Influence of internal sulfate attack on cement paste properties: contamination by pyrite

Abstract

The objective of this work was to evaluate the influence of the presence of sulfate in the microstructure and compressive strength of cement pastes. The lack of availability of more suitable aggregates, for reasons of distance or costs, sometimes leads to the use of materials that contain sulfate in their composition, which is harmful to cement mixtures. Currently, there are normative recommendations that limit the content of contaminants in the aggregates. However, there are still divergences as to the content that does not damage the concrete. In order to discuss the levels presented in the standards and the values above those allowed by them, tests were carried out on cementitious compounds contaminated by pyrite in different levels of sulfates (0.0%, 0.5%, 1.0% and 5.0% of SO3). SEM, XRD, compressive strength, ultrasonic pulse velocity and porosity analyses were performed in samples at different ages until 720 days of age. During early ages until the first year, the most contaminated samples presented an increase in their strength (1.0% and 5.0% of SO3). This behavior was explained by SEM, XRD and porosity analyses by filling the pores with products of sulfate attack, such as ettringite. At the end of the tests (720 days) the series that presented the lowest compressive strength, the presence of cracks and large amounts of ettringite was the one that had 5.0% SO3 contamination, proving the importance of a normative limit content.