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Technical Paper
SHOTCRETE & GUNNING MIXES WITH IMPROVED PROPERTIES USING MICROSILICA AS BINDER
Hong Peng and Bjørn Myhre Elkem Silicon Materials, Kristiansand, Norway Abstract
No-cement refractory castables (NCCs) with microsilica-gel binder have been developed for high temperature applications. This paper focuses on the development of microsilica-gel bonded no-cement shotcrete and dry-gunning mixes using two speciality products, SioxX-Flow as deflocculant and SioxX-Set as accelerator. Critical installation properties were first investigated in laboratory-scale, then full-scale shotcreting and dry-gunning trials were carried out by industrial partners. The sprayed panels were cut and used for mechanical and hot-properties evaluations. The results demonstrate that cement-free, shotcreted and dry-gunned microsilica-gel bonded mixes not only exhibit good installation properties and low rebound, but also provide enhanced hot-properties compared to low-cement castables (LCCs).
Furthermore, microsilica-gel bonded NCC exhibit excellent explosion
resistance. Perfect 400kg blocks were cast and fired with no problems using a fast firing program (20 to 850°C at a heating rate of 50°C/hr). This indicates that rapid heating of microsilica-gel bonded NCC shotcrete and dry-gunning mixes are possible.
Keywords: Microsilica-gel bond; no-cement castable (NCC), shotcreting, dry-gunning.
1. Introduction Dry-gunning and shotcrete
and accelerators) is essential to successfully install refractory materials by either dry-gunning or shotcreting. In addition, good practices (such as selection of an appropriate pump, nozzle and experienced installation team) is of the utmost importance.
A number of research papers have been published during the last 30 years, of which most have been focusing on equipment and accelerators, using low-cement castables (LCCs) or ultra-low cement castables (ULCCs)3-7
Shotcreting and dry-gunning of NCCs have not been widely applied due to the challenges of controlling the setting and low green strength.
The first shotcrete (called wet-gunning) method for cement-free refractory castables was demonstrated in 1999 in Japan by Iwasaki et al.8
and in cement plants10 . However, handling, storage and use of Recently,
silica-sol bonded shotcreted NCCs have been used for blast furnace main troughs9
liquid silica-sol are logistic factors that must be dealt with, especially at lower temperatures, which limits the application of silica-sol bonded NCCs. Furthermore, silica-sol bonded NCC, is of course not suitable for dry- gunning installation.
Obviously, a technology based on a “dry-version” silica binder, using microsilica powder, is of great interest. Recent reports disclose that a genuine bond based on microsilica coagulation is created, and that the setting of microsilica-gel bonded castables is caused by cations, a similar set mechanism to colloidal silica11
. installation techniques are used in the
refractory industry due to cost- and time-saving compared to conventional casting (e.g. self-flowing and vibrating)1-2
. For dry gunning, the dry premix
is pneumatically transported through the pipe to the nozzle where water is added. The nozzle operator adds whatever water is needed to get the material to stick to the wall, hence the water content may vary during the operation. The interaction time from the water is added to the mix hits the wall is short. Compared to shotcrete, higher rebound and more dust are expected in dry gunning. Dry gunning is a well-proven installation technique for small jobs and/or hot repairs. The gunning capacity is 0.3-5 m3
per hour.
Shotcreting provides homogenous properties, does not generate dust, has very low rebound and high efficiency, with output up to 10-15 m3
per hour.
The main features of the shotcrete process are summarised as follows: i) A strictly controlled water content is added during a mixing step; ii) The wet- mix is pumped with high pressure pumps through a hose to its application area; iii) A set-accelerator is distributed in the mix by pressurised air at the nozzle; iv) The mix is sprayed and consolidated onto a surface.
It is well known that the mix design (particle size distribution, dispersants January 2018 Issue ENGINEER THE REFRACTORIES 19
In this paper, compositions of bauxite based NCCs for shotcreting and dry- gunning installation were designed using microsilica-gel as binder system in combination with two specialty products, SioxX-Flow and SioxX-Set. SioxX-Flow works as a multi-functional dispersant for microsilica-containing aluminosilicate LCCs, ULCCs and NCCs. It is also suitable for shotcreting application. SioxX-Set is designed for undispersed systems such as gunning mixes, but can also be used as accelerator for LCCs and ULCCs. Our mixes were first tested in the laboratory to check the workability. Then, several full-scale shotcreting and dry-gunning trials were carried out in cooperation with industrial partners. Finally, the mechanical properties, hot-properties, bond mechanism and explosion resistance of NCC shotcrete and dry- gunning mixes were evaluated in comparison to LCCs.
2. Experimental
2.1 Mix design The particle size distributions (PSDs) were calculated using the EMMA program in order to achieve a suitable workability12
. EMMA uses the Andreassen model and is widely used to evaluate and optimize particle . NO-CEMENT REFRACTORY
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