Effect of CFBC ash as partial replacement of PCC ash in alkali-activated material
Introduction
The principal power energy supply worldwide is based on coal powered thermal plants [1]. In a report by World Health Organization (WHO) and United Nations Environment Programme (UNEP), nearly 200 million tons of SO2 is emitted by thermal power plants [2]. To control this huge amount of SO2 emission various countries have adopted circulating fluidized bed combustion (CFBC) technology [3], [4], [5]. The CFBC boiler is injected with limestone or dolomite for capture of SO2, for improved efficiency of SO2 capture the ratio of Ca/S is often kept in range of 2.0–2.5 [6]. As a result, the amount of fly ash generated from CFBC boiler with same capacity is 40% more than the conventional pulverized coal combustion (PCC) boiler. In South Korea the share of fluidized bed combustion boilers in coal-fired power plant market grew from around 2% in the early 1990 s to 10% in the mid-2000s, and it has been on a gradual rise due to continued expansion and new investments [7]. Due to absence of proper disposal techniques, however, the huge amount of CFBC ash generated in South Korea is being used as a landfilling material [8].
Compared to conventional coal based fly ash, CFBC ash is not considered suitable as supplementary cementitious material for Portland cement concrete due to higher amount of SO3 [9]. The ASTM C618 [10] has limited the SO3 content in CFBC ash up to 5% for use as pozzolana. The higher amount of SO3 leads to volume changes in latter ages of cement concrete. On the other hand, studies have reported utilization of CFBC ash to develop low strength material [11], [12], [13]. In addition, studies have been carried out to develop CFBC ash based alkali-activated materials [11], [14], [15], [16], [17], [18], [19], [20]. Chindaprasirt and Rattanasak developed geopolymer mixes by blending various ratios of CFBC ash and conventional coal fly ash [14]. They reported that use of conventional coal fly ash and CFBC bottom ash can help in providing good workability and compressive strength (35–44.0 MPa) in CFBC blend mixes [14]. Jang et al. utilized CFBC fly ash and bottom ash to produce low strength materials by alkali activation with cement or sodium carbonate as activator [11]. They concluded that CFBC fly ash and bottom ash are suitable for to develop low strength mixes, however, are prone to self-destruction due to formation ettringite and gypsum [11]. Topcu and Toprak investigated the properties of CFBC bottom ash based geopolymer [15]. It was reported that use of CFBC bottom ash leads high thermal stability and dense microstructure [15]. Xu et al. adopted alkali-fusion pretreatment along with metakaolin to develop geopolymer system [16]. It was reported that metakaolin was efficient in consuming excess alkali required for fusion [16]. Li et al. also reported that alkali-fusion pretreatment can be effectively used to enhance the reactivity of CFBC fly ash to produce value added geopolymer composites [17]. Chindaprasirt et al. observed that blend of CFBC fly ash and silica fume can be used to develop geopolymer system [18]. The addition of silica fume enhanced the durability of geopolymer against acid attack [18]. Park et al. reported that addition of 5% sodium carbonate showed rapid strength development in alkali activated CFBC ash mix [19]. Liu et al. developed foam geopolymer concrete incorporating CFBC fly ash, incomplete alkali reaction along with low atomic ratio of Si/Al and Si/Na were reported [20]. Few studies have reported that use of steam/heat curing in case of CFBC blend mixes can counter its self-destructing volume expansion [6], [21]. Additionally, its use in blended blast furnace slag mixes to develop concrete have yielded promising results [22], [23]. The use of alkali activated CFBC mixes have been recommended in low strength materials applications [11], [12], autoclaved bricks [24], aerated concrete [21] and foam concrete [6] along with few recommendations for structural use [22], [23].
In the present study the physical, chemical and microstructural properties of conventional coal fly ash partially replaced by CFBC ash, activated with sodium hydroxide and sodium silicate are investigated. An experimental program was designed to partially replace conventional coal fly ash with CFBC and vary the molarity of sodium hydroxide solution. The variation of molarity in alkali activator solution was done to observe the changes in polymerization process. The compressive strength, mineralogical and microstructural properties were examined to assess the effect of partially incorporation of CFBC ash. For microstructural and mineralogical analysis mercury intrusion porosimetery (MIP), X-ray diffraction (XRD), Fourier transformation infrared spectroscopy (FT-IR) and 29Si nuclear magnetic resonance (NMR) were employed.
Section snippets
Materials and mix details
The conventional coal fly ash was obtained from Dangijn thermal power plant and the CFBC fly ash was obtained from the Gunsan power plant in South Korea. Table 1 presents the chemical composition determined by XRF of the two fly ashes used in the study. The XRD spectra of conventional PCC fly ash and CFBC fly ash is shown in Fig. 1. Three different alkali activator solution were prepared by mixing 4M, 5M and 6M NaOH solution and Na2SiO3 with mass ratio of 1:1. The alkali activator solution was
Compressive strength
The compressive strength of CFBC ash-incorporated alkali-activated material is shown in Fig. 2. From Fig. 2 it can be observed that incorporation of CFBC ash have negligible effect on compressive strength of 4M and 5M alkali activated mixes. This can be attributed to the lower molarity of alkali activating solution (4M and 5M) which was unable to promote the compressive strength development of CFBC ash mixes. Similar observations regarding use of CFBC ash for development of alkali activated low
Concluding remarks
This study investigated the effect of incorporation of CFBC fly ash on the strength development of alkali-activated material produced by conventional PCC fly ash. Three different alkali activating solution of molarity 4M, 5M and 6M were used. The compressive strength along with microstructure and polymerization products were examined to provide in depth understanding. The following conclusions are drawn from the results.
- 1)
The effect of up to 10% CFBC ash was negligible on the compressive strength
CRediT authorship contribution statement
Salman Siddique: Conceptualization, Methodology, Formal analysis, Investigation, Writing - original draft, Writing - review & editing. Jeong Gook Jang: Conceptualization, Methodology, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Korea government (MSIT) (No. 2018R1C1B6002093 and 2019M3D8A2113254).
References (83)
1 – Air pollution
- et al.
Sulfation phenomena in fluidized bed combustion systems
Prog. Energy Combust. Sci.
(2001) - et al.
Observation of simultaneously low CO, NOx and SO2 emission during oxy-coal combustion in a pressurized fluidized bed
Fuel
(2019) - et al.
Influence of the loop seal fluidization on the operation of a fluidized bed reactor system
Powder Technol.
(2019) - et al.
Utilization of circulating fluidized bed fly ash for the preparation of foam concrete
Constr. Build. Mater.
(2014) - et al.
Characteristics of CFBC fly ash and properties of cement-based composites with CFBC fly ash and coal-fired fly ash
Constr. Build. Mater.
(2014) - et al.
Utilization of circulating fluidized bed combustion ash in producing controlled low-strength materials with cement or sodium carbonate as activator
Constr. Build. Mater.
(2018) - et al.
Circulating fluidized bed combustion ash as controlled low-strength material (CLSM) by alkaline activation
Constr. Build. Mater.
(2017) - et al.
Potential use of stockpiled circulating fluidized bed combustion ashes in controlled low strength material (CLSM) mixture
Constr. Build. Mater.
(2010) - et al.
Utilization of blended fluidized bed combustion (FBC) ash and pulverized coal combustion (PCC) fly ash in geopolymer
Waste Manage.
(2010)
Properties of geopolymer from circulating fluidized bed combustion coal bottom ash
Mater. Sci. Eng.: A
Low-reactive circulating fluidized bed combustion (CFBC) fly ashes as source material for geopolymer synthesis
Waste Manage.
Synthesis of geopolymer composites from blends of CFBC fly and bottom ashes
Fuel
Strength and resistance to sulfate and sulfuric acid of ground fluidized bed combustion fly ash–silica fume alkali-activated composite
Adv. Powder Technol.
Binder chemistry of sodium carbonate-activated CFBC fly ash
Mater. Struct.
Utilization of circulating fluidized bed fly ash in preparing non-autoclaved aerated concrete production
Constr. Build. Mater.
Compressive strength development and durability of an environmental load-reduction material manufactured using circulating fluidized bed ash and blast-furnace slag
Constr. Build. Mater.
Use of circulating fluidized bed combustion fly ash and slag in autoclaved brick
Constr. Build. Mater.
Effect of fly ash characteristics on delayed high-strength development of geopolymers
Constr. Build. Mater.
Effects of NaOH concentrations on physical and electrical properties of high calcium fly ash geopolymer paste
Cem. Concr. Compos.
The mechanical properties of fly ash-based geopolymer concrete with alkaline activators
Constr. Build. Mater.
Evaluation of sodium content and sodium hydroxide molarity on compressive strength of alkali activated low-calcium fly ash
Cem. Concr. Compos.
The effect of different Na2O and K2O ratios of alkali activator on compressive strength of fly ash based-geopolymer
Constr. Build. Mater.
An XRD study of the effect of the SiO2/Na2O ratio on the alkali activation of fly ash
Cem. Concr. Res.
Alkali-activated, cementless, controlled low-strength materials (CLSM) utilizing industrial by-products
Constr. Build. Mater.
Effects of slag substitution on physical and mechanical properties of fly ash-based alkali activated binders (AABs)
Cem. Concr. Res.
Alkali-activated natural pozzolan/slag mortars: a parametric study
Constr. Build. Mater.
Performance of blended metakaolin/blastfurnace slag alkali-activated mortars
Cem. Concr. Compos.
The pore structure and permeability of alkali activated fly ash
Fuel
Examining the potential of calcined oyster shell waste as additive in high volume slag cement
Constr. Build. Mater.
Impact of microstructure on the performance of composite cements: why higher total porosity can result in higher strength
Cem. Concr. Compos.
Dissolution mechanism of fly ash to quantify the reactive aluminosilicates in geopolymerisation
Resour. Conserv. Recy.
Influence of the processing temperature on the compressive strength of Na activated lateritic soil for building applications
Constr. Build. Mater.
Influence of clay minerals and associated minerals in alkali activation of soils
Constr. Build. Mater.
Understanding the relationship between geopolymer composition, microstructure and mechanical properties
Colloid. Surf. A-Physicochem. Eng. Asp.
Characterisation of fly ashes. Potential reactivity as alkaline cements
Fuel
Determination of the bulk modulus of hydroxycancrinite, a possible zeolitic precursor in geopolymers, by high-pressure synchrotron X-ray diffraction
Cem. Concr. Compos.
The evolution of strength and crystalline phases for alkali-activated ground blast furnace slag and fly ash-based geopolymers
Cem. Concr. Res.
Effects of drying techniques on the crystal structure and morphology of ettringite
Constr. Build. Mater.
Kinetic study of ettringite carbonation reaction
Cem. Concr. Res.
Carbonation of calcium sulfoaluminate mortars
Cem. Concr. Compos.
Cited by (22)
An AHP based sustainability assessment of cement mortar with synergistic utilization of granite cutting waste
2024, Journal of Building EngineeringCesium immobilization of high pH and low pH belite-rich cement under varying temperature
2024, Journal of Hazardous MaterialsRheology, mechanics, microstructure and durability of low-carbon cementitious materials based on circulating fluidized bed fly ash: A comprehensive review
2024, Construction and Building MaterialsModulation of the workability and Ca/Si/Al ratio of cement-metakaolin cementitious material system by using fly ash: Synergistic effect and hydration products
2023, Construction and Building MaterialsUsing calcium carbide residue to prepare ecological alkali activated slag composites: Effect of anion type
2023, Ceramics InternationalMechanical properties, durability and microstructure of cementitious materials with low-calcium circulating fluidized bed fly ash
2023, Construction and Building Materials