BACKGROUNDPaddy husk ash (PHA) is a commonly available biowaste rich in silica which when coated with oligomeric aluminium hydroxyl species becomes a good adsorbent for the treatment of fluoride-bearing water. This is due to the high affinity of Al3+ ions towards F- ion and the good coating of cationic oligomers over the silica surface. To control the serious environmental problem of post-adsorption disposal of the sludge produced in the defluoridation process, studies were made to determine whether they can be further used as a raw material or additive for cement preparation. RESULTSThe surface modified PHA was used as a good adsorbent for fluoride and the resulting sludge was used as a cement clinker additive. The clinker formed was found to have the required cementitious phases. Also, on addition of the sludge the clinker formation temperature was lowered from conventional 1450 to 1350 degrees C, as the presence of Al-F coating over PHA worked as mineraliser. It was observed that the fluoride ion was immobilized in the cement matrix in a form that was difficult to leach out. CONCLUSIONOligomeric aluminium activated PHA acts as a good defluoridising adsorbent which when used for cement clinker formation immobilises the fluoride ion and reduces the clinkerization temperature. (c) 2017 Society of Chemical Industry