Advanced Cement Based Materials最新文献

²⁷Al MAS NMR spectra of synthetic calcium aluminoferrites, Ca₂Al_xFe_2−xO₅ with x = 0.93, 1, 1.33, reveal only a few percent of the expected intensity for the ²⁷Al central transition, indicating that the calcium aluminoferrite phase in Portland cements can barely be observed by ²⁷Al MAS NMR. This result supports the use of ²⁷Al MAS NMR for quantitative analysis of the tricalcium aluminate phase in Portland cements.

The paper industry in Western Europe generates around 6 million tons/year of sludges, which contain about 60% dry matter mainly composed of cellulose fibers, kaolinite, and calcite. The present study deals with an original way of utilizing such wastes: the production of metakaolin by calcining paper sludge in the temperature range of 700°C to 800°C. After calcination, pastes containing 50% calcium hydroxide and 50% burnt sludge were hydrated and the lime consumption investigated by differential thermal analysis. The results show that a very reactive pozzolan is produced by calcining paper sludge at 700°C or 750°C for 2 or 5 hours. Despite a smaller kaolinite content, the burnt paper sludge exhibits more pozzolanic activity than commercially available metakaolins, especially at early ages. Thermodesorption analyses show that this higher activity is due to the presence of superficial defects that occur during the sludge calcination.

Concrete is a composite, and its properties depend on the properties of the component phases and the interaction between them. It is known that the interfaces are the weakest link in concrete, playing a very important role in the process of failure. This process is strongly related with the characteristics of the aggregates (especially coarse aggregates) and with the relative differences in strength between matrix and inclusions. This paper analyzes the mechanical behavior of high strength and conventional concretes prepared with coarse aggregates having significant differences in strength, shape and surface texture, porosity and absorption, and interface bond strength. Two different gravels and two different crushed stones were used. Concrete mixtures with water/cement ratios of 0.30 and 0.50 were designed. The effects of aggregate type and strength level on concrete failure mechanism, including tensile and compressive strength, stiffness, energy of fracture, and crack pattern, are discussed.

Four different series of mortars were cast to study the influence of the microstructure on the transport coefficient of tritiated water. Test parameters included type of binder, water/binder ratio (0.25 and 0.45), and sand volume fraction (0% and 50%). Mercury intrusion porosimetry was used to characterize the pore structure of all mixtures. The effective tritiated water diffusion coefficients were determined using a simple diffusion test. The test results show that the reduction of the water/binder ratio and the use of silica fume significantly contribute to reduce the transport properties. The test results also indicate that aggregates modify both the microstructure and the transport properties of mortars. The diffusion coefficient of tritiated water was found to decrease with an increase of the sand volume fraction. The increased tortuosity of the matrix induced by the presence of aggregates thus appears to be more important than the influence of the interfacial transition zone. Results also clearly underline the influence of the preparation technique prior to a mercury intrusion experiment.

Self stress generated in polymer impregnated gypsum (referred as GPC) when it is composed is estimated, and its influence on flexural strength is discussed. The estimation of the self stress is based on measured values of shrinkage caused by polymerization of impregnated monomer and elastic modulus of dried gypsum base just before impregnation. The effect of this self stress on flexural strength of GPC is investigated. It was found that the following equation is valid to predict flexural strength of GPC (б_b) in terms of the self stress as a variable: б_b = б_gb + V_p (б_p − б_sp), where б_gb = flexural strength of gypsum base, V_p = specific volume of polymer, б_p = tensile strength of polymer, and б_sp = self stress generated in polymer phase. If extremely low water-gypsum ratio is adopted to prepare gypsum base, cracking is observed just after polymerization preceding any flexural loading. For somewhat higher water-gypsum ratio, specimens are not cracked, but their flexural strength is decreased after polymer impregnation. The self stress corresponding to this case turns out to be higher than the tensile strength of polymethyl methacryrate used for the impregnation. Since prediction of б_sp in the equation is based on tri-axial compressive strain of gypsum base that is within its elastic region, б_sp in polymer phase should positively exist. Even for this condition, the validity of the equation seems to be maintained, although the value in the parenthesis of the equation becomes negative. Based on this fact, an unstable physical state where one phase of a composite material is stressed beyond its macroscopic strength as an individual material owing to the crack arresting effect of the other phase (gypsum in this case) has been postulated. This state is designated as a “superstressed” state, taking its resemblance with supercooling or supersaturation into consideration.

X-ray microanalyses have been made on a suite of nine oilwell cement clinkers. Elemental data were obtained on alite, belite, and ferrite phases and, for two clinkers only, also on aluminate. The alite and belite compositions are broadly similar to those previously reported for other portland cements. Guest ion concentrations suggest that several charge-balancing substitutions occur. Bulk MgO and SO₃ levels determine the Mg and S contents of both phases, but for Fe and Al the substitution levels are not strongly correlated with bulk composition. For ferrites, the data are generally consistent with the results of Bergstrom et al. (Adv Cem Res 1992,4, 141–147): the Mg content varies widely, is controlled by the bulk MgO, and is coupled with Si in a charge-balancing substitution for Fe. Estimates of the total phase assembly using the directly determined mineral compositions are compared with the predictions of a modified Bogue calculation (similar to that of Taylor, Adv Cem Res 1989, 2, 73–77). The agreement is variable and some refinements to the method of calculation are indicated.

A theory based on thermodynamics will be presented by which the pressure in the pore structure of wet porous materials can be deduced during freezing. The pore structure is partly filled with liquid and inert gases such as air. The theory is based solely on thermodynamic relationships; no knowledge of the real geometry of the pore system or the degree of liquid filling in the void space is needed. The only inputs needed in the theory are relative humidity and temperature measured in the sample chamber during the freezing. The validity of the theory will be compared with the test results of mortar samples frozen and thawed in a low temperature calorimeter. During the cooling from 20 to −70°C and subsequent heating of the sample, the strains, heat capacity, and ice evolution of the samples were measured simultaneously in the calorimeter. Two of the three mortar samples were produced using an air-entraining admixture.

The effect of the geometry of woven fabrics on the bond between monofilament polyethylene yarns and cement matrix was studied in the present work. The fabrics were all plain weave, with varied fills density: 5, 7, or 10 fills per cm; the warps’ density was kept constant at 22 warps per cm. The interfacial bond was evaluated by pullout tests. To characterize the influence of the fabric’s geometry on bond performance, the influence of different parameters of the fabric’s geometry that may affect bond were separated: (1) pullout of a single crimped yarn untied from the fabric to characterize the influence of the shape of the individual crimped yarn; (2) pullout of a single yarn from free fabric (not embedded in the cement matrix); and (3) pullout of a yarn from a fabric embedded in the cement matrix. Straight yarns were also tested for comparison. It was found that the woven fabric provided a considerably better bond to the cementitious matrix than the bond of a single straight yarn. The crimped geometry of the yarn in the fabric was found to have a significant influence on increasing the bond between the woven fabric and the cementitious matrix.

Two analytical results are presented that are of use to concrete material technologists. Using a model of concrete in which the aggregates are spherical, but with an arbitrary size distribution, a result from statistical geometry can be used to accurately give the total interfacial transition zone (ITZ) volume for any width ITZ and any volume fraction of aggregates. In reality, the ITZ contains a gradient of porosity and therefore a gradient of properties. When only a small volume fraction of aggregates is present (called the dilute limit), it is possible to analytically solve for the effect of the ITZ on the overall concrete properties. This calculation can be carried out for the effective linear elastic moduli, linear electrical conductivity/ionic diffusivity, and linear thermal/moisture shrinkage/expansion. The details of the calculation are summarized and applications described.