Showing papers on "Cement published in 1996"

The use of limestone in portland cement: a state-of-the-art review

Abstract: Data from published literature and laboratory tests regarding limestone additions to portland cement are reviewed. Emphasis is placed on additions of 5% or less. The effects of interground and blended limestone on the particle size distribution of cement and workability of mortar and concrete are reported. Hydration behavior is examined with regard to chemistry, heat evolution, microstructure, and setting time. The properties of hardened mortar and concrete made with limestone portland cement are examined and compared to those made with non-limestone portland cements - including compressive and flexural strength, volume stability, durability (permeability, carbonation, freeze/thaw resistance, sulfate and chloride resistance, and alkali-silica reaction), and interactions with mineral and chemical admixtures. Quality control of the limestone, limestone portland cement, and concrete is also discussed. In general, the addition of up to 5% limestone does not affect the performance of portland cement. Strengths of cements with limestone can be optimized by grinding to an appropriate particle size distribution. An optimized portland cement containing up to 5% limestone can exhibit improvements in workability as compared to the same cement without limestone. Energy requirements for both pyroprocessing and comminution are reduced by the addition of limestone.

Porosity reduction in bone cement at the cement-stem interface

Abstract: The fatigue failure of bone cement, leading to loosening of the stem, is likely to be one mode of failure of cemented total hip replacements. There is strong evidence that cracks in the cement are initiated at voids which act as stress risers, particularly at the cement-stem interface. The preferential formation of voids at this site results from shrinkage during polymerisation and the initiation of this process at the warmer cement-bone interface, which causes bone cement to shrink away from the stem. A reversal of the direction of polymerisation would shrink the cement on to the stem and reduce or eliminate the formation of voids at this interface. We have investigated this by implanting hip prostheses, at room temperature or preheated to 44°C, into human cadaver femora kept at 37°C. Two types of bone cement were either hand-mixed or vacuum-mixed before implantation. We found that the area of porosity at the cement-stem interface was dramatically reduced by preheating the stem and that the preheating temperature of 44°C determined by computer analysis of transient heat transfer was the minimum required to induce initial polymerisation at the cement-stem interface. Temperature measurements taken during these experiments in vitro showed that preheating of the stem caused a negligible increase in the temperature of the bone. Reduction of porosity at the cement-stem interface could significantly increase the life of hip arthroplasties.

Engineering behavior of cement-treated bangkok soft clay

Abstract: This paper presents the physical characteristics, index properties, unconfined compressive strength and compressibility characteristics of cement-treated soft Bangkok clay. The cement-treated samples have reduced water contents and the reduction ceased after 30% cement content. The change in the liquid limit were insignificant while the plastic limit significantly increased. The unconfined compressive strength increased from 15 kPa to 35 kPa with cement content of 10% to 20% for curing time of 28 days. The undrained behavior of cement-treated clays corresponded to 3 zones, namely: Zone I (untreated), Zone II (treated), and Zone III (softening part). Phase transformation was manifested by the abrupt change in the curvature of the stress paths. Higher cement content as much as 10% to 25% was found effective in reducing the compressibility of the clay. For cement-treated clays, the coefficient of consolidation increased by 12 to 19 times with the highest increase at 15% cement content. The reduction of compression index reach optimum also at 15% cement content.

Kinetic study of the setting reaction of a calcium phosphate bone cement

Abstract: The setting reaction of a calcium phosphate bone cement consisting of a mixture of 63.2 wt % alpha-tertiary calcium phosphate (TCP)[alpha-Ca3(PO4)2], 27.7 wt % dicalcium phosphate (DCP) (CaHPO4), and 9.1 wt % of precipitated hydroxyapatite [(PHA) used as seed material] was investigated. The cement samples were prepared at a liquid-to-powder ratio of: L/P = 0.30 ml/g. Bi-distilled water was used as liquid solution. After mixing the powder and liquid, some samples were molded and aged in Ringer's solution at 37 degrees C. At fixed time intervals they were unmolded and then immediately frozen in liquid nitrogen at a temperature of TN = -196 degrees C, lyofilized, and examined by X-ray diffraction as powder samples. The compressive strength versus time was also measured in setting samples of this calcium phosphate bone cement. The crystal entanglement morphology was examined by scanning electron microscopy. The results showed that: 1) alpha-TCP reacted to a calcium-deficient hydroxyapatite (CDHA), Ca9(HPO4)(PO4)5O H, whereas DCP did not react significantly; 2) the reaction was nearly finished within 32 h, during which both the reaction percentage and the compressive strength increased versus time, with a strong correlation between them; and 3) the calcium phosphate bone cement showed in general a structure of groups of interconnected large plates distributed among agglomerations of small crystal plates arranged in very dense packings.

IR and NMR Analyses of Hardening and Maturation of Glass-ionomer Cement

Abstract: It has been reported that the silicate phase as well as the cross-linking of the polycarboxylic acid by aluminum and calcium ions played an important role in the hardening of glass-ionomer cement. The objective of this study was to investigate the structural change during hardening of the cements by means of infrared (IR) spectroscopy and solid-state nuclear magnetic resonance (NMR) spectroscopy and to confirm the role of the silica phase in the hardening of the cement. For that purpose, we measured the change in compressive strength of an experimental glass-ionomer cement, two commercial glass-ionomer cements, and a polycarboxylate cement and carried out 29Si and 27 Al NMR analyses of the cement samples after the strength measurement. In the IR spectra during hardening, a characteristic band of the silicate network around 1000 cm-1 shifted toward high frequency with time. The spectrum after hardening was similar to that for a hydrated amorphous silica structure. The 27Al NMR analysis showed that Al3+ ion...

Effects of ground granulated blast furnace slag (ggbs) on the strength and swelling properties of lime-stabilized kaolinite in the presence of sulphates

Abstract: The use of ground granulated blast furnace slag (ggbs) is well established in many cement applications where it provides enhanced durability, including high resistance to chloride penetration, resistance to sulphate attack and protection against alkali silica reaction (ASR). The use of ggbs in soil stabilization is, however, still a novel process in the UK although it has been used in South Africa. This paper reports on efforts to extend the use of ggbs to highway and other foundation layers by determining the beneficial effect which it has on the reduction of expansion due to the presence of sulphates. The paper describes the results of laboratory tests on lime-stabilized kaolinite containing different levels of added sulphate to which different amounts of ggbs have been added. The tests determine the strength development of compacted cylinders, moist cured in a humid environment at 30°C, and the linear expansion of these moist cured cylinders on soaking in water. The results illustrate that small additions of ggbs to sulphate containing clays which are stabilized with lime reduce substantially their expansion when exposed to water and have no significant deleterious effect on strength development.

Mechanical effects of stem cement interface characteristics in total hip replacement

Abstract: Stem cement debonding is 1 of the most common forms of fixation failure and is thought to be a prelude to gross loosening of a total hip reconstruction. However, the immediate consequences of debonding remains a matter of controversy. The dynamic effects of stem cement debonding in total hip reconstruction were analyzed using 3-dimensional finite element techniques. Stem cement interface conditions were assumed as completely bonded or unbonded, with or without friction. The dynamic effects were accounted for, as presented by the stance and swing phases of the gait cycle. It was found that both cyclic micromotions at the stem cement interface and stresses in the cement mantle were effectively reduced by friction. The friction cases produced failure probabilities of the cement mantle that were relatively close to the one generated by the bonded stem. The probability of mechanical failure of the cement bone interface decreased after debonding and decreased more with reduced stem cement friction. These results show that, although a firm and lasting bond between stem and cement may be desirable for preventing cement failure, the mechanical effects of a debonded stem are less detrimental than were assumed earlier. For straight tapered stem shapes subjected to the loading conditions described, a polished stem may be desirable for the cement bone interface mechanics.

Investigation of lignite-based bottom ash for structural concrete

Abstract: A pilot study was undertaken on the use of lignite-based bottom ash as a fine aggregate in structural grade concrete. Bottom ash is combined with portland cement, crushed limestone coarse aggregate, and water to produce concretes with a uniform slump of 100 mm (4±¼ in.). Four different mixes with cement contents ranging from 297 to 475 kg/m³ (500 to 800 lb/yd³) are utilized. The bottom ash concretes are studied for fresh properties, hardened characteristics, and long-term durability. The engineering characteristics of mixtures made with bottom ash are compared with those of conventional concretes in order to evaluate the effectiveness and suitability of bottom ash as a viable fine aggregate in portland cement-based mixtures. Laboratory test results conclude that the inclusion of bottom ash increases the demand for mixing water in obtaining the required workability. As a consequence, both fresh and hardened properties are impacted, particularly for mixtures of low cement content. When a water-reducing admixture is used, the engineering properties of bottom ash concretes are similar, and in most cases superior, to those of control concretes.

Some effects of cement and curing upon carbonation and reinforcement corrosion in concrete

Abstract: Experimental data are presented to illustrate the effects of cement type and curing upon the depth of carbonation and reinforcement corrosion in cover concrete after exposure for 18 months at 20°C and 60% relative humidity. Three curing periods (1, 3 and 28-days) and 17 cements, with various proportions of granulated blastfurnace slag or limestone, were used to make concretes, at 0.59 water/cement ratio, with 28 day strengths in the range 26 to 46 MPa. The depth of carbonation after 18 months was 64% greater than after 6 months and was affected more by cement type than by curing. The depth of carbonation increased when Portland cement clinker was replaced by 19% or more of limestone or granulated blastfurnace slag. The depth of carbonation after 18 months correlated better with the air permeability of cover concrete, initial weight loss (an indicator of moisture diffusion rate in cover concrete) or the cube strength 8 days after the end of curing than it did with 28-day cube strength. The rate of reinforcement corrosion increased steeply when the carbonation front approached the reinforcing steel, and it was still increasing after the carbonation front had completely passed the reinforcement. For a given unneutralised remainder (i.e. cover depth minus the depth of carbonation), curing had little effect upon the rate of corrosion but higher rates were observed when the cement contained granulated blastfurnace slag. The results were broadly consistent with a simple engineering strategy in which the rate of carbonation was related to the air permeability of cover concrete, and the rate of any subsequent reinforcement corrosion was largely dependent upon moisture conditions, without any obvious influence of the cover depth or the permeability of the cover concrete. The results also suggested that estimation of the rate of reinforcement corrosion could be improved by taking account of the cement type and treating the unneutralised remainder as a variable.

Initial hydration reactions and mechanisms of delayed ettringite formation in Portland cements

Abstract: The aluminate hydration reactions in Portland cements were followed by X-ray diffraction. Using a computerized focusing Huber-Guinier diffractometer, the patterns were obtained from the pastes directly, without preliminary drying or grinding. It is shown that the widely accepted theories on the hydration of C3A in the presence of calcium sulfate cannot be applied generally to commercial Portland cements. Only in cases of pastes nearly free of CO2 will the ettlingite formed in the course of the induction period transform to monosulfate. In cement pastes containing more than about 0.5% CO2 the conversion is prevented and monosulfate is replaced by hemicarbonate or monocarbonate. The quantity of CO2 sufficient to replace monosulfate by carbonated AFm phases depends on the ratio of SO3 to C3A and the amount of available Al2O3. The results clearly show that with cements or concretes containing monosulfate the risk of delayed ettringite formation as a result of carbonation reactions should be taken into account. The interlayer sulfate groups in the crystal structure of monosulfate can be replaced by carbonate. This exchange results in increasing sulfate concentration in the pore solution and thus leads to delayed ettringite formation.

The effect of cement composition and fineness on expansion associated with delayed ettringite formation

Abstract: A large number of laboratory ground cements were prepared from five production clinkers. Grind variables included cement SO 3 , source of added SO 3 and specific surface area. Mortar prisms were cast as described in EN 196-1 except that the prisms were 16 × 16 × 160 mm with stainless steel pins cast in their end faces. Sets of prisms were cured for 12 h at 70 °, 80 ° and 90 °C in addition to the standard storage at 20 °C. After demoulding at 24 h the prisms were stored in water at 20 °C and lengths and weights monitored for periods up to ~ 1800 days. Expansions typical of DEF were normally only observed in prisms cured at 90 °C. The magnitude of the expansion increased with fineness and passed through a maximum at a cement SO 3 content of ~ 4%. No expansions were obtained with a low C 3 A sulfate resisting clinker but there was little difference in the behaviour of the other clinkers. Natural gypsum, natural anhydrite and desulfurisation gypsum gave similar expansions. Adding KOH or K 2 SO 4 dissolved in the mixing water accelerated and increased expansions greatly. Some mortars containing added alkali expanded after curing at 80 °C.

Tensile Behavior of Cement‐Based Composites with Random Discontinuous Steel Fibers

Abstract: In this paper, the tensile properties of cement-based composites containing random discontinuous steel fibers are reported. Direct tensile tests were performed to study the effects of fiber length (hence fiber aspect ratio), interfacial bonding, and processing conditions on composite properties. Composite tensile strength and ductility are highlighted and discussed.