Microstructure Of Interfacial Transition Zone Research Articles

Tensile strength and failure mechanism of rock–cement sample: Roles of curing temperature, nano-silica and rock type

Understanding the tensile strength and failure mechanism of rock–cement interfacial transition zone (ITZ) is of vital significance to the sealing integrity of cement sheath under downhole condition. Taking advantage of multiple techniques, i.e., digital image correlation (DIC), nano-indentation, XRD-Rietveld analysis, 29Si MAS solid NMR, and SEM-EDX, this study is devoted to investigating the impacts of curing temperature, rock type, and the addition of nano-silica (NS), on the tensile strength and failure mechanism of rock–cement sample. The experimental results show that both the curing temperature and the addition of NS leads to the formation of more C-S-H, which densifies the ITZ microstructure and responsible for high tensile strength of rock–cement samples; the tensile strengths of shale-cement samples are consistently higher than those of sandstone-cement sample; the crack velocities for rock–cement samples under three-point bending tests are approximately 1 mm/s, the crack velocities for rock–cement samples are slowed down when the NS is incorporated in cement paste, but they are independent on the rock type and curing temperature.

Study on the damage mechanism of hydraulic concrete under the alternating action of freeze-thaw and abrasion

This study investigates the damage mechanism of hydraulic concrete under the alternating action of freeze–thaw (F) and abrasion (W) in cold regions. To this end, three groups of working conditions were established with a constant water-binder ratio. The mass loss and relative dynamic elastic modulus (RDEM) of each cycle were evaluated using the fast-freezing method and underwater method. The phase composition and relative content change of hydration products, pore characteristic parameters and pore size distribution, morphology and microstructure of interfacial transition zone (ITZ) were analyzed by X-ray diffraction (XRD), mercury intrusion porosimeter(MIP), scanning electron microscopyn (SEM) and image recognition technology, respectively. The results indicate that the alternating effect will accelerate the mass loss of concrete and the reduction of the RDEM compared with the single factor. Following four consecutive alternating cycles, the mass loss rates of working conditions W and F group were found to be 4.61 % and 97.66 % lower, respectively, than those of working condition F-W group. The RDEM damage coefficient (γ < 1) was also observed. At the micro level, the freeze-thaw effect will result in the formation of a considerable number of micropores and the expansion of microcracks, with the abrasion effect continuing to promote this process. In the process of alternating action, the dissolution of Ca(OH)2 by external water through microcracks and interconnected pores is accelerated. Concurrently, the pores and microcracks in the ITZ expand, the morphology is roughened, and the microstructure is complicated. The interfacial bonding ability and compactness of different material components decrease. Under the action of external disturbance, cement paste and aggregate fall off, resulting in mass loss and a decreased RDEM.

Leaching-induced deterioration of shear bonding strength and micro-Vickers hardness of the ITZ modified by micro-SiO2 and nano-SiO2

The interfacial transition zone (ITZ) serves as a critical juncture between aggregate and cement paste in concrete, particularly vulnerable to calcium leaching. This study investigates the impact of water-cement (w/c) ratio and micro-SiO2 and nano-SiO2 content on the deterioration of shear bonding strength and micro-Vickers hardness of the ITZ during leaching. Analysis of ITZ micro-structure and elemental composition is conducted using backscattering electron microscope and energy dispersive spectroscopy (BSEM/EDS), and a deterioration model for the shear bonding strength of the leached ITZ is established. Results indicate that during leaching, shear bonding strength and micro-Vickers hardness of the ITZ are approximately 28 % and 67 % of those of the cement paste, respectively. A linear relationship between the shear bonding strength of the ITZ and its micro-Vickers hardness is observed when employing the same type of admixture. Mechanical properties and leaching resistance of the ITZ diminish progressively with increasing w/c ratio. Conversely, they exhibit an initial enhancement followed by decline with increasing micro-SiO2 and nano-SiO2 contents, with optimal contents of 8 % and 2 %, respectively. Micro-SiO2 and nano-SiO2 are found to enhance long- and short-term leaching resistance of cement-based materials, respectively. Additionally, the shear bonding strength of the ITZ decreases linearly with increased degree of area leaching. These findings enhance our understanding of the deterioration behavior of the ITZ and provide an important foundation for studying the durability changes of concrete subjected to calcium leaching.

Effect of Recycled Concrete Aggregate Addition on the Asphalt Mixtures Performance: ITZ Area, Microstructure, and Chemical Analysis Perspectives

The importance of environmental consciousness and sustainability is increasing among transportation governing bodies worldwide. Many government bodies are concerned with maximizing the usage of recycled substances in road construction. Therefore, assessing the effect of recycled materials consumption is essential, mainly when designing new ‘green’ pavement types. The primary objective of this study is to investigate the impact of different treatments on improving the interfacial transition zone (ITZ) of coarse recycled concrete aggregate (CRCA) and its application in asphalt mixes. Such an aim is accomplished by enhancing its physical and mechanical characteristics, as well as its microstructure. The surface morphology, chemical composition, and intermix phases of the ITZ area and calcium silicate hydrate (CSH) compounds for CRCA were evaluated using scanning electron microscopy (SEM), an energy-dispersive X-ray analyzer (EDAX), and X-ray diffraction analysis (XRD). The performance of asphalt mixtures that included treated and untreated CRCA was also examined using different tests. It was found that heat treatment is an effective technique for enhancing the ITZ. However, cracks were seen in the mortar of CRCA when exposed to high temperatures (500 °C), which adversely affects the characteristics of the mortar. Acid treatment appeared to be an effective approach for improving the ITZ area. Nevertheless, the treatment that used acetic acid, a weak acid, was more effective than HCl acid, a strong acid. The outcomes revealed that the ITZ microstructure is significantly enhanced under different treatment types; however, microstructure improvements mainly included increased surface homogeneity and CSH compounds and a reduced Ca/Si ratio. It was also found that the asphalt mixtures with different proportions of untreated CRCA exhibited enhanced resistance to rutting. Furthermore, their tensile strength ratio (TSR) values were above the minimal level requirements. Moreover, the asphalt mixture with 30% CRCA, which was treated with various treatment methods, demonstrated a significant improvement in the mixtures’ mechanical properties; therefore, its application is highly successful and an environmentally friendly solution.

Effect of nanosilica and fiber on mechanical properties and microstructure of recycled coarse aggregates road concrete

Recycled coarse aggregate has its own characteristics of large porosity, high water absorption and high crushing index, which adversely affects the mechanical strength and durability of recycled aggregate concrete. In this study, the mechanical properties, durability and microstructure of recycled aggregate concrete (RAC) modified with nano-SiO2 (NS) and fibres were investigated to prepare road RAC, using NS polypropylene fibres (PPF) and basalt fibres (BF) as test variables. Compared with RAC only incorporating NS, NS and fibre synergism can significantly enhance the mechanical properties of recycled concrete, compressive strength increased by 27–51 %, flexural strength increased by 12 %, and abrasion resistance increased by 25–72 %. The synergistic effect of NS and PPF significantly improves the frost resistance of recycled concrete. NS has a volcanic ash effect which can react with calcium hydroxide to produce a large number of C-S-H in the secondary hydration reaction, making the internal structure of the cement matrix denser. Interfacial transition zone microstructure is more compact, thus interfacial transition zone porosity is effectively reduced. The hydration products are attached to the fibre surface, which enhances the bonding ability between the fibers and matrix. Therefore, the fibres effectively inhibit the generation and development of internal micro-cracks. NS optimises the internal pore structure of recycled aggregate concrete and enhances the interfacial transition zone, and NS synergistically with PPF can improve the strength and durability of various mechanical properties of recycled concrete. The research results provide the scientific basis for the application of road RAC.

Two scale models for fracture behaviours of cementitious materials subjected to static and cyclic loadings

This study employs a lattice fracture model to simulate static and fatigue fracture behaviour of Interfacial Transition Zone (ITZ) at microscale and mortar at mesoscale. The heterogeneous microstructure of ITZ and mesostructure of mortar are explicitly considered in the models. The initial step involves calibrating and validating the microscopic model of the ITZ through micro-cantilever bending tests. Subsequently, this validated ITZ model serves as a constitutive law to simulate the fracture behavior of mortar at the mesoscale using an uncoupled upscaling method. The influence of microstructural features, such as w/c ratio and microscopic roughness, on the fracture behaviour of ITZ is investigated. Moreover, the effect of ITZ properties and stress level on the fracture performance and fatigue damage evolution of mortar is also studied. The simulation results for both the ITZ and mortar demonstrate good agreement with experimental results. The proposed two models provide insights into the fracture mechanisms and fatigue damage evolution in cementitious materials subjected to static and cyclic loadings.

Improved concrete ITZ and performance via pre-soaking of inert recycled red brick aggregates in carbonation solution

Compared to recycled concrete aggregate (RCA), the high-water absorption of recycled red brick aggregate (RBA) has limited its utilization in concrete production. However, this study takes the advantage of its high-water absorption capacity as a carrier for carbonation solutions, aiming to compensate the strength loss by enhancing the interfacial transition zone (ITZ) properties of incorporated concrete. The effects of carbonation solutions (e.g., CO2 and NaHCO3) by varying the CO2 injection times (10 s, 60 s and 600 s) and NaHCO3 dosages (1%, 3% and 5%) on the physical (i.e. density and water absorption) and mechanical (i.e. compressive and tensile bond strengths) properties of concrete, in particular focus on the ITZ changes, were systematically investigated. In addition, the microstructure changes of concrete ITZ were studied using thermogravimetric analysis (TGA), scanning electron microscope-backscattered electron (SEM-BSE) image analysis and mercury intrusion porosimetry (MIP). The results indicated that it was feasible to enhance the ITZ properties by using CO2 solution as the pre-soaking agent of RBA. The adoption of CO2 solution could significantly improve the bond strength of ITZ by up to 22.5% and resulted 8.6% improvement in 28-day compressive strength. The CO2 solution contributed to the formation of CaCO3 can accelerate the cement hydration of surrounding matrix and hence densified the microstructure of ITZ. For the case of pre-soaking with NaHCO3, although the release of solution from RBA could improve the early strength of concrete, the later strength significantly reduced due to the effect of sodium ions in NaHCO3 on the growth of outer C–S–H gel.

Evaluation of mechanical strength, gamma-ray shielding characteristics, and ITZ microstructural properties of heavyweight concrete using nano-silica (SiO2) and barite aggregates

This research assessed the mechanical performance, gamma-ray shielding, and interfacial transition zone (ITZ) microstructural properties of concrete mixes that incorporate barite coarse aggregates and nanosilica. To achieve this goal, several concrete mixes containing normal coarse aggregate (NC), barite aggregate (BC), and nano-silica/barite aggregates (NSBC) were fabricated. Several properties such as workability, mechanical strength, electrical resistivity (ER), ultrasonic pulse velocity (UPV), chloride migration (RCPT), and gamma shielding were comparatively assessed for these concrete mixtures. In addition, analytical characterizations viz., x-ray diffraction (XRD), Fourier transform infrared (FTIR), scanning electronic microscopy (SEM), and energy dispersive x-ray (EDX) were employed to study the ITZ microstructure at mortar/aggregate interface of concrete mixtures. The results showed that barite concrete incorporating nanosilica (NSBC) exhibited higher density and improved mechanical properties compared to both normal concrete (NC) and barite concrete (BC). Specifically, concrete mixture containing barite aggregate and 3% nanosilica (3NSBC) content exhibited improved impermeability and reduced concrete porosity. The gamma-ray shielding properties for 3NSBC were superior among all concrete mixtures. After 28 days, 3NSBC exhibits higher compressive strength (18.26% and 14.76%) and greater shielding capacity (12.82% and 6.66%) compared to NC and BC, respectively. The analytical characterizations results have demonstrated that nanosilica helps prevent the accumulation of calcium hydroxide in the ITZ by utilizing it to form calcium silicate hydrate. This leads to the development of a more compact and denser microstructure in the ITZ of nano-silica-based barite concrete (NSBC). Furthermore, it was revealed that, the incorporation of nanosilica exceeding 3% led to agglomeration, resulting in a weaker ITZ within the concrete leading to overall reduction in concrete properties.

Influence of Nano-SiO2 Content on Cement Paste and the Interfacial Transition Zone.

Nano-SiO2 (NS) is widely used in cement-based materials due to its excellent physical properties. To study the influence of NS content on a cement paste and the interfacial transition zone (ITZ), cement paste samples containing nano content ranging from 0 to 2% (by weight of cement) were prepared, and digital image correlation (DIC) technology was applied to test the mechanical properties. Finally, the optimal NS content was obtained with statistical analysis. The mini-slump cone test showed that, with the help of superplasticizer and ultrasonic treatment, the flowability decreased continuously, as the NS content increased. The DIC experimental results showed that NS could effectively improve the mechanical properties of the cement paste and the ITZ. Specifically, at the content level of 1%, the elastic modulus of cement paste and ITZ was 20.95 GPa and 3.20 GPa, respectively. When compared to that without nanomaterials, the increased amplitude was 73.50% and 90.50%, respectively. However, with the further increase in NS content, the mechanical properties decreased, which was mainly caused by the agglomeration of nanomaterials. Additionally, the NS content did not exhibit a significant effect on the thickness of the ITZ, and its value was maintained at 76.91-91.38 μm. SEM confirmed that NS would enhance the microstructure of both cement paste and ITZ.

Mechanism underlying effect of expansive agent and shrinkage reducing admixture on mechanical properties and fiber-matrix bonding of fiber-reinforced mortar

Expansive agent (EA) and shrinkage reducing admixture (SRA) are utilized to reduce shrinkage and risk of cracking in concrete. EA compensates shrinkage by initial expansion, and SRA reduces surface tension in the pore fluid. Although EA and SRA effectively reduce shrinkage, they can impair micro-structure of concrete at high contents. The shrinkage reduction effect of EA and SRA is well known; however, there is limited knowledge about their negative effect on microstructure and fiber matrix interfacial transition zone (ITZ). The current study explores the effect of using 10 % CaO-based EA, 2 % SRA, and their combination on mechanical, shrinkage, and microstructural properties of fiber-reinforced mortar (FRM) made with 0.5 % steel fiber. The hydration products are characterized by X-ray diffraction and derivative thermogravimetric. Backscatter scanning electron microscopy, energy dispersive spectroscopy (EDS), and micro-hardness experiments are employed to characterize the microstructure of fiber-matrix ITZ at two distances of 50 and 100 μm surrounding the fibers. Experiment results indicate that the quality of ITZ and fiber-matrix bonding is impaired by using either EA or SRA. The highest drop is obtained by the combination of EA and SRA. The element analysis by EDS line and point scanning reveals the presence of longer silicate chain and stronger fiber-matrix ITZ in the reference mixture. Micro-hardness measurements confirm higher mechanical properties at the ITZ between 50 and 100 μm surrounding the fibers. Thermodynamic simulations and phase assemblages interpret the influence of EA and SRA on mechanical properties and shrinkage.

Effects of shrinkage reducing admixture and internal curing agent on shrinkage and creep of high performance concrete

The shrinkage and creep behavior of track beam concrete have the significant influence on the stability and safety of train operation. While superabsorbent polymer (SAP) and shrinkage reducing agent (SRA) are two commonly used admixtures in such high-performance concrete. So, the shrinkage and creep behavior of concrete with SRA or SAP were studied in this paper. Moreover, The microstructure of the interfacial transition zone (ITZ) observed by the scanning electron microscope (SEM) was discussed for the improvement of the mechanical properties of the concrete after the creep test. The experimental results indicate that SRA has a positive effect on reducing the shrinkage and creep deformation of concrete, and it is beneficial to the improvement of long-term strength and elastic modulus with little influence on creep recovery. In contrast, SAP reduces shrinkage, but increases the creep of concrete with less creep recovery and higher residual stress. Meanwhile, the fib 2010 model was revised to improve the ability to predict the creep of concrete containing SAP and other admixtures. Besides, it was found concrete after creep test has better mechanical properties, and the improvement of sample with SAP is the most obvious. The analyses of SEM also conformed that the concrete after creep test has a higher degree of hydration, and the denser ITZ microstructure.

Research on the durability of nano-SiO2 and sodium silicate co-modified recycled coarse aggregate (RCA) concrete

Compared with natural aggregate (NA), recycled coarse aggregate (RCA) is of lower strength and higher porosity due to the attached mortar, which limits the application of RCA. In this paper, the nano SiO2-sodium silicate mixed solution (NMS) was prepared and used to modify RCA by improving the properties of the attached mortar. Three groups of concrete with NA, RCA, and modified RCA (MRCA), respectively, were prepared. The modification effects of NMS on the concrete were illustrated in regards to the mechanical properties and durability (impermeability, chloride ion resistance, and freezing-thawing resistance). Results indicated that the concrete with the MRCA had higher mechanical properties, better durability properties, and lower air void content. Then, the surface morphology of the MRCA and interfacial transition zone (ITZ) in concrete were both examined by scanning electron microscope (SEM) to understand the modification effect of NMS. It showed that the NMS can fill the pores and repair the cracks on the attached mortar by filling effect (brought by nano SiO2) and film-forming effect (brought by sodium silicate). Moreover, nano SiO2 and sodium silicate could also participate in secondary hydration reaction to form C-S-H gel and strengthen the microstructure of ITZ. The coupling effect both result in the improved mechanical and durable properties of the concrete prepared with MRCA.

Microstructure and micromechanical properties of interfacial transition zone in green recycled aggregate concrete

Green recycled aggregate concrete (GRAC) is considered one of the sustainable building materials, which is made up of recycled aggregates (RAs) and green binding materials (alkali-activated materials, i.e., AAMs). However, the GRAC has not been used in building engineering due to its uncertain reaction mechanisms, especially the formation of interfacial transition zone (ITZ) between RAs and AAMs. Thus, the objective of this paper is to systematically investigate the microstructure and micromechanical properties of ITZ in GRAC. Backscattered electron microscopy (BSE) and energy dispersive spectroscopy (EDS) were used to investigate the microstructure and ITZ, whereas nanoindentation was used to study the micromechanical properties. The results indicate that the elastic modulus of ITZ in GRAC incorporated with crushed concrete (34 GPa) is higher than that with unbound stone (22 GPa) and crushed brick (24 GPa). This is due to the presence of C-A-S-H gels with superior micromechanical properties and compacted microstructure. Additionally, the elastic modulus and hardness of ITZ were found to be higher than the paste matrix in GRAC regardless of RAs due to the desired bonding between paste and RAs. It signifies the great potential of utilizing industrial by-products as well as construction and demolition waste to produce low-carbon and sustainable GRAC.

Evaluation of the ITZ modification efficiency via aggregate surface coating with nano SiO2 (NS) and its influence on properties

In order to effectively refine the porous microstructure of interfacial transition zone (ITZ) region in cement-based materials, the nano SiO2 (NS) was introduced into the mixture via aggregate pre-immerging in NS suspension to directly regulate the formation of ITZ microstructure. For comparison, a corresponding content of NS was directly added to the mixture during the casting process. The macro mechanical properties and micro pore structures of the specimens with varying modification methods and NS contents were assessed. The ITZ microstructures (in terms of porosity distribution and nano mechanical properties) were quantitatively characterized via the combination of BSE-SEM and nano indentation techniques. The results demonstrate that both modification methods could refine the microstructure of the bulk paste and ITZ. The directing adding method can more effectively modify the microstructure of the bulk paste with certain NS dosage (no more than 3%). At higher dosage, due to the agglomeration of the NS, the difference between two methods was not obvious. For ITZ modification, the aggregate pre-immerging method has more profound effect, from the aspects of reducing the ITZ thickness and decreasing the porosity of region in the extreme vicinity of aggregate surface (0–10 µm).

Cement-Based Materials Modified by Colloidal Nano-Silica: Impermeability Characteristic and Microstructure.

Colloidal nano-silica (CNS) was used to improve the mechanical and impermeability characteristics of mortar in this study. The samples were prepared with 0%, 1%, 2% and 3% (solid content) CNS addition. The mechanical strength and permeability of each mixture was studied, and the mechanism behind was revealed by hydration heat evolution, XRD, DSC-DTG, 29Si MAS-NMR and SEM-EDS analysis. The compressive strength and impermeability characteristics of mortars incorporating CNS were significantly improved. The experimental results demonstrated that the incorporation of CNS promoted the early hydration process of cement, thus increasing the polymerization degree of hydrated calcium silicate, decreasing the porosity, and improving the microstructure of mortar. Furthermore, 3% CNS decreased the Ca/Si ratio of the interfacial transition zone (ITZ) from 3.18 to 2.22, thus the enrichment of CH was reduced and the density and strength were improved. This was mainly because of the high pozzolanic activity of CNS, which consumed plenty of calcium hydroxide and converted to C-S-H. Besides, nanoscale CNS and C-S-H particles filled the voids between hydrates, thus refining the pore size, increasing the complexity of pores, and improving the microstructure of ITZ which contributed to the improvement of the impermeability.

Comparison of lightweight and normal-weight aggregate concrete

In this experimental study, lightweight aggregate concrete (LWAC) and normal-weight aggregate concrete (NWAC) were compared within the contexts of the size of calcium hydroxide (CH) crystals in interfacial transition zone (ITZ), compressive strength, and oven-dry density. Six LWAC and NWAC mixtures were prepared for this study. Thirty-six images obtained from Scanning Electron Microscope (SEM) were used to determine the size of CH crystals in ITZ of LWAC and NWAC. Eighteen test specimens (three for each of the six LWAC and NWAC mixtures) were prepared in 150x300 mm sizes and in the form of cylinders for the compressive strength tests and also eighteen test specimens in 100x100x100 mm sizes and in the form of cubes for the oven-dry density tests. It was determined that the size of CH crystals in ITZ of LWAC is 8.43% less than (on average), compressive strength of LWAC is 39.09% more than (on average), and oven-dry density of LWAC is 10.97% less than (on average) the NWAC’s that has the same volumetric proportions of ingredients. The findings of this study show that lightweight aggregate that has high particle density, angular shape, rough surface texture, and a structure that enables chemical reaction with CH crystals will be beneficial for the ITZ microstructure and properties of concrete. It is considered that these properties should be taken into consideration in the selection of lightweight aggregate for structural concrete production.

Study of the Interfacial Transition Zone Characteristics of Geopolymer and Conventional Concretes.

The properties and performance of geopolymer at different length scales have been intensively studied, but only limited studies on geopolymer have investigated the zone located between paste and aggregates, which is called the interfacial transition zone (ITZ). The microstructure of ITZ and its nanomechanical properties in geopolymer concrete are examined in this study. Fly ash-based geopolymer has great potential to be an alternative to traditional concrete. To this end, scanning electron microscopy (SEM) and nanoindentation tests were performed to investigate the microstructural characteristics and nanomechanical properties of the ITZ, and the results were compared with the ITZ of traditional concrete. Results show that traditional concrete demonstrated a weak ITZ with pores and microcracks, while the geopolymer concrete microstructure did not present weak ITZs in the vicinity of aggregates. More pores and crack were observed in the ITZ in traditional concrete. Further, a considerable amount of fly ash particles, that appear to be unreacted or partially reacted in the matrix phase, was observed. Based on the nanoindentation results, 58% of the microstructure is composed of unreacted or partially reacted fly ash particles. The results of nano- and microscale tests will enhance the understanding of how concrete behaves and performs at large scales.

Occurrence and migration laws of water in circulating fluidized bed bottom slag mortar and their influences on mortar properties

The low utilization rate of circulating fluidized bed bottom slag (CFBBS) leads to its serious stacking and induces severe environmental pollution. Fortunately, the CFBBS can be recycled to the maximum extent as aggregate in mortar or concrete. Due to the porous structure of CFBBS, the occurrence and migration of water between CFBBS and the cement paste have some special laws, which are meaningful to the applications of CFBBS in mortar or concrete. In this paper, the water occurrence and migration laws of CFBBS in pure water, fresh mortar, and hardened mortar were explored systematically. The relationships among the effective water-cement ratio, the total water-cement ratio, and the time after adding water in CFBBS mortar were discussed, and the influences of the effective water-cement ratio on the fluidity and compressive strength of mortar were studied. The strengthening mechanism of CFBBS on the interfacial transition zone (ITZ) of mortar was revealed by micro test technology. The results showed that in the fresh stage, water migrated from the cement paste to CFBBS, which reduced the effective water-cement ratio and led to the decrease of mortar fluidity and the increase of compressive strength; in the hardened stage, however, water migrated from CFBBS to the hardened paste, which maintained the internal humidity of mortar, increased the hydration degree of cement, and improved the microstructure of ITZ.

Proposing a three-phase model for predicting the mechanical properties of mortar and concrete

The interfacial transition zone (ITZ) around aggregates is known as the weakest zone in mortar and concrete, which highly influences the mechanical properties. In this study, the influence of ITZ on mechanical properties such as Young’s modulus and Poisson’s ratio of cementitious materials is evaluated using a multi-scale model (developed in five hierarchical levels: nano-scale cement hydrates∼ scale of concrete). In the proposed model, the microstructure of mortar/ concrete is considered as a three-phase material: fine/ coarse aggregates, ITZ of aggregates and bulk paste/ mortar. The primary input is the microstructure of cement paste and ITZ, which are predicted as the function of curing time using coupled cement hydration-thermodynamic model. The ITZ volume fraction is analytically computed based on aggregate particle size distribution. Multi-level homogenization methods (based on three-phase sphere model for two-phase composite material) are finally implemented to predict the effective properties of mortar and concrete. Here, the equivalent matrix consisted of fine/ coarse aggregate, bulk paste/ mortar and ITZ are obtained with the first and second levels homogenization procedures for mortar/ concrete. To validate the predictability of the models for mortar and concrete, predicted values are compared with independent experimental data sets. The results show a reasonable agreement with experimental results of Poisson’s ratio and Youngs modulus (in the error range of 5 GPa). The influences of ITZ on properties of mortar and concrete are also discussed based on the outcomes.

Micromechanical analysis of interfacial transition zone in alkali-activated fly ash-slag concrete

This paper systematically investigates the micromechanical properties of interfacial transition zone (ITZ) in alkali-activated fly ash-slag (AAFS) concrete using nanoindentation, backscattered electron microscopy and energy dispersive spectroscopy. Results indicate that the micromechanical properties of ITZ depend on the chemical composition of reaction products and its microstructural characteristics. The ITZ with high proportion of N-C-A-S-H and C-A-S-H gels tends to have high elastic modulus because of their superior micromechanical properties. The formation of reaction products would refine the microstructure of ITZ and improve its elastic modulus. The evolution of micromechanical properties of ITZ can be divided into three stages: (i) accelerated growth stage via fast chemical reactions (<12 h); (ii) stationary stage via stable chemical reactions (12 h-7 d); (iii) decrement stage via microcrack propagation (7 d-28 d). ITZ is not the weakest region in AAFS concrete due to its desired micromechanical properties and compact microstructure compared to paste matrix.