Abstract
Recently, there has been increased use of calcium-nitrite and calcium-nitrate as the main components of chloride- and alkali-free anti-freezing agents to promote concrete hydration in cold weather concreting. As the amount of nitrite/nitrate-based accelerators increases, the hydration of tricalcium aluminate (C3A phase) and tricalcium silicate (C3S phase) in cement is accelerated, thereby improving the early strength of cement and effectively preventing initial frost damage. Nitrite/nitrate-based accelerators are used in larger amounts than usual in low temperature areas below −10 °C. However, the correlation between the hydration process and strength development in concrete containing considerable nitrite/nitrate-based accelerators remains to be clearly identified. In this study, the hydrate composition (via X-ray diffraction and nuclear magnetic resonance), pore structures (via mercury intrusion porosimetry), and crystal form (via scanning electron microscopy) were determined, and investigations were performed to elucidate the effect of nitrite/nitrate-based accelerators on the initial strength development and hydrate formation of cement. Nitrite/nitrate-AFm (aluminate-ferret-monosulfate; AFm) was produced in addition to ettringite at the initial stage of hydration of cement by adding a nitrite/nitrate-based accelerator. The amount of the hydrates was attributed to an increase in the absolute amounts of NO2− and NO3− ions reacting with Al2O3 in the tricalcium aluminate (C3A phase). Further, by effectively filling the pores, it greatly contributed to the enhancement of the strength of the hardened cement product, and the degree of the contribution tended to increase with the amount of addition. On the other hand, in addition to the occurrence of cracks due to the release of a large amount of heat of hydration, the amount of expansion and contraction may increase, and it is considered necessary to adjust the amount used for each concrete work.
Highlights
In concrete construction when the outside temperature is below −10 ◦ C, the effect can be achieved by adding a large amount of anti-freezing agents [3,4,5,6,7,8]
This study focuses on the early age, clarifies the hydration reaction mechanism of cement when a large amount of a nitrite/nitrate-based accelerator is added, and examines the effect of NO2 − and NO3 − ions on strength development
The setting test was performed by the same method as that specified in JIS A 1147 [21], and the setting characteristics of the cement paste were grasped
Summary
It is known that hydrates, such as nitriteAFm (Ca4 Al2 [OH]12 [NO2 ]2 ·xH2 O) and nitrate-AFm (Ca4 Al2 [OH]12 [NO3 ]2 ·xH2 O), are formed through reactions between the aluminate in cement and anions in the nitrite/nitratebased accelerator [13,14,15] These are effective for developing the initial strength of concrete and reducing the initial frost damage. This study focuses on the early age (mixing ~24 h), clarifies the hydration reaction mechanism of cement when a large amount of a nitrite/nitrate-based accelerator is added, and examines the effect of NO2 − and NO3 − ions on strength development.
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