Zeolites in ceramic suspensions : a study of rheological effect
Molecular sieves with low silicon to aluminium ratio exhibit high ion-exchange capacity and
fast exchange kinetics for the removal of divalent cations, such as Mg2
+ and Sr>. Owing
to their unique structure and properties and the similarity of their composition to many
ceramic materials, zeolites are also promising materials for use in the ceramics industry. The
processing of modem ceramics formulations, e.g. glaze and coating suspensions. generally
requires constant rheological characteristics over time. Glaze suspension rheology. however.
usually changes with time due to the continual dissolution of the solid fraction, i.e. the frit.
This study, therefore, evaluates the suitability of zeolites as ion exchangers and rheological
stabilisers for the glaze industry.
It is important that the zeolites in question are able to withstand the conditions of high shear
and temperature experienced during ball milling without loss of crystallinity. This is
detennined in this study using a variety of techniques including TGA, XRD, XRF. SEM.
FTIR, zeta potential and particle size measurements. Other important properties of zeolites
include their surface charge characteristics, ion exchange capacity and rheology modifying
behaviour. These properties are evaluated using zeta potential, ICP. and rheology
Zeta potential measurements show that the surface charge properties of zeolites are a function
of the silica and alumina components at the surfaces of the respective structures. i.e. the silicon
to aluminium ratio.
TGA, XRD, XRF, FTIR, ICP and zeta potential results show that the crystallinity of each
zeolite analysed is independent of ball milling, whereas, ion exchange capacity decreases significantly after milling. The ion exchange capacity of each zeolite, however, is sufficient to
cause a marked decrease in the total divalent ion concentration in the liquid phase of each
glaze suspension after milling. The monovalent ion concentration in the liquid phase of each
suspension may increase or decrease depending on the type of zeolite employed. The
changing rheology of each suspension, on zeolite addition, also varies depending on the initial
composition of the glaze components, the liquid phase ion concentration and the type of
Time resolved measurements show that the rheology of each suspension, in the presence of
zeolite, continues to change up to eight weeks after milling due to further dissolution of the
This study, therefore, concludes that although zeolites are effective at modifying the liquid
phase ion concentration and rheology of concentrated glaze suspensions, they are unable to
completely stabilise suspension rheology over time. The ion exchange ability of zeolites.
however, may still prove to be of benefit to the glaze industry, especially in the removal of
Ca2+ and Mg2+ ions from hard water sources.