Microstructural changes in self-compacting concrete by sulphuric acid attack

摘要

Five different Self-Compacting Concrete (SCC) compositions, composed of three different types of mineral additions (limestone powder, quartz powder and fly ash) and two types of cement (CEM I 52.5 R HES and CEM III/A 42.5 N LA), were exposed to a continuous immersion in a H2SO4-solution (pH ~ 1.7) for 26 weeks. Rate of attack was estimated by means of mass variation versus time. Microstructural degradation was investigated using polarizing light microscope and scanning electron microscope. SCC composed of CEM I and limestone powder gives better resistance to sulphuric acid attack compared to the traditionally vibrated concrete (TC) and SCC compositions with fly ash and quartz powder due to the buffering effect of calcium carbonate (CaCO3) under acidic conditions. The extent of damage is greater in SCC incorporating CEM I-fly ash, CEM I-quartz powder and CEM III/A-limestone powder despite the presence of CaCO3 in the latter. With the inward movement of the acid attack from the surface, two distinct zones are observed. The outmost attacked zone indicates severe erosion due to decalcification of the hydrated cement matrix under low pH and contains gypsum crystals formed extensively in the form of small or large inclusions. This is followed by a transition zone acting as the border which separates the attack zone from the relatively sound part of the concrete. Gypsum has been found to be the only reaction product formed independently of the type of mineral addition, type of cement and water-to-cement ratio. Despite densification of the pore structure of SCC by incorporating fly ash and quartz powder to achieve lower permeability, low resistance to acid attack has been established. This is due to the negative consequences of pore densification creating less space to accommodate stresses induced by the growth of relatively large gypsum crystals. The fact that SCC with CEM I and limestone powder having higher porosity indicates better performance under sulphuric acid conditions, points to a combined role of pore structure and chemical effect of the mineral addition on the ultimate resistance of SCC to sulphuric acid attack. Contrary to the general statement that durability of concrete is improved with reduced permeability, this aspect can induce negative consequences under sulphuric acid conditions.