Carbonation of High-Ca Fly Ashes under Flue Gas Conditions: Implications for Their Valorization in the Construction Industry

Kwon Rausis,Agnieszka Ćwik,Katarzyna Zarębska,Ignasi Casanova

doi:10.3390/cryst11111314

Kwon Rausis, Agnieszka Ćwik + Show 2 more

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https://doi.org/10.3390/cryst11111314

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Abstract

The utilization of high-calcium fly ashes (HCFA) from coal-fired power plants in the construction industry is problematic, since their high free lime contents can lead to durability problems. In this research, the carbonation of a high-CaO fly ash has been carried out using simulated flue gas and concentrated CO2, with the aim to assess the valorization potential of such materials in the construction industry. The results show that, at 7 bars total pressure, an up to 36% carbonation efficiency can be achieved in just 30 min when pure CO2 is used; a comparable result with flue gas requires about 4 h of reaction. On the other hand, experiments carried out at atmospheric pressure show significantly different carbonation efficiencies depending on the CO2 concentration of the gas used. All experiments resulted in a substantial reduction in the original free lime content, and after reaction times of 4 h (at atmospheric pressure) and pressures of 7 bars (for any reaction time >30 min), the final free lime values were low enough to comply with the requirements of European Standards for their utilization as additions in cement.

Highlights

The utilization of Coal Combustion Products (CCPs) in Europe is a subject of the requirements for different standards imposed by the national authorities
X-ray fluorescence spectroscopy (XRF) analyses were performed on a UniQuant machine from Thermo Fisher ScientificTM using the fusion bead method
Portlandite XRD peaks [38] were identified in samples carbonated for 0.5 h at atmospheric pressure using flue gas or concentrated CO2, as well as after 4 h in the reaction with flue gas

Summary

Introduction

The utilization of Coal Combustion Products (CCPs) in Europe is a subject of the requirements for different standards imposed by the national authorities. One of the main products among the CCPs is fly ash—inorganic residue remaining after combustion. Fly ash is divided mainly into two groups: siliceous fly ash, containing 70 wt.% of SiO2 + Al2 O3 + Fe2 O3 , and high-calcium calcareous fly ash (HCFA), with SiO2 + Al2 O3 + Fe2 O3 between 50 and 70% [1]. HCFA is usually rich in calcium oxide (10–50%) and due to this, the utilization possibilities of this waste are limited. With the worldwide production of 675 mln tonnes, 80% of siliceous fly ash and only 20% of HCFA are utilized [2].

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