Performance of Cement Paste with Denitrified Fly Ash Containing NH₄HSO₄

Area of Science:

• Materials Science
• Civil Engineering
• Environmental Science

Background:

• Denitrification in coal-fired power plants produces fly ash containing ammonium bisulfate (NH₄HSO₄).
• Application of this fly ash in cement and concrete can lead to undesirable properties like retarded setting, reduced compressive strength, and volume expansion.
• Understanding the impact of NH₄HSO₄ on cement hydration is crucial for managing its use.

Purpose of the Study:

• To investigate the influence of NH₄HSO₄-containing fly ash on the properties of fly ash cement paste.
• To compare the effects of NH₄HSO₄ with NaHSO₄ in cement hydration.
• To determine the relationship between NH₄HSO₄ content and the performance of fly ash cement paste.

Main Methods:

• Preparation of cement pastes with varying contents of fly ash containing NH₄HSO₄.
• Use of pastes with NaHSO₄ as control samples.
• Evaluation of cement paste properties including fluidity, setting time, compressive strength, drying shrinkage, hydration heat, and calcium hydroxide content.

Main Results:

• NH₄HSO₄ significantly reduces cement paste fluidity and prolongs setting time.
• Compressive strength decreases, while drying shrinkage increases with higher NH₄HSO₄ content.
• Increased porosity and larger pore sizes were observed in cement paste with higher NH₄HSO₄ content.
• Total heat released during hydration and calcium hydroxide content were also affected.

Conclusions:

• Ammonium bisulfate in fly ash adversely affects key properties of cement paste.
• Higher concentrations of NH₄HSO₄ exacerbate negative impacts on fluidity, setting time, strength, and durability.
• The findings highlight the need for careful consideration when using denitrified fly ash in cement applications.