Formulations and methods for extruded fiber cement products

A method of manufacturing a fiber cement building article, the method comprising:

• forming the fiber cement building article from a fiber cement composite formulation using an extruder, said fiber cement composite formulation comprising:
• between 30% and 60% by dry weight of a cementitious binder having a particle size distribution between 1 μm and 150 μm, the cementitious binder comprising cement and at least 5% by dry weight of limestone and;
• a first quantity of silica having a particle size distribution between 1 μm and 250 μm;
• between 5% and 30% by dry weight of microparticulate cured fiber cement material having a particle size distribution between 1 μm and 600 μm, said microparticulate cured fiber cement material comprising a second quantity of silica, wherein a total amount of silica in the fiber cement composite formulation that includes the first and second quantities of silica comprises between 20% and 50% of the dry weight of the fiber cement composite formulation, and wherein a ratio of the cement to the total amount of silica in the fiber cement composite formulation is between 0.7 and 1.5;
• between 0.5% and 10% by dry weight of cellulose fibers;
• between 0.2% and 4% by dry weight of polypropylene fibers;
• a viscosity-enhancing admixture;
• an air-entraining admixture;
• and
• curing the fiber cement building article.

A fiber cement composite formulation comprising:

• between 30% and 60% by dry weight of a cementitious binder having a particle size distribution between 1 μm and 150 μm, the cementitious binder comprising cement and at least 5% by dry weight of limestone;
• between 20% and 50% by dry weight of silica, wherein a ratio of the cement to the silica in the fiber cement composite formulation is between 0.7 and 1.5;
• between 0.5% and 10% by dry weight of cellulose fibers;
• and
• between 0.2% and 4% by dry weight of polypropylene fibers.