From a material viewpoint, modern concrete's frequent propensity to plastic shrinkage cracking can be attributable to a combination of low water-binder ratio use and ever-changing properties of binding materials. To obtain a better understanding of this phenomenon, this paper explores the effects of cement fineness and alkali content on the plastic shrinkage cracking of concrete manufactured with two water-binder ratios. Results indicate that within the range 275-385m 2 /kg, cement specific surface area is approximately directly and inversely proportional to hydration rate and evaporation rate respectively; a trend generally leading to higher plastic shrinkage and resulting areas of plastic cracking. Similar effects were observed for alkali contents which resulted in increased levels of plastic shrinkage. Furthermore, while decreasing crack tendency was noted as alkali content increased from 0.4 to 0.8% by mass of cement, further increases in alkali content caused significant decreases of compressive strength and slump; thereby lowering overall concrete performance. It is also found that plastic shrinkage cracking is closely related to the kinetics of plastic shrinkage. In summary, the experimental programme confirmed that cement with relatively low surface area (less than 340 m 2 /kg) and low alkali content (less than 0.8%) is preferred for modern concretes with minimal plastic cracking problems.
The damage process of plain and blended cement mortars subjected to sulfate attack under electrical field was investigated in this paper. The deterioration of mortars was investigated by measuring the loss of compressive strength. To identify the changes of microstructure and mineral phases after the test, scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction analysis were performed on the selected samples. The results indicated that compared with sulfate attack alone, the combination of sulfate attack and electrical field accelerated the damage process of mortars, which can be explained through two different mechanisms. On the one hand, Ca 2+ ions directionally moved and then leached out from mortars under the electrical field, leading to the dissolution of portlandite as well as the decomposition of C-S-H gel at the later stage. On the other hand, the electrical field accelerated the migration of sulfate ions and then they reacted with hydration products to form massive ettringite and gypsum, which resulted in the microcracks and strength loss of mortars. Moreover, the deterioration of the mortar blended with fly ash was still visible in spite of its better chemical resistance. The strength loss of limestone powder incorporated mortar increased in comparison with Portland cement mortar as a result of an increase in porosity.
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