The research examined herein classifies initial and final set times, for samples of ternary mortars composed of CEM 1 (52.5) and Ground Granulated Blast Furnace Slag (GGBS). The samples tested comprised of different CEM 1 and GGBS proportions. The mixes used, ranged from 100% CEM1 where this component was replaced at 10% increments by mass terminating at a 20% CEM1 content. With a reduction in cement content, the balance of the total required binder was being made up with GGBS. The ternary mortar cubes were tested for initial and final set times at average ambient room temperature (19.7 – 22.2 °C) and temperatures of 5 and 40 degrees Celsius. The findings highlighted the additional time required for initial and final set times at reduced temperatures and it also highlighted the further additional time for initial and final set times when GGBS is used as a cement replacement in progressively increasing quantities. Initial and final set times at 40 degrees Celsius were faster than ambient and 5 degrees Celsius, however the difference between initial and final set times was much reduced at 40 degrees Celsius. Both temperature and cement replacement affected the compressive strength at a curing period of 28 days, however GGBS is known to take longer than CEM1 to achieve a given strength development of say 90% of the final or ultimate strength. The two factors of temperature and cement replacement have a significant impact on setting times.