Showing papers on "Cement published in 2003"

Alkali-Activated Cements and Concretes

Abstract: Introduction 1. Alkaline Activators 2. Cementing Components 3. Hydration and Microstructure of Alkali-Activated Slag Cement 4. Properties of Alkali-Activated Slag Cement Pastes and Mortars (Under Both Atmospheric Pressure and Autoclave Conditions) 5. Properties of Alkali-Activated Slag Cement Concrete 6. Durability of Alkali-Activated Slag Cement and Concrete 7. Mix Design of Alkali-Activated Slag Cement Concrete 8. Alkali-Activated Portland Cement Based Blended Cement 9. Alkali-Activated Lime-Pozzolan Cement 10. Other Alkali-Activated Cementitious Systems 11. Applications of Alkali-Activated Cement And Concrete 12. Standards and Specifications

Physical and pozzolanic action of mineral additions on the mechanical strength of high-performance concrete

Abstract: Pozzolans play an important role when added to Portland cement because they usually increase the mechanical strength and durability of concrete structures. The most important effects in the cementitious paste microstructure are changes in pore structure produced by the reduction in the grain size caused by the pozzolanic reactions pozzolanic effect (PE) and the obstruction of pores and voids by the action of the finer grains (physical or filler effect). Few published investigations quantify these two effects. Twelve concrete mixtures were tested in this study: one with Portland cement (control), nine mixtures with 12.5%, 25% and 50% of replacement of cement by fly ash, rice husk ash and limestone filler; two with (12.5+12.5)% and (25+25)% of fly ash and rice husk ash. All the mixtures were prepared with water/binder ratios of 0.35, 0.50, and 0.65. The compressive strength for the samples was calculated in MPa per kg of cement. The remaining contents of calcium hydroxide and combined water were also tested. The results show that the pozzolanic and physical effects have increased as the mineral addition increased in the mixture, being higher after 91 days than after 28 days. When the results for the same strength values are compared (35 and 65 MPa), it was observed that the filler effect (FE) increased more than the pozzolanic effect. The PE was stronger in the binary and ternary mixtures prepared with rice husk ash in proportions of 25% or higher.

Development of vegetable fibre–mortar composites of improved durability

Abstract: The primary concern for vegetable fibre reinforced mortar composites (VFRMC) is the durability of the fibres in the alkaline environment of cement. The composites may undergo a reduction in strength and toughness as a result of weakening of the fibres by a combination of alkali attack and mineralisation through the migration of hydration products to lumens and spaces. This paper presents several approaches used to improve the durability performance of VFRMCs incorporating sisal and coconut fibres. These include carbonation of the matrix in a CO2-rich environment; the immersion of fibres in slurried silica fume prior to incorporation in the ordinary Portland cement (OPC) matrix; partial replacement of OPC matrix by undensified silica fume or blast-furnace slag and a combination of fibre immersion in slurried silica fume and cement replacement. The durability of the modified VFRMC was studied by determining the effects of ageing in water, exposure to cycles of wetting and drying and open air weathering on the microstructures and flexural behaviour of the composites. Immersion of natural fibres in a silica fume slurry before their addition to cement-based composites was found to be an effective means of reducing embrittlement of the composite in the environments studied. Early cure of composites in a CO2-rich environment and the partial replacement of OPC by undensified silica fume were also efficient approaches in obtaining a composite of improved durability. The use of slag as a partial cement replacement had no effect on reducing the embrittlement of the composite. © 2002 Published by Elsevier Science Ltd.

Mineralogical and microstructural characterisation of alkali-activated fly ash/slag pastes

Abstract: A mechanical, mineralogical and microstructural characterisation of the cement pastes obtained by alkaline activation of fly ash/slag mixtures cured at different temperatures has been carried out. The pastes obtained were characterised by XRD, FTIR, MAS NMR, SEM/EDX, atomic absorption and ion chromatography, also the insoluble residue in HCl was determined. The results obtained have proved the existence of two different reaction products in those activated pastes. The average atomic ratios in the main reaction product were Ca/Si∼0.8, Al/Ca∼0.6, Si/Al∼2–3. Such analysis corresponds to calcium silicate hydrate rich in Al, which includes Na in its structure. Other reaction product which was detected in the pastes as result of fly ash activation, was an alkaline aluminosilicate hydrate with a three-dimensional structure.

Engineered Steel Fibers with Optimal Properties for Reinforcement of Cement Composites

Abstract: Although steel fibers have been used in cement and concrete composites for more than four decades, most of the steel fibers on the market today have been introduced prior to 1980. This is in sharp contract to the continuous progress and development in the cement matrix itself. Following a brief summary of the main properties and limitations of steel fibers used in cement based composites, this paper describes the rationale and technical background behind the development and design of a new generation of steel fibers for use in cement, ceramic and polymeric matrices. These fibers are engineered to achieve optimal properties in terms of shape, size, and mechanical properties, as well as compatibility with a given matrix. They are identified as Torex fibers. Typical tests results are provided and illustrate without any doubt the superior performance (2 to 3 times) of Torex fibers in comparison to other steel fibers on the market. The new fibers will advance the broader use of high performance fiber reinforced cement composites in structural applications such as in blast and seismic resistant structures, as well as in stand-alone applications such as in thin cement sheet products.

Cement Manufacture and the Environment Part II: Environmental Challenges and Opportunities

Abstract: Summary Construction materials account for a significant proportion of nonfuel materials flows throughout the industrialized world. Hydraulic (chiefly portland) cement, the binding agent in concrete and most mortars, is an important construction material. Portland cement is made primarily from finely ground clinker, a manufactured intermediate product that is composed predominantly of hydraulically active calcium silicate minerals formed through high-temperature burning of limestone and other materials in a kiln. This process typically requires approximately 3 to 6 million Btu (3.2 to 6.3 GJ) of energy and 1.7 tons of raw materials (chiefly limestone) per ton (t) of clinker produced and is accompanied by significant emissions of, in particular, carbon dioxide (CO2), but also nitrogen oxides, sulfur oxides, and particulates. The overall level of CO2 output, about 1 ton/ton clinker, is almost equally contributed by the calcination of limestone and the combustion of fuels and makes the cement industry one of the top two manufacturing industry sources of this greenhouse gas. The enormous demand for cement and the large energy and raw material requirements of its manufacture allow the cement industry to consume a wide variety of waste raw materials and fuels and provide the industry with significant opportunities to symbiotically utilize large quantities of by-products of other industries. This article, the second in a two-part series, summarizes some of the environmental challenges and opportunities facing the cement manufacturing industry. In the companion article, the chemistry, technology, raw materials, and energy requirements of cement manufacture were summarized. Because of the size and scope of the U.S. cement industry, the article relies primarily on data and practices from the United States.

Strength development of ternary blended cement with limestone filler and blast-furnace slag

Abstract: The benefits of limestone filler (LF) and granulated blast-furnace slag (BFS) as partial replacement of portland cement are well established. However, both supplementary materials have certain shortfalls. LF addition to portland cement causes an increase of hydration at early ages inducing a high early strength, but it can reduce the later strength due to the dilution effect. On the other hand, BFS contributes to hydration after seven days improving the strength at medium and later ages. Mortar prisms in which portland cement was replaced by up to 20% LF and 35% BFS were tested at 1, 3, 7, 28 and 90 days. Results show that the contribution of LF to hydration degree of portland cement at 1 and 3 days increases the early strength of blended cements containing about 5–15% LF and 0–20% BFS. The later hydration of BFS is very effective in producing ternary blended cements with similar or higher compressive strength than portland cement at 28 and 90 days. Additionally, a statistical analysis is presented for the optimal strength estimation considering different proportions of LF and BFS at a given age. The use of ternary blended cements (PC–LF–BFS) provides economic and environmental advantages by reducing portland cement production and CO2 emission, whilst also improving the early and the later compressive strength.

Autogenous Deformation and Internal Curing of Concrete

Abstract: High-performance concrete (HPC) is generally characterized by a low water/binder ratio and by silica-fume addition, which guarantee a low porosity and a discontinuous capillary pore structure of the cement paste. Modern concretes possess some highly advantageous properties compared to traditional concrete, such as good workability in the fresh state, high strength, and low permeability. However, they have also shown to be more sensitive to early-age cracking than traditional concrete. Early-age cracking mainly occurs due to the fact that the deformations of the concrete member are restrained by adjoining structures. In addition, internal microcracking may occur, due to restraint offered to the shrinking paste by the non-shrinking aggregates. A main source of early-age deformations in HPC is autogenous deformation. Autogenous deformation is the self-created deformation of a cement paste, mortar or concrete during hardening. In traditional concretes it is negligible compared to drying shrinkage. However, the low water/binder ratio and the addition of silica fume in HPC cause a significant drop of the internal relative humidity (RH) in the cement paste during sealed hydration and the occurrence of autogenous shrinkage. Despite the growing interest in autogenous shrinkage, no consensus has been reached in the scientific community about its mechanisms neither about measuring methods. Moreover, different strategies aimed at limiting the autogenous shrinkage are debated at the moment. In this thesis, autogenous deformation of cement pastes, Normal Weight Concrete (NWC), and Lightweight Aggregate Concrete (LWAC) were measured. Both Portland and Blast Furnace Slag (BFS) cement were studied. A model for calculating self-desiccation shrinkage of cement paste was proposed and validated with experiments. Shrinkage of NWC was derived with a composite model and early-age expansion of LWAC, a puzzling phenomenon up to now, was explained. Finally, transport of water from saturated lightweight aggregates (LWA) to hardening cement paste was measured with x-ray absorption.

2 – Calcium aluminate cements

Abstract: In terms of the length of time it has been produced, the volume produced and the breadth of applications, calcium aluminate cements are by far the most important class of non-Portland cements. Calcium aluminate cements are a relatively large family with a range of compositions which varies much more widely than Portland cement. Today the two major markets for calcium aluminate cement are in castable refractories and in dry mix mortars for special construction applications, which together account for around 80% of consumption. The use in technical concretes, for example, sewer linings, rapid repair, etc. is rather small. Calcium aluminate cements are known for their rapid strength gain, especially at low temperatures, superior durability across several categories and high temperature resistance. Their ability to consume water rapidly during hydration makes them a preferred component in building chemistry applications as this contributes to construction expediency. CACs are highly versatile materials that can be used as the full binding material, or as is more common, a component of a blended system where the contribution is based on the final desired properties.

Industrial trial production of low energy belite cement

Abstract: The Portland cement industry consumes large amounts of energy and produces huge quantities of carbon dioxide, which contribute to global warming, the so-called “Greenhouse Effect”. Industrial trials are reported for the production of belite cements (≈3000 t) at lower temperatures and with lower lime saturation factors than for ordinary Portland cement. Belite cements with reasonably good properties have been made on an industrial scale from limestone, burnt clay, volcanic ash, pyrite ash and gypsum. A rapid rate of cooling improves the hydraulic activity, and also the physical–mechanical properties by stabilising reactive forms of belite. Such low energy cements provide a cheap alternative to Portland cement with properties that are acceptable for many applications and the additional benefit of possible improved durability.

Development of Bottom Ash as Pozzolanic Material

Abstract: This research studies the potential of using bottom ash from the Mae Moh power plant in Thailand as a pozzolanic material. Bottom ash, which was rarely used in concrete due to its inactive pozzolanic reaction, improved its quality by grinding until the particle size retained on Sieve 325 was less than 5% by weight. Bottom ashes before and after being ground were investigated and compared for their physical and chemical properties. The bottom ashes were used to replace portland cement type I in mortar and concrete mixtures. The results indicated that the particle of bottom ash was large, porous, and irregular shapes. The grinding process reduced the particle size as well as porosity of the bottom ash. Compressive strengths of mortar containing 20–30% of bottom ash as cement replacement were much less than that of cement mortar at all ages, but the use of ground bottom ash produced higher compressive strength than the cement mortar after 60 days. When ground bottom ash was used at a 20% replacement of cemen...

Sugar cane bagasse fibre reinforced cement composites. Part I. Influence of the botanical components of bagasse on the setting of bagasse/cement composite

Abstract: Various bagasse fibre/cement composites have been prepared, the fibres having a random distribution in the composites. The influence of different parameters on the setting of the composite material has been studied: (1) botanical components of the fibre, (2) thermal or chemical treatment of the fibre, (3) bagasse fibre content and (4) added water percentage. This study shows a retarding effect of lignin on the setting of the composite, for small amount of heat-treated bagasse (200 °C) the behaviour of the composite is closely the same as the classical cement or cellulose/cement composite.