Andrew Smith
Andrew Smith, Programme Leader, Clay, Cement & Concrete Technologies, Mineral Products Centre, University of Derby, Enterprise Centre, Bridge Street, Derby, DE1 3LD, UK, a.smith5@derby.ac.uk
ABSTRACT
Sulfate attack of mortars due to the chemical reaction between a source of water-soluble sulfate from an external source, such as clay bricks, groundwater or contamination, and the tri-calcium aluminate (C3A) component of the cement clinker phases, in the presence of water, is known and widely reported in the scientific literature. In the UK, over the past 5 years or so, a number of cases have been reported and investigated that have been attributed to sulfate attack, normally with the primary source of the sulfate being the clay bricks. Closer inspection of the evidence suggests that this is not a form of “classic” sulfate attack, but may result from different chemical or physical processes. This paper presents a number of examples investigated by the author that relate to situations where damage, in the form of surface erosion of the mortar joint, predominantly in vertical, cavity wall construction of domestic dwellings within 5 years of construction, has been claimed to be attributed to sulfate attack. However, upon further investigation the damage observed and the evaluation of the mortar itself, chemically and mineralogically, suggest that the damage results from a different form of deleterious process. Whilst water-soluble sulfates and Ettringite have been detected throughout the mortar joint, only the surface 2-10mm of the mortar joint are affected. In addition, bricks which are known to have low to very low water-soluble sulfate content exhibit the same type and appearance of mortar damage between them, as those with higher sulfate content. The paper presents the analytical data, and proposes that the damage results from the deposition of crystalline salts within the pore structure, some derived from the mortar itself, and potentially the damage is enhanced by the aerated pore structure of the mortar.
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