This study investigated the potential of utilizing bio-based residues in formulated magnesium and calcium phosphate cement binders. In the first phase, we used monopotassium phosphate and magnesium oxide to produce magnesium phosphate binder. In the second phase, we used unslaked lime, calcium silicate and monopotassium phosphate to produce calcium phosphate binder as a second type of the phosphate binder. These two binders were developed to produce durable low and medium density composite panel boards. The properties of the panels were tested to products technical specifications. The production process was optimised by means of variable optimization using the response surface methodology. The use of an industrial coal-fired plant fly ash was also investigated. The rationale was to reduce the total cost of production of the binder and increase the generation of more binder, while technically complementing the properties of the basic material. To our best knowledge, this is the first time that this binder system has been optimised for such particleboards.
Based on the objectives of this study, the following conclusions can be drawn;
We investigated the application of bio-based residues including pine sawdust, sugarcane bagasse, hemp hurds, papermill sludge and waste paper in magnesium phosphate cement binder to produce composite boards. The variables considered were the binder ratio of monopotassium phosphate (KH2PO4)to magnesium oxide (MgO) and the amount of lignocellulosic fibres
required to achieve the maximum desirable properties. The effect of fly ash on the composite properties was also studied. The optimum composite manufacturing processes for making
146 durable products within the experimental design was found to contain a ratio of KH2PO4/MgO
= 2.6, 10% fly ash, wood/inorganic ratio of 0.96 for pine; KH2PO4/MgO = 2.0, 8% fly ash,
fibre/inorganic ratio of 3.34 for paper sludge; KH2PO4/MgO = 2.3, 7% fly ash, fibre/inorganic
ratio of 2.92 for waste paper; KH2PO4/MgO = 3.0, 20% fly ash, fibre/inorganic ratio of 0.83 for
bagasse; KH2PO4/MgO = 4.83, 28% fly ash, fibre/inorganic ratio of 0.83 for hemp fibres.
We demonstrated that it is feasible to utilize alien wood waste including black wattle, long- leaved wattle, rooikrans and port jackson willow in the magnesium phosphate cement binder. We concluded that the presence of bark in processed wood does not negatively affect composite properties. When bark of black wattle (Acacia mearnsii) was added to the composite, maximum strength properties was obtained at a bark loading of 50% of the total wood content. Further studies on bush encroachment species using the optimized conditions established for this study resulted in maximum composite properties. In general, the physical properties of the composites met the minimum requirements for cement bonded particleboards (EN 634-2, 2007).
The application of unslaked lime modified with calcium silicate was also investigated. To our best knowledge, calcium phosphate cement has not been used in large scale productions involving lignocellulosic fibres. Some residues were selected for this study including hardwood (black wattle), softwood (slash pine), bagasse, hemp hurds, paper mill sludge and office waste. Similar optimization procedure was conducted in this study. In addition, the ratio of the alkali minerals between calcium silicate and unslaked lime was optimised. The use of fly ash was also investigated. The physical and mechanical properties of the produced composite panels were tested to products technical specifications. The physical properties of the proposed products met the minimum requirements for use in dry and humid conditions according to EN 634-2 (2007). The achieved strength properties of the panels were adequate for such applications as wall finishes, ceilings and partitions.
The preceding studies demonstrated the feasibility of incorporating bio-based residues into magnesium phosphate and calcium phosphate cement binders to produce panel boards. It was concluded that the phosphate cement binder is robust enough to accommodate the variability in these residues and is not affected by the raw material composition. In addition, we investigated the use of fly ash and concluded that, while fly ash can be incorporated to reduce binder cost and the overall cost of the production process, at high quantities, it proved to have a negative effect on strength properties. The variables considered i.e. binder ratio, fly ash and lignocellulosic fibre contents have significant effects on the physical and mechanical properties of the boards to varying degrees within the experimental design (p < 0.05). As applied in cement bonded boards, the micromechanical properties of the composites could be improved by modifying the hydrophilic fibre surfaces. In the same way, we treated the lignocellulosic fibres before composite development to allow flexibility in material application and potentially improve the composite properties.
147 The fourth objective describes the surface treatment of some selected lignocellulosic fibres. Based on the results of the preceding studies, we selected black wattle, slash pine and bagasse fibres for chemical treatments. These fibres were subjected differently to acetylation, alkalization and hot water extraction. The treated and untreated fibres were used in the manufacturing of composite panels using the established optimum conditions. The effect of the treatments on the fibres was studied using the FTIR, SEM and the HPLC. The physical and mechanical properties of the composites were tested to product technical specifications. Finally, µCT was used to visualize and analyse the phase volume distribution in both treated and untreated fibre composite panels. In the bagasse panels, the water absorption was 54.61% for untreated, 48.74% for hot water extracted, 42.21% for acetylated and 36.44% for alkalized panels. This represents a percentage improvement of 11, 23 and 33% respectively. The study concluded that alkali treated fibres had the best effect overall for all measured properties.
The utilization of bio-based industrial residues in composite product development promises to reduce the dependence on wood fibres, reduce the environmental impact of waste disposal and bring economic potential to developing countries. In addition, the development of environmentally friendly composite materials can help to reduce the carbon footprints of current composite product making process. With a small capital investment, satisfactory phosphate bonded composite materials can be produced on a small scale using mostly unskilled labour. However, technology can be introduced to increase the manufacturing output if the market for such composite materials increases.