Accelerating admixtures are added to concrete either to increase the rate of early strength development or to shorten the time of setting, or both. Chemical compositions of accelerators include some of inorganic compounds such as soluble chlorides, carbonates, silicates, fluosilicates, and some organic compounds such as triethanolamine.
Among all these accelerating materials, calcium chloride is the most common accelerator used in concrete. Most of the available literature treats calcium chloride as the main accelerator and briefly discusses the other types of accelerators. However, growing interest in using "chloride-free" accelerators as replacement for calcium chloride has been observed. This is because calcium chloride in reinforced concrete can promote corrosion activity of steel reinforcement, especially in moist environments. However, the use of good practices, i.e. proper proportioning, proper consolidation, and adequate cover thickness can significantly reduce or eliminate problems related to corrosion.
Calcium chloride has been used in concrete since 1885 and finds application mainly in cold weather, when it allows the strength gain to approach that of concrete cured under normal curing temperatures . In normal conditions, calcium chloride is used to speed up the setting and hardening process for earlier finishing or mold turnaround.
Compressive and flexural strengths of concrete are substantially improved at early ages by using calcium chloride. Laboratory tests have indicated that most increases in compressive strength of concrete resulting from the use of 2% of calcium chloride by weight of cement range from 400 to 1,000 psi (2.8 to 6.9 MPa) at 1 through 7 days, for 70 F (21 C) curing . Long-term strength is usually unaffected and is sometimes reduced, especially at high temperatures .
Nonchloride Accelerators Although calcium chloride is an effective and economical accelerator, its corrosion-related problem limited its use and forced engineers to look for other options, mainly nonchloride accelerating admixtures. A number of compounds ncluding sulfates, formates, nitrates, and triethanolamine ave been investigated. These materials have been researched and successfully used in concrete. Triethanolamine (N(C2H4OH)3) is an oily, water-soluble liquid with a fishy odor and is produced by the reaction between ammonia and ethylene oxide. It is normally used as a component in other admixture formulations and rarely, if ever, as a sole ingredient .
Calcium formate is another type of nonchloride accelerator used to accelerate the setting time of concrete. At equal concentration, calcium formate (Ca[OOOCH] 2) is less effective in accelerating the hydration of C3S than calcium chloride and a higher dosage is required to impart the same level of acceleration as that imparted by CaCl2 (Ramachandran 1984). An evaluation study of calcium formate as an accelerating admixture conducted by Gebler (1983) indicated that the composition of cement, in particular gypsum (SO3) content, had a major influence on the compressive strength development of concretes containing calcium formate. Results showed that the ratio of C3A to SO3 should be greater than 4 for calcium formate to be an effective accelerating admixture; and that the optimum amount of calcium formate to accelerate the concrete compressive strength appeared to be 2-3% by weight of cement (Gebler 1983). Calcium nitrate and calcium thiosulfate are also considered accelerators.
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