Nature and behaviour of cement bonded particleboard: structure, physical property and movement

The paper records work carried out to analyse the structure and determine the behaviour of cement bonded particleboard (CBPB). The structure was quantified with respect to the structural parameters (distribution, size, shape and occupied area) of and interaction between individual components (pore, wood particle and cement paste), and the movement was analysed with respect to the structure of CBPB and the nature of the cement paste and wood particles. The results showed that: (1) The volume of CBPB occupied by the detectable pores is less than 1%, the areas occupied by wood particles are about 42% and 38% in vertical and horizontal surfaces respectively, and those by cement paste about 58% and 62%. (2) Orientation, size and shape of wood particles are very different between across the thickness and along the transverse directions of CBPB. (3) Equilibrium moisture content (EMC) and density have been complicated by the penetration of cement paste into wood particles and interfacial region between wood and cement paste. (4) CBPB was unstable under both constant and changing environmental conditions: exposure to a constant environmental condition resulted in an increase in mass and decrease in dimensions. Under a single change in relative humidity, the changes both in mass and dimensions on both adsorption and desorption consisted of two distinct stages: a significant change in the early stage of exposure and a gradual change in the later stage. Cycling under changing environmental regimes resulted in corresponding changes in mass and dimensions, and both reversible and irreversible behaviour occurred, giving rise to a series of displaced hysteresis loops which are very dissimilar to those for other materials. (5) The movement of CBPB has been attributed to the combined effects of moisture reaction, carbonation and degradation of CBPB, and all of these parameters gave rise to the development of incompatible stresses, which aggravated the above effects. Both mass and dimensional changes were essentially Fickian and non-Fickian. Models have been developed and able to predict both mass and dimensional changes effectively and efficiently. (6) The change of CBPB also reflected the change of wood particles and cement paste, and the strain and sorption of the CBPB have been successfully quantified in terms of moduli and volume/mass concentration of the wood particles and cement paste (the rule of mixtures).