Assessing the Setting Times and Strength Development of Concrete containing Pozzolanic Materials

Setting time is considered as an identification boundary of fresh and hardened of concrete. Initial set refers to a change from a fluid to a rigid state, accompanied by a rapid temperature rise in the body of concrete structures. The early hydration of C3A and C3S in presence of gypsum plays an important role in concrete rheology, and setting points of concrete. The final set, on the other hand, express the start of strength development. Knowledge of the initial setting time will help not only in determining the time available for the transportation, placement, and consolidation of concrete; but also in identifying the effectiveness of various admixtures and pozzolanic materials. The use of different types of pozzolanic materials has allowed to reduce the carbon dioxide emission per ton of produced cement. In the present work, the effects of silica fume, fly ash and slag on the setting times and strength development of different concrete mixtures are examined. The setting times were conducted accordance to ASTM C403. The Concrete specimens were prepared with three water cement ratios of 0.35, 0.45, and 0.55. Silica fume at 7%, fly ash at 15% and 25% and slag at 25% and 35%, by weight of cement, were used as replacement for cement. The results showed that the setting times of the concrete increased as water-cement ratio increased and the lowest setting times corresponded to the specimens with 0.35 water-cement ratio. The setting time of the specimens with water cement ratios of 0.45 and 0.55 increased by 10% and 18%, respectively, compared to that for 0.35. Using silica fume, fly ash and slag, as replacement for cement, retarded the setting times of the mixtures. The more replacement contents led to increase of setting times. The setting time of the fly ash specimens at 15% and 25% increased by 6% and 12%, whereas addition of slag at 25% and 35% retarded the setting time by 11% and 18%, respectively, compared to those for reference specimens with 0.45 water-cement ratio. Silica fume caused a slight increase of setting time by 5.5% compared to those without pozzolan materials. These may attributed to the low cement content in the pozzolanic specimens which resulted to the low friction surface between cement particles and on the other hand enhancement of effectiveness of the super-plasticizer in the mixtures which led to the increase of setting times. The results also indicated that silica fume, as a very fine supplementary cementitious material, enhanced both the early and later age strength. Fly ash and slag, as replacement for cement, increased the later stage strength of the specimens, but had a negative impact on the early- age strength. This may attributed to the low pozzolanic activity of these cementing materials in the early age, however in the later ages which calcium hydroxide content increased in the mixtures the secondary induced gel enhanced the compressive strength. A power relationship was proposed for the early age strength and the final setting time relationships.