Low-temperature curing strength enhancement in cement-based materials containing limestone powder

With the ongoing sustainability movement, the incorporation of limestone powder in cementitious binders for concrete in the U.S. has become a subject of renewed interest. In addition to accelerating the early age hydration reactions of cementitious systems by providing additional surfaces for nucleation and growth of products, limestone powder is also intriguing based on its influence on low-temperature curing. For example, previous results have indicated that the utilization of limestone powder to replace one quarter of the fly ash in a high volume fly ash mixture (40–60% cement replacement) produces a reduction in the apparent activation energy for setting for temperatures below 25 °C. In the present study, the relationship between heat release and compressive strength of mortars at batching/curing temperatures of 10 and 23 °C is investigated. For Portland-limestone cements (PLC) with limestone additions on the order of 10%, a higher strength per unit heat release is obtained after only 7 d of curing in lime water. Surprisingly, in some cases, the absolute strength of these mortar cubes measured at 7 d is higher when cured at 10 °C than at 23 °C. Solubilities vs. temperature, reaction stoichiometries and enthalpies, and projected phase distributions based on thermodynamic modeling for the cementitious phases are examined to provide some theoretical insight into this strength enhancement. For a subset of the investigated cements, thermogravimetric analysis, quantitative X-ray diffraction, and scanning electron microscopy are conducted on 7-d paste specimens produced at the two temperatures to examine differences in their reaction rates and the phases produced. The strength enhancement observed in the PLC cements is related to the cement hydration products formed in the presence of carbonates as a function of temperature.     

废玻璃粉对水泥基材料流动性能的影响

为探究废玻璃粉对水泥胶砂和混凝土2种水泥基材料流动性能的影响,测试在不同废玻璃粉粒径和质量分数条件下水泥胶砂的流动度和屈服应力,研究在不同水胶比条件下废玻璃粉的质量分数对混凝土坍落度的影响。结果表明:随着废玻璃粉粒径和质量分数的增大,水泥胶砂流动度和流动速率皆呈现减小的趋势,而屈服应力逐渐增大;废玻璃粉质量分数为10%时,粒径范围为38～53μm对水泥胶砂流动度的增强效果最好;碎石、卵石混凝土坍落度随着废玻璃粉质量分数的增大而减小,随着水胶比的增大而增大,碎石混凝土的坍落度总体要好于卵石混凝土的;废玻璃粉质量分数为10%、水胶比范围为0.40～0.50时,2种混凝土坍落度达到较优状态。     

A discussion on improving hydration activity of steel slag by altering its mineral compositions

This study aims to investigate the ways to improve the cementitious properties of steel slag. The results show that the cementitious phase of steel slag is composed of silicate and aluminate, but the large particles of these phases make a very small contribution to the cementitious properties of steel slag. RO phase (CaO-FeO-MnO-MgO solid solution), Fe(3)O(4), C(2)F and f-CaO make no contribution to the cementitious properties of steel slag. A new kind of steel slag with more cementitious phase and less RO phase can be obtained by removing some large particles. This new steel slag possesses better cementitious properties than the original steel slag. The large particles can be used as fine aggregates for concrete. Adding regulating agent high in CaO and SiO(2) during manufacturing process of steel slag to increase the cementitious phase to inert phase ratio is another way to improve its cementitious properties. The regulating agent should be selected to adapt to the specific steel slag and the alkalinity should be increased as high as possible on the premise that the f-CaO content does not increase. The cooling rate should be enhanced to improve the hydration activity of the cementitious phase at the early ages and the grindability of steel slag.     

Influence of silica fume content on microstructure development and bond to steel fiber in ultra-high strength cement-based materials (UHSC)

The use of silica fume can significantly enhance mechanical properties of concrete given its beneficial filling and pozzolanic effects. In this study, a simple and effective double-side pullout testing method was adopted to characterize the interfacial bond properties, which include pullout load-slip relationship, bond strength, and pullout energy, of steel fiber-matrix in ultra-high strength cement-based material (UHSC) with 0–25% silica fume by the mass of binder. The effects of silica fume content on flowability, heat of hydration, compressive and flexural strengths, hydration products, and pore structure of matrix at different curing time were evaluated as well. Backscatter scanning electron microscopy (BSEM) and micro-hardness measurement were used to examine the quality of interfacial transition zone (ITZ) around the fiber. In terms of the results, the optimal silica fume content could be in the range of 15%–25%. UHSC mixtures with these dosages of silica fume showed significant improvement in pullout behavior. Its bond strength and pullout energy at 28 d could increase by 170% and 250% compared to the reference samples without any silica fume. The microstructural observation verified the findings on the macro-properties development. Formation of more and higher strength of hydration products and refinement of ITZ around the fiber ensured higher micro-hardness, and thus improved the bond to fiber.     

水泥基吸波材料的性能研究及微观分析?

通过双层设计使水泥基吸波材料尽可能与空间波阻抗相匹配,同时具有良好的电磁波损耗特性,实验测试了水泥基吸波材料的吸波与力学性能,并对吸波材料进行了微观分析.研究结果表明,水泥基吸波板在2~18 GHz频率范围内低于－10 dB的有效带宽分别为3．7和10．8 GHz,水泥基吸波材料的力学性能优于基准材料,掺入0．5％碳纳米管可有效提高水泥净浆的抗压强度.     

Environmental impacts of utilizing waste steel slag aggregates as recycled road construction materials

Slag is an industrial waste generated during the steelmaking process. Electric arc furnace slag (EAFS) and ladle furnace slag (LFS) are both produced at different stages of steelmaking process, respectively, in electric arc furnaces and refining ladle furnaces. As part of this research, an extensive suite of engineering and environmental tests were undertaken on steel slag aggregates to evaluate their potential usage as road construction materials. The engineering assessment included particle size distribution, hydrometer, organic content, flakiness index, Atterberg limits, particle density, water absorption, pH, minimum and maximum dry densities with a vibrating table, modified compaction, California bearing ratio (CBR) and Los Angeles abrasion tests. In addition, a suite of environmental tests comprising total and leachable heavy metal tests were undertaken on both types of steel slag aggregates. From an environmental perspective, EAFS and LFS were found to pose no environmental risks for use as aggregates in roadwork applications. The engineering properties of LFS aggregates with its satisfactory geotechnical and environmental results, particularly its high CBR values, indicated that the material was ideal for usage as a construction material in roadwork applications such as pavement base/subbases and engineering fills. EAFS, with its comparatively lower CBR value, was found to be only suitable to use as a construction material for pavement subbases and engineering fills. The usage of steel slag aggregates in roadwork applications would bring about a practical end-of-life alternative for their sustainable reuse and possibly divert large amount of these waste materials from landfills.     

Microstructural changes induced in Portland cement-based materials due to natural and supercritical carbonation

Supercritical carbonation of Portland cement binders was studied to analyse the influence of the type of cement on carbonation at high CO₂ pressures (CO₂ at 20 MPa and 318 K) and to improve the understanding of the effects on the microstructure and physicochemical properties of binders. The results were compared with those obtained in a natural exposure. Microstructural properties of supercritically and atmospherically carbonated Portland cement binders were examined using the complementary analytical techniques of FTIR, TG-DTA, and BSEM-EDX. Supercritically carbonated binders showed a microstructure based on a more polymerized and lower Ca form of CSH gel, formed by decalcification of high-Ca form of CSH gel. Results suggested that during the treatment at artificially intensified conditions, the crystallized calcium carbonate came mainly from the carbonation of the CSH gel, and at atmospheric conditions, from the carbonation of the portlandite phase.     

Defect dynamics and damage of cement-based materials,studied by electrical resistance measurement

Defect dynamics, as studied by DC electrical resistance measurement during repeated compression of cement paste, mortar and concrete in the elastic regime, are characterized by defect generation that dominates during the first loading, defect healing that dominates during subsequent loading, and defect aggravation that dominates during subsequent unloading. The interface between sand and cement, that between silica fume and cement, and that between coarse aggregate and mortar contribute to the defect dynamics, particularly the defect healing. Electrical resistance measurement is also effective for monitoring damage, which causes the resistance to increase. Defect generation results in an irreversible increase in the baseline resistance as stress cycling progresses, whereas defect healing results in a reversible decrease in the resistivity upon compression within a stress cycle. Defect generation is relatively significant in the early cycles and diminishes upon cycling. As the cumulative damage increases, the extent of defect healing within a cycle also increases.     

Temporal and spatial development of drying shrinkage cracking in cement-based materials

In optical microscopy and acoustic emission (AE) experiments, the development in time and space of drying shrinkage cracks in hardened cement paste and a cement composite containing mono-sized glass particles has been studied. The results indicate two types of restraint, i.e. self-restraint of the specimen/structure, and aggregate restraint. In plain cement paste cracks initially develop perpendicular to the drying surface, but may subsequently grow parallel to the drying surfaces. In composites containing glass particles, aggregate restraint causes continuous growth of microcracks. AE monitoring has revealed that drying shrinkage cracks develop almost instantaneously at the onset of drying. Several AE parameters elucidate the details of the process. The two techniques are complementary: AE monitoring is suitable for temporal development assessment, whereas optical microscopy is very well suited for measuring crack geometry.     

Drying, moisture distribution, and shrinkage of cement-based materials

The aim of this study was to develop a new method to determine hygral diffusion, film, and shrinkage coefficients of cement-based materials. These coeffcients are required for the numerical simulation of shrinkage strain and total deformation of concrete elements and structures using the finite element method. Both an experimental approach to determine the time-dependent relative humidity in the pore system of concrete and a numerical method to determine material coefficients on the basis of experimental data are described in this paper. The hygral diffusion coefficient can be expressed as function of moisture content and as function of relative humidity. An experiment is carried out with sliced specimens measuring 150×100×3 mm. Each specimen is prepared by piling up 11 slices and sealing the outer surfaces with aluminum sheet. The distribution of relative humidity is estimated by measuring the shrinkage strain on each slice at arbitrary drying times. An inverse analysis is then used to obtain the diffusion coefficient from the measured relative humidity distribution. A numerical approach based on the weighted residual method and on a nonlinear least squares method is proposed on the basis of the experimental results.

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Résumé L'objectif de cette étude était de développer une nouvelle méthode visant à déterminer les coefficients de diffusion hygrométrique, de pellicule et de retrait de matériaux à base de ciment. Ces coefficients sont nécessaires dans la simulation numérique de la force de retrait et de la déformation totale des éléments bétonnés et des ouvrages en utilisant la méthode de l'élément fini. Aussi bien l'approche expérimentale de détermination de la distribution de l'humidité relative dépendante du temps dans le système des pores du béton, que l'approache numérique de détermination des coefficients de matériaux sur la base des données expérimentales seront décrites dans cet article. Le coefficient de diffusion hygrométrique peut être exprimé à la fois comme fonction de la teneur en humidité et comme fonction de l'humidité relative. Une expérience a été conduite à l'aide d'échantillons en tranches fines mesurant 150×100×3 mm. Chaque échantillon a été préparé en empilant 11 tranches, scellées sur les bords par une feuille d'aluminium. La distribution de l'humidité relative a été calculée en mesurant la force de retrait pour chaque tranche à des temps de séchage arbitraires. Une contre-analyse a été ensuite utilisée pour obtenir les coefficients de diffusion à partir d'une mesure de la distribution de l'humidité relative. Une approache numérique fondée sur une méthode de pesée résiduelle et sur la méthode des moindres carrés non linéaires a été ensuite proposée sur la base des résultats expérimentaux.

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Editorial Note Prof. Folker H. Wittmann is a RILEM Senior Member.     

Grey correlation analysis between strength of slag cement and particle fractions of slag powder

The particle size distributions of slag powder were investigated by Laser Scatter equipment. The influence of particle fractions of slag powder on the compressive strength of slag cement composed of 50% slag powder and 50% Portland cement was also studied by the method of grey correlation analysis. The results indicated that the volume fraction of particles 5–10 μm had a maximum positive effect on the mortar compressive strength of slag cement at 7 d and the volume fraction of particles 10–20 μm had a maximum positive effect on the mortar compressive strength at 28 d, whereas the volume fraction of particles larger than 20 μm had a negative effect on the mortar compressive strength at 7 and 28 d.     

Restrained shrinkage cracking in steel fibre reinforced and rubberised cement-based mortars

The paper focuses on the combined effect of Steel Fibre Reinforcement and of Rubber aggregates on the resistance to restrained shrinkage cracking of cement-based mortars. The kinetics of restrained shrinkage cracking of a control mortar is compared both to the one incorporating a single fibre content as reinforcement and to the above fibre reinforcement combined to rubber aggregates substitution. Two rates of substitution were considered in the case of Steel Fibre Reinforced and Rubberized Mortar (SFRRM). The used rubber aggregates are obtained by grinding used tyres, a way that may address the demand for the conservation of a clean environment by recycling an industrial by-product. Fibre-reinforced mortar was based on the control mortar and one metal fibre content was studied: 40 kg/m³ (0.5% by volume). The used fibres have a high bond with the cementitious matrix. SFRRM composites were cast using two contents of rubber aggregates: 20 and 30% by volume replacing mineral aggregates and the one fibre content reported here above (40 kg/m³). Tests were conducted using ring type specimens to simulate restrained shrinkage cracking according to ASTM C 1581-04 standard. Additionally, after the cracking occurred, the development of the crack widths was measured by video-microscope. Ring tests demonstrated that the SFRRM exhibit high strain capacity prior to macro-cracking localization and the effectiveness of rubber aggregates along with their positive synergistic effect when combined with fibre reinforcement to improve resistance to shrinkage cracking. It is a promising solution to improve the durability of large surface area such as pavements and thin bonded cement-based overlays, whose durability is often limited by shrinkage cracking.     

Hardening and properties of cement-based materials incorporating heavy metal oxides

The subject of the study was the influence of oxides PbO, ZrO₂ and Cr₂O₃ immobilized in cement matrix. The obtained results show the positive effect of the present oxides in hydrating cement on compressive strength development and the quality of the formating pore structure of the resulting material. A surprising effect showing the paralysis of the positive effect of heavy metal oxide admixture has been shown by the increasing of the added quantity of PbO (from 1 to 4%) and the used w/c ratio (0·7 instead of 0·4). The increase of the quantity overcoming the optimum of the reaction product of the interaction between hydrating cement and PbO added for the cause of the paralysis is supposed. The explanation of the adverse effect and the identification of the reaction product in question needs more detailed study.     

Effects of sand aggregate on ultrasonic attenuation in cement-based materials

Ultrasonic wave attenuation measurements have been successfully used to characterize the microstructure and mechanical properties of inhomogeneous materials; these ultrasonic techniques have the potential to provide for the in-situ characterization of microstructure changes in cement-based materials due to damage. Recent research has applied acoustic scattering models to quantitatively predict ultrasonic attenuation for evaluating the air void content in hardened cement paste. The objective of the current research is to investigate the influence of sand aggregate on the ultrasonic attenuation as a first step towards a full simulation of more realistic microstructures in real concrete. Hardened cement paste samples containing sand aggregate of varying volume fractions are considered. The research employs an independent scattering model and a self-consistent effective medium theory to predict the scattering-induced attenuation of longitudinal ultrasonic waves by the sand inclusions distributed in the cement paste matrix. The predicted attenuation coefficients are compared with measured ones. It is observed that at low volume fractions, both models provide a good estimate of the total attenuation in the specimens. These results indicate that it is possible to use a physics-based model to quantify the effect of sand aggregate on ultrasonic attenuation.     

Hydration monitoring of cement-based materials with resistivity and ultrasonic methods

Two test setups, the electrical resistivity and ultrasonic techniques, were used to monitor the hydration process of cement-based materials. In the electrical resistivity method, a non-contacting device was used. In the ultrasonic method, a wave was transmitted and measured by the embedded piezoelectric ultrasonic transducers, which had good coupling with the surrounding materials. The focus of the study was to detect the setting and hardening behaviors of cement paste during the first 7 days of hydration using the above techniques. Immediate after placing the cement paste into the mould, the measurement started and continued throughout the hydration process. The obtained resistivity and ultrasonic data were used to interpret the hydration process of the specimens. The correlation of two techniques was also studied. The results illustrated that both electrical resistivity and ultrasonic techniques were effective to accurately monitor the hydration of cement pastes. The resistivity method was able to study both the chemical reaction and physical change during hydration, while ultrasonic method was sensitive to physical change of cement only.     

Corrosion protection of cement-based building materials by surface deposition of CaCO3 by Bacillus pasteurii

Bacterially induced calcium carbonate precipitation was used as a novel and environmentally friendly approach to produce a protective layer on the surface of cement-based materials in the study. The physical and chemical properties of the obtained layer were examined. X-ray diffraction analysis characterized the composition of the deposited layer and scanning electron microscopy displayed the morphology of particles. The results showed that both bacterial activity and the method of adding Ca²⁺ and urea had a profound effect on the properties of the calcium carbonate layer. A capillary water absorption test was carried out to evaluate the ability of the protective layer to improve the resistance to water penetration. Experimental results indicated that the calcium carbonate layer, obtained under the conditions of high bacterial activity, appropriate concentration of Ca²⁺, and adding Ca²⁺ before urea to the reaction mixture, could greatly improve the water penetration resistance of the specimen surface. This type of treatment has the potential to conserve and consolidate cement-based materials.     

聚丙烯纤维化学接枝改性增强水泥基复合材料的界面结合

以过氧化苯甲酰(BPO)为引发剂,采用二步化学接枝法对聚丙烯纤维进行接枝丙烯酸改性,利用正交分析法研究了引发温度、BPO浓度、接枝温度、接枝时间及丙烯酸(AA)浓度对纤维接枝率的影响,并评价了改性前后纤维与水泥基体的界面结合性能.结果表明:上述因素对纤维接枝率的影响大小为引发温度＞ BPO浓度＞接枝时间＞接枝温度＞AA浓度,最佳反应条件为引发温度90℃,BPO 4.50×10-2 mol/L,接枝时间60 min,接枝温度75℃,AA 1.4 mol/L,此时纤维接枝率达13.12％;经化学接枝改性后,聚丙烯纤维表面亲水性和粗糙度增大,与水泥基体的界面结合得到增强,纤维掺量为0.05％(体积分数)时,聚丙烯纤维增强水泥砂浆的抗开裂性能比增大了26.6％,抗塑性收缩开裂性能显著增强.     

Effects of a combined admixture of slag powder and thermally treated flue gas desulphurization (FGD) gypsum on the compressive strength and durability of concrete

In this paper, the effects of a combined admixture of slag powder and thermally treated flue gas desulphurization (FGD) gypsum on the compressive strength and durability of concrete were explored. The gypsum was baked at a proper temperature of 200°C for 60 min; it was then mixed with slag powder to form the type-G slag powder in which SO₃ content was controlled at the optimum ratio of 3.5%. Concretes containing the type-G slag powder or ordinary slag powder were further contrasted on the compressive strength and durability. The results showed that the compressive strength, resistance to chloride, and resistance to gas permeability of concrete with the type-G slag powder were higher than that with ordinary slag powder. Additionally, there were no negative effects on concrete against sulfate attack. The combination of the two byproducts, slag powder and thermally treated FGD gypsum, provided synergistic benefits above that of ordinary slag powder alone.