Synthesis and mechanical properties of metakaolinite-based geopolymer

doi:10.1016/j.colsurfa.2005.01.016

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Volume 268, Issues 1–3, 31 October 2005, Pages 1-6

https://doi.org/10.1016/j.colsurfa.2005.01.016 Get rights and content

Abstract

Metakaolinite-based geopolymer has been synthesized at about 20 °C from metakaolinite under activation of NaOH solution (4–12 mol/L) and sodium silicate solution. The reaction product was compressed at 4 MPa in mould and then cured at 65 °C in oven for 10 h. Flexural strength, compressive strength and apparent density of the material were systematically measured. The effect of concentration of NaOH solution on the mechanical and chemical properties of the geopolymer was investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and infrared spectrometry (IR). Results show flexural strength, compressive strength and apparent density of the geopolymer increase along with the increase of concentration of NaOH solution within 4–12 mol/L. Storage time in air has slight effect on flexural strength while almost has no effect on the compressive strength and apparent density. XRD and IR analyses indicate the geopolymer material is composed of amorphous phase and semi-crystalline phase, containing adsorbed atmospheric water and small amount of inert ingredients originated from the metakaolinite. Content of the amorphous phase in the geopolymer increased along with the increase of NaOH concentration. SEM results show the synthesized geopolymer maintained layer structure of metakaolinite particulates, thus it was thought that the geopolymeric reaction mainly occurred at the surface of the microflake of metakaolinite particulates. But this presumption need further study. Strength data of the material show such a colloids reaction based on metakaolinite could make building materials with good quality.

Introduction

Davidovits initially reported geopolymer in 1978 [1]. As a kind of inorganic material with ceramic-like properties, geopolymeric materials are attracting increasing interest as ecologically friend fireproof building materials [2]. Geopolymeric reaction was applied to reuse some waste products with pozzolanic properties, for example, fly ash [3]. Geopolymer were used to immobilize toxic metal [4], [5]. Due to its cage structure, geopolymer is even thought to be a valid approach to solidify radioactive element.

Usually, geopolymer can be synthesized at ambient temperature or slightly above, by activating aluminosilicate using the activator composed of aqueous hydroxide and silicate. It is a colloid reaction with low water content in the system. Aluminosilicate used in the reaction is mostly in solid powder form while the activator can be easily constituted by NaOH or KOH solution and sodium silicate solution or potassium silicate solution. According to Davidovits, the geopolymer consist of polymeric Si single bond OAl framework, with SiO₄ and AlO₄ tetrahedron linked alternately by sharing all the O atoms, and the geopolymeric reaction can be expressed as below [1]:In formula (1) (Si₂O₅, Al₂O₂) is an emphasized form to stand out the IV-fold coordination of Al instead of the conventional aluminosilicate oxides form of (2SiO₂, Al₂O₃). SiO₂ in formula (1) is in monomer or polymer form which is come from silicate solution. Formula (2) shows the final backbone of the geopolymer and charge balance in the matrix.

According to the reaction process, two factors have direct effect on final product, namely, the aluminosilicate and activator. Properties of the solid aluminosilicate will directly affect the dissolution process and the subsequent reaction [6], while the liquid activator will partially or completely dissolve the solid raw material and determine break and recombination of the aluminosilicate structure, polycondensation and charge balance in the reaction system.

Kaolinite has been widely used as starting material to synthesize geopolymer [5], [7], [8], [9], [10], [11], [12]. Derived from kaolinite by a series of treatment, including calcination, remove of Fe and bleaching, metakaolinite is a kind of artificial pozzolanic material with higher reaction activation than kaolinite and has been extensively used in industry. Barbosa synthesized potassium polysialate (K-PS) and potassium polysialate disiloxo (K-PSDS) geopolymer from metakaolinite, and found this kind of geopolymer has excellent thermal stability, especially, the K-PS shows little sign of melting up to 1400 °C [2]. Unfortunately, few have been reported on the effect of the activator on the mechanical and chemical properties of the geopolymer materials. In present work metakaolinite is chosen as the starting material to synthesize geopolymer, and the effect of the liquid activator on the mechanical strength and chemical properties of the geopolymer is systematically investigated. Findings reported in this article would help to reveal the feasibility of using geopolymer materials as building material with higher mechanical strength than traditional cement product.

Section snippets

Materials

Analytical grade metakaolinite and NaOH and industrial sodium silicate solution distilled water were used in the experiment. Chemical composition of the metakaolinite was determined on a JSM-5600LV scanning electron microscope (SEM) coupled with a KEVEX energy dispersive spectrometer (EDS), and the result is shown in Table 1.

Methods

The mass ratio of the solid material and liquid activator for the geosynthetic reaction is optimized as 3:1 based on a series of experimental tests, focusing on the

X-ray diffraction analysis

Fig. 1 shows XRD patterns of metakaolinite and the reaction products after stored in air for 7 days. Activation in the experiment was composed of sodium silicate solution (NaO₂·3.2SiO₂) and NaOH solution of various concentrations.

XRD patterns show that some undissolvable ingredients (quartz and mullite) in the metakaolinite remain in all the reaction products.

The broad band in the XRD pattern of the metakaolinite (between 18° and 25°) indicates that the metakaolinite has mixed semi-crystalline

Conclusion

The metakaolinite-based geopolymer material synthesized through activating metakaolinite using NaOH solution and sodium silicate solution mainly is amorphous and contains absorbed atmospheric water. Its layer framework is similar to the flake-like structure of the metakaolinite particulates, which indicates that the main structure, especially the flake structure of the metakaolinite particulates remained during the process of geopolymerization, curing and molding. The colloid reaction in the

Acknowledgements

The authors wish to acknowledge the financial support of Natural Science Foundation of China (NSFC, China No. 50432020). Prof. Jiazheng Zhao, Dr. Jinfang Zhou and Engineers Li He and Jie Yan at the State Key Laboratory of Solid Lubrication, are gratefully acknowledged for carrying out SEM, XRD and IR analyses.

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Synthesis and mechanical properties of metakaolinite-based geopolymer

Abstract

Introduction

Section snippets

Materials

Methods

X-ray diffraction analysis

Conclusion

Acknowledgements

Appl. Geochem.

Miner. Eng.

Miner. Eng.

Mater. Lett.

Cement Concrete Res.

Chem. Eng. J.