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NitCalNitCal® is a raw material sold to concrete admixture companies and widely used in formulations of chloride-free set accelerating admixtures. Cement type dependency Examination of the set accelerating efficiency of NitCal in 10 different Portland cements revealed that there was no correlation between the set accelerating efficiency and C3A-content as initially assumed. However, a correlation was found with the belite, C2S, content. This correlation was also confirmed by a study of five other cement pastes at 5°C. The efficiency of NitCal as set accelerator decreases with increasing alkali content of the concrete fluid since both hydroxides and sulphates will precipitate active calcium from the solution. Mechanism A study was conducted to find the reason for the extreme difference between set accelerating efficiency of NitCal with different cement types. The study concludes that when adding NitCal to concrete the mechanism for setting is two-fold: An increased Calcium concentration leads to a faster super-saturation of the fluid with respect to Calcium Hydroxide, Ca(OH)2, which determines the onset of alite C3S, hydration. A lower sulphate concentration leads to slower/less formation of ettringite that shortens the onset of aluminate, C3A, hydration. The belite content vs set accelerating efficiency correlation is understandable since belite can incorporate a portion of the total alkalies in its structure and consequently prevent them from taking part in the early fluid chemistry since belite is a slow reacting mineral. NitCal is a most effective set accelerator for Portland cements with high belite content and and low alkali content. The reduced induction period and setting time can be utilised in several ways:
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