Influence of Hardening Accelerating Admixtures on Properties of Cement with Ground Granulated Blast Furnace Slag

Ground granulated blast furnace slag (GGBFS) is a by-product of steel manufacturing process. As such it is commonly used in concrete technology as component of blended cements or addition for concrete. Utilization of GGBFS conforms to idea of sustainability. Unfortunately, slag may lead to delay of setting and hardening of concrete. It is undesirable effect for manufacturing of precast elements or executing of concrete works during lower temperature period. Attempt to acceleration of hydration process of such cements by hardening accelerating admixtures (ACC) was made. Paper shows results of compressive strength, hydration heat and initial setting time examinations. Portland cement (CEM I 52,5R) and cement with addition of 35% of GGBFS (which corresponds to normative range of CEM II/B-S) were tested. Tests have involved four accelerating admixtures with different chemical base (calcium formate, C-S-H crystal seeds, calcium nitrate and trietanolamine). Initial setting time was measured by Vicat apparatus. Compressive strength of mortars was measured after 12, 24, 48 hours and 7 and 28 days of curing in climatic chamber. Hydration heat tests were conducted using isothermal calorimeter TAM AIR III during 72 hours after cement contact with water. Tests were conducted in 20 °C. Water-binder ratio of mortars for compressive strength tests and cement pastes for hydration heat examinations was equal 0,5. Research had shown that all accelerators cause shortening of initial setting time by 40-50% depending on admixtures chemical base. The most efficient was accelerator based on C-S-H crystal seeds. Increase of hydration heat exhalation rate was observed. Summary hydration heat after 72 hours was greater in presence of hardening accelerating admixtures. Those admixtures caused also the increase of early compressive strength of standard mortars. Higher percentage growth of compressive strength was observed for cement with GGBFS addition. Calcium nitrate, calcium formate and C-S-H crystal seeds based admixtures were the most efficient for Portland cement mortars and the latter for cement with GGBFS addition.