EMPLOYEE WAS PERFORMING MAINTENANCE REPAIR WORK ON A LOWER DAMPER OF THE RAW MILL BY-PASS DUCT (EQUIP #423LD3). EMPLOYEE ACCIDENTALY STEPPED INTO A PILE OF HOT DUST MATERIAL, CAUSING BURN TO T OP OF LEFT FOOT. EMPLOYEE DID NOT THINK THIS MATERIAL WOULD BE HOT ENOUGH TO CAUSE INJURY BECAUSE THIS SYSTEM WAS SHUT DOWN SEVERAL DAYS BEFORE.