Screening Inorganic Additives for Ameliorating the Inhibition of Hydration of Portland Cement by the Heartwood of Acacia mangium

摘要

The suitability of Acacia mangium for the manufacture of cement-bonded wood composites is adversely affected by the presence of heartwood tannins ― water soluble polyphenolic extractives that strongly inhibit the hydration of Portland cement. Once the inhibitory extractives have been removed from the wood by soaking it in fresh water, wood-cement composites such as wood-wool cement boards (WWCBs) can be manufactured from A. mangium. However, if a sufficient supply of fresh water is not consistently available, alternative strategies are needed to ameliorate the effect of polyphenols in A. mangium wood on cement hydration. Certain compounds such as calcium chloride are commonly used in the manufacture of wood-cement composites to reduce the inhibitory effect of heartwood extractives on the hydration of cement. It is well known that calcium chloride achieves this effect by accelerating the hydration of cement. This alone is not sufficient to overcome the inhibition of cement hydration caused by heartwood polyphenols in species like A. mangium. Compounds that accelerate cement hydration and also chelate or chemically modify polyphenols to prevent them from interfering with cement hydration may be more effective than calcium chloride at improving the compatibility of A. mangium wood with cement. This study used laboratory-scale cement hydration tests to screen a wide range of soluble and insoluble inorganic additives to identify those most effective at strengthening the hydration of Portland cement. The ability of compounds to form insoluble chelates with inhibitory heartwood polyphenols from A. mangium was tested, as was their capacity to strengthen the hydration of cement containing A. mangium heartwood. The results showed that compounds with the ability to strongly accelerate cement hydration and form insoluble chelates with inhibitory heartwood tannins were the most effective at reducing the inhibitory effect of A. mangium heartwood on cement hydration. Most of the compounds were chlorides and nitrates, including SnCl_4, AlCl_3, (NH_4)_2Ce(NO_3)_6 and FeCl_3. These compounds accelerate cement hydration and also contain cations such as Sn~(4+), Al~(3+) and Fe~(3+) which can strongly chelate heartwood tannins in A. mangium. Accordingly, these compounds were more effective than calcium chloride (CaCl_2,), which, although it accelerated cement hydration, did not form insoluble chelates with heartwood tannins. The treatment of wood with compounds that can accelerate cement hydration and chelate heartwood extractives may facilitate the manufacture of wood-cement composites from fresh A. mangium wood, a goal that hitherto has not been possible to achieve using conventional cement-hardening additives.