Durability of self-compacting concrete with coal bottom ash as sand replacement material under aggressive environment

Concrete is a major building material use for numerous purposes in construction including the concrete structures that exposed to aggressive environment or seawater such as coastal berthing facilities, breakwaters, retaining walls, tidal barriers, dry docks, container terminals, off-shore floating docks and drilling platforms. As these concrete structures that usually exposed to an aggressive environment, it is expected to require a minimum level of repair or maintenance during their service life. The ingress of aggressive agents, in particular chloride and sulphate ions, can lead to corrosion of reinforcing steel bars and thus cause a reduction in strength and subsequent decrease in the service life of concrete structures. In context to resist this type of premature deterioration, concrete must be proportioned to achieve high durability in aggressive environments. The incorporation of coal bottom ash as sand replacement material was possible to help self-compacting concrete in designing mix proportion which contains extra fine particles content and fewer amounts of coarse aggregates. On the other hand, self-compacting concrete was developed to respond to the need for a self-compacting concrete with improved durability. In this study, six replacement levels were considered for self-compacting concrete: 0%, 10%, 15%, 20%, 25% and 30% by volume. The essential workability properties of the fresh selfcompacting concrete containing coal bottom ash were prepared and evaluated by the test of slump flow, L-box and sieve segregation resistance. Later, the effect of coal bottom ash subjected to sodium chloride (NaCl), sodium sulphate (Na2SO4) and seawater through cyclic wetting and drying was also investigated. The durability performance of the coal bottom ash self-compacting concrete exposed to aggressive environment was evaluated through compressive strength, Rapid Chloride Permeability Test (RCPT), chloride penetration by Rapid Migration Test (RMT) and carbonation depth test. In addition, microstructural changes that occur in specimens due to aggressive environmental effects were identified through X-ray diffraction (XRD) techniques and Scanning Electron Microscopy (SEM). Test results show that coal bottom ash can be acceptably used as a fine aggregate replacement material in order to achieve good durability of concrete. The self-compacting concrete containing 10% coal bottom ash replacement showed excellent durability to chloride, sulphate and seawater attack. The test results also indicate that the amount of calcium hydroxide (Ca(OH)2) in the 10% coal bottom ash concrete was slightly lower than that of control sample due to the pozzolanic reaction of coal bottom ash and cement. The combination of compressive strength, XRD and SEM analysis leads to the identification of Friedel’s salt, ettringite, gypsum, calcium hydroxide formations in specimen