Cement hydration, effect calcium formate

The addition of finely ground calcium caibonate (calcite) to calcium aluminate cement results in the formation of calcium aluminocarbonate hydrate (C ACh j) as the product of hydration, instead of CAHg and CjAHg. At sufficiently high additions of the carbonate conversion of the cement may be avoided, and hence the strength loss associated with it may be prevented (Trivino, 1986). The long-term effectiveness of this measure is questionable, however. 

Calcium nitrite has been used as a corrosion inhibitor in concrete. It accelerates the hydration of cement. In Fig. 18, its acceleration effect on Type V cement is obvious from the larger amoimt of heat developed compared to that by the reference.It appears to be a better accelerator than calcium formate. 

The addition of fly ash to cement results in the formation of decreased amounts of calcium hydroxide in the hydration product. This is attributable to the dilution effect and to the consumption of calciiun hydroxide by the pozzolanic reaction with the fly ash. In Fig. 1, the amount of calcium hydroxide formed at different times of hydration in cement containing fly ash is given. The amount of Ca(OH)2 estimated by TG was found to be lower in samples containing fly ash. With the increase in the amount of fly ash, less calcium hydroxide was formed because of the pozzolanic reaction and dilution effect. Even at 60% fly ash, some lime was present in the mortar, and the pH was found to be 13.5. At this pH value, the passivity of steel is assured. It can also be observed that there is more lime at 60% fly ash than at 75% slag addition. 

Spmng and Rechenberg [132] state that in the pastes of cements rich in alkalis C3A dissolves quickly because of the reduced Ca(OH)2 solubility. At low gypsum content the quick set can be observed as a result of the hexagonal calcium aluminate hydrates formation In the presence of gypsum excess the similar effect can be the consequence of ettringite formation. 

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. 

Particulates in the form of different calcium aluminate hydrates, e g., CAHio, C2AH8, and CjAHg, or pre-hydrated CAC/CH (H-CAC/CH) have varying effects on ettringite formation and expansion of expansive cement pastes. 

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