2018 Volume 72 Issue 1 Pages 25-32
This study was conducted from the viewpoint of chemical composition and Al bonding state of the sample in order to clarify the promotion mechanism of the secondary ettringite formation due to the coexistence of C-S-H. As a result of mixing the synthesized monosulfate and the various coexisting materials using sodium sulfate solution, when C-S-H coexisted with monosulfate, the amount of the secondary ettringite formation remarkably increased regardless of drying conditions. This promotion phenomenon of the secondary ettringite formation due to the coexistence of C-S-H did not occur when the calcium hydroxide and the amorphous silica coexisted at the same C/S ratio as C-S-H. On the other hand, only when C-S-H coexisted, the peak of the four-coordinate Al indicating the formation of C-A-S-H was confirmed in the 27bAl NMR spectra. Therefore, when a similar experiment was carried out with C-A-S-H coexisting sample, it was confirmed that ettringite was formed the same amount as the case of C-S-H coexistence, whereas a large amount of monosulfate remained. From these facts, it was considered that ettringite is likely to form from four-coordinate Al and C-S-H promotes the secondary ettringite formation in order to convert six-coordinate Al to four-coordinate Al by substituting Al in the silicate chain.
