Showing papers on "Cement published in 1992"

Computer simulation of the diffusivity of cement-based materials

Abstract: A digital image-based model of the microstructure of cement paste, coupled with exact transport algorithms, is used to study the diffusivity of Portland cement paste. The principal variables considered are water∶cement ratio, degree of cement hydration and capillary porosity. Computational methods are described and diffusivity results are presented, which are found to agree with the available experimental measurements within experimental error. Model cement pastes prepared with different water∶cement ratios, and having different degrees of hydration, are found to have diffusivities that lie on a single master curve when plotted as a function of capillary porosity. Concepts from percolation theory are used to explain quantitatively the dependence of diffusivity on capillary porosity. The effect of silica fume addition on diffusivity is also examined.

High temperature wellbore cement slurry

Abstract: A cement slurry is provided which is useful to cement wellbores in high temperature service. A high alumina cement is used. The density of the slurry is relatively low as a result of using a low density aggregate. In a preferred embodiment, the low density aggregate is graphite. The graphite additionally imparts a significantly improved thermal and electrical conductivity to the hardened cement.

Microstructure and microanalysis of hardened cement pastes involving ground granulated blast-furnace slag

Abstract: The microstructure and composition of hardened cement pastes of a wide range of blends of ground granulated blast-furnace slag with ordinary Portland cement have been studied, using techniques of transmission electron microscopy with microanalysis combined with electron microprobe analysis. Throughout the range, a calcium silicate hydrate gel (C-S-H) is the dominant cementing phase, present in the “inner product” within the space originally occupied by either slag grains or alite or belite grains originating from the Portland cement, or in the “outer product” in the originally water-filled spaces. The morphology and composition of the outer product C-S-H and the composition of inner product C-S-H change with blend composition. Inner product of slag grains contains C-S-H of the same composition as the outer product C-S-H, intimately mixed with a Mg, Al-rich hydroxide phase whose fineness shows considerable variation. Inner product C-S-H of alite or belite does not differ significantly in Ca∶Si ratio from that of slag. The reduction of Ca∶Si ratio of all forms of C-S-H with increasing slag loading may have implications for the pH-buffering capacity of blends of large slag loading.

Universal water-based medical and dental cement

Abstract: Dental cement containing water, acid-reactive filler, water-miscible acidic polymer, an ethylenically-unsaturated moiety, photoinitiator, water-soluble reducing agent and water-soluble oxidizing agent. The cement has three curing modes, namely an acid-filler ionic reaction, a photoinitiated crosslinking reaction and a redox-initiated crosslinking reaction. The cement cures well in thick layers, and can be used without a dental curing light or with a light that is weak or defective.

Effect of composition, environmental factors and cement-lime mortar coating on concrete carbonation

Abstract: The paper describes the physicochemical processes of concrete carbonation and presents a simple mathematical model for the evolution of carbonation in time, applicable under constant relative humidity higher than 50%. The model is based on fundamental principles of chemical reaction engineering, and uses as parameters the ambient concentration of CO2, the molar concentratrations of the carbonatable constituents, Ca(OH)2 and CSH, in the concrete volume, and the effective diffusivity of CO2 in carbonated concrete. The latter is given by an empirical function of the porosity of hardened cement paste and of relative humidity, derived from laboratory diffusion tests. The validity of the model for OPC or pozzolanic cement concretes and mortars is demonstrated by comparison of its predictions with accelerated carbonation test results obtained in an environment of controlled CO2 concentration, humidity and temperature. The mathematical model is extended to cover the case of carbonation of the coating-concrete system, for concrete coated with a cement-lime mortar finish, applied either almost immediately after the end of concrete curing or with a delay of a certain time. Parametric studies are performed to show how the evolution of carbonation depth with time is affected by cement and concrete composition (water/cement or aggregate/cement ratio, percentage OPC or aggregate replacement by a pozzolan), environmental factors (relative humidity, ambient concentration of CO2), the presence and the time of application of a lime-cement mortar coating and its composition (water/cement, aggregate/cement and lime/cement ratios of the mortar, percentage OPC or aggregate replacement by a pozzolan).

Influence of Silica Fume on the Early Hydration of Portland Cements Using Impedance Spectroscopy

Abstract: Electrical properties of hydrating portland cements (PC) and portland cements containing silica fume were studied from 5 min to 90 days Cement pastes with water to solids ratios (w/s) of 030, 035, and 040, as well as silica fume to portland cement ratios (s/c) of 005, 010, and 020, were made and impedance was measured within the frequencies of 13 MHz to 5 Hz The impedance spectra exhibit electrode arcs at low frequencies and bulk material arcs at high frequencies The bulk resistance of the paste increases with increasing silica fume content and/or decreasing water content The conductivity of pore fluid from PC pastes increases rapidly with time during the early stages and then remains constant, while that of the silica fume pastes increases then decreases sharply

Situ prepared calcium phosphate composition and method

Abstract: Setting times of calcium phosphate cement compositions are enhanced by the addition of phosphate or carbonate lubricant compositions. Methods for preparing the cement compositions are provided.

Effects of cement, cement space, marginal design, seating aid materials, and seating force on crown cementation

Abstract: An evaluation of the effects of a die spacer, the seating force, the marginal design, seating aid materials, and the cement type during cementation was conducted. Two stainless steel dies were used: one with a 1 mm shoulder and the other with a shoulder and a 65-degree bevel. Ten stone dies were produced from each metal die and half were painted with four layers of die spacer. The crowns were waxed on the dies and cast in a nonprecious alloy, and the seating of crowns was measured with a micrometer before and after cementation. Comparisons were made between zinc phosphate and glass ionomer cements under two seating forces of 5 and 30 lb using an orangewood stick or E-Z-bite seating aid. ANOVA and the Newman-Keuls test revealed that the use of a die spacer, a heavier force of 30 lb, and glass ionomer cement significantly improved crown seating. The beveled preparation led to superior crown seating when the heavier force or glass ionomer cement was used. The orangewood stick and bite device had a similar effect on crown seating.

Zeolite Formation in Class F Fly Ash Blended Cement Pastes

Abstract: Zeolite formation in Class F fly ash blended cement pastes is under investigation. A Na–P type zeolite and Zeolite Y were synthesized from Class F fly ash and NaOH solution after 2 days of aging at room temperature and 6 days of curing at 80°C. However, no zeolites formed when KOH was used. In additional experiments, a Na–P type zeolite, Zeolite Y, and chabazite developed in cement pastes blended with Class F fly ash and NaOH solution which had been aged 2 days at room temperature and then cured 6 days at temperatures ranging from room temperature to 90°C. Seeding the pastes with natural zeolites was also investigated.

Adhesive bone cement containing hydroxyapatite particle as bone compatible filler

Abstract: Acrylic bone cement containing hydroxyapatite (HA) as a filler was developed using 4-methacryloyloxyethyl trimellitate anhydride (4-META) to promote adhesion both to bone and HA. The mechanical strengths of the cement did not decrease significantly with increasing HA in the cement by 4-META. However, strengths decreased with increasing HA content in the absence of 4-META. Scanning electron micrographic examination of fractured surfaces of the cement clearly showed that the HA particles adhered to the matrix resin when 4-META was added. Thus, it was important to maintain the original mechanical strengths for 4-META. The HA particles along the surface increased with increased HA content in the cement. The cement adhered to bone with a tensile bond strength was higher than 10 MPa.

Well completion and remedial methods utilizing cement-ladened rubber

Abstract: A method of forming in a desired location in a well bore a dual-state composite of a hardened hydraulic cement and a solid rubber which features the use of a composition consisting essentially of a mixture of a slurry of a hydraulic cement and a vulcanizable rubber latex.

The effect of cement on the strength of granular rocks

Abstract: Relatively neglected by the oil industry in the past, the rock mechanics approach is now routinely applied to production problems such as subsidence or wellbore stability. As a result, there is a growing need for a better understanding of the strength of porous rocks. In addition to porosity, composition and amount of cement are two important parameters controlling the strength of granular rocks. In order to investigate their effect we prepared synthetic granular materials in which the amount and composition of cement could be varied. Synthetic materials are more reproducible and better characterized than natural rocks but do not always possess properties realistically imitating those of natural rocks. Here, we propose procedures to fabricate satisfactory materials that could also be fruitfully used in other experimental studies. We performed a series of triaxial tests on samples of cemented and un-cemented material. We observed that cement, even in very small quantity, significantly increased the strength of granular materials. Our results suggest that the amount of cement was less important than the location where it was deposited. Strengthening was due to precise deposition of the cement at the grain-to-grain contacts, thus preventing sliding and rotation of grains.

Resorbable surgical cements

Abstract: A new surgical cement is disclosed. This surgical cement comprises a hardened cement formed from a mixture comprising a cementing component selected from the group consisting of calcium components having a solubility in water at 25° C. in the range of about 0.5 × 10 -2 M to about 20 × 10 -2 M and mixtures thereof; a setting component selected from the group consisting of water soluble salts of polyfunctional carboxylic acids containing 2 to about 10 carbon atoms, water soluble dibasic phosphate salts and mixtures thereof; and water in an amount effective to form a paste from the mixture which paste hardens into said hardened cement which is biocompatible.

Phase Evolution during Autoclaving Process of Aerated Concrete

Abstract: The reactions were investigated that occur when lime, cement, and quartz sand are mixed together and molded, then treated at 180°C under saturated steam pressures to produce autoclaved aerated concrete. The hydrothermal treatment of mixtures gives Ca-rich C-S-H with varying Ca/Si ratios as an initial product, which reacts further with silica dissolved from quartz to form 1.1-nm tobermorite with increase of curing time. During autoclaving, the composition of C-S-H and tobermorite as a binder continues to change until after 8 h, when the Ca/(Al + Si) ratio becomes constant at 0.8. As the reaction proceeds, the number of micropores increases, and the strength also increases due to the binder effect of the tobermorite. However, the total pore volume does not change, remaining constant values.

Cement and concrete

Abstract: To produce lunar cement, high-temperature processing will be required. It may be possible to make calcium-rich silicate and aluminate for cement by solar heating of lunar pyroxene and feldspar, or chemical treatment may be required to enrich the calcium and aluminum in lunar soil. The effects of magnesium and ferrous iron present in the starting materials and products would need to be evaluated. So would the problems of grinding to produce cement, mixing, forming in vacuo and low gravity, and minimizing water loss.

Role of Silica Fume in Compressive Strength of Cement Paste, Mortar, and Concrete

Abstract: Controversy exists as to why silica fume increases the strength of concrete when it is used as a partial replacement for cement. Some evidence supports the view that the increase in strength is due to an increase in the strength of the cement paste constituent of concrete. However, contradictory evidence exists that shows no increase in the strength of cement paste, but substantial increases in concrete strength, when silica fume is used. The latter evidence is used to support the theory that silica fume strengthens concrete by strengthening the bond between cement paste and aggregate. This study is designed to explain the contradictory evidence and establish the role played by silica fume in controlling the strength of concrete and its constituent materials. These goals are accomplished using cement pastes, mortars, and concretes with water-cementitious material ratios ranging from 0.30 to 0.39. Mixtures incorporate no admixtures, a superplasticizer only, or silica fume and a superplasticizer. The research demonstrates that replacement of cement by silica fume and the addition of a superplasticizer increases the strength of cement paste. It also demonstrates that cement paste specimens, with or without silica fume, can exhibit reduced strength compared to other specimens with the same water-cementitious material ratio if the material segregates during fabrication, thus explaining some earlier experimental observations. The segregation of cement paste is caused by high superplasticizer dosages that do not cause segregation of concrete with the same water-cementitious material ratio. Concrete containing silica fume as a partial replacement for cement exhibits an increased compressive strength because of the improved strength of its cement paste constituent. Changes in the paste-aggregate interface caused by silica fume appear to have little effect on the uniaxial compressive strength of concrete.

Geotechnical properties of a chemically stabilized soil from Malaysia with rice husk ash as an additive

Abstract: The stabilization of Malaysian soil by mixing with rice husk ash, a locally available waste material, to improve its engineering properties is described. Stabilizing agents, i.e. cement and lime, were added to produce the reaction products which are responsible for the enhancement of the engineering properties. Based on the strength development, it seems that lime is the more effective stabilizing agent. However, the cheap waste material can be used as partial replacement for the more expensive cement in the cement-treatment of the soil. A durability study was carried out to evaluate the effectiveness of this stabilization method.