- Original article
- Open Access
Effect of moisture content on manufacturing cement-bonded particleboard using supercritical CO2
© The Japan Wood Research Society 2011
- Received: 10 June 2011
- Accepted: 5 September 2011
- Published: 21 December 2011
This study examined the effects of moisture content (MC) on the manufacture of cement-bonded particleboard (CBP) using supercritical CO2 in the curing process. Significant correlations were found between MC and the performance of CBP: the internal bond strength, modulus of rupture, and modulus of elasticity values of CBP achieved their maximums, when the MC of boards was approximately 30%. This finding indicated that during the curing phase of manufacturing CBP, a MC of about 30%, which is nearly equal to the water–cement (w/c) ratio of about 0.34, contributes to improved mechanical properties. However, the mechanical properties decreased when the MC was below 30%, which had a negative effect on board performance, indicating that carbon dioxide could not fully react and no carbonation occurred during the curing process. Maintaining a MC of approximately 30% as an ordinary condition of the cement required in the curing of CBP could promote the reaction of carbon dioxide to form calcium carbonate (CaCO3), which leads to increased final strength of CBP. Both X-ray diffractometry and thermal gravimetry observation agreed well with these results and clarified that the increase of CaCO3 content caused by carbonation with increased MC of boards contributed to improving the mechanical properties of CBP.
- Cement-bonded particleboard
- Moisture content
- Water–cement (w/c) ratio
- Supercritical CO2
Cement-bonded particleboard (CBP) consists of cement, wood particles, water, and chemical admixture. When all of the materials are mixed, cement powder upon contact with water undergo a hardening reaction (curing) that bonds the wood particles together. In this process, water is essential for cement to harden by hydration  and using the optimum amount of water in the presence of moisture content (MC) during the manufacture of CBP is important. Geimer et al.  and Simatupang et al.  showed that the amount of water is a key factor in the production of wood–cement composites with carbon dioxide injection on the cement setting process, but excessive water has a detrimental effect on the properties of cement-bonded material . In the field of cement-based materials, Boutouil and Levacher  investigated the effect of high initial water content and hydrated cement paste on the mechanical performance and porosity. Furthermore, Qi and Cooper  examined the effects of wood fiber/cement ratio, water/cement ratio, and carbon dioxide injection time on the properties of wood–cement composites. This research showed that the wood fiber/cement ratio, the water/cement ratio and their interaction significantly influenced the carbonation reaction and strength of wood–cement composites subjected to 4 min of carbon dioxide injection during the setting process. In general, the carbonation degree was affected more by the water/cement ratio, while the strength of composites was affected more by the wood/cement ratio. Water takes part in the curing of cement by carbon dioxide, a process in which carbon dioxide dissolves in water to form carbonic acid (H2CO3). In the latter case of cement hydration, under the influence of carbon dioxide, the formed calcium hydroxide is carbonized to calcium carbonate (CaCO3). These phases are mainly responsible for strength development of cement .
Qi and Cooper  clarified that the injection of CO2 gas during pressing is one of the methods to reduce the pressing (setting) time of cement, but the resulting board properties are almost the same as those produced by the conventional method , and there is the disadvantage that at least 14 days are required to achieve complete curing . In our previous study , we found that the performance of CBP can be changed by the curing treatment with supercritical CO2. The major advantage of using the supercritical CO2 treatment was the rapid curing of cement. During a shorter treatment time (30 min), the mechanical properties [internal bond (IB) strength, modulus of rupture (MOR) and modulus of elasticity (MOE)] of CBP achieved the maximum values, which indicated that supercritical CO2 greatly accelerated the curing process and enhanced the mechanical properties of the boards. The dimensional stability was also improved significantly when boards were treated with supercritical CO2 for a shorter time. With a longer time span of treatment, IB, MOR and MOE values decreased, with the resulting negative effect on board performance. We concluded that supercritical CO2 treatment over a longer time span leads to degradation of CBP because of the carbonation of cement.
As a related issue, the effects of MC on the manufacture of CBP by supercritical CO2 treatment during the curing process have not yet been revealed. During the cement hydration process, di-calcium silicate, Ca2SiO4 (C2S) and tri-calcium silicate, Ca3SiO5 (C3S) are hydrated to form calcium silicate hydrate, Ca3Si2O7·3H2O (CSH) and calcium hydroxide, Ca(OH)2 (CH), respectively . When carbon dioxide is added to cement, calcium carbonate, CaCO3 is formed [8, 9, 12]. In this process, water is necessary to promote the reaction of CO2. Therefore, in relation to the manufacture of CBP, the presence of these boards which equals to a sufficient water–cement (w/c) ratio during the curing process is important, because it will affect the final performances.
The objectives of this study were to investigate the effect of MC on the manufacture of CBP using supercritical CO2 in the curing process and to clarify the optimum conditions of MC.
Mixtures with proportionally equal amounts of particles of Japanese cypress (Chamaecyparis obtusa Endl) and Japanese cedar (Cryptomeria japonica D. Don) were used to manufacture CBP. The average size of the wood particles was 3.3 mm (L), 0.7 mm (W), and 0.1 mm (T). Ordinary Portland cement of Osaka Sumitomo was used as a binder; supercritical CO2 was used as a curing accelerator. To produce supercritical CO2, CO2 gas was maintained in the liquid phase by passing the gas through a condenser. The liquid CO2 was then pumped into the reaction cell surrounded by a water jacket set at 40°C at a pressure of 10 MPa.
Manufacture of CBP
The w/c ratios ranged from 0.07 to 0.34 under the assumption that the fiber saturation point of wood was 28%. According to our previous research finding  that during a shorter treatment time supercritical CO2 greatly accelerated the curing process and enhanced the mechanical properties of CBP, we used a curing treatment with supercritical CO2 for 30 min. For the treatment with supercritical CO2, the specimens were placed in a reaction cell surrounded by a water jacket set at 40°C with a pressure of 10 MPa, followed by conditioning at ambient temperature before further testing. For comparison, the additional treatments were (1) curing treatment for 28 days at room temperature (conventional), in which the CBP was wrapped with a polyvinylchloride (PVC) film immediately after clamping, followed by drying in an oven at 80°C for 10 h and conditioning at room temperature for 1 week; (2) neither conventional curing nor supercritical CO2 curing treatment (control), in which the CBP was immediately dried at 80°C for 10 h after clamping, followed by 1 week of conditioning as already described; and (3) conventional board treatment, in which the CBP was cured for 28 days at room temperature, but followed by 30 min of supercritical CO2 treatment.
Evaluation of CBP properties
The mechanical and dimensional properties of the boards were tested in accordance with the Japan Industrial Standard (JIS) A 5908 (1994). The boards were cut into 50 × 210 mm samples for the static bending test and 50 × 50 mm samples for the IB strength, thickness swelling (TS), and water absorption (WA) tests. Four test samples were prepared from each treatment group for the foregoing tests. The static bending test was conducted using a three-point bending test over an effective span of 180 mm (15 times the board thickness) at a loading speed of 10 mm/min. The chemical changes and the mineral decomposition of the CBP as the effects of MC on manufacturing using supercritical CO2 were examined using X-ray diffractometry (XRD) and thermal gravimetry analysis (TGA).
Powdered samples of 100-mesh pass taken from an IB test specimens were examined by XRD analysis using a method widely applied to the analysis of cement hydration [13, 14]. Step scan was measured using XRD (RINT 2100; Rigaku) at 40 kV and 40 mA; 2θ ranged from 5.0° to 40.0° when scanning at 0.02° and 2.0°/min. The amounts of unreacted clinkers taken at 2θ = 32.3° and 32.6°, calcium hydroxide at 2θ = 18.0°, and calcium carbonate at 2θ = 29.4° and 36.0° for the samples were determined and compared.
Powdered samples of 100-mesh pass were examined by a thermogravimetric analyzer (TGA 2050; TA Instrument, Japan). Thermal properties of the specimens were observed from room temperature to 800°C at a heating rate of 10°C/min under nitrogen flow.
Effect of MC on manufacturing CBP using supercritical CO2
Chemical changes and thermal properties
The MC affects the performance of CBP during manufacturing that includes the use of supercritical CO2 in the curing process. The IB, MOR and MOE values of the CBP treated with supercritical CO2 with MC of about 30% achieved maximum values, indicating that this level of MC, equaling a w/c ratio of about 0.34, which is an ordinary condition of the cement required in the curing of CBP, and could promote the reaction of carbon dioxide to form calcium carbonate (CaCO3), leading to an increase in the final strength of CBP. When CBP was manufactured using supercritical CO2 with MC below 30%, i.e., with insufficient water and a w/c ratio below 0.34, the carbon dioxide could not fully react and CBP did not carbonate successfully. This condition had a negative effect on the CBP performance. Both XRD and TGA observation clarified the increase of CaCO3 content caused by carbonation with increased MC of boards, and this phenomenon contributed to improving the mechanical properties of CBP.
This paper is a part of the outcome of the JSPS Global COE Program (E-03): in search of Sustainable Humanosphere in Asia and Africa. The authors thank the Directorate of Higher Education; Ministry of National Education-Indonesia for their finance support by DIKTI scholarship.
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