Binders to be used in combination with wood - Special Inorganic Cements - page 343

the conventional production of these products, blends of wood chips or flakes with
cement and limited amounts of water are kept under compression at elevated temperature
until the material attains the needed cohesion and sufficient strength. Flakes and chips
from only a limited number of wood species are suitable as a reinforcing material, as in
many other species the water-soluble oligosaccharides present in the wood may interfere
with the hydration of the cement. It may also be sensible to store the wood chips for
several months before use, to lower the content of undesired constituents by spontaneous
biodegradation reactions taking place in the material.
To accelerate the process of hardening it has been suggested that carbon dioxide should
be injected into the spreadable mix of wood, cement, and water, or that ammonium,
sodium or potassium carbonate should be added (Simatupang et al., 1995). Under these
conditions the cement sets in a very short time, owing to the formation of calcium
carbonate. Effective acceleration of the hardening process may also be achieved by the
use of a fast-setting cement produced by combining Portland cement with limited
amounts of calcium aluminate cement (see section 10.10.1).
To improve the strength properties it has been suggested that silica fume should be
added to the starting mix (Lin et al., 1994).
Wetting-drying and freezing-thawing tests have confirmed the acceptable performance
of Portland cement-bound wood fiber reinforced materials (Parviz et al., 1994). In using
such products in combination with other construction materials one has to bear in mind,
however, the relatively large longitudinal changes of the wood-based board in the course
of wetting and drying. The resistance of wood fiber-cement composites to moisture
changes may be improved, and the gradual loss of strength due to temperature cycling
may be reduced, by pretreatment of the wood fibers with an aqueous acrylic emulsion or
aqueous alkylalkoxysilane (Blankenhorn et al., 1999).
A combination of Portland cement and clay has also been suggested for use as a binder
in the production of wood chip based lightweight concrete (Bouguerra et al., 1998)
As an alternative to Portland cement, alkali-activated blended cements (see section
8.5 and 9.1.5) have also been suggested as a binder in wood-based materials (Lin et al,
1994). The hardening of these binders is little affected by the quality of the wood
employed.
REFERENCES
Blankenhorn, P.R. et al. (1999) Temperature and moisture effects on selected properties
of wood fiber-cement composites. Cement and Concrete Research 29, 737-741.
Bouguerra, A. et al. (1998) Effect of microstructure on the mechanical and thermal
properties of lightweight concrete prepared from clay, cement and wood aggregates.
Cement and Concrete Research 28, 1179-1190.
Deng, Y., Furuno, T., and Uehara, T. (1998) Improvement on the properties of gypsum
particle board by adding cement. Journal of Wood Science 44, 98-102.
Fan, M.Z. et al. (1999) Dimensional stability of cement-bonded particleboard. Cement
and Concrete Research 29, 923-932.
Lin, X. et al. (1994) Approaches to improve properties of wood fiber reinforced
cementitious composites. Cement and Concrete Research 24, 1558-1566.