THE INFLUENCE OF EXPOSURE CONDITIONS ON DELAYED ETTRINGITE FORMATION IN MORTAR SPECIMENS
Amde, Amde M
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The objectives of this research were to experimentally study the influence of exposure conditions on delayed ettringite formation (DEF) in mortar specimens, and to explore the complex processes of expansive cracking associated with DEF. Different exposure conditions were investigated while other parameters such as water-to-cement ratio, fine and coarse aggregates, cement and curing conditions were kept constant in order to study the influence of storage conditions on mortar damages associated with DEF. Mortar bars and cubes were prepared using Portland cement ASTM Type III, Frederick sand and ordinary tap water. The specimens were steam-cured and then subjected to the Duggan heat cycle to introduce microcracks. Control batches of mortar mixes with no additional potassium (K2O) content and sets with additional potassium increasing the overall potassium level to 1.5% K2O by weight of cement were prepared. Increasing the potassium level has a deleterious effect on the specimen expansion and causes deterioration in compressive strength. Length change measurements according to ASTM C490 and weight changes of the mortar bars were constantly monitored during the study. The pH value, Potassium ion [K+], Sodium ion [Na+] and Calcium ion [Ca2+] concentrations of the storage solutions were also monitored to analyze the amount of alkali being leached into the water. Scanning electron microscope (SEM) equipped with high-energy dispersive analysis X-ray was used to identify materials present in the cavities, transition zones and cracks to determine failure mechanism of the specimens. X-ray computed tomography (X-ray CT), a non-destructive method was employed to reveal the internal crack patterns. Ettringite crystals are mainly found in the cavities, in the interface between the aggregate and cement paste. Mortar bars subsequently stored in exposure conditions 1 (water pH maintained at 12.5) and 2 (plain water) after the Duggan heat cycle, clearly showed expansion against weight change results to be positive linearly correlated over the storage period. This was again observed in the mortar specimens with the higher potassium (K2O) content. Results from the petrography analyses of the expanded mortar specimens revealed delayed ettringite formation (DEF) without detecting alkali-silica reaction (ASR). The results also show that deleterious expansions caused by DEF prevails in exposure conditions with high potential leaching of alkali hydroxide to the storage solution.