The present invention relates to a filler aggregate composition comprising filler particles of ground calcium carbonate (GCC); a pretreatment agent selected from polyvinylamine (PVAM) and cationic polyacrylamide (CPAM), or a mixture thereof; and a nanofibrillar cellulose (NFC); wherein said filler aggregate composition is in the form of a slurry, wherein said pretreatment agent has an average molecular weight below 6000000 g/mol and a has a charge density below 7 meq/g, determined at pH 7, and the nanofibrillar cellulose is present in an amount of about 0.1-40% of the dry weight of the filler particles. Further it also relates to its method of production, and its use in stocks, paper and paper board.