Elsevier

Ceramics International

Volume 42, Issue 9, July 2016, Pages 11496-11499
Ceramics International

Short communication
Enhancing the green mechanical strength of colloidal silica-bonded alumina castables using a silane coupling agent

https://doi.org/10.1016/j.ceramint.2016.04.045Get rights and content

Abstract

The use of colloidal silica as a binder for refractory castables has attracted great interest in recent years. Nevertheless, the resultant low green mechanical strength of colloidal silica-bonded castables has hindered their application in relevant areas. A silane coupling agent, 3-methacryloxypropyltrimethoxysilane (CH2=C(CH3)COOC3H6Si(OCH3)3), normally called KH-570, has been added to colloidal silica-bonded alumina castables to enhance their green mechanical strength. The results show that KH-570 has a positive effect on the green mechanical strength improvement of colloidal silica-bonded alumina castables. The reaction between hydration products of KH-570 and the solid particles of colloidal silica (confirmed by Fourier transform infrared spectroscopy (FTIR)) could promote the formation of an organic-inorganic interpenetrating network structure, which is the main reason for the green mechanical strength improvement. Meanwhile, the water consumption by KH-570 hydration and the hydration products of methanol accelerate the gelling process of colloidal silica. This also has some positive influences on the early strength enhancement.

Introduction

Calcium aluminate cement (CAC) has been used as the most common binder for many types of refractory castables [1]. However, there are some drawbacks related to the use of CAC as a binder, especially for refractory castables containing SiO2 fume [2]. In recent years, increasing numbers of researchers have focused on the use colloidal silica or alumina as a binder instead of CAC [3], [4]. Compared to CAC, colloidal silica is a CaO-free binder and allows for fast drying. Therefore, using colloidal silica as a binder can significantly shorten the processing time. Thus, colloidal silica has been widely used as an alternative to overcome the drawbacks of CAC for refractory castables. However, the low green mechanical strength of colloidal silica-bonded refractory castables may cause severe damage during demoulding and transportation, thereby narrowing the range of application of these materials.

Aiming to minimize the above drawbacks, many publications are focused on studying the impact of setting agents [5]. Magnesium oxide (MgO), magnesium hydroxide (Mg(OH)2), magnesium chloride (MgCl2), magnesium sulfate (MgSO4), calcium oxide (CaO), calcium hydroxide (Ca(OH)2), calcium chloride (CaCl2) and calcium sulfate (CaSO4) are commonly used as setting agents because of their solubility range and basicity. All of these setting agents promote the early mechanical strength of castables by accelerating the three-dimensional network establishment of colloidal silica via gelling or coagulation mechanisms. However, these additives always lead to rapid setting of the whole system and reduce the suitable working time. Consequently, these additives deteriorate the flowability and cause more inconvenience in practice. Some reports in the literature considered the use of CAC as the setting agent of colloidal silica [6]. However, the presence of CaO in the composition may be deleterious to the refractoriness of some ceramic systems. Moreover, if castables contain CAC, careful curing and drying processes should be performed; such processes prolong the whole processing time of castables. Published works have also addressed the use of CAC and/or hydratable alumina (HA) as setting agents [6]. Although the combination of CAC and HA has been proved to be an effective setting agent for colloidal silica-bonded refractory castables, some problems remain that require careful consideration. On the one hand, the hydration of HA [7], [8] forms a gel layer on the surface of the HA particle immediately after being in contact with water, resulting in a much less permeable structure and leading to a higher likelihood of explosive spalling during water dry-out [9]. On the other hand, it is better for high-performance refractory castables to avoid CAC utilization.

It is a substantial challenge to find a new setting agent for colloidal silica-bonded castables to improve the green mechanical strength of such castables, thereby widely extending their industrial application. In this context, the effect of KH-570 as a setting agent was investigated in this study.

In the KH-570 molecule (shown in Fig. 1), there are two different types of chemical structures, and KH-570 can be expressed using the general formula Y–R–SiX3, in which Y is the organic end CH2=C(CH3)COO–, R is –CH2–, and X is the hydrolysable inorganic end –Si(OCH3)3. On the one hand, the X group is capable of hydrolyzing and yielding silanols (Si–OH), which can react with the inorganic substrate. On the other hand, the Y group could be absorbed by colloidal silica particles, affecting the desperation of the system. FTIR analysis was performed on colloidal silica and colloidal silica with KH-570 separately to explore the reason why KH-570 has a positive effect on the green mechanical strength improvement of colloidal silica-bonded refractory castables.

Section snippets

Experimental procedure

The raw materials included brown fused alumina (3 mm ≤D≤5 mm, 95.2 wt% Al2O3, Sanjiang, China), white fused alumina (1 mm≤D≤3 mm, D≤1 mm, 99 wt% Al2O3, Sanmenxia, China), white fused alumina powder (D≤0.074 mm, 99.2 wt% Al2O3, Sanmenxia, China), α-Al2O3 micro-powder (GW-1, 4 μm, Dengfeng, China), and micro-silica (951U, Elkem materials, Norway). The colloidal silica binder used in the experiments was JN-30 (Qingdao, China), and its features are shown in Table 1. KH-570 (synthesized by Hubei University)

Results and discussion

Fig. 2 shows the CMOR of alumina castables after curing for 24 h at 50 °C and drying at 110 °C for 24 h. As shown in Fig. 2, the CMOR of CS is lower than that of the CAC-ref. However, with KH-570 as a setting agent, the CMOR of CS+KH-570 is significantly improved after curing at 50 °C or drying at 110 °C compared to the CS ones and almost reaches the level of the CAC-ref. Therefore, it is clear that the addition of KH-570 can significantly enhance the green mechanical strength of colloidal

Conclusion

As clearly indicated by the experimental results, the appropriate addition of KH-570 significantly improved the green mechanical strength of alumina castables almost to the level of the CAC-bonded ones. The effect of KH-570 may be attributed to the accelerated setting process of colloidal silica via its hydration.

The addition of KH-570 influences the setting of alumina castables bonded by colloidal silica not only through its hydration process via a gelling mechanism but also through the

Acknowledgments

The authors thank Xiaochao Yan for his kind assistance with the FTIR tests.

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Postal address: Wuhan University of Science and Technology, Hengpin Avenue 947#, Qingshan District, Wuhan 430081, People's Republic of China.

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