Showing papers on "Cement published in 1990"

Durability of concrete incorporating high-volume of low-calcium (ASTM Class F) fly ash

Abstract: Research on structural concrete incorporating high volumes of low-calcium (ASTM Class F) fly ash has been in progress at CANMET since 1985. In this type of concrete, the cement content is kept at about 150 kg/m3. The water-to-cementitious materials ratio is of the order of 0·30, and fly ash varies from 54 to 58% of the total cementitious material. A large dosage of a superplasticizer is used to achieve high workability. This paper presents data on the durability of this new type of concrete. The durability aspects considered are: freezing and thawing cycling; resistance to chloride ion permeability; and the expansion of concrete specimens when highly reactive aggregates are used in the concrete. The investigations performed at CANMET indicate that concrete incorporating high volumes of low-calcium fly ash has excellent durability with regard to frost action, has very low permeability to chloride ions and shows no adverse expansion when highly reactive aggregates are incorporated into the concrete.

Ash from Oil-Palm Waste as a Concrete Material

Abstract: The palm‐oil industry produces large amounts of solid wastes. Shell and fiber wastes are used extensively as fuel for steam production in palm‐oil mills. After combustion, a large quantity of ash is produced and creates problems of disposal. The feasibility of using the shell and fiber ash as a construction material is studied. The material differs from PFA from coal‐fired power plants in that it has a higher content of residual organic, a higher alkali content, is coarser. The experimental results indicate that no significant effects of ash addition on the segregation, shrinkage, water absorption, density, and soundness of cement. The workability of concrete blended with the shell and fiber ash is good, and setting times are well within the requirements of both American and British standards. The shell and fiber ash is only weakly pozzolanic, the decrease in compressive strength of concrete is almost proportional to the amount of ash in the blended cement, except when only 10% ash is used. The results sh...

Immobilization mechanisms in solidifiction/stabilization of cadmium and lead salts using portland cement fixing agents

Abstract: The authors have investigated the behavior of Cd and Pb salts toward cement-based solidification using TCLP leaching tests, conduction calorimetry, and solid-state NMR as a function of time. Concentrations of Cd in leachates are very low, while Pb concentrations are considerably higher and would represent a serious threat to groundwater. The Cd/cement system involves Cd(OH){sub 2}, while provides nucleation sites for precipitation of calcium silicate hydrate (C-S-H) gel and calcium hydroxide, resulting in Cd being in the form of the insoluble hydroxide with a very impervious coating. On the other hand, the Pb/cement system involves hydroxide, sulfate, and nitrate mixed salts, which retard cement hydration reactions by forming an impervious coating around cement clinker grains. However, as pH in the cement pore waters undergoes fluctuations during the progress of hydration, the Pb salts undergo solubilization and reprecipitation on leachable surfaces of the cement matrix.

The carbonation of hardened cement pastes

Abstract: The microstructure of hardened pastes of C3S and a C3S/silica fume blend have been examined by transmission electron microscopy before and after partial carbonation in pure CO2 at a relative humidity of 72·6%. Fibrillar outer product C-S-H and the outer regions of inner product C-S-H gel carbonate without change of morphology to silica gel. Calcium carbonate is formed mainly in outer product regions as microcrystalline vaterite or calcite. Residual CH embedded in vaterite microcrystals is observed in the carbonated C3S pastes. It is proposed that outer product C-S-H and CH surfaces carbonate relatively rapidly, compared with inner regions of CH or the inner regions of inner product C-S-H, leading to a substantial level of carbonation of thin slices (0·4 mm) of paste within one day, with little further carbonation in the next few days. The implications for the theory of the kinetics of cement paste carbonation are discussed.

Stabilizing Compacted Clay against Chemical Attack

Abstract: Concentrated organic chemicals have been shown to cause large increases in the hydraulic conductivity of compacted clay. Mechanical and chemical methods of stabilizing four different types of compacted clay against chemical attack are investigated. Mechanical stabilization using large compactive effort (modified Proctor compaction) or application of a compressive stress ≥\N 10 psi (70 kPa) is found to render a compacted clay invulnerable to attack by concentrated organic chemicals under laboratory-test conditions. Attapulgite, a clay mineral having little electrical charge, was found to be relatively unaffected (compared to more common clay minerals such as kaolinite, illite, and smectite) by concentrated organic chemicals. Addition of approximately 7% (by weight) of lime, portland cement, or lime plus sodium silicate greatly improved the ability of compacted clay to resist attack by concentrated organic chemicals; in some cases the amended soils were less permeable to concentrated organic chemicals than the unamended soils were to water.

Sulphate attack on concrete

Abstract: The types of disruption observed during sulphate attack on mortars or concretes, and the chemical processes involved, are summarized. Concrete mix design and binder type have an important influence on concrete performance in sulphate environments. The present UK Codes of Practice (based on BRE Digest 250) reflect the technical information available for various binders currently used in the UK. Sulphate-resisting Portland cement is a purpose-made cement designed to resist sulphates. The chemical factors giving rise to its sulphate resistance are well understood, and its performance has been uniformly good in experimental trials throughout the world. This cement is the standard by which other binders are judged, and should continue to be the preferred binder for concretes expected to withstand sulphate concentrations in ground waters greater than 3000 mg SO4− −/l. Although partial replacement of ordinary Portland cement (OPC) by various latent hydraulic binders can lead to improved sulphate resistance, the ...

Mechanisms of hydration reactions in high volume fly ash pastes and mortars

Abstract: This paper describes investigations of high-volume fly ash (HVFA)-Portland cement (PC) binders, the physical and chemical properties of which have been characterized up to 365 days of curing Physical investigations were made of compressive strength development, pore structure by porosimetry, and morphology by scanning electron microscopy Chemical examination was conducted for solid phase composition and degree of hydration by X-ray diffraction and thermal analysis, and for pore-fluid composition by high pressure extraction and analysis Up to 365d the cement in the HVFA pastes is not fully hydrated However, the ash participates in both early (sulpho-pozzolanic) and late (alumino-silicate) hydration reactions In addition to the usual products of cement hydration, ettringite (AFt) has been identified as a product of the early hydration of the fly ash It has not been possible to identify long term hydration products of fly ash which appear to be non-crystalline A two-step mechanism for pozzolanic reaction between fly ash and Portland cement has been proposed involving: (a) depolymerization/silanolation of the glassy constituents of the ash by the highly alkaline pore fluids, followed by (b) reaction between solubilized silicate and calcium ions in solution to form CSH

Studies on mechanics of development of physical and mechanical properties of high-volume fly ash-cement pastes

Abstract: High-volume fly ash concrete for structural applications was developed at CANMET. In this concrete fly ash to ‘total cementitious material’ was maintained over 55%. The purpose of this work was to investigate, by the use of similar paste mixtures of the same fly ash and cement, the mechanism by which the mechanical properties were developed. Mechanical property-porosity relations, pore size distribution, permeability, degree of hydration and Ca(OH)2 content measurements were made. It was observed that the fly ash-cememt reaction occurred relatively early at 3 to 7 days and it was concluded that the cement matrix and residual unreacted fly ash form a good mechanical bond.

Variation in powder/liquid ratio of a restorative glass-ionomer cement used in dental practice

Abstract: Variation in powder/liquid ratio of a restorative glass-ionomer cement used in dental practice

Method of producing shaped articles of fiber/binder mixtures

Abstract: Paper or cellulose pieces forming a fiber raw material are wetted with water prior to comminution so that the comminution into fibers is effected with the moistened raw material. The moisture content of the fibers makes up at least part of the water of hydration required to completely set the binder which is mixed with the fibers. The binder can be plaster (gypsum) or hydraulic cement. The mixture is pressed to the desired shape and heat may be applied.

The Mechanism for Erosion of Glass-ionomer Cements in Organic-Acid Buffer Solutions:

Abstract: The behavior and mechanism for erosion of glass-ionomer cements in organic-acid buffer solutions were studied as a function of time, pH, and citric-acid concentration. In acidic solutions, the dissolution of the cement was controlled by the diffusion of the eluted species in the cement matrix, which depended on H+ ion concentration. In citric-acid solutions, the dissolution of the cement was controlled by both the diffusion and the surface reaction between the acid anion and the eluted species. Contribution of the latter reaction was larger with the increase in the acid concentration.

High-Strength and Flowing Concrete with a Zeolitic Mineral Admixture

Abstract: Zeolitic mineral admixture (ZMA) is made of the finely divided powder of natural zeolite with a bit of other agent such as triethanolamine. When ZMA is used to displace about 10% (by mass) of the ordinary portland cement (OPC) (strength grade No. 525) in concrete and mixed with a suitable amount of superplasticizer (W/C = 0.31 to 0.35), then a high-strength concrete with compressive strength of about 80 MPa and a slump of about 180 mm can be obtained. The strength of this concrete is about 10 to 15% higher than that of the corresponding concrete mixed with pure OPC, and its bleeding decreases greatly. It also results in no segregation or separation of the mix, and thus it satisfies the requirement of pumping concrete in construction. The ZMA is suitable not only for the OPC but also for the slag portland cement. The strengthening effect of the ZMA is somewhat similar to that of silica fume. But the cost is only two thirds that of OPC. Thus, when ZMA is used to displace a certain amount of the cement in the concrete, the cost of the concrete thus made will be 3 to 5% cheaper than that of the corresponding concrete with pure cement. The ZMA can increase the amount of micropores (d 938 A) in the cement paste. Hence, the strength of concrete is increased and its other properties are also improved. Furthermore, ZMA can raise the SiO2/CaO weight ratio in the transition zone to increase its C-S-H phase and decrease its calcium hydroxide content. Thus, the structure of the transition zone is improved. Consequently, the strength and resistance to permeability of the concrete are increased.

Influence of cement composition on the corrosion of reinforcement and sulfate resistance of concrete

Abstract: Performance data based on accelerated corrosion-monitoring and exposure site tests indicate that cement type, reflecting particularly the C3A content, significantly affected concrete durability with respect to corrosion of reinforcing steel. On average, Type I cement (C3A=9.5%) performed 1.7 times better than Type V cement (C3A=2.8%) in terms of time of initiation of corrosion. Accelerated sulfate-rsistance tests show that a 20% microsilica blended with Type I 14% C3A cement performed 1.4 times better against sulfate attack than a Type V portland cement with 1.88 % C3A. Also, sulfate deterioration data indicate that, in addition to the C3A content, the C3S/C2S ration of the cement has a significant effect on the sulfate resistance of the cement. The sulfate deterioration of a Type I portland cement (C3A-11.9%) with a C3S/C2S of 7.88 was found to be 2.5 times the deterioration for another Type I portland cement (C3A=9.3%) with a C3S/C2S of 2.57 after 150 days of exposure to an accelerated sulfate test. Significnat retrogression of strength was observed on immersion in sulfate solution even for a Type V(C3A-1.88%) sulfate-resisting cement that had a high C3S/C2S ratio of 5.28.

Some engineering properties of slag concrete as influenced by mix proportioning and curing

Abstract: This paper presents a simple method to obtain a 50 MPa 28-day strength concrete having 50 and 65 % by weight cement replacements with slag having a relatively low specific surface. The method produces slag concrete with strengths comparable to ordinary portland cement concrete from 3 days onward. The compressive and flexural strengths and the elastic modulus of these two concretes as affected by curing conditions are then presented. Prolonged dry curing is shown to adversely affect tensile strength and elastic modulus, and to create internal microcracking, as identified by pulse velocity mearurements. High swelling strains at high slag replacement levels show the need for longer wet curing for such concretes. The results emphasize that even 7-day wet curing was inadequate for high levels of slag replacement, and that continued exposure to a drying environment can have adverse effects on the long-term durability of inadequately cured slag concrete.