Showing papers on "Cement published in 1997"

Setting Reaction and Hardening of an Apatitic Calcium Phosphate Cement

Abstract: The combination of self-setting and biocompatibility makes calcium phosphate cements potentially useful materials for a variety of dental applications. The objective of this study was to investigate the setting and hardening mechanisms of a cement-type reaction leading to the formation of calcium-deficient hydroxyapatite at low temperature. Reactants used were alpha-tricalcium phosphate containing 17 wt% beta-tricalcium phosphate, and 2 wt% of precipitated hydroxyapatite as solid phase and an aqueous solution 2.5 wt% of disodium hydrogen phosphate as liquid phase. The transformation of the mixture was stopped at selected times by a freeze-drying techniques, so that the cement properties at various stages could be studied by means of x-ray diffraction, infrared spectroscopy, and scanning electron microscopy. Also, the compressive strength of the cement was measured as a function of time. The results showed that: (1) the cement setting was the result of the alpha-tricalcium phosphate hydrolysis, giving as a product calcium-deficient hydroxyapatite, while beta-tricalcium phosphate did not participate in the reaction; (2) the extent of conversion of alpha-TCP was nearly 80% after 24 hr; (3) both the extent of conversion and the compressive strength increased initially linearly with time, subsequently reaching a saturation level, with a strong correlation observed between them, indicating that the microstructural changes taking place as the setting reaction proceeded were responsible for the mechanical behavior of the cement; and (4) the microstructure of the set cement consisted of clusters of big plates with radial or parallel orientations in a matrix of small plate-like crystals.

Cement and Concrete

Abstract: Preface. Acknowledgments. History of inorganic (calcareous) cements, mortars and concretes. Production of Portland cement. Hydration of cement - setting reactions. Mortar. Concrete. Standard tests for cements, cement pastes, mortars and concrete. Some additives (admixtures) used in mortar and concrete. High performance concrete. Physical behaviour of concrete after pouring. Reinforced and prestressed concrete. Fibre reinforced cement and concrete. Deterioration of cement and concrete. Durability and protection of concrete. Resistance of concrete to fire. Special cements and concretes. Index.

Resilient well cement compositions and methods

Abstract: The present invention provides improved compositions and methods for sealing pipe in a well bore. The compositions which harden into highly resilient solid masses having high strengths are basically comprised of a hydraulic cement, an aqueous rubber latex, an aqueous rubber latex stabilizing surfactant and silica hydrophobicized with silicon oil.

Cement and Concrete

Abstract: Preface. Acknowledgments. History of inorganic (calcareous) cements, mortars and concretes. Production of Portland cement. Hydration of cement - setting reactions. Mortar. Concrete. Standard tests for cements, cement pastes, mortars and concrete. Some additives (admixtures) used in mortar and concrete. High performance concrete. Physical behaviour of concrete after pouring. Reinforced and prestressed concrete. Fibre reinforced cement and concrete. Deterioration of cement and concrete. Durability and protection of concrete. Resistance of concrete to fire. Special cements and concretes. Index.

Fly Ash in Concrete: Production, Properties and Uses

Abstract: 1. Uses of Fly Ash in Cement and Concrete 2. Effect of Fly Ash on the Properties of Fresh Concrete 3. Effect of Fly Ash on the Structural Properties of Hardened Concrete 4. Admixtures in Fly Ash Concrete 5. Miscellaneous Opportunities for Fly Ash Use 6. Fly Ash Usage in Waste Management 7. Special Problems Including Use Constraints 8. Types and Properties of Fly Ash 9. Effect of Fly Ash on the Durability of Concrete 10. Applications of Fly Ash in Special Concretes

The structure of the calcium silicate hydrate phases present in hardened pastes of white Portland cement/blast-furnace slag blends

Abstract: The C-S-H gels present in both water- and alkali-activated hardened pastes of white Portland cement/blast-furnace slag blends have been studied by solid-state 29Si magic angle spinning nuclear magnetic resonance (NMR) spectroscopy and analytical transmission electron microscopy (TEM). Structural data are obtained by NMR for the semi-crystalline C-S-H gels in the alkali-activated systems and extended to the nearly amorphous gels in the water-activated systems by peak broadening; unambiguous chemical analyses are determined in the TEM. The following conclusions apply to both the semi-crystalline and nearly amorphous C-S-H gels: (1) aluminium substitutes for silicon at tetrahedral sites; (2) aluminium only substitutes for silicon in the central tetrahedron of pentameric silicate chains; (3) the results strengthen confidence in dreierkette-based models for the structure of C-S-H. Compositional similarities suggest that these conclusions will be true for OPC/slag blends, and possibly also for OPC/pulverized fuel ash blends indicating that the same structural model applies to C-S-H gels in a wide range of hardened cement pastes.

Properties of some cement stabilised compressed earth blocks and mortars

Abstract: Findings from an on-going investigation into the effects of soil properties and cement content on physical characteristics of compressed earth blocks and soil mortars are presented. A series of test blocks were fabricated using a range of composite soils, stabilised with 5% and 10% cement, and compacted with a manual press. Results for saturated compressive strength, drying shrinkage, wetting/drying durability, and water absorption testing are presented in the paper. In conjunction with the block tests, workability and compressive strength characteristics of suitable soil: cement and cement: lime: sand mortars were also studied. Mortar consistency was assessed using cone penetrometer and slump tests. Water retention properties of the mortars were also measured. For a given compactive effort, the strength, drying shrinkage, and durability characteristics of the compressed earth blocks improved with increasing cement and reducing clay content. Slump testing proved the most reliable means of assessing soil: cement mortar consistency. Both the flow table and cone penetrometer tests were found to be unsuitable. Water retention properties of soil: cement mortars appear well-suited to typical unit water absorption characteristics. Mortar strengths were closely related to cement and clay contents, but as expected were less than the average unit strengths.

Effects of hydroxypropyl methylcellulose and other gelling agents on the handling properties of calcium phosphate cement

Abstract: The calcium phosphate cement (CPC) used in this study was formed by combining equimolar amounts of tetracalcium phosphate (TTCP) and dicalcium phosphate anhydrous (DCPA). This powder, when mixed with water, sets to a hard cement in about 30 min. However, the water-based CPC paste is not highly cohesive and is vulnerable to washout until hardening occurs. The objectives of this study were to investigate the effects on handling properties, washout resistance, cement hardening behavior, and mechanical properties of adding several gelling agents to CPC paste. Aqueous solutions that contained a mass fraction of 2-4% of hydroxypropyl methylcellulose (HPMC), carboxyl methylcellulose (CMC), chitosan acetate, and chitosan lactate were used as cement liquids. Hardening time was measured by the Gilmore needle test; resistance to washout was evaluated by the disintegration of the cement specimen in water with agitation; and mechanical strength was evaluated by the measurement of diametral tensile strength and compressive strength. Handling properties were greatly improved by the addition of HPMC, CMC, chitosan acetate, and chitosan lactate. Hardening time was retarded by the additions of HPMC and CMC, and mechanical strength was weakened by the addition of either the chitosan lactate or the chitosan acetate.

Foamed well cement compositions, additives and methods

Abstract: The present invention relates to improved foamed well cement compositions, additives and methods. The additives, which foam and stabilize a cement slurry containing freshwater or saltwater, are basically comprised of an aqueous solution of an alpha-olefinic sulfonate surfactant and a betaine surfactant.

Mechanical Properties of Modified Reactive Powder Concrete

Abstract: Original Reactive Powder Concrete (RPC) - in form of a superplasticized cement mixture with silica fume, steel fibers and ground fine quartz was studied in comparison with a modified RPC where a graded natural aggregate (max size 8 mm) was used to replace the fine sand and/or part of the cementitious binder. Original and modified RPC were manufactured at a plastic-fluid consistency, cast by vibration and cured at three different conditions: a) room temperature; b) steam-curing at 90 C; c) high pressure steam-curing at 160C. The addition of the graded aggregate does not reduce the compressive strength provided that the quality of the cement matrix, in terms of its water-cement ratio, is not changed. This result is in contrast with the model proposed to relate to high compressive strength level of RPC (200 MPa) to the absence of coarse aggregate. Both the original and modified RPC (with coarse aggregate addition) perform better - in terms of higher strength and lower drying shrinkage or creep strain - when they are steam cured rather than cured at room temperature. This improvement was related to a more dense microstructure of the cement matrix, particularly in the RPC specimens steam cured at 160 C. The main purpose of the present investigation was to modify RPC including some coarse aggregate in the mixture and to study the influence of the coarse aggregate on the properties of cement mixtures in terms of required mixing water, compressive and flexural strength, shrinkage, swelling and creep.

Measurement of Polyethylene Wear in Acetabular Components Inserted with and without Cement. A Randomized Trial

Abstract: We measured the three-dimensional wear of polyethylene after total hip arthroplasty with a titanium metal-backed Mallory-Head prosthesis that was inserted with cement in sixty-nine patients (sixty-nine hips) and with a press-fit titanium metal-backed Mallory-Head prosthesis that was inserted without cement in seventy patients (seventy hips) A modular titanium femoral head was used in all of the hips The patients in the present study were part of a larger double-blind randomized trial comparing the result of total hip arthroplasty performed with cement with that of the same procedure performed without cement in 250 patients The criterion for inclusion in the study of polyethylene wear was a minimum duration of follow-up of four years, which was met by 148 patients As adequate radiographs for digitization were not available for nine patients, 139 patients were included in the present study The age of the patient, the postoperative Harris hip score, the diameter of the femoral head, the thickness of the liner in the polar region of the acetabular component, and the duration of follow-up were similar for the two groups The mean rate of volumetric wear of the polyethylene was significantly greater in the prostheses that had been inserted without cement than in those that had been inserted with cement (1551 cubic millimeters per year compared with 985 cubic millimeters per year; p = 0000008) Thirty-four (49 per cent) of the seventy hips in which the prosthesis had been inserted without cement had evidence of osteolysis on radiographs, compared with twelve (17 per cent) of the sixty-nine hips in the other group (p = 00002) Osteolysis was associated with an increased rate of polyethylene wear only in the hips in which the prosthesis had been inserted without cement

The effect of cement composition and ph of environment on sulfate resistance of portland cements and blended cements

Abstract: This paper presents an investigation into the sulfate resistance of Portland cements and blended cements. Four Portland cements of different characteristics and blended cements containing fly ash, ground granulated blast furnace slag and silica fume were used in this work. The performances of binders were evaluated in sulfate solutions maintained at different pH levels ranging from 3 to 12 using expansion of mortar prisms (ASTM C 1012) and strength development of mortar cubes. The results indicate that sulfate resistance of cementitious materials is dependent on its composition and on the pH of the environment. Portland cement with low C3A and low C3S performed well in all sulfate solutions. Blended cements containing silica fume and fly ash (particularly at 40% replacement) showed a more superior performance than any of the Portland cements used. For slag blended cement, this can be achieved when the replacement percentage is higher than 60%.

A practical procedure for the analysis ofmoisture transfer within concrete due to drying

Abstract: The prediction of moisture transfer within concrete is necessary in order to evaluate its durability, because moisture transfer is related to shrinkage cracks and the transfer of chloride ions. Moisture transfer within concrete during drying has been analysed by many investigators using a non-linear diffusion equation. In the present paper, a comprehensive analytical procedure using a diffusion equation is established by comparing calculated results with experimental data for a wide range of concrete mix proportions. These experiments involved observing the effect of one-face drying and six-face drying on the water content profiles and mass decreases in prismatic specimens. The values of the coefficient required for the analysis are reported for a wide range of water/cement ratios, and the effect of these coefficients on the calculated results is discussed.