Cement-Bonded Wood Composites: General Properties

The term composite can be defined as a new material created by combining two or more different materials using various adhesives. Wood-based composites produced with mineral or synthetic-based adhesives contain many products with different properties (Maloney, 1996; Youngquist, 1999). These products are not limited to sheet form but may consist of molded products, large-sized lumber, and products combining certain materials with wood (for example, plastic-reinforced). Wood-cement boards (WCB) were first produced commercially in Switzerland in 1970. From the time of their initial production to the present day, thousands of tons per year of cement-bonded particle boards and fiber boards have been produced, utilizing biomass and cement's binding properties. The primary reason these products have gained such wide acceptance is their simple and effective production processes, as well as their use in construction as a renewable and durable building material (Moslemi, 2001; Simatupang and Geimer, 1990; Youngquist, 1999).

2- Production Process

Typically, wood-cement composites are produced by mixing wood particles (fiber, particles) and Portland cement along with the addition of certain chemical substances. The suspension created by this mixture forms a matrix structure, and the resulting boards are called cement-bonded wood boards, reinforced with wood and bonded with cement. These products are generally produced with 10-70% wood and 90-30% adhesive (cement), depending on the properties expected from the material (Buchmayer, 1999; Jorge, et al. 2004; Simatupang and Geimer, 1990). Fundamentally, the production process of cement-bonded wood composites comprises four different stages. These are: • Raw material preparation (wood, cement, and chemical substances), • Draft formation, • Pressing, • Drying and conditioning. The major difference from composite materials produced using synthetic adhesives is that these materials require a very long curing time during production (up to 28 days). The general cement-wood-based composite production process is summarized in Figure 1.

Figure 1. Cement-wood-based composite production process

Five different types of Portland cement are commercially available (Types I-V). They are differentiated according to their chemical composition and properties. In fact, concrete produced from these cements has very similar properties with minimal differences. These five different Portland cement types contain approximately 75% calcium silicate (Ca₃SiO₄ and Ca₂SiO₅) and 18% calcium aluminate minerals (Ca₃Al2O₆ and Ca₄Al₂Fe₂O₁₀) by weight (Lea, 1970; Taylor, 1997). When wood combines with cement, numerous complex and uncontrollable reactions occur simultaneously. Literature provides more detailed information on the bonds formed between wood, cellulose, and cement during cement curing (Li, et al., 2005; Krüger, et al. 2009; Doudart de la Grée, et al. 2014 and 2015). Wood-cement-based composites show greater resistance to degradation and better performance against combustion and atmospheric conditions compared to materials produced with wood and other synthetic adhesives, as reported extensively in literature. However, cement used as a binder has a higher specific gravity than thermoset adhesives, and consequently, wood-based products manufactured with cement are heavier due to the need for more adhesive (cement) per unit area (Jorge et al. 2004; Van Elten, 1996).

3- Properties of Cement-Bonded Wood Composites (CBWC)

Generally, the properties of products produced depend on the amount and characteristics of cement used. Recently, wood-cement-based composites have begun to be preferred in many countries for construction work as environmentally friendly and renewable products to reduce material costs (Moslemi, 2001; Semple and Evans 2004). Some important properties of cement-bonded wood-based composite materials are provided comparatively in Table 1.

Table 1. General properties of cement-bonded wood composites

4- Conclusion

Materials traditionally produced with synthetic adhesives have limited use in outdoor environments. In this regard, the production of cement-based wood composite materials for outdoor structures and construction work has become more widespread. The greatest problem in bond formation between cement and wood is that certain wood species have high sugar content, making their interaction with cement limited and unsuitable for production. However, by using appropriate wood species, useful and low-cost products can be produced in this regard. Additionally, many wood species that have not been effectively utilized in construction work can be utilized through this approach. The most important aspect here is that the bonding mechanism between cement and wood must be fully elucidated. Prof. Dr. H. Turgut Şahin - Isparta University of Applied Sciences / Faculty of Forestry - Department of Forest Industry Engineering

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