Chloride diffusion in ordinary, blended, and alkali-activated cement pastes and its relation to other properties

Chloride transport into cementitious materials is critical from the viewpoint of protection of reinforcement. This paper is part of a larger study of the characteristics and performance of alkali-activated cementitious materials (AAC) whose properties equal or exceed those of normal Portland cement-based materials. Steady state chloride diffusion studies have been made of pastes of Type I Portland cement, and its blends with different proportions of ground granulated blast-furnace slag. Very substantial reductions in diffusion rates have been found with increased proportion of slag. In addition, alkali activation has been shown to reduce the diffusion rate by at least a factor of two. Other properties determined include: density, porosity, pore size distribution (Hg), BET (N₂) surface area, shrinkage, compressive and flexural strength, leaching, alkali-aggregate reaction, and freezing and thawing resistance. Comparisons with results of previous studies and with other blending components are discussed.     

An investigation into the influence of inter-aggregate spacing and the extent of the ITZ on properties of Portland cement concretes

Conventionally-designed concretes were prepared with different sand particle size distributions, so as to systematically vary the extent of aggregate-cement paste interface and the mean spacing between sand grains. The range of fineness modulus of the sands fully encompassed the range of sands normally used in concretes. The concretes were batched at w/c ratios of 0.30 and 0.50 and cured for various periods before carrying out determinations of mechanical properties and of “rapid chloride permeability”. The conventional notions of the effect of the ITZ on concrete properties would predict that a reduction in strength and an increase in chloride permeability would accompany increased ITZ interfacial area and closer spacing between sand grains. In general, no such influence was found. It appears from this research that the traditional notions of the adverse influence of the ITZ on the properties of conventional concretes may not be accurate, within the realms of conventional concrete and typical inter-aggregate spacings.     

矿渣粉颗粒群参数对矿渣活性和混合水泥性能的影响

以混合材料的填充效应和微集料效应为基础,测试了不同颗粒分布矿渣粉制备的水泥的物理性能,研究了矿渣粉颗粒群参数对水泥使用性能和混合水泥抗压强度比的影响。结果表明,矿渣粉颗粒分布宽、粗大化有利于混合水泥使用性能的提高;同时,颗粒堆积密度也影响混合水泥抗压强度比的高低,但主要对早期作用明显,而后期的抗压强度比则与矿渣粉的比表面积直接相关。     

Pozzolanic properties of reject fly ash in blended cement pastes

Low-grade fly ash (reject fly ash, r-FA), a significant portion of the pulverized fuel ash (PFA) produced from coal-fired power plants and rejected from the ash classifying process, has remained unused due to its high carbon content and large particle size. But it may be used in certain areas, such as in solidification and stabilization processes of hazardous waste and materials for road base or subbase construction, which require relatively lower strength and reactivity. It is therefore necessary to extend research on the properties of r-FA and explore its possible applications. This paper presents experimental results of a study on the mechanical and hydration properties of cementitious materials prepared by blending r-FA with ordinary Portland cement (OPC). Parallel mixes were also prepared with the good ash [i.e., classified fine fly ash (f-FA)] for comparison. Selective chemical activators were added to the mix to study the effects of the activators on the properties of the blend system. The results show that r-FA generally has a lower rate of hydration than f-FA particularly at the early stage of hydration. Adding Ca(OH)₂ alone almost had no effect on accelerating the hydration of r-FA. But adding a small quantity of Na₂SO₄ or K₂SO₄ together with Ca(OH)₂ significantly accelerated the hydration reaction. The results of the compressive strength measurement correlated nicely with the degree of hydration results. It was also found that water-to-binder ratio (w/b) was an important factor in affecting the strength development and the hydration degree of r-FA pastes.     

The influences of siliceous waste on blended cement properties

The influences of siliceous waste on the properties of fly ash and blast furnace slag cement were studied, and its optimum mixing amount in blended cement was determined. The strength, setting time, resistance to chemical attack, dry shrinkage, and impermeability of blended cement mixed with siliceous waste were also investigated by different experiments. The measurement of pore size distribution for hardened cement pastes made by Poremaster-60 was recorded and analyzed in this article.     

Pore structure characterization of cement pastes blended with high-volume fly-ash

The pore structure of cement pastes incorporating fly-ash was evaluated during their hydration process through gravimetry method, mercury intrusion porosimetry (MIP) and nitrogen adsorption/desorption (NAD) methods. The pore structure of samples is characterized by the total porosity, pore size distribution (PSD), pore internal surface area as well as characteristic pore sizes. The correlation between the hydration process and the formed pore structure is investigated. The results indicate that: (i) w/b ratios have determinant impact on all characteristics of pore structure; (ii) fly-ash replacement ratio can influence the pore structure significantly at early age but this influence becomes less important with sample age by fly-ash hydration process; (iii) the total porosity and specific surface area are well correlated with the chemical kinetics of hydration through hydration degree or the formed gel/space ratio but the critical pore size is rather independent on the chemical kinetics.     

On shrinkage and structure changes of pure and blended Portland concretes

This research investigates the long‐term shrinkage and Relative Mass Loss (RML) of mature Portland concretes (pure CEMI and blended CEMV/A), at temperatures of 20°C and 80°C. When placed at 80°C and at relative humidities (RH) below 50‐60%, concrete shrinkage increases with very slow stabilization kinetics by several hundreds of μm/m, while RML remains of about 0.2%. The origins of this continued shrinkage, simultaneously with limited RML, are investigated through the changes in (i) the pore structure of the concretes (by Mercury Intrusion Porosimetry and nitrogen adsorption) and in (ii) their solid phases (by TGA/DTA, FTIR spectroscopy coupled to DVS, quantitative X‐Ray Diffraction by Rietveld analysis, and ²⁹Si and ²⁷Al MAS NMR). While the pore structure coarsens during continued shrinkage, several phase transformations occur, namely ettringite decomposition and changes in the silicate chain length of the C–A–S–H. While these structural changes are documented, their relationship with shrinkage/RML and to the pore structure is novel.     

Investigation on key properties controlling early-age stress development of blended cement concrete

Age dependent mechanical and kinetic properties including Young's modulus, early-age creep, autogeneous and thermal deformations, and heat of hydration were investigated for concrete made of blended cements. These are among the key properties that control the early-age cracking behavior in hydrating concrete members. Among the main goals of the investigation were to provide the experimental data and to study the effect of adding mineral additives such as fly ash (FA), ground granulated blast furnace slag (GGBF), and silica fume (SF) on the aforementioned properties. The age-dependent behavior of Young's modulus, creep compliance, and autogeneous shrinkage as functions of heat of hydration were modeled. We emphasized on mathematical modeling the viscoelastic properties of concrete. The equations obtained can be used as inputs needed to calculate the early-age stress development in concrete members.     

Effect of some superplasticizers on the mechanical and physicochemical properties of blended cement pastes

Blended cement pastes made of Portland cement and fine sand (known in Egypt as El-Karnak cement) were made using a water–cement ratio of 0.25 by weight. Three pastes containing admixture (water-soluble condensates) were also prepared using a water–cement ratio of 0.25 and condensate (superplasticizer) content of 0.25% by the weight of cement; the superplasticizers used are Na-phenol sulfonate formaldehyde, Na-polystyrene sulfonate, and Na-ß-naphthol sulfonate formaldehyde condensates. All pastes were cured for various time intervals within the range of 0.02–90 days. Compressive strength tests, hydration kinetics, X-ray diffraction analysis, thermal analysis, and surface properties were studied and related as much as possible to the pore structure of the hardened pastes. © 1995 John Wiley & Sons, Inc.     

偏高岭土、矿渣和赤泥对高性能混合水泥性能影响的研究

通过用硅酸盐水泥、偏高岭土等混合材、膨胀剂及减水剂等制备混合水泥的研究发现：偏高岭土和赤泥的加入使水泥的凝结时间缩短,矿渣的加入延长了水泥的凝结时间;偏高岭土和矿渣对水泥的胶砂流动性影响较小,赤泥的加入使得水泥胶砂流动性显著降低;适量偏高岭土的加入对水泥的3d和28d强度均有增强作用,适量矿渣的加入使水泥抗折强度降低,抗压强度增大;少量的赤泥对水泥强度特别是早期强度有一定的增强作用,但掺量超过20％后水泥强度迅速降低;偏高岭土对水泥微膨胀的产生有促进作用,矿渣和赤泥对水泥微膨胀有抑制作用。用80％～90％的硅酸盐水泥、10％～20％的偏高岭土以及少量的膨胀剂和减水剂能够制备出具有较优流动性、较高的强度以及微膨胀的高性能混合水泥。     

Identification of microstructural characteristics in lightweight aggregate concretes by micromechanical modelling including the interfacial transition zone (ITZ)

An approach which combines both experimental techniques and micromechanical modelling is developed in order to characterise the elastic behaviour of lightweight aggregate concretes (LWAC). More than three hundred LWAC specimens with various lightweight aggregate types (5) of several volume ratios and three different mortar matrices (normal, HP, VHP) are tested. The modelling is based on iterative homogenisation process and includes the ITZ specificities experimentally observed with scanning electron microscopy (SEM). In agreement with experimental measurements, the effects of mix design parameters as well as of the interfacial transition zone (ITZ) on concrete mechanical performances are quantitatively analysed. Confrontations with experimental results allow identifying the elastic moduli of LWA which are difficult to determine experimentally. Whereas the traditional empirical formulas are not sufficiently precise, predictions of LWAC elastic behaviours computed with the micromechanical models appear in good agreement with experimental measurements.     

立窑水泥与回转窑水泥配制混凝土耐久性能的研究

用立窑和回转窑生产的５２５号Ⅱ型硅酸盐水泥配制Ｃ３０混凝土来对比它们的耐久性能。实验结果表明：优质立窑水泥配制混凝土的抗渗性、抗碳化和抗海水腐蚀性能不如回转窑水泥,而抗冻性、收缩性、钢筋锈蚀性能与回转窑水泥相近。此外,４２５号普通硅酸盐水泥中混合材的品种、质量对混凝土耐久性能影响很大,石灰石作混合材可显著改善立窑水泥配制的混凝土的耐久性能,而烧粘土作混合材则严重损害混凝土的耐久性。     

Slag cement utilization: Rheological properties and related characterization

Mortars prepared from slag-cement blends have been shown to exceed or equal the strengths of portland cement mortars at 7 to 28 days. Such cements have further potential advantages in their performance with regard to durability, and in their early stage properties which govern workability. The early stage rheological properties including mortar flow, yield stress, and viscosity were investigated, combined with zeta potential measurements, and used to explain the excellent workability. The working time of slag cement may be extended to later ages compared with portland cements through use of superplasticizers.     

Hydration and properties of novel blended cements based on cement kiln dust and blast furnace slag

The aim of the present paper is to address the key technical issues pertaining to the utilization of cement kiln dust (CKD) as an activator for ground granulated blast furnace slag (GGBFS) to create nonconventional cementitious binders for concrete. The relatively high alkaline content of CKD is the predominant factor preventing its recycling in cement manufacture. However, it was observed that depending on the water-soluble alkali and sulfate compounds, CKD could provide the environment necessary to activate latent hydraulic materials such as GGBFS. Binary blends containing slag and CKDs from different sources were characterized and compared in terms of the rates of heat evolution and strength development, hydration products, and time of initial setting. A study of the effects of the influencing factors in terms of soluble alkali content, particle size, and free lime content was undertaken. The results confirm the dependence of the dissolution rate of slag on the alkalinity of the reacting system, and the importance of the optimum lime content on the rate of strength gain.     

Strength properties of nylon- and polypropylene-fiber-reinforced concretes

The strength potential of nylon-fiber-reinforced concrete was investigated versus that of the polypropylene-fiber-reinforced concrete, at a fiber content of 0.6 kg/m³. The compressive and splitting tensile strengths and modulus of rupture (MOR) of the nylon fiber concrete improved by 6.3%, 6.7%, and 4.3%, respectively, over those of the polypropylene fiber concrete. On the impact resistance, the first-crack and failure strengths and the percentage increase in the postfirst-crack blows improved more for the nylon fiber concrete than for its polypropylene counterpart. In addition, the shrinkage crack reduction potential also improved more for the nylon-fiber-reinforced mortar. The above-listed improvements stemmed from the nylon fibers registering a higher tensile strength and possibly due to its better distribution in concrete.     

Mechanical and adhesion properties of polymer-modified cement mortars in relation with their microstructure

In this study, cement mortars were modified with a commercial polymer admixture. The aim of this study is to investigate the influence of the polymer content on the mechanical and adhesion properties of the mortars and to relate these properties with mortars’ microstructure. A series of mortars were produced with various polymer/cement/water/aggregate ratios. The adhesion properties of the mortars to clay bricks were tested with a simplified tensile testing measurement. The microstructure of mortars, as well as interfaces, were evaluated by Scanning Electron Microscopy (SEM). It was found that with high polymer content, large size hardened particles are formed, reducing the compressive strength of the mortars. Polymer addition enhances the adhesion between the mortar and brick. The mortar microstructure at the interface affects the adhesion properties and the mode of failure.     

Behavior of blended cement mortars exposed to sulfate solutions cycling in relative humidity

In recent years, several cases of damage to concrete structures due to sulfate exposure have occurred essentially in the above ground parts of structures. Such distress, often characterized by white efflorescence and surface scaling, is driven by salt crystallization in pores and/or repeated reconversions of certain sulfates between their anhydrous and hydrated forms under cycling temperature and relative humidity (RH). However, the effect of the water/cementitious materials ratio (w/cm), pozzolanic additions, and other parameters on the durability of cement-based materials under such exposure conditions is still misunderstood. In this study, 12 cement mortars having different w/cm (0.30, 0.45, and 0.60) and made with ordinary portland cement (OPC) or OPC incorporating 8% silica fume, 25% class F fly ash, or 25% blast furnace slag were made. Standard bars from each of these mortars were submerged in both 10% magnesium sulfate (MgSO₄) and 10% sodium sulfate (Na₂SO₄) solutions; their expansion and surface degradation was monitored for up to 9 months. In addition, cylinders made from these 12 mortars were partially submerged in 50-mm-deep 10% MgSO₄ and 10% Na₂SO₄ solutions. Half of the cylinders were maintained under constant temperature and RH, whereas the others were subjected to cycling RH. The effect of the w/cm and mineral additions on the classic chemical sulfate attack and development of efflorescence was investigated, and the results are discussed in this article.     

Microstructural characterization and wear properties of silver and gold nanoparticle doped K-Mg-Al-Si-O-F glass-ceramics

In ascertaining the effects of silver (Ag) and gold (Au) nanoparticles on crystallization of boro-alumino-silicate system; the K₂O-MgO-Al₂O₃-SiO₂-B₂O₃-F glasses doped with/without 0.2?wt% Ag- and Au- content were melt-quenched at 1550?°C. Doping of nanoparticles considerably increased the glass-transition temperature and softening point but decreased the thermal expansion. A sharp crystallization exotherm in differential scanning calorimetry (DSC) is observed at 750?°C (?±?1?°C) for glass without nanoparticle and that broadened to 800–855?°C when contains nanoparticle. Opaque glass-ceramics were derived from the glasses by controlled heat-treatment at 1050?°C with predominant crystalline phase fluorophlogopite (KMg₃AlSi₃O₁₀F₂) mica. Traces of Ag- and Au- particles were also identified from X-ray diffraction (XRD) technique. The activation energy (E_c) of crystallization (344?±?17?kJ/mol) is decreased to 233 (?±?12) and 307 (?±?15) kJ/mol (Kissinger method) on doping with Ag- and Au- nanoparticles, respectively. Compact microstructure (FESEM) composed of rock like and plate-like mica crystals are developed in base glass-ceramic and that gets restructured to interlocked type morphology in presence of Ag- nanoparticle. Significant microstructural change induced by nanoparticle addition caused the decrease in microhardness (4.31–4.66?GPa) and increase in thermal expansion. Friction and wear testing under reciprocative sliding (using WC-Co ball) exposed that the average coefficient of friction (COF) is 0.60?±?0.2 for all glass-ceramics at 20?N load and 10?Hz frequency. At a lower load of 5?N, the average COF value is increased from 0.69 to 0.92 on use of Au-nanoparticle. A Similar trend was also observed at 10?N load as COF increased from 0.62 to 0.78.     

Heterogeneous microstructure of yttrium hydride and its relation to mechanical properties

The goal of this study is to investigate the properties of yttrium hydride materials in relation to the microstructure, especially its homogeneity. High-throughput nanoindentation mapping was used to evaluate hardness distribution. Raman spectral imaging demonstrated its sensitivity to the presence of YH2 and impurities. Raman peak position maps were correlated with residual stress in the specimens. Electron backscatter diffraction mapping provided phase distributions with correlation to high-energy X-ray diffraction analysis. The experimental mapping data were combined and analyzed using unsupervised machine learning cluster procedures. The machine learning analysis revealed that yttrium hydride specimens contained a major δ-YH2 − x phase component and minor α-Y and δ-YH2 − x components with significant residual stress. The minor phase fraction decreased with increasing nominal H/Y ratio, which affected the nanoindentation and Vickers hardness. The multimodal mapping procedures described herein affect developing important microstructure–property relationships, as well as correlations in heterogeneity and mechanical properties.