Sulfate Attack of Mortars Containing Recycled Fine Aggregates

Abstract

From both environmental and sustainability points of view, it is imperative for engineers to effectively use the increasing amounts of concrete waste available. This article reports on a study of the durability of mortar specimens made with recycled fine aggregate exposed to external sulfate attack. The replacement level and the type of the recycled fine aggregate were the primary variables. The mortar specimens were immersed in a 5% sodium sulfate solution for 360 days and regularly monitored for visual damage, variation of expansion, and compressive strength. At the end of 360 days, the products of sulfate attack and the mechanism of attack were investigated through x-ray diffraction and scanning electron microscopy. Results show that the use of recycled fine aggregates up to a maximum of 50% replacement level had a beneficial effect on durability against sulfate attack, in terms of strength loss and expansion. Mortar specimens incorporating 100% replacement level, regardless of type of recycled aggregate, suffered severe deterioration. The water absorption of the recycled fine aggregate was a major factor controlling expansion, strength loss, and the intensity of damage due to sulfate attack. The microstructural studies indicated that the primary cause of deterioration of the mortar specimens with recycled aggregate was the formation of thaumasite and gypsum due to sulfate attack. The authors hypothesize that incorporation of recycled fine aggregate possibly led to the production of a relatively weak interfacial transition zone, particularly when high replacement levels were employed. The authors conclude that, when recycled fine aggregates with high water absorption are used in concrete, the limiting level of replacement is 50%. Care in the design of concrete mixtures should be taken even when such concretes are used in normal exposure conditions.