Polyamide fibers, microsilica and Portland cement composites and method for production

A plastic cementitious composition which cures to a hardened cementitous composite which consists essentially of in admixture:

• (a) Portland cement;
• (b) superplasticizer in a ratio by weight of between about 0.02 and 0.20 of the superplasticizer to the Portland cement, wherein the superplasticizer reduces the amount of water necessary to provide the plastic cementitious composition;
• (c) discontinuous polyamide fibers having a diameter between about 8 and 15 microns and a length between about 6 and 65 mm in a ratio by volume of between about 0.0005 and 0.04 of the fibers to the cementitious composition, wherein the fibers have been dispersed in, and thinly coated with, microsilica particles and part of the superplasticizer from part (b) in water prior to being provided in the admixture and wherein the particles of the microsilica have a size of between about 0.05 and 1 micron;
• (d) water in a ratio by weight of between about 0.25 and 0.7 of the water to the Portland cement;
• (e) fly ash substituting for up to 45% by weight of Portland cement;
• and
• (f) aggregates with maximum sizes up to 25 mm filling up to 75% of the volume of the composite, wherein the plastic cementitious composition consists essentially of amounts of the water, the aggregates, the fibers, the superplasticizer, the cement and the fly ash so as to be castable, wherein the microsilica regards alkaline deterioration of the polyamide fibers over time in the rigid hardened cementitious composite and wherein the rigid hardened cementitious composite exhibits greater flexural loading and tensile strength than a similar composite with the fibers and without the microsilica.

In a method of preparing a plastic cementitious composition which cures to a hardened cementitious composite incorporating in a mixture of Portland cement, a superplasticizer, discontinuous polyamide fibers, water and up to 75% of the composite volume of aggregates and wherein fly ash may be substituted for part of the Portland cement, wherein the superplasticizer reduces the amount of water necessary to produce the improvement which consists essentially of:

• (a) thinly coating the fibers in a part of the water and in part of the superplasticizer with microsilica, and a specific fraction of the aggregates, needed for fiber dispersion so that the fibers are uniformly dispersed and have a thin coating of the microsilica, the water and the superplasticizer;
• and
• (b) mixing the microsilica coated and uniformly dispersed fibers in the remainder of the mixture, wherein the plastic cementitious composition consists essentially of amounts of the water, the aggregates, the fibers, the superplasticizer, the cement and the fly ash so as to be castable in a mold wherein the microsilica retards alkaline deterioration of the polyamide fibers over time in the rigid hardened cementitious composite and wherein the rigid hardened cementitious composite with the microsilica exhibits greater flexural loading and tensile strength than a similar composite with the fibers and without the microsilica.