Structural Flakeboards and Composites 3057 



surface. Mats 1 ,300 mm wide and 3,250 mm long are placed on steel cauls and 

 stacked in clamp-cradles closed under pressure. The clamp-cradle is then re- 

 moved from the press and transported to a slightly heated curing chamber where 

 it remains for 10 hours. The press is then employed a second time to permit 

 removal of locking pins from the clamp-cradle. The partially cured boards are 

 removed, trimmed to size, and further cured for 14 days at ambient temperature. 

 Finally they are conditioned to 12 percent moisture content (Eraser 1977). 



THE CEMENT-WOOD BOND^s 



Softwoods have been favored for cement-particleboards because some hard- 

 woods inhibit bonding. Ahn and Moslemi (1980) discussed some of the bonding 

 mechanisms involved. They observed that in a wood-portland cement- water 

 system, various sugars such as mannose, galactose, fructose, glucose, xylose, 

 and arabinose are present and that as wood particles dry these sugars migrate to 

 particle surfaces where some of them inhibit cement hydration and interfere with 

 formation of cement crystals. 



Type I Portland cement contains oxides of silicon, aluminum, iron, and 

 calcium, as follows: 



Compound Proportion 



Percent 



Tricalcium silicate (3 CaO • Si02) 48 



Dicalcium silicate (2 CaO • Si02) 27 



Tricalcium aluminate (3 CaO • AI2O3) 12 



Tetracalcium aluminoferrite 



(4 CaO . AI2O3 • FejOj) 8 



Other elements comprise the balance of cement but are not essential to bond 

 formation. 



All four major components hydrate on contact with water, but at different 

 rates. Hardened cement develops a crystalline structure that intergrows and 

 interlocks; at contact points crystals bond to each other. Calcium chloride 

 promotes crystal formation and bonding, while sugars adversely affect it. Some 

 softwoods such as western hemlock (Tsuga heterophylla (Raf.) Sarg.) and grand 

 fir {Abies grandis (Dougl. ex D. Don) Lindl.) form strong bonds when mixed 

 with cement, while many hardwoods produce weak bonds. 



To study the cement- wood bond, Ahn and Moslemi (1980) utilized grand fir 

 and Type 1 portland cement with calcium chloride dihydrate as an accelerator 

 and anhydrous D-glucose and sucrose as inhibitors. Modulus of rupture of cured 

 specimens nearly quadrupled with addition of 3 percent calcium chloride, but 

 addition of glucose or sucrose greatly reduced modulus of rupture. Trends were 

 similar in modulus of elasticity. 



*^Text under this heading is condensed from Ahn and Moslemi (1980). 



