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A method of forming a high strength cement in a permeable starting material, the method comprising the step of combining the starting material with effective amounts of (i) a urease producing micro-organism;(ii) urea; and (iii) calcium ions and wherein the effective amount of the urease producing organism provides a urea hydrolysis rate, under standard conditions, of 0.5-50 mM urea hydrolysed. min-1.