The Mechanism of corrosion of MgOCaZrO₃–calcium silicate materials by cement clinker

Abstract

The chemical reactions involved in the corrosion of MgOCaZrO₃–calcium silicate materials by cement clinker were studied using a hot-stage microscope up to 1600 °C. The phases formed at 1500 °C were characterized by RLOM and SEM–EDS of the crystalline phases conducted near the reaction front and on unreacted refractory area.

The general corrosion mechanism of attack on MgOCaZrO₃–calcium silicate materials involves a mechanism of matter diffusion of the liquid clinker phase through the grain boundaries and pores into the refractory substrate. The liquid phases in the clinker mainly enriched in calcium, iron and aluminium are rapidly diffused and preferentially react with magnesium spinel, calcium zirconate and magnesia, which are the major constituents in the refractory substrates. The dissolution of the CaZrO₃ refractory phase produces the enrichment with zirconium of the liquid phase increasing its viscosity and hindering the liquid phase diffusion.

Introduction

Nowadays the burning zone of rotary cement kilns is exposed to alkali salts and some waste by-products such as rubber or other hazardous products of animal origin, these materials enhance the corrosion process of the kilns refractory.

Generally for a better corrosion resistance, the MgO-based materials were adopted as the main components of refractory bricks because they are hard-wearing towards the liquefied cement materials at high temperatures. MgOMgAl₂O₄ bricks were actually used mainly in the burning zone of rotary cement kilns. These conventional materials, however, show an inadequate performance due to problems associated with corrosion resistance and their easily developing hot points.1, 2, 3 Good alternatives for replacing the MgOMgAl₂O₄ materials until now used, are the MgOCaZrO₃–calcium silicate composite materials due to their enhanced refractoriness, high mechanical properties and excellent corrosion resistance against alkali, earth alkali oxides and basic slags.⁴

Kozuka et al.4, 5 have studied the behaviour of MgO/CaZrO₃ materials as a refractory materials in rotary cement kilns. The post-mortem analyses presented in these papers proved that the bricks showed superior corrosion resistance and coating adherence, but they peeled off easily in areas which underwent high mechanical stresses. Hitherto, there is only one work that describes the corrosion behaviour of MgO/CaZrO₃ materials with cement clinker, which was carried out by Serena et al.⁶ These authors have investigated the corrosion behaviour in particular of 80 wt.% MgO–20 wt.% CaZrO₃ materials versus a clinker of Portland cement.

In contrast with the preliminary research works aforementioned, it was found that the use of natural raw materials constituted by major elements such as, Mg, Ca, Si and Zr is an attractive route for the production at low cost of the MgO-based high temperature structural materials. The present authors in previous works7, 8, 9 found the mechanisms involved in the reaction sintering process for MgCa(CO₃)₂/ZrSiO₄ mixtures, and demonstrated that the reaction sintering is a feasible way to obtain MgOCaZrO₃–calcium silicate dense composites with fine-grained microstructure. These mixtures were prepared by using mineral dolomite (MgCa(CO₃)₂) and zircon (ZrSiO₄) raw materials. The composition of the mixtures prepared was tailored on the basis of the information supplied by the quaternary system MgOCaOZrO₂SiO₂10, 11, 12 in order to obtain MgOCaZrO₃–β-Ca₂SiO₄ and MgOCaZrO₃Ca₃Mg(SiO₄)₂ composites.

In the present work, we have explored the possibility of using the composites mentioned above that belong to the quaternary system MgOCaOZrO₂SiO₂, as refractory materials for the burning zone in rotary cement kilns.

The MgOCaZrO₃–β-Ca₂SiO₄ composition is located in the solid-state compatibility plane MgOCaZrO₃Ca₂SiO₄ and lies on the connecting line zircon-dolomite Fig. 1a. The formation of the first liquid starts at 1750 °C; this temperature corresponds to the invariant point of the subsystem MgOCaZrO₃Ca₂SiO₄. From the quaternary section ZrO₂SiO₂CaO/MgO = 1:1 mol, it can be stated that the composition considered is located in the primary phase field of MgO7, 8 (Fig. 1b).

In accordance with Fig. 1a, the MgOCaZrO₃Ca₃Mg(SiO₄)₂ composition also lies on the connecting line MgCa(CO₃)₂/ZrSiO₄ and is located in the solid-state compatibility plane MgOCaZrO₃Ca₃Mg(SiO₄)₂. This composition is also located in the primary phase field of MgO and their invariant point is low (1550 °C) (Fig. 1b). Therefore, the temperature of formation for the first liquid of compacts containing Ca₃Mg(SiO₄)₂ within this compatibility field, is lower than that of those formulated with β-Ca₂SiO₄.

The study described here, was focused to reveal details of the reaction behaviour at the interface of cement clinker/MgOCaZrO₃–calcium silicate materials. In addition, microstructural characteristics of the reaction interface between cement clinker/MgOCaZrO₃–calcium silicate materials, after corrosion treatments, were observed on specimens treated at 1500 °C for reaction intervals up to 360 min. The conditions under which different phases are formed at the interface and in the bulk of the MgOCaZrO₃–calcium silicate materials, were discussed taking into account the phase equilibrium data of the systems MgOAl₂O₃CaO and MgOCaOSiO₂ZrO₂ and compared with MgO and MgAl₂O₄ ceramic materials with similar density (≈98%) and grain size (≈4 μm).