The influence of fabrication effects on the strength of fired clay products.
A study has been made of the enhancement of the mechanical strength
of bricks fabricated from five Sri Lankan Quaternary and post-Quaternary
brick clays with the objective of identifying and optimising those factors
which control the quality and performance characteristics of fired clay
products of this type.
Mineralogical investigations have shown that the clays are
predominantly kaolinitic. Of the accessory minerals, feldspars and gibbsite
are the chief constituents.
The experimental programme involved the development of feasible
processing techniques for clay bodies, the establishment of optimum heat
treatments for their firing, and the testing and evaluation of material
properties of the fired products. The microstructures of fired materials have
been characterized using optical and electron microscopical techniques, as
well as X-ray diffraction, electron probe microanalysis and chemical
analysis. A limited study was also made of the durability of laboratory fired
specimens. Methods of strength enhancement included use of the reactions of
phosphates with natural clays, use of mineralizers to induce mullitization
and surface coating by an efflorescence process.
A kinetic analysis based on the first order kinetics is proposed for the
estimation of optimum firing conditions for kaolinitic clays.
The study has shown that surface coating of bricks increases the load
at the elastic limit by up to 30% and the ultimate failing load by 19% in the
clays examined. The measured increases in modulus of rupture and modulus of
elasticity are over 33% and 40% respectively.
A fabrication technique which requires the incorporation of
phosphates has been developed. This provides the possibility of lowering the
peak temperature of firing to 500°C. Flexural strength increase of up to 60%
over the normally fired unbonded specimens can be achieved using this
technique. Relevant compatibility relations in the ternary system Si02-
P205-AI203 at 500'C are proposed.The presence of an optimum amount of mineralizer in a clay body may
alter its sintering characteristics resulting in an increase in modulus of
rupture up to 55%. However, uncontrolled additions exceeding 4 wt% cause
deleterious effects. Microstructural analysis provides evidence that liquid
phase sintering, development of mullite, development of pores and bloating
are the dominant strength determining features in these clays. An empirical
equation correlating the functional relation between modulus of rupture,
mullite content and porosity is proposed. Mechanisms of strength
development are discussed in the light of these findings.