A mixer for mixing glass fibres into a relatively dilute aqueous cement slurry, e.g. for use in forming glass fibre reinforced cement products on an asbestoscement making machine of the Hatschek or Bell type, comprises an annular chamber (10) with a tangential slurry inlet (14), a helical rising floor (21), a coaxial conical outlet (15) disposed to receive slurry flowing over the inner wall (12) which is lower than the outer wall (11), and a fibre inlet (16) above the conical outlet (15) so that the fibres are mixed with the slurry as it passes down the wall of the conical outlet (15) in a vortex motion.