THE INFLUENCE OF FABRICATION EFFECTS ON THE STRENGTH OF FIRED CLAY PRODUCTS

Abstract

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.