Effect of Nano-SiO2 and Silica Fume on Durability of High Strength Concretes under Sulfuric Acid Attack

Concrete is one of the most practical and durable construction materials due to economic aspects، abundance of the constituents، adaptability with the environment and its high compressive strength and durabity. High strength concretes are used in the most substructure constructions such as bridges، dams، towers، silos and etc.. The selection of proper aggregates and the constituents affects on the concrete properties such as strength، permeability، and durability. Therefore، optimization of mix designs is necessary to produce high strength and low permeability concretes. Although most of the concrete structures are constructed for 50 years sevice life، however، some of them are sometimes damaged due to sulfuric acid attack. The rate of concrete corrosion in sulfuric acid is a process of dissolution، sedimentation and leaching depends on the chemical compounds of cement، paste reactivity، aggregates type and their grading curves and the proportion of concrete components. Moreover، wetting and drying cycles increases the acid penetration and consequently the rate of corrosion. Remarkable concrete damages have been reported in Australia، Japan، the countries around Persian Gulf، Germany، South Africa and U. S. A.. Therefore، It will be necessary to repair or replace the damaged structure after the attack and it would be very expensive and creates many social problems. Nevetheless، several reports have been published to investigate plain concretes durability in acidic environments، but first، the results are paradoxical for example، silica fume effect against sulfuric acid attack and second، their studies have been conducted on the native construction materials such as fly ash or blast furnace slag. Therefore، it seems necessary to investigate concrete durability against sulfuric acid attack by using new materials such as nano-SiO2. In the present study، durability of concretes containing silica fume، nano-SiO2 and ultra fine Quartz powder was investigated under sulfuric acid attack. Concrete samples of four mix designs، including Silica Fume، nano-SiO2 and ultra fine Quartz powder as cement filler، were investigated to find optimum content of Silica Fume and nano-SiO2. Also to obtain a high strength concretes، ideal grading curve of aggregates and cementitious materials were used which lead to decrease cement consumption. Results show that the very low porosity concretes were obtained based on water absorption test results in comparison to plain concretes. Also، the 28 days compressive strength of all mixtures were obtaied more than 500 kg/cm2 using no more than 325 kg/m3 cementitious material content. Moreover، the durability of concretes against sulfuric acid attack، which is increased by silica fume and nano-SiO2، directly related to the 90 days water absorption. In addition، After 6 months of immersion in sulfuric acid solution with pH of 1. 0، the mixture containing 8% silica fume، 2% nano-SiO2 and 25% ultra fine Quartz powder showed the best performance.