Acid-base cements (ABCs) are formed by reaction of a mild acid with a suitable base. The most common examples of inorganic acid-base cements are phosphate cements, oxy-chloride and oxy-sulfate cements. Among these, phosphate cements have been researched the most and several products have been developed that range from dental cements, ceramics for stabilization of radioactive and hazardous waste streams, specialty cements for oil field applications, resins for paints and coatings and adhesives, road patching materials and many more. Among these, few have been commercialized.
There have been attempts to use inorganic cementatious binders for producing natural fiber composites. For example, use of thin pastes of Portland cement or calcium aluminate cement has been reported as an adhesive. However, these cementatious binders are extremely alkaline, and the paste prepared from these materials is very thick and cannot effectively impregnate the cells and capillaries of the cellulosic structure of the wood/fiber and likely cure prior to impregnating the fiber. As a result, the loading of these cementatious binders in the fibers is very low. Therefore, fiber content essentially improves the properties of the cement, rather than the cement being used as an adhesive to load large amount of fiber.
Other previously reported inorganic composites comprise the reaction product of phosphoric acid and a silicate or aluminate of calcium, magnesium, or zirconium and are used with various fillers to produce a gypsum-board-like core that is wrapped in recycled paper. This method of producing wall boards and other similar products has several drawbacks. Namely, the use of phosphoric acid solution at very low pH leads to corrosion of equipment and handling problems. This is especially true when the products are produced in large scale. This approach also uses sparsely soluble, powdered precursor components. Calcium silicate or other silicates or aluminates, being slightly soluble at ambient temperature do not completely react, resulting in most of the silicate remaining un-reacted in the composite product as particulate matter. Thus, the wall board produced by this previously reported method is essentially a composite of cellulose, cement, and variable amounts of un-reacted precursor powder. This incomplete reaction produces composite products with low performance attributes than is expected. While the previously reported method may be adequate for production of gypsum-like boards that exhibits low mechanical performance, it is not suitable for production of high quality and versatile fiber reinforced composites.
Moreover, the inorganic acid component of the previously reported methods is limited to phosphoric acid as the only effective acid-phosphate for the reaction employing a silicate precursor. It is generally known that it is difficult to react acid phosphates (such as sodium and potassium dihydrogen phosphate, NaH2PO4 and KH2PO4 respectively) and also alkaline phosphates (such as dipotassium phosphate, K2HPO4), with silicates and aluminates at room temperature, and it is difficult or impossible to work with alkaline phosphates for producing binders. Thus, it is unlikely that previously reported processes will work with any acid phosphate other than phosphoric acid. The above previously disclosed processes mention products (mainly wall boards) with silicates and aluminates, however, they do not mention such products having high loadings of cellulose.
Likewise, it has been reported using Ceramicrete technology for producing cellulosic composites, in which calcined magnesium oxide is reacted with mono potassium phosphate to produce a matrix, in which cellulose forms the reinforcing material. Thus, using Ceramicrete technology for producing cellulosic composites is similar to previously disclosed methods above, where the cementatious material is reinforced by cellulose fibers, and as a result, the loading of the fibers is low.
Methods of mixing solid calcined oxide or calcined minerals (e.g., calcium silicate) with inorganic acidic component precursor solution together with fiber in one step is not efficient, and presents technical and manufacturing problems of providing reproducible pulp affecting reproducibility of article formation. It is believed that because of the rapid acid-base reaction between the inorganic acidic component and the solid or calcined alkaline oxide component, there is insufficient time for the fibers to mix and to wet out.