Abstract:
Nano-structured calcium silicate consists of randomly stacked nano-sized platelets that make up an open framework structure of macropores that resembles a house of cards. This structure affords the material the desirable physical properties of a large pore volume and a highly accessible surface area that exceed many other silicas and silicates. The material is possibly related to other disordered calcium silicate hydrates at an atomic level, although it is the
macro-structure and the potential of performing chemistry upon its surface that is of great interest. Due to the novelty of nano-structured calcium silicate, little was known about it
before this work. The focus of this study has therefore been placed upon characterising the material and determining the conditions that allow the pore volume and surface area to be maximised. The material is prepared through an
initial precipitation from the reaction of a calcium salt with monomeric silica, followed subsequently by self-ordering on both an atomic-scale and on a
macro-scale to develop the porous framework. The framework of the material has been found to collapse due to forces created from surface tension during the removal of water from the pores upon drying. The result of this collapse is a
substantial reduction in both the surface area and pore volume of the material. Three different methods have been developed to maintain the structure with each modification producing a material that is suitable for different applications. A reinforcing process following the development of the open framework whereby additional silica is polymerised upon the structure strengthens the material so that the forces resulting from the removal pore water are unable to cause collapse of the framework. This material is therefore
able to be repeatedly re-wet and dried without any detrimental effect to the pore volume or surface area of the material. The replacement of water within the pores with 2-ethoxyethanol, that has a low surface tension, and by modifying the material through treatment with acid have also been found to prevent collapse of the structure. Through the knowledge gained of the
development of the nano-structured calcium silicate and of the reaction conditions required for the optimisation of the surface area and pore volume, a semi-continuous process has been devised that has allowed for production of the material on a larger scale. This work also contains details on the formation of nano-structured calcium silicate by using geothermal water from an electricity generation plant as the source of monomeric silica rather than using sodium silicate. Currently, the formation of a scale from supersaturated geothermal water is problematic for the industries that use the fluid and limits the use of the resource. The removal of monomeric silica from geothermal water as a result of producing nanostructured calcium silicate prevents the formation of the scale and therefore allows a greater proportion of the thermal energy in the fluid to be potentially utilised.
