Examinations of the structure of highly polymerized alkali containing C-S-H phases

Abstract

Cement is an inorganic hydraulic binder widely used in civil engineering. By the reaction of cement with water calcium-silicate-hydrate (C-S-H) gel is the principal hydration product [1]. For example, buildingmaterials based on Portland cement could contain up to 70 wt% C-S-H gel. Therefore, the structure of the C-S-H gel is responsible for the mechanical properties of the hardened cement paste. The chemical compostion of C-S-H gel varies between the molar CaO/SiO2 (C/S) ratio of 0.5 in older and partly carbonated hardend cement paste and 2.2 in fresh ones. Reaction products (alkali containing C-S-H gels) of the deteriorative alkali-silica-reaction (ASR) show an even lower Ca content. Additionally, these gels have a significant amount of alkalis. Their formation processes and the reaction mechanism of ASR are still unknown and a subject of investigation. To get insights into the structure and formation processes of alkali-C-S-H gels, samples with differerent chemical compositions have been synthesized with the agate-ball milling technique. A part of the gels was converted into their crystalline analogues by hydrothermal treatment. Due to their low crystallinity the gels were investigated with complementary spectroscopic and diffraction methods. Synchrotron based measurements at the diffraction-, XAFSand IR-beamlines at ANKA (Angstromquelle Karlsruhe) were performed. By comparison of the gels and crystalline alkali-C-S-H with XAFS measurements at the Ca absorption edge, it turned out that the Ca environment is stronger affected upon hydrothermal treatment in Ca poor samples than in samples with a higher C/S ratio. Similar results for theK environment were achieved fromXAFS measurements at the K absorption edge. Data from IR spectroscopy show distinct changes in the region of Ca polyhedra vibrations around 240 cm [2]. This result is in good agreement with the XAFS measurements. To point out the role of water in the gels structure the dehydration of C-S-H and alkali-C-S-H was followed in situ with IR-microscopy and the total water loss was analyzed with DTA/TG. The IR data reveal a structural reorganization of the gel. Especially the bands in the Si-O-Si stretching region show distinct changes during the drying process whereas bands attributet to Ca polyhedra vibrations remain largely unchanged.