Sustainably Produced Nano-structured Calcium Silicate (CaSil) - A Versatile Material and its Industrial and Environmental Remediation Applications

J.H. Johnston
Victoria University of Wellington,
New Zealand

Keywords: nanostructured calcium silicate, calcium silicate, geothermal, environmental remediation, fertilizer, building, paper, mining, rubber, plastic, paint

Summary:

Nano-structured calcium silicate is a remarkable inorganic material. The simple chemically stoichiometric form of calcium silicate, CaSiO3, belongs to the inosilicate group, crystallizes in the triclinic system and occurs naturally as the mineral wollastonite. Our proprietary nano-structured calcium silicate (CaSil) material [1] can be represented by the general formula CaSiO3-x(OH)2x.yH2O, where the Ca:Si mol ratio can vary from about 0.3 to about 2, and the OH content varies accordingly to maintain charge balance - a typical example of the CaSil composition is where x≈2. The CaSil material comprises non-planar platelets a few hundred nanometres in size by a few nanometres thick, which are stacked together in a unique open framework structure forming discrete particles of about 1-5 microns in size. The structure is considered to comprise SiO44- silicate tetrahedra linked together to form inosilicate chains, where the apical oxygens are bound to Ca2+ ions on the surface of the nano-thick platelets, which then either bond to adjacent chains or to further OH groups or H2O molecules. This provides short range order in the structure as shown by broad peaks in the CaSil X-ray diffraction pattern, occurring at 29.5, 32 and 49.6 o2θ (Cu Kα). This nano-structure provides the CaSil material with a high surface area of up to about 400 m2 g-1 and a liquid absorbency of up to 400 g oil g-1. The nano-structured calcium silicate material can be prepared by the reaction of dissolved silicate species with Ca2+ under specific reactant concentrations and pH conditions, wherein the chemical composition, stoichiometry and the structure and properties of the CaSil can be controlled tightly by the synthesis methodology. The dissolved silicate species, typically H3SiO4- can be from geothermal water or sodium silicate or similar solution, the former affording a sustainable and environmentally attractive method. The synthesis conditions, composition and structure, including the OH groups and Ca2+ entities on the surface of the platelets, provide the opportunity to control the properties of the CaSil material and tailor its uses for a number of applications in the fertilizer, building, paper, paint, polymer, absorbent, mining and environmental remediation industries. Applications include: The recovery and capture of environmentally problematic phosphate and other species from surface waters and effluent streams. A “Green phosphate fertilizer” which recycles phosphate and other nutrients. Lightweight CaSil-cement-based building products with inherent thermal and sound insulation, and fire-retardant properties. As an adsorbent to capture base and heavy metals from mining and industrial waste streams. As an ink absorbent filler in the paper industry to enhance the print and optical properties of paper. As a functional filler in rubber, plastics and paint. We have a pilot plant operating and are currently producing CaSil from a New Zealand geothermal water source that is supersaturated in dissolved silica, for use in the above applications. This paper presents an overview of this versatile nano-structured calcium silicate material, its properties, sustainable production from geothermal water and industrial and environmental remediation applications. New business opportunities are discussed.