ABSTRACT
Sulphate attack of concretes and mortars is a well known problem and is generally associated with the formation of the sulphate bearing minerals gypsum and ettringite. Over the last 30 years identification of a third sulphate attack reaction product thaumasite has increased. Thaumasite is a calcium silicate carbonate sulphate hydrate. Consistently wet, cold environments along with readily available supplies of calcium silicate, sulphate ions and carbonate ions provide the optimum conditions for thaumasite formation. This paper describes a site investigation of a housing estate where sulphate attack of brickwork mortar had occurred in underground brickwork retaining walls resulting in the formation of thaumasite. This investigation shows that the deterioration of the brickwork was due to water penetration problems and mobile sulphates which are formed by a base exchange reaction between the alkalis from the mortar and relatively insoluble calcium sulphate in the bricks. These sulphates led to efflorescence, delamination of the brick faces due to crypto-efflorescence and sulphate attack of the mortar producing thaumasite, reducing mortar strength to zero at locations of attack. Results of laboratory analysis using optical microscopy, electron microscopy and X-ray diffraction have been included.
