J. Cent. South Univ. Technol. (2009) 16: 0049-0052

DOI: 10.1007/s11771-009-0008-4

Preparation and mechanical properties of polypropylene fiber reinforced calcined kaolin-fly ash based geopolymer

ZHANG Zu-hua(张祖华)¹, YAO Xiao(姚 晓)^{1, 2}, ZHU Hua-jun(诸华军)¹,

HUA Su-dong(华苏东)¹, CHEN Yue(陈 悦)¹

(1. College of Materials Science and Engineering, Nanjing University of Technology, Nanjing 210009, China;

2. State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology,Nanjing 210009, China)

Abstract: To improve the environmental benefits and solve the problems of large shrinkage and high brittleness, the partial replacement of calcined kaolin by fly ash as a raw material for geopolymer synthesis and the influences of polypropylene (PP) fiber on the mechanical properties and volume stability were investigated. The results show that compressive strength of the geopolymer containing 33.3%(mass fraction) fly ash by steam curing at 80 ℃ for 6 d is improved by 35.5%. The 3-day compressive strength, flexural strength and impacting energy of geopolymers containing 0.05%PP fiber increase by 67.8%, 36.1% and 6.25%, while the shrinkage and modulus of compressibility decrease by 38.6% and 31.3%, respectively. The results of scanning electron microscopy (SEM) and the appearances of crack growths confirm that PP fiber can offer a bridging effect over the harmful pores and defects and change the expanding ways of cracks, resulting in a great improvement of strength and toughness.

Key words: polypropylene fiber; calcined kaolin; fly ash; geopolymer; preparation; mechanical properties

1 Introduction

Geopolymers belong to a range of inorganic polymeric materials formed by activating silica-aluminum rich minerals, tailings and industrial wastes with alkaline or alkaline-silicate solution at ambient or higher temperature [1]. The source aluminosilicates are usually pretreated and then undergo geopolymerisation, which may include three main steps: (1) dissolution of source materials; (2) transportation or orientation of alumina and silicate species; (3) polycondensation [2]. For the possibility of an effective way to treat solid wastes and by-products, the preparation of geopolymers attracts worldwide concentration [3-4].

It is not a resource and energy saving process to prepare geopolymer only with calcined kaolin as raw material, neither do the generated products have ideal performances [5]. Much work has confirmed that the industrial waste fly ash can be used as a raw material to undergo geopolymerisation [6-9]. Moreover, the impact resistance of geopolymer can be improved by adding 10% fly ash into metakaolin as compound raw materials [9]. Although PVA fiber [9], basalt fiber [10] and metal wires[11] have been used to increase the strength and toughness of geopolymers, the barriers for further application are still the large shrinkage and high brittleness, which would result in catastrophic failure during service. Polypropylene (PP) fiber has been extensively used in reinforced cement and concrete industries due to its high toughness and anticorrosion, however, there is few researches about PP fiber modifying geopolymer.

In this work, both calcined kaolin and fly ash were used as raw materials to prepare geopolymers, and the optimum admixture ratio was studied. PP fiber was added into the optimized starting material system to reinforce geopolymers. Its effects on the fluidity and setting time of fresh pastes, as well as on the strength, roughness and the volume stability of hardened geopolymers were investigated.

2 Experimental

2.1 Materials

The used super fined kaolin (from Fujian Province, China) was calcined at 900 ℃ for 6 h. The selected calcination temperature and the mechanism behind the activation for kaolin can be seen in Ref.[12]. Grade Ⅱfly ash is the disposal of Huaneng Power Station (Nanjing, China). The main chemical compositions of calcined kaolin and fly ash as determined by X-ray fluorescence (XRF) are shown in Table 1, and the parameters of particles are listed in Table 2.

Table 1 Compositions of calcined kaolin and fly ash (mass fraction, %)

Table 2 Particle parameters of calcined kaolin and fly ash

The alkaline activators used in this study were sodium hydroxide in flake form (NaOH with 98% purity) and sodium silicate solution (w(Na₂O)=9.98%, w(SiO₂)= 27.10%, w(H₂O)=62.92%). Sodium hydroxide and water were mixed into sodium silicate solution to adjust the mole ratio of SiO₂ to Na₂O of 1.2 and water content of 70%. The activator was prepared at least 24 h prior to use.

Polypropylene fiber (Sheyang Qingjie Chemical Fiber Co. Ltd) has an average diameter of 10 μm, a length of 3 mm, a tensile strength of 550 MPa, an elastic modulus of 13.5 GPa, a fracture extensibility of 37.5%, a density of 1.36-1.40 g/cm³ and a high acid and alkaline resistance. Distilled water was used throughout experiments.

2.2 Methods

It was found that PP fiber agglomerated if it was mixed with dry raw materials firstly, resulting in poor dispersing and large amount of bubble. So PP fiber should be mixed with alkaline activator solution homogenously, and then the solution was held in the curing room to keep at 20 ℃ before it was mixed with raw materials. The activator, with or without PP fiber, was mixed with raw material at a constant liquid/solid ratio of 0.9 mL/g. The fluidity and setting time of fresh paste were measured by fluidity measurement and the Vicat needle method, respectively. The paste was filled in moulds and cured under standard condition for 1 d before another steam curing (SC) at 80 ℃ or air curing (AC) at 20 ℃ for 1, 3 and 6 d. The compressive strength and modulus of compressibility of Φ25.5 mm×25 mm cylinders, the flexural strength of 10 mm×10 mm×60 mm specimens, as well as impacting energy and linear shrinkage ratio, were measured.

To analyze the fiber reinforced mechanism, cracking appearances and fracture surfaces of compressive specimens were taken with digital camera (Canon) and SEM (JSM-5900, Japan Electron Company), respectively. The samples were sputtered with Au before SEM observation.

3 Results and discussion

3.1 Optimizing admixture ratio of source materials

Table 3 shows that as the fly ash content increases, fluidity of paste increases, which is due to the smaller specific surface of fly ash. Less activator liquid for wetting powder of the same mass results in the increase of fluidity of raw material system containing fly ash. Setting time is also prolonged, which confirms that fly ash is less active than 900 ℃-calcined kaolin. The 6-day compressive strength of geopolymers of SC and AC with 33.3% fly ash content is improved by 35.5% and 46.8%, respectively. But the compressive strength of geopolymers with 50.0% and 66.7% fly ash decreases during the latter 3-day curing. The large fly ash particles, as listed in Table 2, may play the role of aggregate during geopolymerisation. In addition, the particles of fly ash are also served as the nucleation sites of geopolymerisation reaction, which promotes the formation of geopolymer products [9]. These two particle functions make the geopolymer stronger. Besides, fly ash comprises glassy as well as crystalline (often mullite and quartz) phases. The glassy phases are made up of silicon, aluminum and iron oxides, which can also be activated by alkaline solutions. But the activation process is much slower than that of calcined kaolin. When exessive fly ash is used, the compressive strength development is slow, so 66.7% calcined kaolin with 33.3% fly ash is an optimum raw material system for strength and setting time.

Table 3 Effects of fly ash on slurry and geopolymer

3.2 Influences of PP fiber on properties of geopolymers

As shown in Table 4, when PP fiber content (mass fraction) increases linearly, fluidity declines correspondingly and the interval between initial and final setting time is reduced (tested at 80 ℃) while there is a little effect on the final setting time.

Table 4 Effects of PP fiber content on slurry and geopolymer

Generally, geopolymers containing fiber have higher strength than pure ones. The compressive strength of PPf3 is increased by 67.8% and 19.5% for 1 and 3 d,respectively. Flexural strength and impacting energy, the direct toughness indicating parameters, are increased by 36.1% and 6.25% for 3 d. Though flexural strength and impacting energy of PPf4 are higher, its compressive is lower even than that of pure ones, suggesting that excessive PP fiber may have a negative effect on the structural integrity of geopolymer.

The linear shrinkage ratio of geopolymers and modulus of compressibility of geopolymers are decreased by adding PP fiber, as shown in Fig.1. The decline of shrinkage ratio of PPf3 and PPf4 is 38.6% and 44.6%, respectively, while the decrease of modulus of compressibility may have a maximum point (31.3% for PPf3 according to Fig.1).

Fig.1 Effects of PP fiber content on modulus of compressibility and shrinkage ratio

3.3 Mechanism of PP fiber reinforced geopolymer

Based on the mechanism of fiber reinforced cement-based composites [13], the supposed mechanism of PP fiber reinforcing geopolymer includes the following three aspects: modifying the basic structure, inhibiting the growth of micro cracks, and changing the expanding ways of cracks.

It can be concluded from previous researches that the basic geopolymer structure includes the formed amorphous geopolymeric gel, residual unreacted raw material particles and varied pores [6, 9, 14]. As shown in Fig.2, PP fiber can offer a bridging effect over the harmful pores (Fig.2(a)) or defects (Fig.2(b)) by embedding its two thrums in geopolymeric matrix and can deform elastically when geopolymer is affected by inside and outside forces rather than the deformation of harmful pores, so the roughness and strength is enhanced largely.

Fig.2 Bridging effect of PP fiber on harmful pore (a) and defect (b) (A: Harmful pore; B: PP fiber; C: Defect)

Geopolymer shrinks badly during curing, which easily results in the formation of microcracks and even visible cracks [15]. But the probability of formation of microcracks during plastic stage is reduced for PP fiber-absorbed activator on surface may plug up the leakage passageway or make it more flexure. It is relatively hard for the hardened geopolymer to shrink because of the existence of PP fiber. The inside stress generated by the self-shrinkage will be consumed by the deformation of fiber, stopping the formation of microcracks at a certain extent decreasing the inner defects.

Unlike the geopolymer without PP fiber that a sudden and explosive failure will form when the brittleness of geopolymer is tested, most of the load would be passed to fiber, which is higher of strength and modulus, by the interface between fiber and geopolymer. There are three different expanding ways for crack tips: moving around the fiber, elongate it or tensile failure the fiber, either of which needs more fracture energy. As a result, the crack expanding is changed even stopped. Fig.3 shows that specimens 2 and 3 have interlaced cracks, while specimen 1 has flat fracture surface. The through crack in specimen 4 suggests that excessive fiber may be hard to disperse and flaws are formed in matrix of geopolymer. When geopolymer is impacted or sheared, the impacting energy will be consumed or the shearing stress will be relaxed due to the tensile strain of PP fiber on the rupture surface. Meanwhile, the impacting energy is partially transformed into pull-out energy (Fig.4). The random dispersed fiber hinders the expanding of cracks, thus improving the toughness and strength of geopolymers.

Fig.3 Different crack growths of geopolymers (specimens 1, 2, 3 and 4 contain 0%, 0.25%, 0.50%, and 0.75% of PP fiber)

Fig.4 Rupture surfaces without fiber (specimen 1) and with fiber (specimens 2, 3, and 4)

4 Conclusions

(1) Industrial waste fly ash can be used to partially replace calcined kaolin as a raw material to synthesize geopolymer. Proper addition of fly ash will increase the fluidity of fresh paste, prolong its setting time and improve compressive strength of hardened geopolymer.

(2) PP fiber can improve the quality of geopolymer considerably by changing the basic structure of geopolymer matrix and self deformation even tensile failure.

(3) The usage of fly ash and PP fiber in geopolymer synthesis suggests a feasible approach to further enhancing the environment benefits and solving the problems of large shrinkage and high brittleness.

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Foundation item: Project(2006AA06Z225) supported by the National High-Tech Research and Development Program of China

Received date: 2008-04-11; Accepted date: 2008-06-17

Corresponding author: YAO Xiao, Professor, PhD; Tel: +86-25-83587253; E-mail: htm@njut.edu.cn

(Edited by YANG You-ping)