Degradation of concrete members exposed to aggressive sulphuric acid environments is a key durability issue that affects the life cycle performance and maintenance costs of vital civil infrastructures [1]. Groundwater, chemical waste, sulphur bearing compounds in backfill, acid rain in industrial zones and biogenic acid in sewage systems are the main sources of sulphuric acid affecting concrete structures. In this research, microstructures of a novel acid resistant concrete (ARC) and a type of conventional concrete (CC), as the reference, have been studied in the laboratory subjected to accelerated testing. For this purpose, ARC and CC, were immersed in 7% (by volume) sulphuric acid solution. Mechanical properties of both concretes as well as their microstructures were examined after 28 days of curing and then after two, four and eight weeks of exposure to acid. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Xray diffraction (XRD) and X-ray mapping (XRM) were employed to analyse the microstructure of concretes before and after exposure to acidic environment. The results of this analysis revealed interesting facts about the mechanism of sulphur penetration in both concrete samples. In addition, they showed differences in the crack locations and propagation patterns and in interfacial transition zones (ITZ) of concretes, particularly after acid exposure. These changes of microstructures, as was proved in experimental tests, could significantly contribute to changing the ductility of the concretes in modulus of rupture (MOR) test.

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Concrete, which is a widely used material in construction industry, is vulnerable to acid attack from various sources such as ground water, chemical wastes, sewage systems and acid rain. Acid attack degrades the cement in concrete which has high amount of calcium hydroxide and jeopardise the structural performance of concrete structures by eventually causing a premature failure during its expected life time. Various research works have been carried out to increase the chemical resistance of concrete. However, high cost, special curing conditions requirements and uncertainty of structural performance have raised the need for further research. In this research a novel sustainable acid resistant concrete was developed by reducing the Portland cement in concrete and its replacement with supplementary cementitious materials including fly ash, slag and silica fume as well as additions of some activators to make a hybrid concrete having commonality to conventional and geopolymer concretes. Mechanical, structural and microstructural properties of the acid resistant concrete (ARC) were evaluated and compared to conventional concrete (CC) before and after immersion in high acidic environments (13% by weight sulphuric acid solution).The mechanical properties and structural performance of the ARC showed quite promising results compared to conventional concrete even after eight weeks of immersion in high acidic environment in terms of loss of strength and performance. Table 1 summarises the depth of penetration of acid in ARC and CC after different periods of immersion in acid. The acid affected layer of CC is totally removed after exposure to acid. However, the ARC external layer remained intact in terms of diameter. The acid also was shown to penetrate into the ARC to a certain depth and then ceased further progression.Characterisation analysis including scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray mapping (XRM) were used to obtain a better understanding about the mechanism of acid attack on two types of concretes. In the XRM analysis, four main elements, calcium, silica, sulphur and iron were investigated at the interface of the acid affected and non-acid affected areas of the ARC and CC samples.Elemental XRM image and pseudo coloured image of the acid exposed ARC sample are shown in Figure 1. The acid affected layer is rich with sulphur ( Figure 1a) which is due to penetration of sulphuric acid. The Ca/Si ratio of this layer is much lower than the intact area due to decalcification of hydrated concrete phases. The pseudo coloured map reveals both calcium and sulphur (light blue area) are present that are the main elements of gypsum. Three main areas are observed in this image: Area 1 which is the acid affected layer and has not been totally decomposed. Area 2 that seems to have higher silica and area 3, which is the intact area and mostly calcium. The Ca/Si ratio map also shows lower ratio for the affected layer compared to the intact area. This high amount of silica at the interface...