement. Constr. Build. Mater. 25, 2911e2917. Ding, Z., Li, Z., 2005. High-early-strength magnesium phosphate cement with fly ash. ACI Mater. J. 102, 375e381. Fan, S.J., Chen, B., 2014. Experimental study of phosphate salts influencing

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