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Accelerating

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. Calcium chloride (CaCl2) is a byproduct of the Solvay process for sodium carbonate manufacture.

CaCO3 + 2NaCI Na2CO3 + CaCI2
limestone brine solution

Calcium chloride is available in two forms. Regular flake calcium chloride (ASTM D 98 Type 1) contains a minimum of 77% CaCl2; concentrated flake, pellet, or granular calcium chloride (ASTM D 98 Type 2) contains a minimum of 94% CaCl2 (ACI Comm. 212 1963). A 29% solution of CaCl2 is the most frequent form of liquid product commercially available. In solid or liquid form, the product should meet the requirement for ASTM C 494, Type C and ASTM D 98 (Admixtures and ground slag 1990).

Calcium chloride has been used in concrete since 1885 (Rixom and Mailvaganam 1986) and finds application mainly in cold weather, when it allows the strength gain to approach that of concrete cured under normal curing temperatures (Rixom and Mailvaganam 1986). In normal conditions, calcium chloride is used to speed up the setting and hardening process for earlier finishing or mold turnaround.

Effects of calcium chloride on concrete properties are also widely studied and quantified. Aside from affecting setting time, calcium chloride has a minor effect on fresh concrete properties. It has been observed that addition of CaCl2 slightly increases the workability and reduces the water required to produce a given slump (Ramachandran 1984) and reduces bleeding. Initial and final setting times of concrete are significantly reduced by using calcium chloride. Effects of calcium chloride on initial and final setting of cement paste are shown in Figure 2.4 (Ramachandran 1984). The total effect of adding calcium chloride depends on dosage, type of cement used, and temperature of the mix.

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 (ACI Comm. 212 1963). Long-term strength is usually unaffected and is sometimes reduced, especially at high temperatures (Admixtures and ground slag 1990).

There is evidence that drying shrinkage of mortar or concrete is increased by using calcium chloride, especially at early ages. The large shrinkage at earlier periods may be attributed mainly to more hydration. Some work has shown that it is possible to reduce drying shrinkage by the addition of sodium sulfate (Ramachandran 1984). At early ages concrete with 2% CaCl2 shows a higher resistance to freezing and thawing than that without the accelerator, but this resistance is decreased with time. It has been found, however, that addition of CaCl2 up to 2% does not decrease the effectiveness of air entrainment (Ramachandran 1984).

Because of its corrosion potential, calcium chloride—especially in prestressed concrete—has been strictly limited in use. ACI Committee 222 (1988) has determined that total chloride ions should not exceed 0.08% by mass of cement in prestressed concrete. British Standard CP.110 strongly recommends that calcium chloride should never be added to concrete containing embedded metals.

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—including sulfates, formates, nitrates, and triethanolamine—have 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 (Rixom and Ramachandran 1986).

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.

Calcium nitrite accelerates the hydration of cement, as shown by the larger amounts of heat developed in its presence. Calcium nitrite and calcium thiosulfate usually increase the strength development of concrete at early ages (Ramachandran 1984).

Recommendations

  1. Verification tests should be performed on liquid admixtures to confirm that the material is the same as that which was approved. The identifying tests include chloride and solids content, ph and infrared spectrometry.
  2. Calcium chloride should not be used where reinforcing steel is present.
  3. Calcium chloride should not be used in hot weather conditions, prestressed concrete or steam cured concrete.
  4. In applications using calcium chloride, the dosage rate should be limited to 2 percent by weight of cement.
  5. Care must be taken in selecting non-calcium chloride accelerators since some may be soluble salts which can also aggravate corrision.

References

Sections of this document were obtained from the Synthesis of Current and Projected Concrete Highway Technology, David Whiting, . . . et al, SHRP-C-345, Strategic Highway Research Program, National Research Council.

ACI Committee 212. 1963. Admixtures for concrete. ACI Journal Proceedings 60 (11):1481-524.

ACI Committee 222. 1988. Corrosion of metal in concrete. ACI manual of concrete practice. Part 1. ACI 222R-85. Detroit: American Concrete Institute.

Admixtures and ground slag for concrete. 1990. Transportation research circular no. 365 (December). Washington: Transportation Research Board, National Research Council

Gebler, S. 1983. Evaluation of calcium formate and sodium formate as accelerating admixtures for portland cement concrete. ACI Journal 80 (5):439-44.

Ramachandran, V. S. 1976. Calcium chloride in concrete. Science and technology. Essex, England: Applied Science Publishers.

Ramachandran, V. S. 1984. Accelerators. In Concrete admixtures handbook: Properties, science, and technology, ed. V. S. Ramachandran. Park Ridge, N.J.: Noyes Publications.

Rixom, M. R., and N. P. Mailvaganam. 1986. Chemical admixtures for concrete. Cambridge, England: The University Press.


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