Effects of binder types on iron sulfide oxidation of aggregates in concrete

There are many types of deterioration that cause concrete structures to be damaged earlier than their expected life. Deterioration due to iron sulfide oxidation is one of the deterioration types that becomes widespread recently. This oxidation takes place in the aggregates that contain iron sulfide minerals. The oxidation is initiated when moisture and oxygen come into contact with these iron sulfide minerals. It causes the map cracking, pop-out of concrete, and rust staining on the concrete structures.Even though deterioration due to iron sulfide oxidation is not a new problem, there are still no proper solution to overcome this issue. Here, a three-part solution is proposed. The solutions are 1) screening of aggregates, 2) use of proper mix proportions, and 3) protection and repair of the concrete structures. This study aims to study the second part of the solution which is the study of the effects of types of binders on expansion and rust stain due to iron sulfide oxidation. The study also includes the microstructural tests to obtain better understanding about the mechanisms of expansion and rust stain due to the iron sulfide oxidation.In this study, five types of cement replacing materials were studied. They were high CaO and low CaO fly ashes, limestone powder, calcined clay, and silica fume. Along with OPC cement. The aggregates used in the study was Pyrrhotite mineral which was very reactive. After the samples underwent the modified accelerated mortar bar test, two evaluations were used i.e. length change to evaluate expansion, and image analysis to evaluate rust staining.From the obtained results, it was obvious that expansion and rust staining due to iron sulfide oxidation was governed by different mechanisms. The expansion involved the amount of calcium hydroxide in the mixtures and pore structure, while rust staining is mainly controlled by the alkalinity level of the concrete system but minorly the pore structure. Chance of iron sulfide oxidation occurrence can be reduced by reducing the penetration of moisture and oxygen into the concrete. This can be achieved by a denser pore structure. For expansion, when the amount of calcium hydroxide in the concrete system is reduced, the expansive products can be reduced. However, this lower amount of calcium hydroxide causes lower alkalinity in concrete which is favorable for formation of rust. The alkalinity level of the concrete system should be kept high to reduce rust staining due to iron sulfide oxidation. It was found from this study that the mixture system that best overcome the problems of expansion and rust staining when compared to a controlled OPC-only mixture is the ternary binder system of low CaO fly ash and limestone powder.