In one of our cement plant we are facing problem of high variability in strength at all ages and more particularly at 28 days.We would like to know how to investigate possible causes of strength variation of the PPC cement mortar cubes (5 Nos.) which are having same Blaine 365 m2/kg,same 25% fly ash (all flyash are having of the same source) and same 4% of gypsum. We have attempted to find out hydrated phases in all 5 mortar crushed cube samples by XRD and found same peak of intensity of Portlandite in X-Ray diffractogram.Please suggest how to investigate causes of variation in strength in the mortar cubes.