No abstract

This investigation was made to examine how the conditions of hydrothermal synthesis influence the crystal structure of xonotlite and its morphology. For synthesis we used amorphous silica fume and SiO₂/CaO molar ratio equal to 1.0, water-solids ratio W/S = 10 and W/S = 15. The samples were cured hydrothermally in the rotation autoclave with mixing rod for 2 and 4 hours at 200 °C. The products were characterized by X-ray diffraction, differential thermal analysis, thermogravimetry, SEM, atomic force microscopy and mercury intrusion porosimetry. The low W/S ratio and mixing intensity had a decisive effect on processes of formation of xonotlite. During the 4-hour synthesis at W/S = 15, well formed xonotlite fiber of 0.2 mm diameter was obtained. Under the same conditions, but at W/S = 10, the obtained xonotlite was poorly formed, with fiber of 0.07 mm diameter. The mercury intrusion porosimetry method was used to examine and compare the surface of synthesized xonotlite. In the case of well formed xonotlite, the surface equals to 8.027 m²/g and that of poorly formed xonotlite 22.328 m²/g. The investigation of pressed xonotlite powder by XRD shows the rise in diffraction peak intensity (0.700; 0.324) nm and the drop in diffraction peak intensity (0.308; 0.184) nm. With tobermorite, there is a rise in diffraction peak intensity (1.13; 0.542) nm.

DOI: http://dx.doi.org/10.5755/j01.ms.19.2.4437

This article analyses the results of the freezing – thawing with deicing salt test where changes in the microstructure of the surface layer in contact with aggressive environment of hardened cement paste produced with and without sodium silicate (hereinafter NTS) admixture were observed after freeze-thaw cycles in the presence of calcium chloride. After
56 cycles of freezing – thawing with deicing salt test micro-cracks and cavities were observed in the microstructure of the surface layer of hardened cement paste with and without NTS admixture. In the case of hardened cement paste with NTS admixture changes in the microstructure of the surface layer are less prominent: the number and size of cavities and micro-cracks are smaller. The test revealed that compressive stress, which before freezing – thawing with deicing salt test was very similar in hardened cement paste with and without NTS admixture (85.4 MPa and 82.8 MPa respectively), changed after 56 cycles of freezing – thawing with deicing salt test as follows: reduced by 39.5 % in concrete without NTS admixture and increased slightly (2.5 %) in hardened cement paste with NTS admixture. Based on the test results the authors arrived at the conclusion that sodium silicate solution can be effectively used to extend the useful life of hardened cement paste exposed to freeze-thaw cycles and affected by CaCl₂.

DOI: http://dx.doi.org/10.5755/j01.ms.19.1.3835