Additives for preparing rheology-modified aqueous fluids

A rheology modified aqueous fluid composition comprising clay, water, magnesium oxide, aluminum oxide hydroxide, sodium carbonate or potassium carbonate, and calcium oxide or calcium hydroxide. The magnesium oxide and aluminum oxide hydroxide are prepared by a specific thermal treatment protocol involving heating of appropriate precursors within a specified temperature range. The pH of the composition is at least about 10. The composition is prepared by admixing the components and, optionally, sodium oxide or sodium hydroxide for pH adjustment as necessary. A substantially dry rheology modification agent comprising the enumerated components, not including the water, is also included. The resulting composition preferably exhibits shear-thinning capability including very rapid thinning upon application of shear forces and also rapid viscosification upon removal of such forces.