Porous Interfacial Transition Zone Research Articles

Fresh and Hardened Properties of Brick Aggregate Concrete with Maximum Aggregate Sizes of 10 mm to 75 mm

The fresh and mechanical properties of concrete made with brick aggregates of eight different maximum aggregate sizes (MAS), i.e., 10 mm, 12.5 mm, 19 mm, 25 mm, 37.5 mm, 50 mm, 63 mm, and 75 mm, were investigated. The other parameters studied were sand-to-aggregate volume ratio (s/a) (0.40 and 0.45), W/C (0.45, 0.50, and 0.55), and cement content (375 kg/m3 and 400 kg/m3). In total, 80 different concrete mixes were studied; the perimeter of the interfacial transition zone (ITZ) along the brick aggregates was quantified with an image-analysis software and the microstructure along the ITZ was investigated using a scanning-electron microscope (SEM) to corroborate the hardened properties of the concrete. Although larger MAS leads to greater slump in concrete, its effect on hardened properties is linked to other design parameters. For a cement content of 375 kg/m3 and W/C of 0.45 and 0.50, the compressive strength of concrete increases (by up to 5%–15%) with increases in MAS of up to 37.5 mm irrespective of s/a (0.40 and 0.45) and then reduces gradually. For all other cases, the compressive strength of concrete is reduced with increases in MAS. The SEM imaging confirmed the presence of weak and porous ITZ and the deposition of ettringite in the voids left by entrapped bleed water under large aggregates. The compressive strength also increased with increases in s/a from 0.40 to 0.45, predominantly for smaller MAS. Correlations between mechanical properties of concrete and stress–strain curves are proposed for different MAS.

Effect of pretreatment on reclaimed asphalt pavement aggregates for minimizing the impact of leachate on cement hydration

Many efforts have been made in recent years to justify the use of reclaimed asphalt pavement (RAP) aggregates in concrete. All previous efforts appear to unanimously report a reduction in concrete performance with varying proportions of RAP usage. The poor performance of RAP aggregates in concrete is attributed mainly to a larger, more porous interfacial transition zone (ITZ) and the cohesive failure of the asphalt. This study hypothesizes that the detrimental impact on the ITZ is attributable to organic compounds leached from the asphalt coating in the high pH pore solution. This study proves the presence of organic compounds in the pore solution and demonstrates that the leachates impair cement hydration. This study also attempted to pretreat the RAP in a sodium hydroxide (NaOH) solution to pre-leach the organic compounds. The pretreatment demonstrated that organic compounds were leached and that NaOH modified the asphalt surface chemistry. However, only a marginal improvement in compressive strength was observed by completing a pretreatment.

The Effect of Recycled Waste Glass as a Coarse Aggregate on the Properties of Portland Cement Concrete and Geopolymer Concrete

A significant amount of waste glass is generated in Australia and around the world, which requires sustainable recycling. The use of recycled glass as aggregates in concrete is one of the many options for recycling. This study investigated the characteristics of ordinary Portland cement (OPC) and geopolymer concretes containing different proportions of recycled glass as a partial replacement of natural coarse aggregate. It was found that the 28-day compressive and tensile strengths of OPC concrete decreased up to 21%, and 7% and of geopolymer concrete decreased by 11–26% and 11–29% with the increase in the recycled glass coarse aggregate. The porosity, sorptivity and chloride permeability of OPC and geopolymer concrete increased and the drying shrinkage decreased due to the use of the recycled glass coarse aggregate. The microstructural analysis revealed the porous interfacial transition zone (ITZ) between the glass coarse aggregate and the paste/mortar matrix led to a decrease in the strength and an increase in the porosity, sorptivity and chloride permeability of the concrete due to the increase in the glass coarse aggregate. However, the mechanical and durability properties of OPC and geopolymer concrete containing 10 to 20% glass coarse aggregate were comparable to the corresponding properties of the control concrete sample containing a natural coarse aggregate.

Modelling hydraulic and capillary‐driven two‐phase fluid flow in unsaturated concretes at the meso‐scale with a unique coupled DEM‐CFD technique

AbstractThe goal of the research was to demonstrate the impact of thin porous interfacial transition zones (ITZs) between aggregates and cement matrix on fluid flow in unsaturated concrete caused by hydraulic/capillary pressure. To demonstrate this impact, a novel coupled approach to simulate the two‐phase (water and moist air) flow of hydraulically and capillary‐driven fluid in unsaturated concrete was developed. By merging the discrete element method (DEM) with computational fluid dynamics (CFD) under isothermal settings, the process was numerically studied at the meso‐scale in two‐dimensional conditions. A flow network was used to describe fluid behaviour in a continuous domain between particles. Small concrete specimens of a simplified particle mesostructure were subjected to fully coupled hydro‐mechanical simulation tests. A simple uniaxial compression test was used to calibrate the pure DEM represented by bonded spheres, while a permeability and sorptivity test for an assembly of spheres was used to calibrate the pure CFD. For simplified specimens of the pure cement matrix, cement matrix with aggregate, and cement matrix with aggregate and ITZ of a given thickness, DEM/CFD simulations were performed sequentially. The numerical results of permeability and sorptivity were directly compared to the data found in the literature. A satisfactory agreement was achieved. Porous ITZs in concrete were found to reduce sorption by slowing the capillary‐driven fluid flow, and to speed the full saturation of pores when sufficiently high hydraulic water pressures were dominant.

The role of carbonated steel slag on mechanical performance of ultra-high performance concrete containing coarse aggregates

Carbonated fine steel slag (CSS) can be used as supplementary cementitious materials (SCM) to enhance the sustainability of ultra-high performance concrete (UHPC). However, the role of CSS on the mechanical performance of UHPC containing coarse aggregates (UHPC-CA) has not been well understood. Here, the influence of cement substitution by CSS on the hydration products, microstructure, mesoscale fracture characteristics and macroscopic strength of UHPC-CA is investigated. Results reveal that the C-S-H content in UHPC increases with the increase of CSS substitution up to 15%. Replacement of cement by CSS results in a more porous interfacial transition zone (ITZ), consequently, a lower mechanical contribution from CA. Both the compressive and the flexural strength decrease with the increase of CSS substitution, and the negative effect is rather limited till 30% CSS substitution. Moreover, due to a more porous ITZ at a higher cement substitution, the ratio of flexural to compressive strength decreases with the increase of CSS substitution.

Quantitative evaluation of interfacial transition zone of sustainable concrete with recycled and steel slag as aggregate

AbstractUtilizing steel slag and recycled concrete as fine aggregate could save the consumption of natural resource and reduce the environmental pollution. In this work, the effect of steel slag aggregate and recycled aggregate on the compressive strength and chloride migration coefficient of three types of concretes were investigated and compared with natural aggregate. The interfacial transition zone (ITZ) microstructure and the connection between aggregate and bulk cement matrix was determined by using a quantitative backscattered electron (BSE) image analysis, N2 adsorption–desorption and X‐ray diffraction analysis. Results showed that the compressive strength and chloride resistance ability of natural aggregate concrete (NAC) and steel slag aggregate concrete (SAC) was highly dependent on aggregate volume content. The performance of recycled aggregate concrete (RAC) were much poorer than that of SAC and NAC. Pore structure and phase analysis confirmed that the improved properties of SAC can be partially attributed to the densified ITZ. Regarding RAC, a porous ITZ was observed in BSE image and pore structure analysis, which caused a poor compressive strength and a large chloride migration coefficient. It was found that the improvement of chloride resistance ability by using supplementary cementitious materials in NAC was more remarkable than that in SAC.

Two-step Homogenization of Poroelastic Properties of a Limestone

This study aims at deriving the effective poroelastic properties of the oolitic limestones based on the Hashin composite sphere assemblage (CSA) micromechanical theory. The microstructure of oolitic limestones generally exhibits an assemblage of grains (oolites) surrounded by a matrix. Grain and matrix are linked via the interfacial transition zone (ITZ). Pores exist in these three material phases (oolite, ITZ and matrix). A two-step homogenization method is proposed. The first step consists of upscaling the properties of each porous phase (i.e. porous oolite, porous ITZ and porous matrix) in which each phase contains two sub-phases including pore and solid. The differential self-consistent scheme is used for the first step. At the second step, the three different porous constituents (oolite, ITZ and matrix) are assembled in a CSA model. A mathematical analogy between thermoelasticity and poroelasticity is used to obtain the effective poroelastic properties. A comparison between the proposed model and test data on the oolitic limestone from Bourgogne (France) helps to calibrate the model parameters and to highlight the role of ITZ phase.

Meso-mechanical modelling of damage in concrete using discrete element method with porous ITZs of defined width around aggregates

The article deals with two-dimensional mesoscopic numerical results of fracture in concrete at the aggregate scale in a beam with a notch under bending. The focus was on the effects of both initial micro-porosity in ITZs and aggregate roughness on the load-deflection curve and propagation and location of micro- and a macro-damage. An improved discrete element method (DEM) was used for concrete to predict its strength, brittleness and fracture. A phase concrete description was used to explicitly take its heterogeneity into account. Concrete included aggregate, mortar, interfacial transition zones (ITZs) and macro-pores. In contrast to existing approaches, ITZs were simulated as porous mortar zones around aggregates with a defined width (without reducing mechanical properties). The real shape and place of aggregates in the beam were established in numerical calculations upon on x-ray micro-CT scans. The findings presented in this paper offer a new perspective as to the understanding of micro-cracking formation in concrete under loading.

Pore Structure Characteristics of RAP-Inclusive Cement Mortar and Cement Concrete Using Mercury Intrusion Porosimetry Technique

Abstract The pore structure characteristics of cement mortar and concrete incorporating reclaimed asphalt pavement (RAP) fine aggregates as part replacements of natural fine aggregates (NAs) were studied using mercury intrusion porosimetry (MIP) technique. NAs were replaced by RAP at 25, 50, 75, and 100 % by volume of total fine aggregates. Mineral admixtures, namely silica fume and activated sugarcane bagasse ash, were incorporated as part replacements of cement as well. MIP technique could identify the mesopores and macropores in the cementitious mixture. Porosity increases with an increase in RAP content in cementitious mixture, owing to larger and porous interfacial transition zone. Total intrusion pore volume increases with an increase in RAP content and is greater than the control mix irrespective of RAP content and mineral admixture. Mesopores and macropores follow a similar trend as total intrusion pore volume, suggesting finer and larger pores in RAP-inclusive cementitious mixtures. Threshold diameters were observed to initially decrease until 50 % RAP content and to increase thereafter for RAP-inclusive cement mortar, suggesting easy penetration of chemical species for higher RAP content mixes. From pore classification studies, entrained air, large capillaries, medium capillaries, and small capillaries were also analyzed. Large capillaries follow a similar trend to threshold diameter, in which the former affects the transport processes in cementitious mixture. RAP-inclusive cementitious mixtures have the ability to resist freeze as well as thaw and salt decay; this is concluded indirectly from pore structure studies. The pore-mass fractal dimension has the ability to describe the pore-solid structure, whereas the pore-surface fractal dimension failed to do so for RAP-inclusive cementitious mixtures.

Supported ITZ Modification Efficiencies via Surface Coating Nanoparticles on Aggregate and its Influence on Properties

In order to modify the porous interfacial transition zone (ITZ) microstructure of concrete more efficiently, a method of coating aggregate surfaces by using several nanoparticles was evaluated in this study. The compressive strength, chloride penetration of sound, and pre-loading samples were assessed in relation to the type of coating materials used (slag, nano-CaCO3, and nano-SiO2) and the designed coating thickness (5, 10, and 15 μm). The ITZ microstructure was quantitatively determined via Backscattered electron (BSE) image analysis. Results showed that the overall performance of concrete is highly dependent on the coating materials and the designed coating thickness. Increasing the coating thickness of slag and nano-SiO2 could improve the chloride penetration resistance but decrease the compressive strength. Using nano-CaCO3 to coat the aggregate leads to a significant reduction in the properties of the so-prepared concrete. Though coating inert fine particles around aggregate could disturb the initial particle packing and modify the ITZ, it is not able to improve the overall concrete properties. Coating aggregate could determine the ITZ microstructure, especially within the region that is around 30 μm away from aggregate surface.

Composition and microstructure of 50-year lightweight aggregate concrete (LWAC) from Nanjing Yangtze River bridge (NYRB)

This paper was designed to reveal the hydration products and to characterize the interfacial transition zone (ITZ) in the lightweight aggregate concrete (LWAC) after long-term serving. The results show that the compressive strength and freeze and thaw resistance is evidently dampened. Only a few C2S circling with ferrite remain unhydrated and monosulfate (AFm) tends to form loose structure to generate small pores where microcracks initiate after loading. Ettringite (AFt), calcium silicate hydrate (C-S-H), and portlandite (CH) precipitate in the inlaid zone (IZ) of LWA. The microcracks initiate from the ITZ of sand, connect with the cracks induced by pores and insert into the inner part of LWA through IZ, whereas the cracks in the LWA without IZ bypass the LWA through ITZ. The major issue for improving performance of LWAC is to modify the surface of LWA to avoid formation of porous ITZ and to improve the structure and property of IZ.

The effect of rough vs. smooth aggregate surfaces on the characteristics of the interfacial transition zone

Due to its higher porosity, the interfacial transition zone (ITZ) between cement paste and aggregates in cementitious composites is often viewed as a weak link in the microstructure. Sectional plane analysis based on back-scattered electron scanning electron microscope (SEM-BSE) images is used to quantify the ITZ porosity gradient, yet not much research has focused on the effect of aggregate surface morphology on the ITZ composition. In this paper, a model concrete specimen with a single spherical ceramic particle acting as an aggregate is used to study the properties of the ITZ and its uneven distribution around the aggregate based on quantitative analysis of SEM-BSE images. A careful treatment of the statistics of the ITZ was employed. The average porosity of the ITZ at a smooth part of the aggregate surface was smaller than that found at a rough part of the aggregate surface. A method is proposed to describe the local surface roughness (SR) at the pixel level and relate this quantity to other ITZ properties. The relationship between the surface roughness and the ITZ porosity gradient within 30 μm of the aggregate surface was analyzed with the “K-means” clustering method. The role of gravity was seen, since more porous ITZ regions tended to form underneath the aggregate than above the aggregate, with intermediate results at the aggregate sides.

Masonry’s Resistance to Driving Rain: Mortar Water Content and Impregnation

Alongside well-researched themes such as water and moisture, the service life and function of masonry veneers are often compromised by precipitation combined with poor design considerations, execution, and selection of materials. Little research has been carried out on the subject of the impact of mortar consistency on masonry’s resistance to driving rain. Water-repellent (WR) impregnation is typically considered a quick fix when problems occur. Wall-panels of 1 m2 built with different flow table values for the mortar have been tested in a driving rain chamber, where both time-lapse videos and the measuring of penetrated water are used to evaluate performance. Subsequently, the panels were impregnated with the most common types of WR products and re-tested. The analysis shows that changing the mortar mix from dry to wet can decrease the penetration of driving rain by a factor of ten. The test results presented in this article show that mortar with low water content gives a porous interfacial transition zone (ITZ), thereby increasing the rate of water penetration. The tested WRs are found to be ineffective in increasing masonry’s resistance to high pressure driving rain. The results, combined with what is already known about WR treatments on masonry, call for careful consideration before applying such treatment. This proves especially true in countries with much driving rain followed by frequent freeze-thaw cycles.

Bonding in cementitious materials with asphalt-coated particles: Part I – The interfacial transition zone

Reclaimed asphalt pavement (RAP), when used as a coarse aggregate, has been shown to reduce bulk concrete strength and modulus. Part I of this study quantifies and compares the interfacial transition zone (ITZ) for mortar with RAP aggregates relative to dolomite aggregates through image analysis of backscattered electron micrographs. The ITZ with RAP aggregates was larger and more porous with less calcium silicate hydrate (C-S-H) and calcium hydroxide (CH) at the asphalt interface compared with dolomite aggregates. The CH morphology was not significantly affected, although the presence of the asphalt layer may be affecting the CH growth. The addition of silica fume reduced the porosity and size of CH particles in the ITZ with RAP, but not sufficiently to be similar to the ITZ of the dolomite mortar. The microstructural changes caused by RAP aggregates, primarily the larger, more porous ITZ, provide strong evidence for the observed reduction in concrete strength and modulus.

Utilisation of steel furnace slag coarse aggregate in a low calcium fly ash geopolymer concrete

This paper evaluates the performance of steel furnace slag (SFS) coarse aggregate in blended slag and low calcium fly ash geopolymer concrete (GPC). The geopolymer binder is composed of 90% of low calcium fly ash and 10% of ground granulated blast furnace slag (GGBFS). Mechanical and physical properties, shrinkage, and detailed microstructure analysis were carried out. The results showed that geopolymer concrete with SFS aggregate offered higher compressive strength, surface resistivity and pulse velocity than that of GPC with traditional aggregate. The shrinkage results showed no expansion or swelling due to delayed calcium oxide (CaO) hydration after 320days. No traditional porous interfacial transition zone (ITZ) was detected using scanning electron microscopy, indicating a better bond between SFS aggregate and geopolymer matrix. Energy dispersive spectroscopy results further revealed calcium (Ca) diffusion at the vicinity of ITZ. Raman spectroscopy results showed no new crystalline phase formed due to Ca diffusion. X-ray fluorescence result showed Mg diffusion from SFS aggregate towards geopolymer matrix. The incorporation of Ca and Mg into the geopolymer structure and better bond between SFS aggregate and geopolymer matrix are the most likely reasons for the higher compressive strength observed in GPC with SFS aggregate.

Investigation of the interfacial transition zone between aggregate-cement paste by AC impedance spectroscopy

Three different types and sizes of coarse aggregate were chosen, and the alternating current (AC) impedance of cement paste samples with and without aggregate was measured at different curing ages. Based on Song’s equivalent circuit model, the electrical properties from the AC impedance results were obtained, and the resistance of connected pores RCCP was used to characterize the microstructure of the interfacial transition zone (ITZ). The results show that the RCCP of concrete sample with aggregate is lower than that of cement paste sample, which indicates that the introduction of aggregate in cement paste makes the ITZ porous. Furthermore, for the same type of aggregate, an increase in particle size leads to a more porous ITZ, which accounts for the “water effect” and a larger aggregate would accumulate a thicker water film around it. In addition, for the same size of aggregate, the physical interaction between aggregate and cement paste is dominant in early age, and the microstructure of the ITZ around limestone aggregate is denser, which mainly depends on its rough surface and high water absorption. However, the microstructures of the ITZ around granite and basalt aggregates are denser in later age, which may be due to their higher chemical activity, and the chemical interaction between them and cement paste resulting in the generation of more hydrates. AC impedance spectroscopy thus proves to be powerful for evaluation of the microstructure of the ITZ.

The Interfacial Transition Zone in Alkali-Activated Slag Mortars

The interfacial transition zone (ITZ) is known to strongly influence the mechanical and transport properties of mortars and concretes. This paper studies the ITZ between siliceous (quartz) aggregates and alkali activated slag binders in the context of mortar specimens. Backscattered electron images (BSE) generated in an environmental scanning electron microscope (ESEM) are used to identify unreacted binder components, reaction products and porosity in the zone surrounding aggregate particles, by composition and density contrast. X-ray mapping is used to exclude the regions corresponding to the aggregates from the BSE image of the ITZ, thus enabling analysis of only the binder phases, which are segmented into binary images by grey level discrimination. A distinct yet dense ITZ region is present in the alkali-activated slag mortars, containing a reduced content of unreacted slag particles compared to the bulk binder. The elemental analysis of this region shows that it contains a (C,N)-A-S-H gel which seems to have a higher content of Na (potentially deposited through desiccation of the pore solution) and a lower content of Ca than the bulk inner and outer products forming in the main binding region. These differences are potentially important in terms of long-term concrete performance, as the absence of a highly porous interfacial transition zone region is expected to provide a positive influence on the mechanical and transport properties of alkali-activated slag concretes.

Mechanism Study of Effect of Coarse Aggregate Size on Permeability of Concrete

The chloride penetration and capillary water absorption of concrete with different water-binder ratios (w/b) and different sizes of coarse aggregate were studied as well as the microstructure of the interfacial transition zone (ITZ). The mechanism of the size effect of coarse aggregate on the permeability of concrete was explored. The results show that the negative effect of porous ITZ in concrete with a lower w/b is more significant, while a larger size of the coarse aggregate can decrease the volume fraction of the ITZ and reduce the content of harmful pores in concrete. Based on this study, the impermeability of concrete with coarse aggregates of 0.19 to 1.00 in. (4.75 to 25.0 mm) is optimal. Conversely, the tortuosity effect of aggregate in concrete with a higher w/b becomes prominent so that aggregate that is too large can shorten the permeation path, increase the porosity and macropore content and the trend of the orientation growth of calcium hydroxide (CH) crystals in the ITZ. Under this condition, aggregate with a size of 0.19 to 0.75 in. (4.75 to 19.0 mm) is preferred.

Influence of the Interfacial Transition Zone and Interconnection on Chloride Migration of Portland Cement Mortar

Available experimental studies on the effect of the interfacial transition zone (ITZ) on transport properties of cement-based composite materials appear to be ambiguous. The main objective of this work was to enhance the understanding of the relationship between ITZ and transport properties of Portland cement-based materials by using both a rapid chloride migration test and theoretical calculations. A densification factor which is related to aggregate volume content was introduced to further determine the transport properties of ITZ. Results indicate that the overall porosity decreased with increasing aggregate volume content due to the dilution effect by impermeable aggregates. The porosity was above the theoretical dilution line obtained from P0×(1-Vagg) for mortars with more than 20% of aggregate, which can be attributed to the presence of high porous ITZ. On the other hand, more porous ITZ was expected to be accompanied by a denser bulk cement matrix, which leaded to a decrease in the porosity of mortar with less than 35% of aggregate. The ITZ effect would dominate the blocking aggregate, densification and tortuosity of bulk paste when aggregate volume content exceeded 0.35. The ratio between the migration coefficient of the ITZ and that of the matrix (DITZ/Dmatrix) increased with aggregate volume content and assumed ITZ thickness. In addition, the influence of ITZ increased with increasing the degree of interconnection slightly until 1.0. Beyond this value, a sudden increase in DITZ/Dmatrix ratio was observed indicating the negative percolation effect when the adjacent ITZ start to interconnect.

Effects of micro-structure characteristics of interfacial transition zone on the compressive strength of self-compacting geopolymer concrete

This paper presents an experimental study of the influence of different superplasticizer dosage on compressive strength and micro-structure characteristics of interfacial transition zone (ITZ) prepared with fly ash based self-compacting geopolymer concrete (SCGC). The correlations between compressive strength development and microstructure of interfacial transition zone were also investigated. Concrete specimens were prepared with different superplasticizer (SP) dosage namely 3%, 4%, 5%, 6% and, 7% and cured at 70°C for duration of 48h. Field emission scanning electron microscope (FESEM) observations revealed that improved performance of concrete was found when the compressive strength increased through formation of dense ITZ between the aggregate and binder matrix at higher SP dosage. There are good correlations between compressive strength and micro-structure characteristics of interfacial transition zone. The FESEM analysis revealed that relatively a loose and porous interfacial zone was found between the binder and aggregate for low SP dosage and theses loose and porous ITZ decreased the performance of concrete by lowering the compressive strength; however, a dense ITZ was found between the aggregate and binder matrix for higher SP dosage that enhanced the concrete performance by increasing the compressive strength.