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.     

Preparation of waste glass powder by different grinding methods and its utilization in cement-based materials

Waste glass, as a hard-to-grind material, was often grinded into powder by ball-mill in order to be used as a supplementary cementitious material in concrete. In this study, three different glass powders (GP) referred to as GPd, GPw and GPe were fabricated by different methods (i.e. dry condition, water condition and ethyl alcohol condition), and the effect of GP on the properties of cement-based materials was systematically evaluated. The results showed that the milling efficiency of GP was significantly affected by grinding condition. Ultrafine GP can be obtained as additional water or ethyl alcohol was added during the grinding process. GP showed low pozzolanic reactivity in the early age, even when the particle size was decreased to about 300 nm. GPd with large particle size and non-absorbent feature could increase effective water-to-cement ratio and thus improved the workability and early strength of specimens containing GPd, while the later strength was lower than that of reference ones due to low pozzolanic reactivity of GPd. In contrast, specimens containing GPw and GPe exhibited higher early mechanical strength than reference specimen due to the filling and acceleration effects of GP. An encouraging result indicated that GPe presented high pozzolanic reactivity at 28 d, which was seldom reported in the literature.     

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。     

水泥基材料中倾斜端钩型钢纤维拔出力学性能计算模型

端钩型钢纤维是结构工程中应用最广泛的钢纤维品类之一,单根钢纤维拔出力学性能对于确定钢纤维混凝土的受拉本构及受拉韧性具有重要意义。为了得到能够有效预测倾斜端钩型钢纤维拔出荷载-端部位移曲线的理论模型,首先将倾斜端钩型钢纤维拔出过程分为完全黏结、脱黏和拔出滑移阶段三种受力状态,考虑不同拔出阶段及基体孔道损伤,建立了钢纤维黏结应力与纤维端部位移之间的关系,同时考虑钢纤维塑性变形、附加摩擦力及纤维拔出角度导致的基体剥落和挤压摩擦效应,建立了一种可以预测倾斜端钩型钢纤维拔出全过程的理论计算模型,在此基础上提出形式简单的简化模型,选取已有试验数据对提出的计算模型进行验证,结果表明:本文提出的两种模型均能够有效预测端钩型钢纤维拔出全过程,具有较高的计算精度且变异系数小,为进一步分析钢纤维对水泥基材料受拉性能的增强作用提供了理论依据。      

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.     

钢渣超微粉/橡胶复合材料的性能及补强-阻燃机制

以磁选热闷渣、未磁选热闷渣、电炉渣和风淬渣作为研究对象,以乙二醇、三乙醇胺和无水乙醇制备钢渣助磨剂,钢渣助磨剂与钢渣进行复合获得钢渣超微粉。将钢渣超微粉与炭黑N220、促进剂、硫磺、 ZnO、硬脂酸、天然橡胶进行复合,制备钢渣超微粉/橡胶复合材料。研究钢渣种类和钢渣助磨剂用量对钢渣超微粉/橡胶复合材料力学性能和阻燃性能的影响。利用XRF、 XRD、 LPSA和FTIR对化学成分、矿物组成、粒度分布和组成结构进行测试。结果表明,以电炉渣制备的钢渣超微粉/橡胶复合材料的力学性能最佳,以磁选热闷渣或未磁选热闷渣制备的钢渣超微粉/橡胶复合材料的阻燃性能最佳。钢渣助磨剂可以减小钢渣超微粉的粒度尺寸,改善钢渣超微粉的粒度分布均匀程度。随着钢渣助磨剂用量的增加,钢渣超微粉的粒度分布均匀程度改善,钢渣超微粉/橡胶复合材料的力学性能提高,阻燃性能降低。     

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.     

Performance simulation and quantitative analysis of cement-based materials subjected to leaching

A 3D numerical modelling platform (MuMoCC) developed in a previous work by the authors is applied in this paper to investigate the effect of leaching of some solid phases of cement paste (portlandite and hydrated aluminates or sulfoaluminate phases) on the mechanical and diffusivity performances of cement paste and mortar. The platform is based on a multi-scale approach and uses two numerical tools. First, NIST’s CEMHYD3D code is used to simulate 3D Representative Volume Elements of cement paste and mortar. Then mechanical and diffusivity behaviour of the numerical materials are simulated using ABAQUS software. The proposed three-dimensional heterogonous model presents at least two advantages. Firstly, it is able to capture the complexity of the random microstructure of cement-based materials. Secondly, only a few parameters have to be fitted compared to the other existing models, which indicates the relevance of the model. The numerical simulations of leached cement paste and mortar performance highlight and quantify the significant effect of portlandite and hydrated aluminate and sulfoaluminate phases’ dissolution on the decrease of elastic modulus and compressive strength and on the increase of ductility and diffusivity. The numerical results show that the leaching of portlandite decreases the compressive strength of a w/c = 0.4 cement paste by a factor of 1.33. The dissolution of portlandite and hydrated aluminates or sulfoaluminate phases involves a decrease by a more important factor (1.86).The leaching of portlandite phase involves an important increase, by a factor of 31, of the effective diffusion coefficient.Using the developed multi-scale modelling and knowing the leaching kinetics values, the mechanical and diffusion performances of cement-based materials can be estimated correctly according to leaching duration.     

聚乙烯醇纤维增强钢渣粉-水泥复合材料基本力学性能及微观结构

通过掺加钢渣粉来制备聚乙烯醇（PVA）纤维增强钢渣粉-水泥基复合材料,从宏微观两个方面研究了这种复合材料的性能。考虑了基体材料的水胶比（0.25和0.35）、不同钢渣粉质量分数（0、30wt%、60wt%、80wt%）,采用抗压强度试验、薄板四点弯曲试验研究了PVA纤维增强钢渣粉-水泥基复合材料的基本力学性能变化规律及其在弯曲荷载作用下的裂缝控制能力,采用扫描电镜观测了破坏后试样的微观结构。结果表明,水胶比和钢渣粉掺量均可明显影响PVA纤维增强钢渣粉-水泥基复合材料的基本力学性能,在低水胶比条件下（水胶比为0.25）,钢渣粉掺量达到80wt%时,试样表现出较高的韧性指数和良好的裂缝控制能力,基本满足工程所需强度要求,水胶比为0.35时钢渣掺量不宜超过60wt%;同时,从节能减排的角度考虑,利用钢渣粉制备PVA纤维增强钢渣粉-水泥基复合材料是可行的。      

水泥基材料离子热电效应与机制

当前水泥基材料热电效应主要通过掺入大量功能填料来增强.但掺量过高的功能填料提高了水泥基复合材料的成本,劣化了其力学性能,阻碍其大范围的应用.本研究发现,由于孔隙溶液中存在大量自由移动的离子,不掺任何功能填料的水泥净浆表现出显著的离子热电效应.本研究通过对比水泥净浆干燥前后的热电压,研究了水泥基材料的离子热电效应,并通过...     

Characterisation of ground hydrated Portland cement-based mortar as an additive to alkali-activated slag cement

The sustainable development of cement manufacturing requires extension of the raw material base, including large-tonnage waste. Hydrated mortar waste is a promising mineral resource for the production of Portland cements and alternative binders, such as alkali-activated slag cement. The influences of ground-hydrated mortar aged for 3 months on the properties of alkali-activated slag fresh and hardened pastes were performed. The results show that the properties are dependent on the concentration (2.5–60%), cement:sand ratio (1:1–3) and fineness (200–600 m²/kg) of the ground hydrated mortar; the alkali activator (sodium carbonate and sodium silicate); and the curing conditions (normal conditions and steam curing). The fresh paste properties that we considered in this study included the water requirement and the setting time; the hardened paste properties we considered were the water absorption, the density, and the compressive strength after 2, 7, 14, 28, 180 and 360 days of ageing. The ground hydrated mortar improved the early strength and the long-term strength of the alkali-activated slag paste and replaced the slag up to 50%. The factors that affecting the strength of the alkali-activated slag cement with ground hydrated mortar as an additive were, in order of influence, alkali activator type > curing conditions > cement:sand ratio > ground-hydrated mortar fineness.     

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.     

Micro- and nano-scale characterization to study the thermal degradation of cement-based materials

The degradation of hydration products of cement is known to cause changes in the micro- and nano-structure, which ultimately drive thermo-mechanical degradation of cement-based composite materials at elevated temperatures. However, a detailed characterization of these changes is still incomplete. This paper presents results of an extensive experimental study carried out to investigate micro- and nano-structural changes that occur due to exposure of cement paste to high temperatures. Following heat treatment of cement paste up to 1000 °C, damage states were studied by compressive strength test, thermogravimetric analysis (TGA), scanning electron microscopy (SEM) atomic force microscopy (AFM) and AFM image analysis. Using experimental results and research from existing literature, new degradation processes that drive the loss of mechanical properties of cement paste are proposed. The development of micro-cracks at the interface between unhydrated cement particles and paste matrix, a change in C–S–H nano-structure and shrinkage of C–S–H, are considered as important factors that cause the thermal degradation of cement paste.     

Accelerated carbonation and performance of concrete made with steel slag as binding materials and aggregates

Steel slag has been used as supplementary cementitious materials or aggregates in concrete. However, the substitution levels of steel slag for Portland cement or natural aggregates were limited due to its low hydraulic property or latent volume instability. In this study, 60% of steel slag powders containing high free-CaO content, 20% of Portland cement and up to 20% of reactive magnesia and lime were mixed to prepare the binding blends. The binding blends were then used to cast concrete, in which up to 100% of natural aggregates (limestone and river sands) were replaced with steel slag aggregates. The concrete was exposed to carbonation curing with a concentration of 99.9% CO₂ and a pressure of 0.10 MPa for different durations (1d, 3d, and 14d). The carbonation front, carbonate products, compressive strength, microstructure, and volume stability of the concrete were investigated. Results show that the compressive strength of the steel slag concrete after CO₂ curing was significantly increased. The compressive strengths of concrete subjected to CO₂ curing for 14d were up to five-fold greater than that of the corresponding concrete under conventional moist curing for 28d. This is attributed to the formation of calcium carbonates, leading to a microstructure densification of the concrete. Replacement of limestone and sand aggregates with steel slag aggregates also increased the compressive strengths of the concrete subjected to CO₂ curing. In addition, the concrete pre-exposed to CO₂ curing produced less expansion than the concrete pre-exposed to moist curing during the subsequent accelerated curing in 60 °C water. This study provides a potential approach to prepare concrete with low-carbon emissions via the accelerated carbonation of steel slag.     

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.     

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.     

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.     

钢渣改性硅酸盐水泥-水玻璃双液注浆复合材料的试验研究

研究了钢渣掺量、单浆液水胶比、双浆液体积比对钢渣改性硅酸盐水泥-水玻璃双液注浆复合材料工作性能影响规律。确定了钢渣改性硅酸盐水泥单浆液的最佳水胶比为0.6~0.8,平均粒径为20.4 μm的钢渣在改性硅酸盐水泥单浆液中的最佳质量分数为50%~80%,钢渣改性硅酸盐水泥单浆液与水玻璃单浆液的最佳体积比范围为4：1~6：1。根据以上参数所配制的钢渣改性硅酸盐水泥-水玻璃浆液硬化后在水中养护3天,早期强度均>40 MPa,软化系数也均>0.8。