2.3 COMPONENTS OF WOOD WOOL CEMENT BOARD
The major components used for the manufacture or production of wood wool cement board are wood, cement, chemical accelerator and water.
2.3.1 WOOD STRANDS
Wood cement composite like wood wool cement board can only use certain wood species whose concentration of simple sugar is low and hence will not inhibit the setting of cement. Generally hardwood are more incompatible with cement than soft woods; this is as a result of the high content of hemicelluloses, phenol substances and sugars in hard woods than in soft woods. This makes hard wood unsuitable for wood cement composite manufacture in their natural state (Weathered wax and TarKow 1964). To minimize the inhibitory effect of simple sugars to cement setting, the shredded wood is soaked in tap water prior to manufacture and when this is done in this manner, compatibility of the wood with cement is greatly enhanced.
2.3.2 CEMENT
Cement is any material that has adhesive and cohesive propertics, and those cement that can be used for the manufacture of wood wool cement boards (WWCBs) are Ordinary Portland cement (OPC) or hydraulic cements, this is because they set and harden upon addition of water. When this type of cement is mixed with water, it forms a paste and gradually stiffens until it turns in to a solid, this process is known as setting and hardening. The cement is said to have set when it has gained sufficient strength and rigidity to support an arbititranly defined pressure, after which it continues for a long time to harden, hence gaming further strength and this process is known as hardening. For complete hydration of a given amount of cement, an amount of water equal to about 25% of that given quantity of cement by weight is used, that is water – cement ratio of 0.25 is needed chemically. However an additional amount of water must be present so as to provide mobility for the water in the cement paste during the hydration process (Author, 1991).
There are two main types of cement marketed in the Philippines; these are Ordinary Portland Cement (OPC) and pozzolan cement. However in Nigeria, Ordinary Portland Cement (OPC) is widely used due to its faster setting time. It has been shown that WWCBs manufactured using Ordinary Portland Cement gave higher bending strength and water resistance than those bonded with pozzolan cement and tested after 28 days (Mallari et al. 1994). Cements differs in their setting time however; this can be greatly increased using chemical accelerators.
2.3.3 CEMENT – SETTING ACCELERATOR
Chemical accelerators are very important in the hydration process cement. When this is incorporated in wood-cement composites, it enhances hydration characteristics of cement and cement-based composites. They rapidly are the cement and minimize the inhibitory effect of low molecular weight extractives found in most wood species and this, increase board strength.
A number of chemical acceleration are calcium chloride and aluminium sulphate because of their availability, efficacy and relative low cost.
In the Philippines, the former is more commonly used because of its higher solubility in water which consequently does hot delay board.
2.3.4 WATER
Generally, in the production of wood wool cement board, pipe borne water is to be used because it is reasonably free from impurities such as suspended solids, organic matter and salts which may cause adverse effect to the setting, hardening and strength durability of material produced.
