Effect of CFBC ash as partial replacement of PCC ash in alkali-activated material

Highlights

• CFBC ash was utilized to replace PCC ash in alkali activated material.

• Incorporation of CFBC ash had a positive effect on strength development of alkali activated material.

• CFBC ash lowers the average pore diameter of alkali activated mixes.

• N-(C)-A-S-H polymerization product was obtained.

• Lower dose of CFBC ash does not result in self destructing nature in alkali activated mixes.

Abstract

The present study investigates the effect of partial replacement of conventional pulverized coal combustion (PCC) fly ash with circulating fluidized bed combustion (CFBC) ash in alkali activated material. The alkali activated material was produced by using sodium hydroxide (4M, 5M and 6M) and sodium silicate solution. The characteristics of alkali activated materials were assessed by evaluating compressive strength, along with microstructure and physiochemical properties. A strong correlation was observed between the mole concentration of sodium hydroxide and compressive strength of alkali activated materials containing CFBC ash. The microstructural and physiochemical results revealed that the calcium rich CFBC ash significantly affects the formation of polymerization products. A hybrid form of polymerization product was formed which resulted in densified microstructure. Furthermore, higher molarity of sodium hydroxide can unlock the polymerization potential of CFBC ash. The findings also revealed that incorporation of lower amount of CFBC ash is an efficient way to reduce the formation of sulfoaluminates in alkali activated materials incorporating CFBC ash.

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.