The effect of chemical treatments of natural fibres on the properties of phosphate-bonded composite products

Abstract

Phosphate-bonded composites are an emerging class of building materials produced from natural fibres and phosphate based cement pastes. They are durable and possess mechanical properties similar to those of Portland cement. However, the moisture absorption of natural fibre can lead to swelling which may result in the reduction in the mechanical strength properties and eventually negatively affect the long-term performance and dimensional stability of the products. This study was aimed at the modification of some properties of selected biomaterial residues in order to enhance the final properties of the phosphate-bonded composite product. Three different treatments were evaluated viz. 1% caustic alkali, 1% acetic anhydride and hot water on natural fibres derived from slash pine, black wattle and bagasse. The effect of the treatment on the fibres was evaluated via HPLC, SEM and FTIR. Further, the performance of the treated fibres was evaluated in composite panels bonded with magnesium phosphate (MgPO4) and calcium phosphate (CaPO4) cement pastes against the controls. The manufactured panels were tested for flexural properties and dimensional stability. In the MgPO4-bonded panels, the MOR increased from 0.55 MPa for untreated bagasse panels to 0.79 MPa for alkalised panels. Similarly, the MOE increased from 150.04 MPa for untreated bagasse panels to 175.65 MPa for alkalised panels. In untreated MgPO4-bonded panels, the mean density was 0.76, 078 and 0.75 g/cm3, while in alkalised panels, the mean density was 0.81, 0.81 and 0.81 g/cm3 for wattle, pine and bagasse panels, respectively. 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 alkalised MgPO4-bonded panels. This represents a percentage improvement of 11, 23 and 33%, respectively. Alkali-treated fibres had the best effect overall for all measured properties.