Impact of low Magnesium Concentrations on Sulphate Resistance of Cement Fly Ash Blends at Practical Relevant Sulphate Attack Conditions

摘要

For many years it has been common knowledge that a chemical attack on concrete due to magnesium sulphate can be more severe than an attack due to sodium sulphate. Therefore, in several national standards specific limit values exist for magnesium beside the limit values for sulphate. As an example, the German standard DIN 4030 defines a concentration of 300 mg magnesium per litre as well as 200 mg sulphate per litre water as the lower limit for the concrete exposure class on chemical attack XA1. Above these concentrations, sulphate resisting cements have to be used for concrete manufacture. In most European countries Portland cements (CEM I) with low C3A content and slag cements (CEM III) with slag content above 65% are defined as sulphate resisting cements. Additionally, in some countries (UK, I, E) pozzolanic cements (CEM IV) are defined as well. These pozzolanic cements can be made with natural pozzolans or fly ashes. Recent laboratory investigations as well as field investigations have shown that concrete samples and mortar samples made with cement/fly ash mixes (cement/fly ash blends) are highly sulphate resistant to sodium sulphate or calcium sulphate attack but not necessarily when attacked by sodium sulphate together with magnesium sulphate. The investigations have been conducted with low sulphate concentration of 1500 mg sulphate and low magnesium concentration of about 150 mg magnesium per litre water. The observations were unexpected; because the magnesium concentration is clearly below the XA1 limit value and much lower than the typical magnesium concentration used in common laboratory investigations, which is typically above 1000 mg per litre. The objective of the current comprehensive research project is the clarification of the reaction mechanisms and the explanation of the observations. Therefore, investigations on concrete and mortar samples have been done or are ongoing. Different parameters, e.g. fly ash content, temperature, pre-storage and storage conditions have been varied. Beside length change and resonance frequency measurements, numerous microstructure investigations have to be done. For this purpose scanning electron microscopy (SEM) combined with energy dispersive X-ray analysis (EDX), quantitative X-ray diffraction, and mercury porosimetry (MIP) are used. First results show the dependence between the reactivity of the fly ash, the porosity and the reaction paths.