Cement composite boards comprising cellulose filaments (CF) and/or CF-containing pulp, are described. The composite boards have at least an improved modulus of rupture when compared with similar board that are free of CF. Methods for producing the CF and/or CF-containing pulp reinforced cement boards are also described. The CF cement composite board comprises: cellulose filaments (CF) and/or CF-containing pulp, and cement, wherein the CF has an aspect ratio of 200 to 5000, comprising a weight % of CF is from 1% to 20% by weight of the composite board. The composite boards herein described may have a modulus of rupture of more than 7 MPa.