Improvements in Drying Behaviour and Explosion Resistance of Microsilica-Gel Bonded No-Cement
By H. Peng, B. Myhre
Microsilica-gel bonded no-cement refractory castables (NCCs) have drawn lots of interest lately, because of their easier handling, storage and transportation compared to silica-sol NCCs.
This paper mainly focuses on dry-out and explosion resistance of microsilica-gel bonded NCCs. The effects on water loss and explosion resistance of microsilica-gel bonded NCCs using different heat-up profiles, sample dimensions and types of drying agents have been studied in detail. The results confirm that the explosion resistance of microsilica-gel bonded NCCs can been further significantly improved by using a specialty drying agent (EMSIL-DRYTM); a perfect 400 kg block was produced with no problems using a fast firing program (20-850 oC at a heating of 50 oC/h).
Silical-sol bonded no-cement castables (NCCs) have been used in the refractory industry for many years. Compared to low-cement castables, they exhibit fast dry-out and excellent high temperature properties. However, the use has been limited due to long set-time/complex set-behaviour and inadequate development of green strength. Handling, storage and use of liquid silica-sol are logistic factors that must be dealt with, especially at lower temperatures [1-5].
Recently, microsilica-gel bonded no-cement castables (NCCs) have drawn increasing attention, not only because of easier handling, storage and transportation thanks to the “all-in-the-bag” solution but also because of improved setting behavior and higher green strength compared to silica-sol bonded NCC [6, 7]. Microsilica powder is introduced as a “dry-version” silica binder to replace silica-sol. 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 se mechanism to colloidal silica [8-10]. The cations not only contribute to the reduction of the net repulsion effect of microsilica, but also bridge with the negatively charged microsilica particles. If, e.g. calcium aluminate cement is used as coagulating agent, Ca2+ (and/or other polyvalent cations) released during dissolution of the cement will react at the negative sites on the microsilica surface to from a three-dimensional network of linked microsilica particles. Therefore, the number of cations controls the set-time.
Based on our long experience and understanding of the characteristics of microsilica and its performance in refractory castables, a speciality product (SioxX®-Zero) was developed for microslica-gel bonded NCCs . By using SioxX-Zero in combination with polyvalent cations, microsilica-gel bonded NCCs attain improved green strength and controllable set-behaviour . It not only provides similar advantages as silica-sol NCCs but also eliminates some of the drawbacks of a two-component system (e.g. frost sensitivity resulting in storage and transportation challenges). The bond system of microsilica-gel bonded NCCs contains only small amounts of bound water. Once the free water is removed, the castables can be fired at very high heating rates. Furthermore, it contains only a minor amount of cement as coagulating agent; hence, the hot properties are better than that of ULCCs and LCCs .
In this paper, bauxite based NCCs with microsilica-gel bond have been chosen to explore the drying behavior and explosion resistance. The following aspects are covered: i) effects of drying agent/anti-explosion agent on flowability, ii) effects of drying agent/anti-explosion agent and sample dimensions on water loss and explosion resistance, and iii) improvement of explosion resistance by anti-explosion agent, EMSIL-DRY.