The invention relates to a clay mineral for use as an agent for reducing the concentration of inorganic phosphate in liquids, in particular in bodily fluids or dialysis liquids, in order to treat hyperphosphatemia, in particular in a renal replacement therapy, wherein before use, the clay mineral is thermally treated at a temperature between 400° C. and 800° C., preferably between 500° C. and 700° C., in particular preferably at 550° C., over a time period of 60 min to 240 min, preferably 90 min to 180 min, in particular preferably 120 min.
