Delayed ettringite formation, the influence of aggregate types, curing conditions, exposure conditions, alkali content, fly ash, and mix water conditioner (MWC)

Considerable work has been done in preventing deterioration of concrete and to improve the durability of concrete structures. Research has been done on delayed ettringite formation (DEF) due to internal sulfate attack (ISA) considering the effects of several factors. Some of these factors include curing conditions, sulfate content, potassium content, magnesium content, w/c ratio, SO₃/Al₂O₃ ratio, delay period, various types of coarse aggregate, entrained air, exposure conditions, equivalent Na₂O etc. Results of these research programs have shown that reducing w/c ratio, moist-curing, decreasing the potassium content, reducing the sulphate content, SO₃/Al₂O₃ ratio, and equivalent Na₂O, drying exposure after heat cycle, entraining air, using limestone and an adequate delay period, reduce the delayed ettringite formation and therefore reduce the deterioration of concrete.; The ettringite crystals were found mainly filling the cavities, in the interface between the aggregate and the cement paste and the cracks. SEM showed the ettringite undergoing a physical transition from an amorphous, gel, phase to a stable and well-formed crystal phase. X-ray microanalysis proved that both phases are chemically identical and consistent with ettringite.; Results of tests indicate that concrete made with natural and reactive sand exhibited extremely large expansion, but crashed sand exhibited small expansions. Extensive interior and exterior cracks were found in all the prisms, water or steam-cured, made with natural and reactive sand. In addition, the concrete prisms completely disintegrated and could be torn apart by hand. It seems that the ASR reaction enhances the formation of DEF and the expansion in the case of using natural sand. It is believed that the rough surface and lower density and lower thermal coefficient of crashed sand are able to inhibit the development of an ettringite band in the paste-aggregate transition zone and suppress the expansion. Steam-cured concrete showed relatively larger expansion than the water-cured concretes. Results demonstrated that steam curing is not the primary factor responsible for the formation of ettringite and the related expansion. The most important factors are the exposure conditions and type of aggregate. Results established that increasing the potassium content and exposure to moist environment have a deleterious effect on concrete expansion, ettringite formation and cause premature deterioration. However, using fly ash and mix water conditioner mitigates the problem of ettringite formation and expansion. Results showed that as fly ash content increased, the expansion and ettringite formation decreased. Increasing the fly ash content has a slightly adverse effect on compressive strength. Using mix water conditioner not only mitigated the problem but also illustrated the highest compressive strength. Microstructure analysis gives support to both the cement paste expansion and the growth of ettringite bands as the mechanism responsible for the observed DEF and expansion. There is evidence that calcium hydroxide and calcium silicate hydrate gel are involved in the expansion and reformation of ettringite. (Abstract shortened by UMI.)...