Non-chloride accelerators

One of the limitations to the wider use of calcium chloride in reinforced concrete is that, if present in larger amounts, it promotes corrosion of the reinforcement unless suitable precautions are taken. The use of calcium chloride is banned in many countries. There is, hence, a continuing attempt to find an alternative to calcium chloride, one equally effective and economical, but without its limitations. A number of organic and inorganic compoimds including aluminates, sulfates, formates, thiosulfates, nitrates, silicates, alkali hydroxides, carbonates, halides, nitrites, calcium salts of acetic acid, propionic acid, butyric acid, oxalic acid and lactic acid, urea, glyoxal, triethanolamine, and formaldehyde have been suggested. However, practical experience and research on these admixtures are limited. The effect of these compounds on the hydration of individual cement compoimds and cement has been widely studied by thermal analysis techniques. 

Triethanolamine of formulaN(C2H40H)3 (TEA for short) is an oily water-soluble liquid having a fishy odor and is produced by reacting ammonia with ethylene oxide. Normally it is used in combination with other chemicals in admixture formulations. 

DTA results of C3A + 25% gypsum mixtures hydrated with 0% and 1% TEA are shown in Fig. 11. The unhydrated mixture exhibits an endothermal doublet between 100 and 150°Cduetoastepwise dehydration of gypsum. Hydration seems to start from the first minute of contact of water with the sample. At 5 minutes, a larger amount of gypsum disappears from 

Isothermal conduction calorimetric curves of C3S hydrated with different amounts of TEA are shown in Fig. In the C3S sample 

The hydration of C2S with 0.5% TEA has been followed by DTA (Fig. Generally, endothermal peaks due to Ca(OH)2 decomposi- 

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The most widely used anodic inhibitors are calcium and sodium nitrite, sodium benzoate and sodium chromate. With the exception of calcium nitrite, no other chemical is available in North America as a proprietary product. Nitrites have been used in the USA for more than 14 years and for nearly 40 years in Europe. Calcium nitrite is marketed as a non-chloride accelerator, as well as a corrosion inhibitor. For 25-30% solids in solution, dosage rates range from 2 to 4% by weight of cement depending on the application [50]. Calcium nitrite has been used in bridges, parking and roof decks, marine and other prestressed concrete structures that are exposed to chloride attack. 

Chin, D. (1987). A Calcium Nitrite-Based Non-Corrosive, Non-Chloride Accelerator, ACI SP-102, 49-77. 

One of the more widely used non-chloride accelerator in North America, the sodium thiocyanate-based multicomponent cold-weather admixture (CWA), is reported to be effective at sub-freezing temperatures, specifically, at ambient temperatures as low as -7°C (20°F) [110]. Data from laboratory studies [110] indicate that the temperature for ice formation in synthetic pore... 

Fig. 7.39 Effect of cement content and a non-chloride accelerating admixture (NCAA) on initial time of set, at concrete and ambient temperatures of 10 C (Nmai [117]).

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 ). 

Exemplary non-chloride accelerators have been described. These are calcium nitrite and calcium formate (17). Calcium nitrite and calcium formate appear to provide additional sources of Ca and may thus reduce the time required to achieve supersaturation of the solution with respect to Ca(OH)2. Calcium nitrite can decompose into calcium oxide and a mixture of nitrogen dioxides according to following equation (17) ... 

Calcium formate, of formula Ca(HCOO)2, is a by-product in the manufacture of polyhydric alcohol, pentaerthritol. It is a powder and has a solubility of about 15% in water at room temperature. It is a non-chloride chemical that is used in practice. Many non-chloride accelerating admixture formulations contain formates. Calcium formate is an accelerator for the hydration of C3S at equal concentration, however, CaCl2 is more effective (Fig. 

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