Technical Paper


and reported about spinel containing calcium aluminate cements. Feng et al. [11] reported about the preparation and application of aluminate cements containing magnesia-alumina spinel.


Development work on composite hydraulic binder combining


microcrystalline spinel (MA) with calcium aluminate phases (CA, CA2) has also been the topic of numerous patents e.g Braniski et al. [12], Koya et al. [13], Falaschi et al. [14].


After first small and medium size industrial of the 1990’s and during the first decade of the 21st


pilot


trials at the end century the first


fully industrially made calcium magnesium aluminate binder was commercialized under the name CMA 72 in the year 2011 by Imerys Aluminates, France. The chemical and mineralogical compositions are shown in Tab. 1. Parr et al [15] and Wöhrmeyer et al [16] discussed the hydraulic properties and the new castable microstructures that can be achieved with this new micro-spinel containing binder (CMA-binder) and documented the positive effect on penetration and corrosion resistance against steel ladle slags. It was demonstrated that through the CMA- bond a more homogeneous distribution of micro-spinel and a finer spinel network in the castable matrix was achieved than with blends of calcium aluminate and spinel powders. While the CMA-grains have a D50 of typically 18 micron, the spinel particles inside the CMA-grains (measured after dissolution of CA and CA2) are typically in the range of 3 micron (Fig. 3)


REFRACTORIES ENGINEER


THE


Fig. 4: Microstructure of MA-CA-CA2 clinker •


• Tab. 1 Chemical and mineralogical composition of CMA-binder (CMA72)


it has hydraulic properties and hardens with water at room temperature


it serves as functional filler due to the micro-spinel crystals and achieves a more homogeneous distribution in the castable matrix than blends of calcium aluminate and spinel powders.


Furthermore, the spinel size and distribution is similar to spinel that is formed in-situ


in Alumina-Magnesia castables, but without the inconvenience that comes with the Magnesia addition to castables e.g. risk of too early stiffening and crack formation during drying, both caused by reaction of Periclase with water resp. steam to Brucite. To overcome the rheology issues and to compensate for a strong permanent linear change associated with the in-situ formation of MA-spinel during firing of an A-M castable, a small amount of silica fume is typically added but with the disadvantage of lowering the refractoriness of those castables. With CMA-binder those difficulties can all be avoided. However, CMA- binder can also be combined with magnesia to create a hybrid A-MA-M system of pre-formed spinel introduced through the CMA-binder and the in-situ formed spinel from the reaction between alumina and magnesia. Hybrid formulations achieve excellent slag penetration and corrosion resistance [16].


Fig. 3: CMA72 grain size grain size distribution and MA-spinel crystallite size distribution inside the CMA72 grains


This CMA-binder is produced by co-sintering CaO-, MgO- and Al2O3- precursor raw materials. During the reaction sintering of the pre-cursor materials a new clinker is formed that consists of micro-crystallites of Magnesium-Aluminate spinel (MA) with close


to stoichiometric


composition and interstitial Calcium Aluminate phases, mainly the hydraulic CA and some CA2. The particularity of this clinker is its spinel size and its homogenous distribution inside the clinker embedded in calcium aluminates (Fig. 4).


Milled to cement fineness (CMA72) with a specific surface area Blaine of >3500 cm2/g, and in contact with water the material achieves a double function:


Auvray et al. [17] demonstrated the difference between a castable microstructure with CMA72 versus a mix of CAC and MA-spinel powder. Pore size, pore size distribution, and MA-spinel distribution are significantly different as shown in Fig. 5. With CMA-bond, pores are smaller and the matrix is build up by a homogeneous network of micro- spinel and some fine CA6 needles (Fig. 6) that form by reaction sintering between CA, CA2 and alumina during the first heat-up of the castable.


CMA-aggregates for refractory bricks and monolithics


Soon after the introduction of CMA-binder to the market, Imerys Aluminates started to investigate as well the potential of CMA as a refractory aggregate (CMA-agg). Based on a composition as shown in Tab. 1 aggregates with different grain sizes e.g. 0-1, 1-3, 3-5 mm were produced (Fig. 7). After successful industrial pilot trials in steel ladles the


www.irengineers.co.uk www.ireng.org


18


July 2019 Issue


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32