Investigation on physical properties, strength and phase evolution of binary cementitious materials made of CFBC ash and lime

https://doi.org/10.1016/j.conbuildmat.2020.120302Get rights and content

Highlights

  • Lime promotes binary cementitious materials hydration.

  • Lime has the temperature effect, charge effect and activator effect.

  • Binary cementitious materials have four stages during hydration process.

  • CFBC ash fineness and lime or slaked lime has the coupling effect for performance.

Abstract

Circulating fluidized bed combustion ash (CFBC ash) is the waste of coal-fired circulating fluidized bed power plant boiler. CFBC ash contains amounts of free lime and anhydrite, which has a self-hardening and expansion property. This paper studies the influence of CFBC ash fineness and lime or slaked lime dosage on the physical properties, strength and phase evolution of binary cementitious materials (lime or slaked lime-CFBC ash). The experimental results show that when lime or slaked lime dosage is below 30%, through CFBC ash getting finer, setting time of binary cementitious materials becomes shorter and compressive strength of that increases, but water requirement of normal consistency of that decreases to some extent. And when CFBC ash fineness is constant, with the increment of lime dosage, setting time of binary cementitious materials becomes shorter and water requirement of normal consistency of that increases, but compressive strength of that declines first and then increases. But when CFBC ash fineness is constant, with the increment of slaked lime dosage, setting time of binary cementitious materials becomes longer and compressive strength of that declines, but water requirement of normal consistency of that increases.

Introduction

The consumption amount of coal per year has exceeded 3.2 billion tons in China [1]. The traditional coal combustion generates SO2 and NOx, which directly causes air pollution [2]. Nowadays, the application of circulating fluidized bed combustion (CFBC) technology significantly reduces SO2 and NOx emissions for improving air pollution [3], [4] (Fig. 1). However, the application of CFBC technology will generate a large amount of CFBC ash [5]. But the pile-up of CFBC ash also causes environmental pollution.

CFBC ash has a great self-hardening property [7], [8], [9], [10], [11], which can be used as self-cementitious materials. At home and abroad in recent years, some researchers [12], [13], [14], [15] have studied the hydration process of CFBC ash, and pointed out the cementitious process basing on three main reactions. Some scholars compared CFBC ash and conventional fly ash, and found that CFBC ash has the potential instead of cementing materials and as an alternative for pozzolan [16], [17], [18].

Many attempts have been made to utilize CFBC ash as a construction material. Shen [19] found that CFBC ash can replace gypsum as cement retarders. Due to the positive contribution of pozzolanic reaction, CFBC ash can be as mineral admixtures in Portland cement [20], [21], [22], [23], [24]. Zahedi used CFBC ash in ordinary concrete [25], Chi used it in roller compacted concrete [26], and Jackson used it as pavement base course material [27] in road construction. Some researchers prepared concrete products containing CFBC ash, such as non-autoclaved aerated concrete [28], foam concrete [29], autoclaved aerated concrete [30], autoclaved bricks [31] and compressed earth bricks [32]. Now, the utilization percentage of CFBC ash in construction material is still low, which is due to its highly exothermic reactions to water, high pH leachates and excessive expansion of solidified materials [33].

For saving Portland cement consumption and reducing environment load, some researchers developed zero-OPC binders, such as geopolymeric materials [34], [35], [36], [37], [38], [39], [40], [41], [42], and using in low-strength construction materials [43], [44], [45], [46], preparation of self-cementing binders [47], [48], [49]. The researchers [43], [44], [45], [46] mainly used blast-furnace slag and CFBC ash to prepare zero-OPC cement. The researchers [47], [48], [49] mainly used an activator and CFBC ash to prepare zero-OPC cement, such as sodium carbonate and calcium hydroxide.

We have successfully used lime as an activator to prepare lime-CFBC ash cementitious materials, which was used for preparing dry-mixed masonry mortar in the literature [50]. That has the potential to reduce cement content, enhance water retention of fresh mortar and improve volume stability of mortar. Experimental results indicated that performances of dry-mixed masonry mortar which was prepared according to an ingredient design (85% ground CFBC ash and 15% lime, binder-sand ratio of 1:2 and sand grading of 1:1) satisfied requirements of the National Standard of the People’s Republic of China (GB/T 25181–2010). Performances of mortar included the 28-day compressive strength up to 10.7 MPa, water-retention rate of 94%, setting time of 6.9 hours and consistency loss rate of 2 hours up to 14.1%. But we have not studied the mechanism of lime-CFBC ash cementitious materials. Unfortunately, no literature can be found in this field of lime-CFBC ash cementitious materials.

So this paper aims at the coupling effect of CFBC ash fineness and lime or slaked lime dosage on the physical properties, strength and phase evolution of binary cementitious materials (lime or slaked lime-CFBC ash). The results may be used to enhance the understanding of the characteristics of CFBC ash and be provided a theoretical basis for further application of CFBC ash in construction and building.

Section snippets

Research objective and scope

The main objective of this study was to investigate the influence of CFBC ash fineness and lime or slaked lime dosage on the physical properties, strength and phase evolution of binary cementitious materials (lime or slaked lime-CFBC ash). Accomplishment of this goal was figured in a two-phase experimental program: Phase I-Physical and mechanical properties of binary cementitious materials and Phase II-Phase evolution of binary cementitious materials (Fig. 2). Major tasks to be performed in

Raw materials

The original CFBC ash (Notation G0) was from Sichuan Province Neijiang Baima Circulating Fluidized Bed Power Plant of China. Two kinds of finer CFBC ash were ground for 20 min and 60 min using Φ500 mm × 500 mm laboratory ball mill, respectively (Notation G20 and G60). Lime and slaked lime was from Sichuan Province Pengzhou Xinjianfeng Building Material CO., LTD. of China. The chemical composition of CFBC ash, lime and slaked lime is shown in Table 1.

Particle size distribution of three kinds of

Physical and mechanical properties

The physical and mechanical properties of different mixes are shown in Table 3.

Conclusion

From the date presented in this study, the following results are obtained.

  • 1.

    When lime or slaked lime dosage is below 30%, by CFBC ash fineness getting finer, setting time of binary cementitious materials (lime or slaked lime-CFBC ash) becomes shorter and compressive strength of that increases, but water requirement of normal consistency of that decreases to some extent.

  • 2.

    When CFBC ash fineness is constant, with the increment of lime dosage, setting time of binary cementitious materials (lime-CFBC ash)

CRediT authorship contribution statement

Xu Xun: Conceptualization, Methodology, Formal analysis, Writing - original draft, Writing - review & editing. Fan Xiaoling: Investigation, Formal analysis. Yang Chenglin: Investigation, Formal analysis.

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.

Acknowledgment

This paper is supported by Nature Fund Project of Southwest University of Science and Technology of China (No.18zx7140), Key Fund Project of Professional Scientific Research Innovation Team of Southwest University of Science and Technology of China (No.14tdfk01) and the Opening Project of State Key Laboratory of Green Building Materials of China (No.2019GBM07).

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