Page 162 - Handbook of Thermal Analysis of Construction Materials

Section 2.0 - Accelerators 145

2.0 ACCELERATORS

An accelerating admixture is a material that is added to concrete
for reducing the time of setting and accelerating early strength develop-
ment. Accelerating admixtures are used in cold water concreting operations
and are components of antifreezing admixtures and shotcreting mixes. In
cold weather concreting there are other alternatives such as the use of Type
III cement, use of higher than normal amount of portland cement Type I or
warming of the concrete ingredients. Of the above, the most economical
method is the use of Type I cement in conjunction with an accelerator. The
advantages include efficient start and finishing operations, reducing the
period of curing and protection, earlier removal of forms so that the
construction is ready for early service, plugging of leaks and quick setting
when used in shotcreting operations.
Many substances are known to act as accelerators for concrete.
These include soluble inorganic chlorides, bromides, fluorides, carbonates,
thiocyanates, nitrites, nitrates, thiosulfates, silicates, aluminates, alkali
hydroxides, and soluble organic compounds such as triethanolamine,
calcium formate, calcium acetate, calcium propionate, and calcium bu-
tyrate. Some of them are used in combination with water reducers. Quick
setting admixtures used in shotcrete applications and which promote setting
in a few minutes may contain sodium silicate, sodium aluminate, aluminum
chloride, sodium fluoride, strong alkalis, and calcium chloride. Others are
solid admixtures such as calcium aluminate, seeds of finely divided
portland cement, silicate minerals, finely divided magnesium carbonate,
and calcium carbonate. Of these, calcium chloride has been the most widely
used because of its ready availability, low cost, predictable performance
characteristics, and successful application over several decades. [1][2] In
some countries the use of calcium chloride is prohibited, in some others,
such as Canada and the USA, the use of calcium chloride is permitted
provided certain precautions are taken. Attempts have continued to find an
effective alternative to calcium chloride because of some of the problems
associated with its use.