Set accelerator

Introduction
NITCAL^® is a raw material sold to concrete admixture companies and widely used in formulations of chloride-free set accelerating admixtures.

The use of NITCAL^® is steadily increasing on a global basis and it has been used in many significant construction projects.

To our knowledge, typically 0.25 % - 1.0 % NITCAL^®(of cement weight) is added when concrete admixture companies are making different set accelerators. It is recommended to use not more than 2% NITCAL^® (of cement weight).

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, rather surprisingly 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:

• Winter casting (to avoid cooling of fresh concrete prior to onset of hardening)
• Production of prefabricated elements (faster re-use of moulds)
• Decreased risk of cracks due to settlements or plastic shrinkage
• Reduced time before concrete decks may be surface smoothed
• Improved early strength

Activate retarded concrete at/near site after long transport or transport in hot climate

A concrete admixture based on NITCAL^®was used to secure the slip forming rate during the construction of the shafts for the Troll Gravity Base Structure (height 369 m) in the North Sea, since calcium nitrate counteracted the retarding effect of the plasticizers used in this high performance concrete.