Sorption of inorganic ions on semi-crystalline materials
SECTION I - Thermal cycling to quite modest temperatures, i.e., 80-100°C, affects markedly the future performance of Portland cement concretes. It is shown that ambient (or near ambient) curing temperatures result in pore fluid low in sulfate, while a brief thermal pulse during the early cure history of the cement paste results in increased sulfate solubilities. The trends observed in the pore fluids of four representative clinkers which have been thermally cycled are attributed to the redistribution with temperature of the sulfate phases, principally ettringite, 3CaO.A12O3.3CaSO4.32H2O. Reformation of ettringite during the post-cooling period in thermally cycled pastes may lead to expansion. This 'potential' for expansion is quantified by calculation of molar volume changes with temperature for four representative clinkers. Similar expansion behaviour is predicted for all clinkers in accord with the similarity in bulk chemical composition. The mechanisms by which 'delayed' ettringite causes expansion are reviewed and suggestions for further work are proposed. SECTION II - An attempt is made to improve the activity of silica sand used in water purification by coating it with a high surface area oxide/hydroxide or iron , applied onto the sand grains using an aluminosilicate gel. The iron-containing bulk gel is characterised in terms of its gelling time and surface area. An alternative formulation based on prolonged gelling time is developed. Coatings on the sand grains of the two formulations are investigated by electron probe micro analysis (EPMA). The ion exchange properties of the gel formulations are evaluated using Mn2+, Fe3+, As5+ and As3+. Very efficient cation uptake properties are exhibited but the capacity of the system for anion uptake is comparatively low. Cation uptake mechanisms are reviewed and removal of Mn2+ and Fe3+ through precipitation of insoluble metal oxides/hydroxides, furnished by high local pH, is discussed. Research work with iron-containing aluminosilicate gel coatings is extended by investigating anion sorption, principally of arsenic species, As(V) and As(III), by crystalline oxides of iron; well-crystalline goethite and poorly-crystalline ferrihydrite have been synthesised and their sorption for arsenic as a function of time and solution pH is reported. Study of the sorptive mechanism points to a two-stage process for anions: an initial rapid step, believed to correspond to surface sorption, followed by a slower step believed due to time taken for migration of sorption species to sites of decreasing accessibility. Conditions for optimisation of capacity and rates of sorption do not however, always correspond to the normal pH of drinking water. Suggestions are made for future research.