A modified form of ASTM C 1260 was used for the experiment. The calcined clay pozzolan was used to replace between 0% and 30% of the PLC for the test. Total cement to aggregate ratio of 1:3 was used in forming mortar bars of size 25 mm × 25 mm × 285 mm using water to cement ratio of 0.5 instead of the specified 0.47 to cater for workability of the pozzolan cement mortar. Three mortar bars were moulded for each specimen of mortar mix. Mortar bars were initially stored in a moist cabinet for 24 h as specified in ASTM C 1260. However, after initial length readings were taken, the specimens were further cured for 27 days in water at 20˚C to enable the pozzolanic reaction proceed to an appreciable rate. The bars were further cured in water at 80˚C for 24 h after which zero readings of the bars were measured using a length comparator recommended in ASTM C 490 [25] . After taking the initial lengths, the bars were placed in 1N NaOH solution at 80˚C and subsequent lengths measurements were taken at 14, 42, 56 and 84 days. After 84 days, samples of some bars were treated with acetone to stop hydration and ground for XRD analysis using a PHILLIPS PW1830 diffractometer. The morphology of the hydrated samples was also studied with an Olympus BX 41 TF optical microscope.