Multiphysics processes in the interfacial transition zone of fiber-reinforced cementitious composites under induced curing pressure and implications for mine backfill materials: A critical review

The mesoscale fiber-matrix interfacial transition zone(FM-ITZ) under induced curing pressure plays a key role in the effectiveness of fiber reinforcement and the engineering application of fiber-reinforced cementitious composites(FRCCs). This critical review establishes the link among induced curing pressure(i.e., external loading condition), multiphysics processes(i.e., internal governing mechanism), and interface behavior(i.e., material behavior) for FRCC materials through analysis of the state-of-the-art research findings on the FM-ITZ of FRCC materials. The following results are obtained. For the mechanical process, the induced curing pressure changes the stress state and enhances multicracking behavior, which can strengthen the FM-ITZ. For the hydraulic process, the strengthened seepage of the FM-ITZ under induced curing pressure weakens the effective stress and exaggerates the deficiency in water retention capacity between the bulk matrix and the FMITZ. For the thermal process, the induced curing pressure causes a steep temperature gradient in the FM-ITZ and thus influences the temperature evolution and thermally-induced microcracks in the FM-ITZ. For the chemical process, the induced curing pressure enhances hydration kinetics and results in the formation of additional hydration products in the FM-ITZ. Moreover, recommendations are proposed on the basis of findings from this review to facilitate the implementation of fiber reinforcement in cemented paste backfill technology.

Influence Mechanism of Curing Regimes on Interfacial Transition Zone of Lightweight Ultra-High Performance Concrete

This study aims to clarify the effects of curing regimes and lightweight aggregate(LWA)on the morphology, width and mechanical properties of the interfacial transition zone(ITZ) of ultra-high performance concrete(UHPC), and provide reference for the selection of lightweight ultra-high performance concrete(L-UHPC) curing regimes and the pre-wetting degree LWA. The results show that, under the three curing regimes(standard curing, steam curing and autoclaved curing), LWA is tightly bound to the matrix without obvious boundaries. ITZ width increases with the water absorption of LWA and decreases with increasing curing temperature. The ITZ microhardness is the highest when water absorption is 3%, and the microhardness value is more stable with the distance from LWA. Steam and autoclaved curing increase ITZ microhardness compared to standard curing. As LWA pre-wetting and curing temperatures increase, the degree of hydration at the ITZ increases, generating high-density CSH(HD CSH) and ultra-high-density CSH(UHD CSH), and reducing unhydrated particles in ITZ. ITZ micro-mechanical properties are optimized due to hydration products being denser.

Effect of the Entrained Air Void on Strength and Interfacial Transition Zone of Air-Entrained Mortar

In order to facilitate the development and application of air entraining agents (AEA) in the high performance concrete, entrained air void structure parameters (air void size range from 10 to 1 600 mu m) of 28 d sifted mortar were measured by image analysis method. The relationship between the air void size distribution and strength of mortar was studied by methods of grey connection analysis and multiple linear regression analysis. The multiple linear regression equation was established with a correlation coefficient of 0.966. The weight of the affection of hierarchical porosity on the compressive strength ratio was also obtained. In addition, the effect of air voids on the paste-aggregate interfacial transition zone (ITZ) was analyzed by microhardness. The results show that the correlation between different pore size range and the compressive strength is negative. The effect of air void size distribution on 28 days compressive strength is different: under the condition of similar total porosity, with the increase of the porosity of the air void size, ranging from 10 to 200 mu m, and the decrease of the porosity, ranging from 200 to 1 600 mu m, the average air void diameter and mean free spacing are decreased; as well as the width of ITZ. On the contrary, the microhardness of the ITZ is increased while the compressive strength loss is decreased.

Quantitative Analysis and Affecting Factors of the Overlapping Degree of Interfacial Transition Zone between Neighboring Aggregates in Concrete

Based on the nearest surface function formula, a quantitative formula to measure the overlapping degree between the interfacial transition zone （ITZ） of neighboring aggregate particles was put forward. The formula was further deduced to quantitatively analyze the influence of the volume fraction of aggregate, ITZ thickness and the maximum aggregate diameter on the overlapping degree between neighboring ITZ. The volume of ITZ was quantitatively calculated in actual concrete by comparing the nearest surface function formula with an approximate method, that is the surface area of the aggregates multiplied by the uniform thickness of the ITZ layers. The results showed that the influencing order of these three factors on the overlapping degree between neighboring ITZ in turn was the interface thickness, aggregate volume fraction and the maximum aggregate diameter; As long as the interface thickness 50 μm and the aggregate volume fraction 50%, the calculated error between two methods mentioned above is about 10 %.

Modeling and Software Development of the Interfacial Transition Zone of Ellipsoidal Aggregate in Cement-Based Composites

The interfacial transition zone (ITZ) between the aggregates and the bulk paste is the weakest zone of ordinary concrete, which largely determines its mechanical and transporting properties. However, a complete understanding and a quantitative modeling of ITZ are still lacking. Consequently, an integrated modeling and experimental study were conducted. First, the theoretical calculation model of the ITZ volume fraction about the rotary ellipsoidal aggregate particles was established based on the nearest surface function formula. Its calculation programs were written based on Visual Basic 6.0 language and achieved visualization and functionalization. Then, the influencing factors of ITZ volume fraction of the ellipsoidal aggregate particles and the overlapping degree between the ITZ were systematically analyzed. Finally, the calculation models of ITZ volume fraction on actual ellipsoidal aggregate were given, based on cobblestones or pebbles particles with naturally ellipsoidal shape. The results indicate that the calculation model proposed is highly reliable.

Quantitative Characterization and Elastic Properties of Interfacial Transition Zone around Coarse Aggregate in Concrete

Backscattered electron images（BSE） obtained by scanning electron microscope was used to quantitatively characterize the microstructure of interfacial transition zone（ITZ） in concrete. Influences of aggregate size（5, 10, 20, and 30 mm）, water to cement ratio（0.23, 0.35 and 0.53） and curing time（from 3d to 90d） on the microstructure of interfacial transition zone between coarse aggregate and bulk cement matrix were investigated. The volume percentage of detectable porosity and unhydrated cement in ITZ was quantitatively analyzed and compared with that in the matrix of various concretes. Nanoindentation technology was applied to obtain the elastic properties of ITZ and matrix, and the elastic modulus of concrete was then calculated based on the Lu ＆ Torquato model and self-consistence scheme by using the ITZ thickness and elastic modulus obtained from this investigation. The experimental results demonstrated that the microstructure and thickness of ITZ in concrete vary with a variety of factors, like aggregate size, water to cement ratio and curing time. The relative low elastic properties of ITZ should be paid attention to, especially for early age concrete.

Nanoindentation Characteristics of Cement Paste and Interfacial Transition Zone in Mortar with Rice Husk Ash

The nanostructure of cementitious materials has important effects on concrete properties. The effects of rice husk ash（RHA） on cement hydration product phases and interfacial transition zone（ITZ） in mortar were investigated from the nano-scale structure perspective. The experimental results indicate that, with the increase of RHA dosages of samples, the volume fraction of high-density calcium-silicate-hydrate（HD C-S-H） in porosity and hydration product phases increases. The volume fractions of HD C-S-H in C-S-H of samples show an increasing trend with the increase of RHA dosages. RHA decreases the thickness of ITZ and increases the matrix elastic moduli of samples, however, the RHA dosoges hardly affect the thickness and elastic moduli.

Nanomechanical Characteristics of Interfacial Transition Zone in Nano-Engineered Concrete

This study investigates the effects of nanofillers on the interfacial transition zone(ITZ)between aggregate and cement paste by using nanoindentation and statistical nanoindentation techniques.Moreover,the underlying mechanisms are revealed through micromechanical modeling.The nanoindentation results indicate that incorporating nanofillers increases the degree of hydration in the ITZ,reduces the content of micropores and low-density calcium silicate hydrate(LD C-S-H),and increases the content of highdensity C-S-H(HD C-S-H)and ultra high-density C-S-H(UHD C-S-H).In particular,a new phase,namely nano-core-induced low-density C-S-H(NCILD C-S-H),with a superior hardness of 2.50 GPa and an indentation modulus similar to those of HD C-S-H or UHD C-S-H was identified in this study.The modeling results revealed that the presence of nanofillers increased the packing density of LD CS-H and significantly enhanced the interaction(adhesion and friction)among the basic building blocks of C-S-H gels owing to the formation of nano-core-shell elements,thereby facilitating the formation of NCILD C-S-H and further improving the performance of the ITZ.This study provides insight into the effects of nano fillers on the ITZ in concrete at the nanoscale.

Sandstone-concrete interface transition zone (ITZ) damage and debonding micromechanisms under freeze-thaw

The sufficient bond between concrete and rock is an important prerequisite to ensure the effect of shotcrete support. However, in cold regions engineering protection system, the bond condition of rock and concrete surface is easily affected by freeze-thaw cycles, resulting in interface damage, debonding and even supporting failure. Understanding the micromechanisms of the damage and debonding of the rock-concrete interface is essential for improving the interface protection.Therefore, the micromorphology, micromechanical properties, and microdebonding evolution of the sandstone-concrete interface transition zone(ITZ) under varying freeze-thaw cycles(0, 5, 10, 15, 20) were studied using scanning electron microscope, stereoscopic microscope, and nano-indentation. Furthermore, the distribution range and evolution process of ITZ affected by freeze-thaw cycles were defined. Major findings of this study are as follows:(1) The microdamage evolution law of the ITZ under increasing freeze-thaw cycles is clarified, and the relationship between the number of cracks in the ITZ and freeze-thaw cycles is established;(2) As the number of freeze-thaw cycles increases, the ITZ's micromechanical strength decreases, and its development width tends to increase;(3) The damage and debonding evolution mechanisms of sandstone-concrete ITZ under freeze-thaw cycles is revealed, and its micromechanical evolution model induced by freeze-thaw cycles is proposed.

Discrete Element Modelling of Damage Evolution of Concrete Considering Meso-Structure of ITZ

The mechanical properties of interfacial transition zones(ITZs)have traditionally been simplified by reducing the stiffness of cement in previous simulation methods.A novel approach based on the discrete element method(DEM)has been developed for modeling concrete.This new approach efficiently simulates the meso-structure of ITZs,accurately capturing their heterogeneous properties.Validation against established uniaxial compression experiments confirms the precision of thismodel.The proposedmodel canmodel the process of damage evolution containing cracks initiation,propagation and penetration.Under increasing loads,cracks within ITZs progressively accumulate,culminating in macroscopic fractures that traverse themortarmatrix,forming the complex,serpentine path of cracks.This study reveals four distinct displacement patterns:tensile compliant,tensile opposite,mixed tensile-shear,and shear opposite patterns,each indicative of different stages in concrete’s damage evolution.The widening angle of these patterns delineates the progression of cracks,with the tensile compliant pattern signaling the initial crack appearance and the shear opposite pattern indicating the concrete model’s ultimate failure.

负载纳米TiO_(2)石英砂功能集料-水泥石的界面粘结性能

光催化水泥基材料是先进建筑功能材料的研究热点之一,但传统内掺法制备的TiO_(2)-水泥基复合材料中TiO_(2)有效利用率低、经济效益差。针对此问题,采用负载法制备纳米TiO_(2)功能集料(QST),并将QST集料负载于普通水泥砂浆表面。研究QST集料对水泥砂浆干缩性能的影响,通过拉拔法测试QST集料与水泥石的界面粘结力;采用SEM、EDS、MIP等微观测试方法,研究纳米TiO_(2)对集料-水泥石界面过渡区水化产物、孔结构的影响。结果表明:与普通石英砂集料相比,表面负载纳米TiO_(2)的石英砂功能集料可减少水泥砂浆的干缩,提高集料-水泥石的界面粘结力。在QST集料-水泥石的界面过渡区,纳米TiO_(2)促进了水泥浆体的水化,减少了界面过渡区Ca(OH)2的富集,细化并改善了界面过渡区的孔结构。

不同化学改性方法提高玉米芯骨料混凝土的性能

玉米芯具有较低的导热系数,可以作为天然的保温材料,用于混凝土砌块中。然而,由于玉米芯的多孔结构,导致玉米芯骨料混凝土的抗压强度偏低,亟需进行改性。为了推广玉米芯的规模化利用,该研究以玉米芯为原料改善混凝土砌块的性能,通过红外光谱、拉曼光谱和扫描电镜等测试手段,探讨了3种不同改性技术对玉米芯骨料混凝土水化产物化学键、分子结构、界面过渡区微观结构、抗压强度和导热系数的影响。结果表明:与环氧树脂改性相比,陶粒法改性和裹浆法改性不仅增加了玉米芯骨料混凝土界面过渡区的水化硅酸钙凝胶含量,同时也降低了界面过渡区的厚度,优化了混凝土砌块的微观结构、提高了混凝土砌块的抗压强度、降低了混凝土砌块的导热系数。其中,陶粒法改性技术的效果尤为明显。界面过渡区厚度水化硅酸钙特征峰由大到小顺序分别为陶粒法改性、裹浆法改性、环氧树脂改性。经过陶粒法改性、裹浆法改性和环氧树脂改性后,玉米芯骨料混凝土的界面过渡区分别为无明显界面过渡区、55~66 μm和93~101 μm之间。和未改性玉米芯骨料混凝土相比,掺30%陶粒法改性玉米芯骨料混凝土的抗压强度的导热系数分别降低了51.5%和32.2%。当掺入不超过15%的改性玉米芯骨料时,混凝土砌块满足国家标准GB/T8239-2014中对抗压强度的要求。为了改善玉米芯骨料混凝土砌块的综合性能,建议掺入不超过15%的陶粒法改性玉米芯骨料。研究结果为玉米芯在混凝土砌块中的大规模利用提供了依据,同时也为进一步改善混凝土砌块的相关性能提供了参考。

铁尾矿基多固废混凝土抗压性能及微观结构分析

针对铁尾矿堆存困难、综合利用率低和活性低的问题,以铁尾矿钢渣脱硫灰为复合掺合料制备多固废混凝土。通过抗压性能测试,研究复合掺合料掺量、铁尾矿细度对混凝土抗压强度的影响,并利用压汞法(MIP)和背散射电子成像技术(BSE)探究混凝土的微观结构。结果表明:复合掺合料的掺入对混凝土早期抗压强度影响较大,掺量小于20%的混凝土28 d抗压强度与无掺合料组抗压强度基本持平,30%掺量的混凝土抗压强度随铁尾矿比表面积的增大而先增大后减小;掺入复合掺合料和减小铁尾矿细度能够改善混凝土孔结构和提高界面过渡区的密实度。

Numerical calculation and influencing factors of the volume fraction of interfacial transition zone in concrete

The determination of volume fraction of interfacial transition zone(ITZ) is very important for investigating the quantitative relationship between the microstructure and macroscopical property of concrete.In this paper,based on Lu and Torquato's most nearest surface distribution function,a calculating process of volume fraction of ITZ is given in detail according to the actual sieve curve in concrete.Then,quantitative formulas are put forward to measure the influencing factors on the ITZ volume fraction.In order to validate the given model,the volume fractions of ITZ obtained by numerical calculation are compared with those by computer simulation.The results show that the two are in good agreement.The order of the factors influencing the ITZ volume fraction is the ITZ thickness,the volume fraction of aggregate and the maximum aggregate diameter for Fuller gradation in turn.The ITZ volume fraction obtained from the equal volume fraction(EVF) gradation is always larger than that from the Fuller gradation for a given volume fraction of aggregate.

老混凝土中骨料-水泥界面过渡区(ITZ)(Ⅰ)——元素与化合物在ITZ的富集现象

利用扫描电子显微镜 (SEM)、电子探针 (EPXM)和电子能谱仪 (EDXA)对骨料颗粒和水泥浆体界面过渡区 (ITZ)进行了研究。研究的重点是老混凝土中骨料 -水泥界面过渡区的微观结构特征和成份分布。结果表明 ,老混凝土由于成熟度高 ,其 ITZ中水化物十分丰富 ,密实度高。ITZ内部和外部的水化产物组成存在一定的差别 ,表现在 Ca、K和 Fe等元素富集于 ITZ,而 Si元素在此区域的含量相对较低。对于特定的元素或物质在 ITZ中的富集现象的研究 。

再生混凝土ITZ结构与性质的研究

采取降低水灰比 ,掺入适量粉煤灰、高效减水剂和对再生集料表面进行处理等措施 ,通过 SEM研究了再生混凝土界面过渡区 (ITZ)的结构与性质。试验结果显示 :降低水胶比 ,掺入 2 0 %的粉煤灰和 2 .5 %高效减水剂 (对水泥 ) ,可使再生混凝土 ITZ结合更加紧密 ,抗压强度得以提高。在同样条件下 ,采用 1% PVA溶液处理过的再生集料 ,有利于提高新拌再生混凝土的流动性 ,同时 。

老混凝土中骨料-水泥界面过渡区(ITZ)(Ⅱ)——元素在界面区的分布特征

利用电子探针 (EPXM)对骨料颗粒和水泥浆体界面过渡区 (ITZ)进行了研究。研究的重点是化学元素在 ITZ附近的分布情况。试验结果表明 :Ca、K和 Fe有不同程度地集中于界面过渡区的趋势 ,而 Si的浓度在界面处相对比较低 ,但随着远离界面其浓度逐渐升高。 Mg、Al等元素在 ITZ的局部出现 ,且有不连续分布的特点。通过对老混凝土中ITZ元素分布的分析 ,可以部分地了解混凝土内部所发生的离子和物质迁移 ,骨料和水泥浆体之间发生的化学反应 。

混凝土ITZ性能及其对混凝土性能影响研究

界面过渡区(ITZ)是指硬化水泥浆(水泥基相)和骨料(分散基相)之间的约50μm的薄层部分。基于当前国内外学者对混凝土界面过渡区性能的大量研究成果,发现材料组分和环境作用对ITZ的力学性能和微观结构影响较大;ITZ微观结构变化影响混凝土材料宏观性能;对于ITZ的改性,目前通用的处理方式是掺合料和外加剂的掺入、对骨料进行预处理以及改善制作工艺。本文在分析混凝土界面过渡区的各项研究现状基础上提出相应的建议和展望。

机制砂砂浆界面过渡区特性研究

研究机制砂砂浆界面过渡区特性对于提升机制砂砂浆强度与耐久性具有重要意义。通过扫描电子显微镜(SEM)、纳米压痕和三维图像相关(3D-DIC)技术观测了河砂砂浆和机制砂砂浆的界面微观结构、微区力学强度和非均匀变形。结果表明,机制砂砂浆的界面过渡区宽度大于河砂砂浆,但砂浆中细骨料颗粒与浆体的界面裂缝却更小,同时机制砂棱角位置易产生微裂纹。机制砂的界面过渡区弹性模量略高于河砂,说明机制砂能一定程度削弱骨料的边壁效应。砂岩机制砂砂浆的收缩变形非均匀性最大,其内部出现较大的拉伸应变,最终引起了更严重的损伤分布,这表明细骨料中刀片状颗粒占比偏多会增大砂浆局部开裂风险。

胶凝砂砾石纳米压痕试验及系统聚类法界面过渡区的研究

针对胶凝砂砾石材料微观力学性能的研究成果较少的问题,以及胶凝砂砾石材料界面过渡区的存在性问题,提出了胶凝砂砾石不同相的确定方法以及界面过渡区尺寸的确定方法.结合纳米压痕试验,绘制微观力学性能图像,采用系统聚类法对测试矩阵内的测试点进行分类.在试验阶段,考虑到胶凝砂砾石的材料特性,完善了“30-2-30”测试制度,通过荷载-深度曲线结合Oliver-Pharr方法求出胶凝材料硬度H为0.18 GPa,折合模量Er为13.13 GPa,砂砾石材料硬度H为8.60 GPa,折合模量Er为84.56 GPa.在计算阶段,采用系统聚类法确定界面过渡区范围,确定其尺寸约为18μm.