Effects of flocculants and sizing agents on bending strength of fiber cement composites

Wood fiber is used to replace asbestos in the manufacture of fiber cement due to its high availability, low cost and good reinforcement properties. The different chemical composition of the cellulose fibers makes its compatibility with the cement much more complex than that of asbestos fibers. In the Hatschek process a suitable flocculant is needed when using cellulose fibers. The right selection of the flocculant is crucial due to its effect on mineral fines retention, dewatering and formation and, as a consequence, on the overall efficiency of the machine. This paper shows how anionic poly-acryl-amides (A-PAM), the most common flocculants used in Hatschek machines, have a negative effect on the bending strength properties of fiber cement sheets. In order to overcome this problem fiber surface treatment, with sizing agents, is proposed in this paper. Sizing with styrene-acrylate copolymers and alkyl ketene dimer produces an increase in bending strength properties.