Advanced Cement Based Materials最新文献

Controlled rate thermal analysis (CRTA) was applied to the thermal curing of a reactive powder concrete. Heating was adjusted to keep the rate of gas evolution constant and in dynamic equilibrium at a pressure of 3 Pa. In this way, all internal gradients were kept constant and homogeneous samples were produced. Six different solid-state transformations were detected below 900°C, four of them occurring below 250°C. Under these conditions, no xonotlite or other crystalline silicate hydrates were detected even up to temperatures of 250°C, contrary to what is observed when heating is carried out in regular, nonkinetically controlled conditions, which generate local hydrothermal water vapor pressures inside of the samples. Our results show that controlled rate thermal treatment in dynamic equilibrium with fixed water vapor pressures is a promising method for the accurate control of hydrate crystallization.

The interaction mechanisms of the cement paste hydration products with chloride were investigated for various systems. Test parameters included water/binder ratio (0.25 and 0.45), type of cement (ASTM type I, III, and V), use of silica fume (6%), and chemical composition of the chloride solution. Powdered cement paste samples were immersed in solutions of different chloride concentrations. The total amount of bound chlorides was determined by measuring the equilibrium chloride concentration of the solution after 3 weeks of immersion. The chemical interaction of chlorides with the hydrated cement paste was also studied by X-ray diffratometry. Test results show that the total amount of bound chlorides increases with the chloride concentration of the solution. This nonlinear relationship can be represented best by a Freundlich isotherm. When expressed on a unit mass of cement gel basis, the total amount of bound chlorides was found to be independent of the water/binder ratio and the type of binder in all cases but one. The chloride binding capacity and the formation of chloroaluminates appear to be attributable not only to the tricalcium aluminate content but also to the total aluminate content of the cement. The use of calcium chloride instead of sodium chloride, as well as the use of a lime solution instead of an alkaline solution, increases the amount of bound chlorides.

Raman spectra of single-phase calcium-silicate hydrate (C-S-H) samples with C/S ratios between 0.88 and 1.45 are consistent with a defect tobermorite model for the structure of these materials, in agreement with previously published nuclear magnetic resonance (NMR) spectroscopic data for the same samples. The Raman spectra of C-S-H samples with C/S ratios <1.0 are very similar to those of 14Å tobermorite. Those of C-S-H samples with C/S ratios ⩾1.0 show substantial concentrations of both Q¹ and Q² Si sites and indicate the possible presence of jennite-like environments. Raman spectroscopy, like infrared and NMR, is a probe of local structure on the atomic nearest neighbor and next-nearest neighbor scale, and thus provides significantly different information than diffraction methods. The Raman spectra of the C-S-H have a rich structure and contain peaks for Si-O stretching, Si-O-Si bending, internal deformation of the Si-O tetrahedra and Ca-O polyhedra, and characteristic peaks at lower frequencies. The spectra of jennite and 14Å tobermorite are quite similar, confirming the result from NMR spectroscopy that jennite has dominantly Q² polymerization. The Raman spectra of 11Å and 14Å tobermorite are also similar, although our sample of 11Å tobermorite has a significant concentration of Q³ Si sites, which indicates cross-linking of the chains.

The primary concern for glass fiber reinforced cement composites (GFRC) is the durability of glass fibers in the alkaline environment of cement. Despite the use of improved alkaline-resistant glass fibers (AR-glass) and pozzolanic materials such as silica fume and fly ash, durability concerns still exist. This report presents an experimental investigation on the hot-water durability of glass fiber reinforced cement composites. Hot-water durabilities of AR-glass fiber reinforced composites in blended cement matrix were compared for their flexural and tensile performance. The different matrices selected were (a) cement; (b) cement + 25% metakaolin; and (c) cement + 25% silica fume. Specimens after normal curing of 28 days were immersed in a hot water bath at 50°C for up to 84 days and then tested in flexure and tension. The results indicate that the blended cement consisting of synthetic pozzolan metakaolin significantly improves the durability of GFRC composite.

Three different series of mortars with variable sand volume contents were cast in two different laboratories to study the influence of the interfacial transition zones (ITZ) on the transport coefficient of chloride ions. The first series was prepared and tested at Laboratoire Matériaux et Durabilité des Constructions (LMDC, Toulouse, France). The water/cement (w/c) ratio was 0.38 and the sand volume fractions were 0, 19, and 57%. The transport properties were investigated using a diffusion test. The two other series were prepared and tested at Centre de recherche interuniversitaire sur le béton (CRIB, Québec, Canada). The w/c ratios were 0.25 and 0.45 and the sand volume fractions were 0, 30, and 50%. The transport properties were assessed using a migration test. The test results indicate that aggregates modify the microstructure and the transport properties of mortars. The transport coefficient of chloride ions was found to decrease with an increase of the sand volume fraction. At the same time, the transport coefficient of the corresponding paste fraction was found to increase. The increased tortuosity of the matrix induced by the presence of aggregates thus appears to be more important than the influence of ITZ. The interconnection of ITZ was not found to lead to a rapid increase of the chloride ion transport coefficient.

The dependence on frequency of the damping maximum of hardened cement paste (hcp) in the region of −90°C has been analysed in a frequency range between 1 kHz and 9 kHz. The bending vibration of an hcp beam has been induced and the eigenfrequency and damping measured. After the Arrhenius-equation the activation energy was calculated. The dispersion of the eigenfrequency and the damping maximum at the same temperature indicate a mechanical relaxation process. It can be attributed to interaction between the pore ice and the internal solid surface of hcp.

The analysis of concrete members under shear action requires reliable material parameters. A new testing method has been developed which yields pure mode II. This has been demonstrated by numerical analysis as well as by analytical treatment. Tests have been performed on a high strength concrete (cylinder compressive strength of 85 MPa) which confirm the theoretical predictions. It is discussed how the new achievements can be used in predicting a size effect in concrete members under shear action.