Chloride diffusion in ordinary, blended, and alkali-activated cement pastes and its relation to other properties

Chloride transport into cementitious materials is critical from the viewpoint of protection of reinforcement. This paper is part of a larger study of the characteristics and performance of alkali-activated cementitious materials (AAC) whose properties equal or exceed those of normal Portland cement-based materials. Steady state chloride diffusion studies have been made of pastes of Type I Portland cement, and its blends with different proportions of ground granulated blast-furnace slag. Very substantial reductions in diffusion rates have been found with increased proportion of slag. In addition, alkali activation has been shown to reduce the diffusion rate by at least a factor of two. Other properties determined include: density, porosity, pore size distribution (Hg), BET (N₂) surface area, shrinkage, compressive and flexural strength, leaching, alkali-aggregate reaction, and freezing and thawing resistance. Comparisons with results of previous studies and with other blending components are discussed.     

Sulfate resistance of portland-pozzolanic cements in relation to strength

Sulfate resistance of Portland-pozzolanic cements, prepared by gradual replacement of Portland cement by pozzolana, were studied in relation to their transverse strengths. Common clay and China clay, after calcination, were used as two different pozzolana. Strengths of samples, after curing separately in water and in 5% MgSO₄ solution, were measured. On sulfate-curing the strength increased remarkably upto a maximum value beyond which it decreased. This was thought to be dependent on the mode of formation of ettringite in the set cement. Strength on water-curing was not much improved. This was due to the non-availability of lime by the reacting pozzolana because of slower rate of diffusion of lime from the hydrating cement.     

Hydration kinetics of cements by Time-Domain Nuclear Magnetic Resonance: Application to Portland-cement-derived endodontic pastes

Time-Domain Nuclear Magnetic Resonance (TD-NMR) of ¹H nuclei is used to monitor the maturation up to 30 days of three different endodontic cement pastes. The “Solid–liquid” separation of the NMR signals and quasi-continuous distributions of relaxation times allow one to follow the formation of chemical compounds and the build-up of the nano- and subnano-structured C–S–H gel. ¹H populations, distinguished by their different mobilities, can be identified and assigned to water confined within the pores of the C–S–H gel, to crystallization water and Portlandite, and to hydroxyl groups. Changes of the TD-NMR parameters during hydration are in agreement with the expected effects of the different additives, which, as it is known, can substantially modify the rate of reactions and the properties of cementitious pastes. Endodontic cements are suitable systems to check the ability of this non-destructive technique to give insight into the complex hydration process of real cement pastes.     

Structural effect of prepared and commercial superplasticizers on performance of cement pastes

The sodium salt of melamine‐phenol formaldehyde sulfonate (MPhFS), melamine formaldehyde sulfonate (MFS), and phenol formaldehyde sulfonate (PhFS) were prepared according to a four‐step reaction procedure. The four steps of the reaction are hydroxymethylation, sulfonation, low pH condensation, and high pH rearrangement. Fourier transform IR and differential scanning calorimetry spectra were used to determine the structure of the synthesized resins. The effects of MFS, PhFS, MPhFS, and commercial superplasticizers on the rheological properties of cement pastes were investigated using a rotating coaxial viscometer. It was found that the prepared resins enhanced the rheological properties of cement pastes more than commercial ones. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 482–487, 2003     

Measurement of concrete E-modulus evolution since casting: A novel method based on ambient vibration

The use of ambient vibration tests to characterize the evolution of E-modulus of concrete right after casting is investigated in this paper. A new methodology is proposed, which starts by casting a concrete cylindrical beam inside a hollow acrylic formwork. This beam is then placed horizontally, simply supported at both extremities, and vertical accelerations resulting from ambient vibration are measured at mid-span. Processing these mid-span acceleration time series using power spectral density functions allows a continuous identification of the first flexural frequency of vibration of the composite beam, which in turn is correlated with the evolutive E-modulus of concrete since casting. Together with experiments conducted with the proposed methodology, a complementary validation campaign for concrete E-modulus determination was undertaken by static loading tests performed on the composite beam, as well as by standard compressive tests of concrete cylinders of the same batch loaded at different ages.     

Deformation and flow of stiff pastes: review of rheology of some soft solids

Abstract

This paper reviews modelling and supporting experimental work that provides insight into the nature of stiff ceramic pastes. These pastes consist of discrete powder particles in a matrix of viscous fluid. At sufficiently high particle volume fractions, such pastes are able to withstand, without excessive deformation, the forces usual during handling of green ceramics. First, the industrial motivation for the study of pastes is examined, paying special attention to difficulties which are often glossed over in the open literature but which the present authors know from first hand experience. Common test methods are then discussed, and theories are outlined that have been developed to explain or generalise the ex perimental results. The penultimate section reviews some of the progress that has been made in applying theory to industrially based problems. The approach pioneered in the discipline of soil mechanics appears particularly promising as a basis for developing models of pastes that give some physical insight into their mechanical behaviour, and so might give a link to mix design. In conclusion a look into the future of the study of pastes is presented.     

Mechanical properties in relation to the microstructure of fibre reinforced portland clinker pastes

Portland clinker was mixed with various water/clinker ratios ranging between 0.7 to 0.2 covering the range for both “normal” and “low” porosity pastes. These pastes were reinforced with five kinds of fibres, all with weight percentages of 0.5 and 3.0. All the samples were tested for compressive and tensile strengths, total porosity, microstructure and degree of hydration. Fibre reinforcement led to an increase in the total porosity and a decrease in the degree of hydration as compared with the neat pastes. Also, compressive strength has to be sacrificed to a certain extent in order to obtain better flexural strength. SEM furnished direct evidence that fibre reinforcement could affect the pore structure, the habit and shape of the hydration products, as well as their spatial distribution. The indication gained is that the mechanical behavior of the mix is due mainly to changes in the physicochemical properties induced by the presence of fibres.     

Gas and solids mixing in a commercial FCC regenerator

Mixing of gas, solids and heat in a commercial FCC regenerator, 5.76 m in diameter, was investigated. This regenerator was operated at a high superficial gas velocity (>1.2 m/s) and high temperature (650–710°C). It is classified as a highly efficient regenerator. The profiles of coke content in the catalyst, gas composition, temperature and pressure were obtained by sampling and instrumental measurement. It was confirmed that a proposed axial dispersion model could be used for a satisfactory prediction of these experimental profiles. The values of gas and solids dispersion coefficients are also presented.     

Carbon black dispersions and carbon-silver combinations as thermal pastes that surpass commercial silver and ceramic pastes in providing high thermal contact conductance

Carbon black dispersions are superior to the best commercial silver and ceramic particle thermal pastes for providing high thermal contact conductance across mating surfaces that are smooth (0.05 μm). For mating surfaces that are rough (15 μm), the combined use of carbon black and silver is more effective than carbon, silver or ceramic pastes. The use of a silver paste to even out the rough surfaces prior to using the carbon black dispersion is more effective than the use of a carbon-silver mixture. For both smooth and rough surfaces, the carbon-silver mixture is superior to silver pastes. The highest conductance attained for rough surfaces is lower than that attained for smooth surfaces.     

Oxidation of bitumen in relation to its recovery from tar-sand formations

Simple chemical reactions are described which bring about modification of the components of a bitumen. This is accomplished by using an oxygen-containing gas and subsequent treatment with alkali solutions of sulphites and/or bisulphites. The resulting water-soluble sulphonated bituminous derivatives have significant emulsifying and dispersing powers and are likely to be of use in extracting the bitumen in situ.     

Preparations of TiO2 pastes and its application to light-scattering layer for dye-sensitized solar cells

Three different structures of TiO₂ electrodes of the dye-sensitized solar cell (DSSC) devices were fabricated with layers of nanoparticles and light-scattering particles, and their photovoltaic performances were investigated when the polymer electrolytes were used. Especially, 20-nm- and 123-nm-TiO₂ pastes were prepared by using sol–gel method, to use for light-scattering layer from the incident light. The best efficiency of 6.03% under AM 1.5 was attained with a multi-layer structure using 123-nm-TiO₂ layer for the light-scattering layer and 9-nm-TiO₂ layer for the dense layer.     

Permanent jet formation in beds of particulate solids

The simple criterion proposed by Chandnani and Epstein (1984, 1986) for stable spouting, d_or/d_p ≤ 25.4, is shown to give a good separation between gas-solid systems which form permanent jets and those that bubble, for a wide range of experimental data reported in the literature. The criterion is suggested as a necessary but not sufficient condition for permanent jet formation in fluidized, spout-fluid and spouted beds.     

Investigation of the microstructural evolution of calcium sulfoaluminate cements by thermoporometry

Thermoporometry was applied to the investigation of the microstructural evolution of cementing systems. A pure calcium sulfoaluminate cement – CSA – and one mixed CSA–Portland system, together with a reference Portland cement were considered. Specimen preparation was carefully optimized in order to minimize any structural damage and the repeatability of results was checked through the utilization of inorganic standard. Nitrogen adsorption/desorption was used for comparison.A wide set of information could be acquired regarding the microstructure of the investigated materials: a) both the CSA and the mixed CSA–Portland system mainly revealed ink-bottle pores; b) a much more rapid development of hydrated structures was observed for all CSA cements than for the Portland cement; c) melting and freezing curves allow to gain information about the pore size distribution and the presence of pore entries of preferential size, about tortuosity and connectivity of the cement microstructure, and about the existence of isolated pores.     

Velocity and momentum of vortex rings in relation to formation parameters

The formation of ring vortices under water has been studied at orifices of diameter 2.54 to 6.35 cm and formation times from 0.04 to 0.68 s, with orifice Reynolds number between 3 × 10⁴ and 3 × 10⁵. Ciné photographs of the formation process were taken, and the initial velocity of the vortices was found in general to be one half of the velocity of flow through the orifice. The translational momentum of the vortex rings was measured by a surge-tube method. It was concluded that the formation process is most efficient when the length of the impulsive jet is not more than two orifice diameters.     

Celsian formation from barium-exchanged geopolymer precursor: Thermal evolution

In this paper, thermal evolution, including element & phase composition and microstructure, of Ba²⁺ exchanged K-based geopolymer precursor (BaGP) were systematically investigated during high-temperature treatment. The results proved that celsian precursor with lower residual alkaline cation content were obtained through amorphous geopolymer than traditional ion-exchanged celsian through crystallized zeolite. With the increase in temperature, weight loss of BaGP was due to evaporation of OH groups and decomposition of BaCO₃. Similar to K-based geopolymer, BaGP showed amorphous structure, and nanometer-sized celsian nucleuses first crystallized from the amorphous BaGP matrix after it was treated at 900 °C. In the treatment temperature range from 1000 to 1400 °C, hexagonal celsian became the main phase. After being treated at 1400 °C, hexagonal celsian grains were clearly noticeable with extra SiO₂ locating between celsian grains. It was therefore concluded that geopolymer precursor technique provides an alternative route for the preparation of celsian ceramics.     

Surface acid property and its relation to SCR activity of phosphorus added to commercial V2O5(WO3)/TiO2 catalyst

To examine the influence of phosphorus on the commercial V₂O₅(WO₃)/TiO₂ SCR catalyst, measurements were carried out by means of infrared and Raman spectroscopy, XPS, and NO reduction measurement as a function of phosphorus loading. Phosphorus added to the catalyst was found to disperse well over the catalyst without a significant agglomeration up to 5 wt% P₂O₅ addition. The number of the hydroxyl groups bonded to the vanadium and titanium species decreased readily with increasing amount of phosphorus. Correspondingly, the hydroxyl groups bonded to the phosphorus species were formed. NH₃ adsorbed on both hydroxyl groups bonded to vanadium and phosphorus as ammonium ions, implying that the P–OH groups formed are also responsible for the Brønsted acidity. The NO reduction activity was found to be decreased with increasing amount of phosphorus; however, the influence of phosphorus was relatively small irrespective of the large amount of phosphorus addition. The deactivation might be caused by the change in the nature of the surface hydroxyl groups as Brønsted acid sites. Phosphorus species might partially wrap the surface V=O and W=O groups, which might also contribute to the deactivation.     

Effect of gypsum on the hydration and strength development of commercial portland cements containing alkali sulfates

The influence of gypsum content on the hydration and compressive strengths was studied on two commercial Portland cements having the same amount of alite, a similar amount of C₃A, but different amounts of readily soluble alkalies and sulfates. The phase analysis of samples hydrated during the 90-days' period was carried out by X-ray diffraction, selective extraction and simultaneous DTA-TG-DTG method. It was observed that all the effects noticed on pure C₃S-$\text{S}$-H system are determinant for the optimum gypsum content, although for the sample containing a higher amount of readily soluble alkalies the influence of the added SO₃ is considerably modified by the delayed ettringite formation caused by the interaction between Al, Fe-bearing phases and $\text{S}$-rich C-(S,$\text{S}$)-H.