Adverse effects of heartwood on the mechanical properties of wood-wool cement boards manufactured from radiata pine wood

Wood-wool cement boards (WWCBs) that are manufactured commercially in Australia from radiata pine occasionally contain localized areas in which there is poor bonding between wood and cement. The cause of this defect, which leads to the rejection of boards before they are sold, is not known, but it has been suggested that it may be due to the use of blue-stained wood or heartwood in the manufacture of boards. In this study, both wood types were tested for their effects on the hydration of Portland cement and the mechanical properties of WWCBs. Blue-stained sapwood slightly retarded the hydration of cement but had no significant (P < 0.05) effect on the mechanical properties of boards. In contrast, heartwood severely retarded cement hydration, and boards made from heartwood had little structural integrity. The appearance of such boards resembled the defective portions of commercially produced boards, and therefore it can be concluded that the defect arises from the inhibitory effect of heartwood on cement hydration. The problem could be eliminated by processing logs from young radiata pine trees, less than 12-15 years old, which will contain little or no heartwood.