Showing papers in "Cement and Concrete Research in 1999"

Alkali-activated fly ashes: A cement for the future

Abstract: The alkali activation of waste materials (especially those coming from industrial and mining activities) has become an important area of research in many laboratories because it is possible to use these materials to synthesize inexpensive and ecologically sound cementlike construction materials. In the present paper, the mechanism of activation of a fly ash (no other solid material was used) with highly alkaline solutions is described. These solutions, made with NaOH, KOH, water glass, etc., have the common characteristic of having a very high OH 2 concentration. The product of the reaction is an amorphous aluminosilicate gel having a structure similar to that of zeolitic precursors. Temperature and time of curing of specimens together with the solution/fly ash ratio are some of the variables that were studied. These variables have been shown to notably influence the development of the mechanical strength of the final product. Mechanical strengths with values in the 60 MPa range were obtained after curing the fly ash at 85 8 C for only 5 h. © 1999 Elsevier Science Ltd. All rights reserved.

Crystallization in pores

Abstract: This review discusses the thermodynamics of crystallization within porous materials and the factors that influence stress development and cracking. The maximum driving force for crystallization is related to the supersaturation for crystals growing in solution, and to the undercooling for crystals growing from a melt. However, the stresses generated on the pore walls depend on other factors, including the pore size, the energy (γcs) of the interface between the pore wall and the crystal, and (for acicular crystals) the yield stress or buckling strength of the crystal. The fact that growing crystals push particles over large distances indicates that γcs is often large. If γcs were small, crystals would tend to nucleate on pore walls rather than pushing them away, and the crystals would propagate through the pore network without resistance. Even when the crystallization pressure is large, the stress existing in a single pore cannot cause failure because it acts on too small a volume. For fracture to occur, the crystals must propagate through a region of the network large enough that the stress field can interact with the large flaws that control the strength. In concrete, growth on this scale requires that the driving force be sufficient to permit the crystals to pass through pores as small as the breakthrough radius (which is the size of the entry into the percolating network of larger pores that controls the permeability of the body).

The nature of C-S-H in hardened cements

Abstract: Calcium silicate hydrates (C-S-H) are the main binding phases in all Portland cement-based systems. This paper considers the morphology, composition, and nanostructure of C-S-H in a range of hardened cements. Inner product (Ip) C-S-H present in larger Portland cement grains typically has a fine-scale and homogeneous morphology with pores somewhat under 10 nm in diameter. Ip from larger slag grains also displays this morphology, but is chemically distinct in having high content of Mg and Al. The hydrated remains of small particles—whether of Portland cement, slag or fly ash—contain a less dense product with substantial porosity surrounded by a zone of relatively dense C-S-H; this has implications for the analysis of porosity and pore-size distributions on backscattered electron images. In cement-slag blends, the fibrillar morphology of outer product (Op) C-S-H is gradually replaced by a foil-like morphology as the slag loading is increased. It seems likely that this change in morphology is largely responsible for the improved durability performance possible with slag-containing systems. The Ca/Si ratio of C-S-H in neat Portland cement pastes varies from ∼1.2 to ∼2.3 with a mean of ∼1.75. The Ca/(Si + Al) ratio of C-S-H in water activated cement-slag pastes (0–100% slag) varies from ∼0.7 to ∼2.4; these limits are consistent with dreierkette-based models for the structure of C-S-H. Al substitutes for Si in C-S-H only in the “bridging” tetrahedra of dreierkette chains; this is true for a range of systems, including blends of Portland cement with slag, fly ash, and metakaolin. These data support Richardson and Groves' general model for substituted C-S-H phases. The bonding of C-S-H to other products of hydration is generally good.

Alkali-activated cements Opportunities and challenges

Abstract: Alkali-activated cements as discussed here are those with compositions falling in the Me2O-MeO-Me2O3-SiO2-H2O system. This paper reviews their history of development and discusses their present status. Currently, there are major opportunities for such cements based upon (a) substantial knowledge of properties and mechanisms; (b) good track record of field performance in various applications and; (c) future orientation as environmentally friendly materials in accord with making use of substantial amounts of by-product and waste materials, thereby consuming less energy and generating less waste. The equivalent performance to Portland cement materials is one target for these cements, but, in many cases, the properties of alkali-activated cements actually are superior. It is important for assuring long-term durability to characterize more fully the complex solid phases, including determining the combined state of alkali in the solid hydration products, and of the residual soluble species in the pore fluids as a function of time.

Modelling chloride diffusion in concrete: Effect of fly ash and slag

Abstract: Data from long-term field and laboratory studies of concrete exposed to chloride environments were analyzed using a chloride transport model developed at the University of Toronto. The results show that the incorporation of fly ash and slag may have little impact on transport properties determined at early ages (e.g., 28 days), but can lead to order of magnitude improvements in the long term. This means that the rate of chloride penetration during the first 6 months or so of exposure is similar for concretes with and without these materials. However, after a few years of exposure, chloride ingress slows to a much-decreased rate in fly ash and slag concretes, leading to dramatic increases in the predicted service life. Predictive models and laboratory test methods for determining chloride ingress should take account of the time-dependent nature of the transport processes in concrete, especially when supplementary cementing materials, such as fly ash or slag, are used.

Alkali-activated slag mortars: Mechanical strength behaviour

Abstract: The objective of the present work is to know the joint influence of a series of factors (specific surface of the slag, curing temperature, activator concentration, and the nature of the alkaline activator) on the development of mechanical strengths in alkaline-activated slag cement mortars. To reach this aim, a factorial experimental design was carried out (a complete 2 3 × 3 1 design) for every age studied (3 to 180 days). Through the variance analysis, the most significant factor on the response turned out to be the alkaline activator nature. The activator used, Na 2 SiO 3 · nH 2 O + NaOH, was the factor that gave the highest mechanical strengths in all tests. The next most statistically significant factor was the activator concentration, followed by curing temperature, and, finally, the specific surface of the slag. The equations of the model describing the mechanical behaviour for flexural and compressive strengths and their relationships for each age studied were established

Chemical stability of cementitious materials based on metakaolin

Abstract: The alkali activation of metakaolin is a way of producing high strength cementitious materials. The processing of these materials has been the subject of numerous investigations. The present paper describes the results of a research project initiated to study the stability of these materials when exposed to aggressive solutions. Prisms of mortar made of sand and alkali-activated metakaolin were immersed in deionized water, ASTM sea water, sodium sulfate solution (4.4% wt), and sulfuric acid solution (0.001 M). The prisms were removed from the solutions at 7, 28, 56, 90, 180, and 270 days. Their microstructure was characterized and their physical, mechanical, and microstructural properties were measured. It was observed that the nature of the aggressive solution had little negative effect on the evolution of microstructure and the strength of these materials. It was also found that the 90-day and older samples experienced a slight increase in their flexural strengths with time. This tendency was most pronounced in those samples cured in sodium sulfate solutions. This behavior may be related to the change in microstructure of the cementitious matrix of the mortars cured longer than 90 days. Some of the amorphous material present had crystallized to a zeolite-like material belonging to the faujasite family of zeolites.

Characterization and identification of equilibrium and transfer moisture properties for ordinary and high-performance cementitious materials

Abstract: In this paper some aspects of equilibrium and transfer moisture properties of high-performance materials are presented and compared with ordinary cement pastes and concretes. First, the equilibrium moisture properties of the hardened materials are described by means of water vapour sorption isotherms, which illustrate the hysteretical behaviour of the materials. Experimental results of drying shrinkage versus relative humidity (RH) are also reported here. These experimental data are in good agreement with the numerical results provided by a thermodynamic modelling based on capillary stresses and hygromechanical couplings. In particular the linearity of the strains-RH curve over a wide range is pointed out in both cases. Isothermal drying process at RH = 50% has experimentally and numerically been studied. After identification of the intrinsic permeability of the materials from experimental weight losses, numerical moisture profiles were compared with gamma-ray attenuation measurements. The influence of the initial moisture state of the materials that results from self-desiccation in particular was pointed out on the evolution of the moisture profiles as a function of time.

Mercury porosimetry of hardened cement pastes

Abstract: Mercury porosimetry was performed on 92 hardened cement paste specimens of water/cement (w/c) ratios 0.3, 0.4, 0.5, 0.6, and 0.7 and curing times of 1, 3, 7, 14, 28, and 56 days. This paper presents the experimental techniques, results, and their possible implications with respect to pore connectivity. As expected, longer curing times and lower w/c ratios resulted in smaller indicated total porosities and smaller threshold pore widths. Longer curing times and higher w/c ratios resulted in greater degrees of hydration. In most of the mercury intrusion results, two peaks could be observed in the differential curves that were identified as the “initial” and “rounded” peaks. The initial peak may correspond to the intrusion of mercury through a connected capillary network, while the rounded peak may correspond to the crushing of interposed hydration products.

Effect of fly ash on Portland cement systems. Part II. High-calcium fly ash

Abstract: A typical low-calcium fly ash was used as additive in mortar, replacing part of the volume either of Portland cement or aggregate. The development of the strength, heat, porosity, bound water, and calcium hydroxide content was measured. In aggregate replacement higher strengths were observed after 14 days, whereas in cement replacement higher strengths were observed after 91 days. The final strength gain was found to be roughly proportional to the content of active silica in the concrete volume. Bound water content and porosity results showed that fly ash reacts with calcium hydroxide, binding small amounts of water. On the basis of the experimental results, a simplified scheme describing the chemical reactions of the low-calcium fly ash in hydrating cement is proposed. Using the reaction stoichiometry, quantitative expressions for the estimation of the chemical and volumetric composition of a fly ash concrete are proposed. The model expressions can be applied in mix design and concrete performance prediction.

Alkali binding in cement pastes: Part I. The C-S-H phase

Abstract: The binding of sodium and potassium into cement paste influences the performance of concrete: for example, alkali balances between solid and paste constituents and pore fluid affect the potential for reaction with alkali-susceptible aggregates. However, quantification of the binding potential into paste solids has proven to be difficult, although much empirical data are available from pore fluid analyses. In this study, single-phase homogeneous C-S-H phases have been prepared at Ca:Si molar ratios of 1.8, 1.5, 1.2, and 0.85 and reacted with six alkali hydroxide concentrations, both NaOH and KOH, between 1 and 300 mM, giving a grid of 48 alkali concentrations and Ca:Si ratios. A steady-state alkali partition is attained in less than 48 h. A distribution coefficient, Rd, was calculated to express the partition of alkali between solid and aqueous phases at 20°C. The numerical value of Rd is independent of alkali hydroxide concentration and depends only on Ca:Si ratio. Approximate reversibility is demonstrated, so the Rd values are constants of a C-S-H over wide ranges of alkali concentration. The trend of Rd values indicates that alkali binding into the solid improves as its Ca:Si ratio decreases.

Alkali activation of Australian slag cements

Abstract: Investigation of alkali activation of Australian slag (AAS) was carried out using sodium silicate, sodium hydroxide, sodium carbonate, sodium phosphate, and combinations of these activators. Compressive strengths in the range from 20 to 40 MPa were achieved for the pastes. The most effective activator was liquid sodium silicate. With this activator, the effect of curing at 60 °C, modulus (M s ) of sodium silicate solution and concentration of alkalis on the compressive strength and setting times have been studied. On the basis of this investigation, a sodium silicate solution with a low Na content and M s = 0.75 is recommended for formulation of AAS concrete.

Protected paste volume in concrete: Extension to internal curing using saturated lightweight fine aggregate

Abstract: One difficulty in the field use of high-performance concrete is the extensive self-desiccation and autogenous shrinkage that may occur due to its low water/cement ratio and the addition of silica fume to the mixture proportions. Several researchers have proposed the use of saturated lightweight aggregates to provide “internal” curing for the concrete. In this communication, simple equations are developed to estimate the replacement level needed to ensure adequate water for complete curing of the concrete. Additionally, a three-dimensional concrete microstructural model is applied to determine the fraction of the cement paste within a given distance from the lightweight aggregate surfaces. The simulation results are compared with analytical approximations developed previously. This new concept for curing is similar to the protected paste volume concept conventionally applied to characterizing air void systems in air-entrained concrete.

Effects of cement particle size distribution on performance properties of Portland cement-based materials

Abstract: The original size, spatial distribution, and composition of Portland cement particles have a large influence on hydration kinetics, microstructure development, and ultimate properties of cement-based materials. In this paper, the effects of cement particle size distribution on a variety of performance properties are explored via computer simulation and a few experimental studies. Properties examined include setting time, heat release, capillary porosity percolation, diffusivity, chemical shrinkage, autogenous shrinkage, internal relative humidity evolution, and interfacial transition zone microstructure. The effects of flocculation and dispersion of the cement particles in the starting microstructures on resultant properties are also briefly evaluated. The computer simulations are conducted using two cement particle size distributions that bound those commonly in use today and three different water-to-cement ratios: 0.5, 0.3, and 0.246. For lower water-to-cement ratio systems, the use of coarser cements may offer equivalent or superior performance, as well as reducing production costs for the manufacturer.

Influence of key parameters on drying shrinkage of cementitious materials

Abstract: Drying shrinkage can be a major cause of the deterioration of concrete structures. The contraction of the material is normally hindered by either internal or external restraints and tensile stresses are induced. These stresses may exceed the tensile strength and cause concrete to crack. The evaluation of the stress distribution in the material requires the knowledge of the “real” free shrinkage deformation. This paper presents the results of a study performed to evaluate this deformation and obtain a better understanding of the behavior of concrete under drying conditions. Shrinkage tests were carried out on cement pastes, mortars, and concretes. The influences of different key parameters were evaluated: relative humidity, specimen size, water/cement ratio, and paste volume. The results indicate that between 48 and 100% relative humidity, the shrinkage of cement paste is approximately inversely proportional to relative humidity. Results also show that the ultimate shrinkage of pastes and mortars measured on 50 × 50 × 400-mm specimens does not differ much from the “real” shrinkage measured on 4 × 8 × 32-mm specimens. Thus, for the specimen dimensions investigated in this study, the existence of a humidity gradient did not affect to a large extent the ultimate shrinkage strain. The influence of the water/cement ratio, within the range investigated (0.35–0.50), was found to be relatively small. Conversely, paste volume was observed to have a very strong influence.

Workability and mechanical properties of alkali activated slag concrete

Abstract: This paper reports the results of an investigation on concrete containing alkali activated slag (AAS) as the binder, with emphasis on achievement of reasonable workability and equivalent one-day strength to portland cement concrete at normal curing temperatures. Two types of activators were used: sodium hydroxide in combination with sodium carbonate and sodium silicate in combination with hydrated lime. The fresh concrete properties reported include slump and slump loss, air content, and bleed. Mechanical properties of AAS concrete, including compressive strength, elastic modulus, flexural strength, drying shrinkage, and creep are contrasted with those of portland cement concrete.

Pore solution chemistry of alkali-activated ground granulated blast-furnace slag

Abstract: The chemical composition and pH of the pore solution extracted from six different ground granulated blast-furnace slag (GGBFS) pastes were determined. The concentrations of Si, Ca, Al, and Mg are functions of the pH of the aqueous phase, with high pH associated with the higher concentrations of Si and Al and the lower concentrations of Ca and Mg. When GGBFS is mixed with an aqueous phase with pH higher than 11.5, the reaction is activated or accelerated. The main hydration product was identified as C-S-H, and hydrotalcite, at later stages of hydration, was observed in the pastes with an aqueous phase of a high pH. The effect of pore solution on the alkali activation of GGBFS is discussed with reference to the hydration products.

Interaction between cement and chemical admixture from the point of cement hydration, absorption behaviour of admixture, and paste rheology

Abstract: Chemical admixtures can improve the properties of concrete. High performance concrete with high strength, superior fluidity, and self-compactibility can be realized mainly because of chemical admixtures. Rheological properties of fresh concrete can be strongly affected by the combination of cement and chemical admixture, method of admixture addition, or the water-cement ratio. Problems in fluidity, such as stiffening and large slump-loss, occasionally happen under a particular combination of cement and admixture. These phenomena are generally called incompatibilities between cement and chemical admixtures. In this study, the interaction between cement and the chemical admixture types lignin sulfonate, naphthalene sulfonate, melamine sulfonate, amino sulfonate, and polycarboxylate, together with the working factors and mechanisms, are discussed from the viewpoint of cement hydration. Although the polycarboxylate type superplasticizer was considered to have better compatibility in combination with different kinds of cement, the authors show that its compatibility is affected by the amount of alkaline sulfates in cement.

Pozzolanic properties of pulverized coal combustion bottom ash

Abstract: The pozzolanic properties of a coal combustion bottom ash were investigated Plain pastes containing equal amounts of calcium hydroxide and bottom ash were prepared and analyzed at different ages for their strength and the calcium hydroxide consumption At early ages, bottom ash does not react with calcium hydroxide Its pozzolanic reaction proceeds slowly and accelerates gradually to become very interesting after 28 days and especially after 90 days The strength activity indexes measured on mortars are sufficiently important to allow the use of bottom ash in concrete When ground for 6 h in a laboratory ball mill, the 28-day strength activity index of bottom ash is increased by 27%

High-performance concretes from calcium aluminate cements

Abstract: Calcium aluminate cements have a radically different chemistry to Portland cements. Due principally to their higher cost, they do not compete directly with Portland cements. Nevertheless, concretes based on these cements have very high performance in specific applications. Two of these are discussed in this article: resistance to acid attack and particularly biogenic corrosion and abrasion resistance in hydraulic structures. Such applications extend the range of applications for cementitious materials.

Effect of elevated temperature curing on properties of alkali-activated slag concrete

Abstract: This investigation is focused on the effect of curing temperature on microstructure, shrinkage, and compressive strength of alkali-activated slag (AAS) concrete. Concrete prepared using sodium silicate and sodium hydroxide as the activator had greater early and flexural strength than ordinary Portland cement concrete of the same water/binder ratio, but it also had high autogenous and drying shrinkage. Heat treatment was found to be very effective in reducing drying shrinkage of AAS concrete and promoting high early strength. However, strength of AAS concrete at later ages was reduced. Microstructural study revealed an inhomogeneity in distribution of hydration product in AAS concrete that can be a cause of strength reduction. Pretreatment at room temperature before elevated temperature curing further improved early strength and considerably decreased shrinkage in AAS concrete.

Use of ternary cementitious systems containing silica fume and fly ash in concrete

Abstract: This paper reports the results from laboratory studies on the durability of concrete that contains ternary blends of portland cement, silica fume, and a wide range of fly ashes. Previous work has shown that high CaO fly ashes are generally less effective in controlling alkali silica reactivity (ASR) and sulfate attack compared with Class F or low lime fly ashes. Indeed, in this study it was shown that replacement levels of up to 60% were required to control expansion due to ASR with some fly ashes. However, combinations of relatively small levels of silica fume (e.g., 3 to 6%) and moderate levels of high CaO fly ash (20 to 30%) were very effective in reducing expansion due to ASR and also produced a high level of sulphate resistance. Concretes made with these proportions generally show excellent fresh and hardened properties since the combination of silica fume and fly ash is somewhat synergistic. For instance, fly ash appears to compensate for some of the workability problems often associated with the use of higher levels of silica fume, whereas the silica fume appears to compensate for the relatively low early strength of fly ash concrete. Diffusion testing indicates that concrete produced with ternary cementitious blends has a very high resistance to the penetration of chloride ions. Furthermore, these data indicate that the diffusivity of the concrete that contains ternary blends continues to decrease with age. The reductions are very significant and have a considerable effect on the predicted service life of reinforced concrete elements exposed to chloride environments.

Differential scanning calorimetry study of ordinary Portland cement

Abstract: The present work involves using differential scanning calorimeter (DSC) in an investigation of the thermal behaviour of hydration products in ordinary Portland cement as a function of age. The two-step loss of water from calcium silicate hydrate, dehydroxylation of calcium hydroxide, and decarbonation of calcium carbonate contribute respectively to the three major endothermic peaks in the DSC curves. Peaks due to the formation of ettringite and iron-substituted ettringite, C 4 AH 13 and Fe 2 O 3 solid solution, were also found. Some DSC observations were supplemented by X-ray diffraction analysis.

The reaction between rice husk ash and Ca(OH)2 solution and the nature of its product

Abstract: In this study it was confirmed that, at temperatures around 40°C and in the presence of water, the amorphous silica contained in rice husk ash (RHA) can react with Ca(OH)2 to form one kind of C-S-H gel (Ca1.5SiO3.5· xH2O). The C-S-H gel looks like flocs in morphology, with a porous structure and large specific surface. The average particle diameter of the reaction product, ranging from 4.8 to 7.9 μm, varies slightly with the condition under which the reaction occurs. When the product is heated, it gradually loses the water that exists in it, but it maintains an amorphous form up to 750°C. Above 780°C it begins to transform to crystalline CaSiO3. One of the main reasons for the improvement of concrete properties upon addition of RHA possibly may be attributed to the formation of more C-S-H gel and less portlandite in concrete due to the reaction occurring between RHA and the Ca2+, OH− ions, or Ca(OH)2 in hydrating cement.

Particle shape analysis of coarse aggregate using digital image processing

Abstract: The particle shape characteristics of the coarse aggregate used can significantly affect the workability, strength, and durability of the concrete produced. However, traditionally, particle shape measurements have to be done in a manual way that is both cumbersome and time-consuming. Herein, digital image processing (DIP) techniques are used to analyze the particle shape characteristics of coarse aggregate. The main particle shape characteristics measured are flakiness and elongation. Twenty-five aggregate samples of different rock type and size have been analyzed and the results are compared to results obtained by the traditional manual method. Strong correlation between the DIP and manual measurement results is achieved and thus the DIP method, which is much faster, may be a better alternative for particle shape measurement. In fact, the DIP method yields more information about the particle shape than the manual method. With the DIP method used, it is possible to measure the mean thickness/breadth and length/breadth ratios of the aggregate directly, rather than just the proportion of flaky or elongated particles according to arbitrary definitions.

Factors influencing properties of phosphate cement-based binder for rapid repair of concrete

Abstract: Phosphate cement-based binders were prepared by mixing MgO powder (M) with NH 4 H 2 PO 4 powder (P) and borax powder (B). Effects of various factors including the amount of retarder B, P/M ratio, fineness of M, and addition of fly ash and environment temperature on setting time and mechanical properties of magnesium phosphate cement-based binder (MPB) were investigated. Results showed that the setting time and early strength of MPB were mainly controlled by the amount of B, the fineness of M, and temperature, but that these factors had little influence on the final strength of the binder. It is interesting that MPB sets and hardens within 1 h, even at −10 °C. A setting and hardening mechanism on MPB is proposed.

Microstructural investigation of innovative UHPC

Abstract: The production of ultra high performance concrete (UHPC) with target strengths greater than 200 MPa has recently been considered for specific structural applications that need this enhanced mechanical performance. The main purpose of developing these innovative UHPC mixtures is to produce high-strength precast concrete elements with excellent durability to serve as both the inner wedges and the outer barrel of a new nonmetallic anchorage system. The anchorage is for post-tensioning applications using carbon fibre-reinforced polymer tendons. The UHPC mixtures examined show very dense microstructures with some unique characteristics. The bond between the micro carbon fibres and the cement paste seems to be very good and the cement paste observed in the vicinity of the fibres was shown to be very dense and homogeneous. The micro carbon fibres seem to govern the strength and postcracking behaviour of these materials.

Concrete using waste oil palm shells as aggregate

Abstract: Concrete with oil palm shells (OPS) as coarse aggregate was investigated for its workability, density, and compressive strength development over 56 days under three curing conditions. The effect of fly ash as partial cement replacement was also studied. Fresh OPS concrete was found to have better workability while its 28-day air-dry density was 19–20% lower than ordinary concrete. Compressive strength after 56 days was found to be 41–50% lower than ordinary concrete. These results were still within the normal range for structural lightweight concrete. Fly ash was found to lower the compressive strength of OPS concrete, which was the opposite of its effect on normal concrete.

Stability and solubility relationships in AFm phases: Part I. Chloride, sulfate and hydroxide

Abstract: Portland cements contain an AFm phase whose anion content is initially dominated by OH − (hydroxy) and SO 4 2− (sulfate); variants such as those based on C 2 ASH 8 may occur in blended cements. In service conditions AF m phases may exhibit anion exchange, principally with carbonate, chloride, and additional sulfate. The chemistry, stability, and crystal chemistry of AF m phases are reviewed briefly. New data are presented on the role of 3CaOAl 2 O 3 0.5CaCl 2 0.5CaSO 4 10H 2 O, which, it is proposed, should be named Kuzel’s salt after its discoverer.