Showing papers on "Cement published in 1986"

Physical structure of hardened cement paste. A classical approach

Abstract: It is explained how the classical model of the structure of hardened Portland cement paste was deduced by T.C. Powers and coworkers from data on water vapor adsorption isotherms. Moreover, it is shown how the fractional volumes of all major constituents in the physical structure of room temperature cured Portland cement paste can be estimated from information on water-cement ratio and degree of hydration of the cement.

Aqueous Solubility Relationships for Two Types of Calcium Silicate Hydrate

Abstract: There are many published values for the concentrations of lime and silica in the aqueous phase which is in contact with calcium silicate hydrate. These have been collected, carefully analyzed, and found often to be apparently inconsistent. When, however, they are interpreted as having come from an aqueous phase which is in near equilibrium with one or another of two possible modifications of calcium silicate hydrate, almost all the data are rationalized. Some important insights emerge for the understanding of the complex processes which occur during the hydration of tricalcium silicate.

Glass-ionomer Cements of Improved Flexural Strength

Abstract: The effect of glass and polyacid composition on cement strength has been investigated with a view to improving the glass-ionomer cements. The flexural strength of glass-ionomer cements was found to be greatly dependent on the glass and polyelectrolyte used to prepare them. Opaque and opal glasses containing crystallites tended to yield cements with high flexural strength. Flexural strength was also found to be increased by increasing the molecular weight of the polyacid. In water-setting systems, cement strength was shown to be critically dependent on the glasslpolyacid ratio.

Chloride concentrations in concrete pore solutions resulting from calcium and sodium chloride admixtures

Abstract: Cement pastes prepared from a “typical” ASTM Type I portland cement at a water/cement ratio of 0.50 were mixed with various calcium chloride (CaCl2) admixture treatment levels, hydrated for various periods, and then subjected to pore solution expression. The expressed pore solutions were then analyzed for chloride and hydroxide ion concentrations. It was found that the concentration of chloride ions drops rapidly but reaches an equilibrium level by about ten days. The equilibrium concentrations attained varied between 20 and 50% of the original concentration in the mix water, the latter value being for the highest CaCl2 treatment level, 2% by weight of the cement. Substituting sodium chloride (NaCl) for CaCl2 as the admixture produced only minor differences in equilibrium concentration. The equilibrium Cl−/OH− ratios for treatments of 0.5% CaCl2 and above were found to be beyond that considered to be the boundary for depassivation, slightly above 0.3. X-ray diffraction results were ambiguous but differential thermal analysis (DTA) trials provided some evidence for the formation of Friedel's salt in these pastes. However, it was considered likely that at least some of the chloride not retained in solution is associated with calcium-silicate-hydrate (C-S-H) gel.

Process for forming a light-weight aggregate

Abstract: A process is disclosed for forming lightweight aggregate pellets having a density of less than about 60 lbs./ft. 2 . The first step of the process is to form an admixture which includes fly ash, cement and filler. By weight percent of the admixture formed, the admixture includes greater than 60% fly ash, 1-25% cement and 1-10% filler. Water and a chemical accelerator are added to the admixture in a pelletizing device. By weight percent of the aggregate formed, about 1 to 18% water is added and between about 0 and 3% chemical accelerator is added by weight of the cement used in the admixture. The aggregate pellets are then removed from the pelletizing device and cured with an oxygen containing gas, such as air, at a temperature of less than about 300° F.

Hydraulic cement slurry

Abstract: The cement slurry of the present invention is gas tight, very low strength retrogression at high temperature, and substantially no tendency to settle or segregate The cement slurry is made with a hydraulic cement, 5-85% microsilica based on the weight of the cement, 5-250% of a high density filler material based on the weight of the cement, 0-5% of a retarder (dry weight) based on the weight of the cement, 0-12% of a thinner (dry weight) based on the weight of the cement, 0-8% of a fluid loss additive (dry weight) based on the weight of the cement, 0-30% silica flour and/or silica sand based on the weight of the cement and water in such an amount that the cement slurry has a density between 195 and 240 g/cm 3

Permeability and Pore Structure of Cement Pastes Containing Fly Ash, Slag, and Silica Fume

Abstract: As part of research to develop a highly durable concrete container for radioactive waste disposal in chloride and sulfate bearing granite groundwaters, a variety of cement pastes were studied. A sulfate resisting portlandcement was used with various replacement levels of Class F fly ash and pelletized blast furnace slag at a water to solids ratio (W/S) = 0.36. Blends with fly ash, slag, and silica fume were also combined with a super water reducer at W/S = 0.25. Results are presented for strength development, permeability to water, and pore size distribution after 7, 28, 91, and 182 days moist curing. As a direct measure of durability, after 91 days moist curing, paste prisms were immersed in both de-ionized water and a synthetic chloride and sulfate bearing groundwater at 70°C. While all three supplementary cementing materials (mineral admixtures) reduced ultimate permeabilities, silica fume was more effective in reducing permeability at early ages. Silica fume was also the most effective in reducing calcium hydroxide contents of the pastes while slag was the least effective; only reducing calcium hydroxide levels by dilution of the portland cement. From preliminary analysis, there does not appear to be a way of accurately predicting permeability from porosity or pore size parameters alone.

Characterization of silica in rice husk ash

Abstract: La decomposition isotherme de balle de riz produit des cendres riches en silice. Les caracteristiques physiques et la reactivite chimique de la silice dans la cendre de balle de riz dependent de la temperature de chauffage et de la duree du palier de cuisson

The influence of contaminating (fossil) carbonate and the variations of delta (super 13) C in mortar dating.

Abstract: The influence of the aggregate in mortar dating is examined. Sample activity as well as isotopic fractionation approach the expected values at lower yields of the preparation reaction of the counting gas. Good results are obtained at low fossil carbonate concentration. 813C cannot give information about this concentration but preliminary visual and chemical analysis of the mortar makes prediction of sample validity possible. INTRODUCTION It is well known that 14C dating does not date archaeologic events but only the materials found in an archaeologic context. Thus, there may be an important interval between the age of the material and the archaeologic event. A good example of this is 14C dating wooden beams used in construction. The 14C dated tree rings can easily be a century older than the building, itself. This problem becomes even more severe if we consider re-use of material. This problem can be avoided by using mortar as a dating material. Traditionally mortars were made essentially of lime and sand. Different limes were used, quicklime and hydraulic lime. Quicklime (CaO) is obtained by burning (CaCO3) over a fire at ca 1000°C. When water is added it forms Ca(OH)2 and hardens by absorption of atmospheric CO2. Hydraulic lime is obtained by burning argillaceous limestone. It contains aluminates and silicates which harden with water. Younger mortars tend to be more hydraulic than older mortars. The hydraulic index framed by Leduc and Chenu (1912) provides a useful parameter to classify mortars. This index is defined as Al203 + Fe203 + S102 soluble CaO + MgO This is, in fact, the ratio between the clay and the lime fractions. Fat limes have values up to 0.1; values > 0.5 indicate that the material is cement rather than mortar. In between are mortars which are more or less hydraulic. Besides lime, mortars also contain sand and aggregate which are used as charge. Their purpose is to enlarge the volume of the mortar and to avoid cracks. Several laboratories have been working on mortar dating since 1964. Some obtained very good results (Delibrias & Labeyrie, 1965); others obtained dates that were far too old (Stuiver & Smith, 1965). In summary, we can say that, theoretically, mortars containing lime ought to be 14C datable since hardening time is short compared to the half-life of 14C. How-

Influence of Type of Cement and Curing on Carbonation Progress and Pore Structure of Hydrated Cement Pastes

Abstract: Different series of cement paste specimens were prepared with ordinary Portland cement, with portland, blast furnace slag cements having slag contents of 30, 50 and 75% by mass, with commercial fly ash cement and with portland cement containing fly ash additions of 10, 20, 30 and 50% by mass. Moist curing of the specimens varied between 3 and 28 days before the pore size distribution and characteristics of the phase composition were analyzed. Subsequent to curing, the specimens were subjected to drying in air of 65% RH with a controlled CO2 content of 0, 0.03 and 2% CO2 by volume. Depth of carbonation, pore size distribution of the carbonated paste, and the phase composition were investigated after 28 days and 6 months of drying, respectively. The results show that carbonation alters the prevailing pore structure of the hydrated paste. Important parameters are the type of cement used and the duration of curing.

Influence of mixing technique on some properties of PMMA bone cement

Abstract: PMMA bone cements (Refobacin-Palacos R, Sulfix 6, AKZ, and CMW bone cement, types I and II), from six different clinics, were investigated in three stages. In the first stage, studies of density, hardness, flexural strength, and compressive strength were made, as well as molecular weight measurements and microscopic investigations. These studies reflected the current state of techniques of application used in operating theaters. They revealed wide variations in the properties of the materials studied. Secondly, a comprehensive study of the process-technology in the laboratory was performed. The following variables were investigated or discussed: mixing vessel, order of the individual components, mixing time, rate of mixing, pressure application on the mixed bone cement, kneading, cement thickness, pouring into the syringe, contact force during polymerization, and preparation quantity. The third stage involved the development and clinical testing of an improved mixing technique. Using this improved mixing technique, all three selected clinics achieved far better results with reduced variability. A comparison between a centrifuging technique after mixing and our improved, but conventional, mixing technique, displays advantages for the latter. The question regarding a correlation between cement specimens of high porosity and early implant loosening could not be answered on the basis of the 43 PMMA bone cement explants investigated (implanted 6 months to 15 years). In some cases, the studies revealed that the bone cement manufacturers should be required to revise and quantify existing instructions for use. The users, on the other hand, should give more consideration to the mixing technique and its consequences.

Process for accelerating hardening of cement with fibre-reinforced and cement-bound plates

Abstract: The invention relates to a process for accelerating hardening of cement in fibre-reinforced and cement-bond plates and solids respectively The essence of the process lies in that a composition known in itself containing water, fibrous material and cement is placed between the pressure plates of a press, compressed, simultaneously with pressing a flange of increased density is formed on the profile to be produced; the medium between the gasproof or quasi gasproof boundary surface is carbonized wih CO 2 gas thereafter pressure is reduced