Influences of coal fly ash containing ammonium salts on properties of cement paste

This paper aims to investigate the influence of coal fly ash containing ammonium salts on properties of cement paste. Fly ash was incorporated at percentage of 20% by weight of the total binder to replace Portland cement. Ammonium hydrogen sulfate (NH₃HSO₄) or ammonium sulfate ((NH₃)₂SO₄) were introduced at percentages of 3.0%–6.0% or 1.5%–3.0% by fly ash weight. Compressive strength, setting time and hydration heat were evaluated on variable blend mixtures. Adsorption behaviors of polycarboxylate-based superplasticizer and air entraining agent on fly ash particles were also evaluated using total organic carbon (TOC) method. Semi-adiabatic calorimetry, X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetry-differential thermal analysis, mercury intrusion porosity and scanning electron microscope measurements were carried out on typical samples. Experimental results showed that the chemical admixtures adsorbed by coal fly ash were increased by the introduction of NH₃HSO₄ or (NH₃)₂SO₄. The addition of 3.0%–6.0% NH₃HSO₄ and 1.5%–3.0% (NH₃)₂SO₄ decreased the 28d compressive strength of fly ash-cement pastes by 4.3%–10.4% and 6.3%–8.9%, respectively. The initial and final setting times were delayed and the early age hydration of Portland cement was also retarded. Moreover, the pore structure was coarsened and porosity was increased for the hardened cement specimens due to the release of ammonia and lower hydration degree. Therefore, more attention should be paid to the application of denitration fly ash to the cement and concrete industry.