Denser Interfacial Transition Zone Research Articles

Novel thermal insulating and lightweight composites from metakaolin geopolymer and polystyrene particles

In this work, new foamed thermal insulation geopolymer composite based on polystyrene particles (PP) and metakaolin was developed. Compressive strength, flexural strength, high temperature resistance and microstructure were evaluated. The experimental results show that compressivestrengthand flexural strength of the thermal insulation geopolymer composite decrease with increasing polystyrene particle content. However, it still exhibits considerable and sufficient strength. The dry density and thermal conductivityalso decrease as polystyrene particle content increases due to the contribution of polystyrene particles with low density. The floatation of the thermal insulation geopolymer composite on water surface indicates the relatively low density and a good quadratic function relationship can be found between thermal conductivity and dry density. Furthermore, the dense interfacial transition zone indicates the high compressive strength and flexural strength of thermal insulation geopolymer composites. The cumulative intrusion volume corresponding to the porosity decreases and the critical pore diametersshift to lower values with addition of polystyrene particles. Geopolymer composites gain strength after exposure around 400°C, and it suffers dramatic strength loss after 800°C temperature exposure especially for the 100% polystyrene particles addition specimen.

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.

Chemical reactivity of lightweight aggregate in cement paste

Quantitative investigation on the chemical reactivity of lightweight aggregate (LWA) in cement paste is important to enrich our understanding of the role played by LWA in influencing the microstructure of the interfacial transition zone (ITZ). In this paper, three types of LWA (YT, NT and FT) were chosen, which are based on shale rock, expanded clay and sintered fly ash respectively, and the ion concentration, calcium hydroxide (CH) content and reaction degree of LWA in cement pastes at different curing ages were studied, the fly ash (FA) was also included for comparison. The results show that, all the types of LWA studied showed chemical activity, which absorb very significant amounts of Ca2+ and OH−, and released significant amounts of Si4+ into cement solution. The CH reduction content of the paste also indicates that the LWA have pozzolanic reactivity, especially in the later period the pozzolanic reaction become the main part of the hydration, but the reaction degree of them is slightly lower than that of FA. In addition, with the increase of the water–binder ratio, the reaction degree of FA and LWA has some increase as well. Although LWA in the form of a powder is more reactive than in the form of a coarse aggregate, the XRD results also show that the CH peak height of the ITZ samples around different types of LWA have a small decrease, in contrast to the bulk cement paste. The results obtained confirm that the dense ITZ around LWA was not only characterized by the water absorbing and mechanical interlocking, the chemical reaction at the interface also has some contribution.

Improving performance of recycled aggregate concrete with superfine pozzolanic powders

Phosphorous slag (PHS), ground granulated blast-furnace slag (GGBS) and fly ash (FA) were used as replacements of Portland cement to modify the microstructure of recycled aggregate concrete (RAC). A new manufacturing method named “W3T4” was proposed to improve the performances of interfacial transition zone (ITZ) between recycled aggregate and mortar. The mechanical properties and the durability of RAC were tested, which show that this new manufacturing method improves the properties of RAC, and the GGBS with finest size makes a great contribution to the performance of RAC due to its better filling effect and much earlier pozzolanic reaction. Combined with GGBS, the effects of PHS on the retardation of setting time can be alleviated and the synergistic effect helps to make a more compact RAC. For the RAC with 25% of the recycled aggregate (RA) replacement and 10% PHS + 10% GGBS additives, the compressive strength increases by 25.4%, but the permeability decreases by 64.3% with respect to the reference concrete made with nature aggregates. The micro-mechanisms of these improvements were investigated by the scanning electron microscope (SEM). The SEM images show that the new manufacturing method, adding superfine pozzolanic powders and super-plasticizer benefits, makes a much denser ITZ in RAC.

Efficiency of mineral admixtures in concrete: Microstructure, compressive strength and stability of hydrate phases

The use of mineral additives in concrete such as fly ash, silica fume, natural pozzolan, metakaolin and calcined clay has become widespread due to their pozzolanic reaction and environmental friendliness. The microstructure characteristics of concrete including pore structure and interfacial transition zone (ITZ) with addition of metakaolin, silica fume and slag were investigated in this study. The experimental results show that the addition of mineral admixtures results in the denser ITZ, optimized pore structure and reasonable pore size distribution especially at later curing stages. Metakaolin presents the most distinct improvement effects on microstructure of concrete. The development of the compressive strength is quantificationally related to the total porosity and average microhardness of the ITZ. Importantly, the influence of metakaolin, silica fume and slag on concrete was analyzed from thermodynamic stability of hydrate phase aspects.

Physico-mechanical properties of high performance concrete using different aggregates in presence of silica fume

Heavy weight high performance concrete (HPC) can be used when particular properties, such as high strength and good radiation shielding are required. Such concrete, using ilmenite and hematite coarse aggregates can significantly have higher specific gravities than those of concrete made with dolomite and air-cooled slag aggregates. Four different concrete mixes with the same cement content and different w/c ratios were designed using normal dolomite aggregate, air-cooled slag by-product and two different types of iron ore aggregates. High performance concrete (grade-M60) can be achieved using superplasticizer to reduce the water/cement ratio; the effect of SF on the performance of concrete was studied by addition of 10% silica fume to the total cement content. The physico-mechanical properties of coarse aggregates and hardened concrete were studied. The results show that, Ilmenite coarse aggregate gives higher physical and mechanical properties than the other aggregates. Also, addition of 10% silica fume developed a stronger and a denser interfacial transition zone (ITZ) between concrete particles and the cement matrix. Crushed air-cooled slag can be used to produce a high-strength concrete with better mechanical properties than corresponding concrete made with crushed hematite and ilmenite. Heavy density concrete made with fine aggregates of ilmenite and air-cooled slag are expected to be suitable as shielding materials to attenuate gamma rays.

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.

Influence of the micro- and nanoscale local mechanical properties of the interfacial transition zone on impact behavior of concrete made with different aggregates

The influence of the microscale local mechanical properties of the interfacial transition zone (ITZ) on macro-level mechanical response and impact behavior is studied for concretes made with copper slag and gravel aggregates. 3D nanotech vertical scanning interferometry, scanning electron microscopy coupled with energy dispersive X-ray micro-analysis, digital image analysis, and 3D X-ray computed tomography were used to characterize the microstructures and the ITZs. It was deduced that a stronger and denser ITZ in the copper slag specimen would reduce its vulnerability to stiffness loss and contribute to its elastic and more ductile response under impact loading. The analysis also indicated that a significant degeneration in the pore structure of the gravel specimen associated with a relatively weaker and non-homogeneous ITZ occurred under impact. Finally, it was also concluded that increased roughness of ITZ may contribute to the load-carrying capacity of concrete under impact by improving contact point interactions and energy dissipation.

Influence of the Interfacial Transition Zone Properties on Chloride Corrosion in Reinforced Concrete-Characterization of ITZ

Influence of the Interfacial Transition Zone Properties on Chloride Corrosion in Reinforced Concrete-Characterization of ITZ

Influence of Nano-SiO2 Addition on Microstructure and Mechanical Properties of Cement Mortars for Ferrocement

Because of their unique physical and chemical properties, nanoparticles have been gaining increasing attention and have been used in many fields to fabricate new materials with novel functions. If nanoparticles are integrated with cement-based building materials, the new materials might possess some outstanding properties. Ferrocement is a type of thin-wall reinforced concrete commonly constructed of hydraulic cement mortar reinforced with closely spaced layers of continuous and relatively small-sized wire mesh. The low level of technical skill required to make ferrocement and the ready availability of its materials make ferrocement suitable for a wide variety of applications. This study investigates the mechanical properties (compressive and flexural strengths) and microstructural properties (as determined by scanning electron microscopy) of the interfacial transition zone between cement pastes and aggregates of mortars applicable for the casting of thin ferrocement elements combining with nanosilica. Study parameters include the ratio of nanosilica to cement in ordinary portland cement mortar mixtures (including 1%, 2%, and 3%) and the water-to-binder ratio (including 0.35 and 0.4). The results show that cement mortars containing nanoparticles have a reasonably higher strength and denser interfacial transition zone than do ordinary portland cement ferrocement mortars, while the flowability of mortars with and without nanoparticles is considered equal.

Influence of internal curing using lightweight aggregates on interfacial transition zone percolation and chloride ingress in mortars

The microstructure of the interfacial transition zone (ITZ) between cement paste and aggregate depends strongly on the nature of the aggregate, specifically its porosity and water absorption. Lightweight aggregates (LWA) with a porous surface layer have been noted to produce a dense ITZ microstructure that is equivalent to that of the bulk cement paste, as opposed to the more porous ITZ regions that typically surround normal weight aggregates. This ITZ microstructure can have a large influence on diffusive transport into a concrete, especially if the individual ITZ regions are percolated (connected) across the three-dimensional microstructure. In this paper, the substitution of LWA sand for a portion of the normal weight sand to provide internal curing (IC) for a mortar is examined with respect to its influence on ITZ percolation and chloride ingress. Experimental measurements of chloride ion penetration depths are combined with computer modeling of the ITZ percolation and random walk diffusion simulations to determine the magnitude of the reduced diffusivity provided in a mortar with IC vs. one with only normal weight sand. In this study, for a mixture of sands that is 31% LWA and 69% normal weight sand by volume, the chloride ion diffusivity is estimated to be reduced by 25% or more, based on the measured penetration depths.

Special issue on scanning electron microscopy of cements and concretes

Quantitative investigation on the chemical reactivity of lightweight aggregate (LWA) in cement paste is important to enrich our understanding of the role played by LWA in influencing the microstructure of the interfacial transition zone (ITZ). In this paper, three types of LWA (YT, NT and FT) were chosen, which are based on shale rock, expanded clay and sintered fly ash respectively, and the ion concentration, calcium hydroxide (CH) content and reaction degree of LWA in cement pastes at different curing ages were studied, the fly ash (FA) was also included for comparison. The results show that, all the types of LWA studied showed chemical activity, which absorb very significant amounts of Ca2+ and OH−, and released significant amounts of Si4+ into cement solution. The CH reduction content of the paste also indicates that the LWA have pozzolanic reactivity, especially in the later period the pozzolanic reaction become the main part of the hydration, but the reaction degree of them is slightly lower than that of FA. In addition, with the increase of the water–binder ratio, the reaction degree of FA and LWA has some increase as well. Although LWA in the form of a powder is more reactive than in the form of a coarse aggregate, the XRD results also show that the CH peak height of the ITZ samples around different types of LWA have a small decrease, in contrast to the bulk cement paste. The results obtained confirm that the dense ITZ around LWA was not only characterized by the water absorbing and mechanical interlocking, the chemical reaction at the interface also has some contribution.