Other Non-Chloride Accelerators

Cement Brand Admixture Setting Time (hr min) Initial Final  

The relative acceleratory effects of calcium chloride, calcium nitrate, and sodium thiocyanate on cement have been studied.The total heat is higher in the presence of all accelerators. Calcium chloride addition results in the maximum amount of heat. Sodium thiocyanate accelerates better than calcium nitrate. Calcium nitrate works well at early stages, but sodium thiocyanate is more effective at later ages. Sodium molybdate and trioxide also have an acceleratory effect. 

Generally, the a hydroxycarboxylic acids are regarded as retarders, but lactic acid behaves differently.In dosages of 0.1 to 0.4%, it reduces both the initial and final setting times of cement. The compressive strength at 28 days is higher than the reference by about 10-15%. 

In cements, incorporation of calcium carbonate is permitted in some countries. In Canada, the maximum limit is set at 5%. Calcium carbonate is not an inert filler. It is known to react with calcium aluminate. In a study of the hydration of tricalcium silicate in the presence of finely divided calcium carbonate, Ramachandran observed that the carbonate acted as an accelerator. Ushiyama, et al.,t examined the effect of carbonates of Na, K, Li, Cs, and bicarbonates of Na, K, and Li on the hydration of alite. Although small amounts retarded the hydration, larger amounts acted as accelerators. 

Alkalis, such as NaOH, and Na salts of carbonate, aluminate, and silicate are known to accelerate the hydration of cement and cause early stiffening. In the hydration of tricalcium silicate with NaOH, there was an acceleratory effect even up to 28 days. After 7 days however, the strength of the reference was higher than that containing NaOH.[ l Evidence was also obtained for the incorporation of Na in the hydrated product. 

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The best non-chloride admixture to date appears to be calcium nitrite marketed as a 20% solids solution, it can be readily added to the mix using the usual dispensers. The admixture has therefore been more readily accepted by the ready-mixed concrete industry than the formate-based admixture. Strength development effects produced in concrete are reported to be comparable to those obtained with calcium chloride [40], Table 7.12 shows a comparison of the results for concrete containing calcium chloride, calcium nitrite, and no admixture. In addition to the improvements in both compressive and tensile strengths, calcium nitrite is an effective inhibitor of chloride-induced corrosion. The material is now finding wide acceptance in bridge and parking deck repair. Other non-chloride accelerators used in Russia and special applications include the alkali-metal carbonates (Na CO, K CO, LiCO ). 

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