Browsing by Author "Yardimci, Mert Yucel"

Now showing 1 - 12 of 12

Citation Count: 7
Accelerating the reaction process of sodium carbonate-activated slag mixtures with the incorporation of a small addition of sodium hydroxide/sodium silicate
(Elsevier Sci Ltd, 2023) Yardımcı, Mert Yücel; Ren, Qiang; Aydin, Serdar; Yardimci, Mert Yucel; De Schutter, Geert; İnşaat Mühendisliği / Civil Engineering
This paper investigates the possibility of accelerating the slowly progressive early reaction process of sodium carbonate-activated slag mixtures by adding a small amount of sodium hydroxide or sodium silicate to the so-dium carbonate activator. The reaction process of sodium carbonate-activated slag mixtures was evaluated by isothermal calorimetry and ultrasonic wave technology. Test results showed that adding a small amount of so-dium hydroxide or sodium silicate to the sodium carbonate activator significantly accelerated the reaction process, resulting in shortened setting times and enhanced mechanical properties. Only 1% Na2O contribution from sodium hydroxide addition can lead to a 2-d compressive strength of 20 MPa for sodium carbonate -activated slag mixtures. In addition, the 28 days compressive strength was significantly higher than the sole sodium carbonate activator. The addition of sodium hydroxide or sodium silicate also considerably influenced the rheological behavior of the sodium carbonate-activated slag mixture, enhancing the structural build-up rate and transforming the shear-thinning behavior to Bingham or shear-thickening behavior. A more well-packed microstructure was observed in the mixtures with the addition of sodium hydroxide; however, more micro -cracks were observed in the mixtures with a small incorporation of sodium silicate.
Citation Count: 18
Early structural build-up, setting behavior, reaction kinetics and microstructure of sodium silicate-activated slag mixtures with different retarder chemicals
(Pergamon-elsevier Science Ltd, 2022) Yardımcı, Mert Yücel; Aydin, Serdar; Yardimci, Mert Yucel; De Schutter, Geert; İnşaat Mühendisliği / Civil Engineering
The utilization potential of citric acid, sodium tetraborate decahydrate and sodium triphosphate pentabasic to control fresh state properties of sodium silicate-activated alkali-activated slag cements (AASC) has been studied. The early structural build-up, reaction kinetics, pore solution chemistry, setting behavior, and hardened properties such as microstructure, pore structure, and mechanical properties of AASC with these chemicals were investigated. Test results showed that early structural build-up, reaction kinetics and setting behavior of AASC can be well controlled by using these chemicals. The citric acid was found the most effective admixture to prolong the setting times of AASC; however, it caused a dramatic decrease in mechanical properties. On the other hand sodium triphosphate pentabasic could provide a slight delay in setting times, while sodium tetraborate decahydrate prolonged the setting times significantly without any detrimental effect on the mechanical properties.
Citation Count: 22
Effect of Ca(OH)₂ Addition on the Engineering Properties of Sodium Sulfate Activated Slag
(Mdpi, 2021) Yardımcı, Mert Yücel; Aydin, Serdar; Yardimci, Mert Yucel; Lesage, Karel; De Schutter, Geert; İnşaat Mühendisliği / Civil Engineering
Alkali-activated slag is considered as a sustainable construction material due to its environmentally friendly nature. To further promote the sustainable nature of alkali-activated slag, a sodium sulfate activator is suggested to be used since it can be obtained naturally and generates lower greenhouse gas emissions. However, the mixtures activated by sodium sulfate exhibit low early strength and very long setting times. This study investigates the effects of calcium hydroxide (Ca(OH)(2)) addition on some engineering properties such as rheology, setting time, mechanical properties, porosity, and microstructure of sodium sulfate activated ground granulated blast furnace slag (GGBFS). Furthermore, the changes of chemical groups in reaction products and phase identification have been evaluated by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction. Test results showed that Ca(OH)(2) addition can substantially increase the reaction rate and the compressive strength at early ages. In addition, the very long setting times of the sodium sulfate-activated mixtures were shortened by the addition of Ca(OH)(2). SEM analysis confirmed that the incorporation of excessive amounts of Ca(OH)(2) could lead to a less well-packed microstructure although the reaction degree of GGBFS remained the same at later ages as compared to the sodium sulfate mixture. It was also revealed that in case of the Ca(OH)(2) addition into sodium sulfate activator, the main reaction products are chain-structured C-A-S-H gels and ettringite.
Citation Count: 0
Effect of temperature on the fresh and hardened state properties of alkali-activated slag/fly ash mixtures
(Springer, 2023) Yardımcı, Mert Yücel; Aydin, Serdar; Yardimci, Mert Yucel; Sun, Yubo; De Schutter, Geert; İnşaat Mühendisliği / Civil Engineering
The effect of ambient temperature on the engineering properties of alkali-activated materials (AAM) needs to be further investigated due to the high variety of activating solutions in the AAM technology. This paper presents the rheological behavior, structural build-up, reaction kinetics and mechanical properties of GGBFS-FA mixtures activated by sodium hydroxide, sodium silicate, sodium carbonate and sodium sulfateinvestigated under different ambient temperature conditions. It was found that the effect of ambient temperature on the rheology and reaction kinetics was highly dependent on the activator type. Under room temperature conditions, the highest and lowest yield stress values were obtained in sodium hydroxide and sodium silicate mixtures, respectively. The increase in temperature did not affect the yield stresses and viscosities of sodium carbonate and sodium sulfate mixtures; however, the yield stresses of sodium hydroxide and sodium silicate mixtures significantly increased. This effect was more pronounced in mixtures with high Ms values. Higher storage modulus values were obtained with an increase in temperature, indicating initial structuration with temperature. The increasing temperature enhanced the compressive strength of alkali-activated GGBFS-FA mixtures. This improvement was more pronounced at early ages for the sodium silicate mixture, and at later ages for the sodium carbonate and sodium sulfate mixtures, while it was very limited in the sodium hydroxide mixture. The SEM images and calorimetric measurements showed the formation of a denser microstructure and enhancement in the exothermic peak with a shorter induction period with an increase in temperature.
Citation Count: 2
Effect of the steel fibre hook geometry on the flexural properties of high strength steel fibre reinforced concretes under static and impact loading
(Gazi Univ, Fac Engineering Architecture, 2019) Yardımcı, Mert Yücel; Aydin, Serdar; Yardimci, Mert Yucel; İnşaat Mühendisliği / Civil Engineering
In recent years, steel fibre producers have produced new generation steel fibres with high strength and multiple hooked-end geometry (4D and 5D). The studies on the mechanical properties of steel fibre reinforced concretes containing 4D and 5D fibres under the static loading conditions are very limited and there is no published study on the performance of these new generation fibres under flexural impact loading yet. In this study, the effects of the dosage and the aspect (length/diameter) ratio of 4D and 5D fibres on the flexural strength and fracture energy of high strength concrete under static and impact flexural loading conditions have been investigated comparatively with the conventional single-hook-end 3D fibres. The experimental results showed that the flexural strength and the fracture energy of fibre reinforced high strength concrete significantly improved with the increase in the fibre volume, and by using multiple hooks-end steel fibres with sufficiently high tensile strength. For the fibres with aspect ratio of 65, significantly higher flexural strength and fracture energy values have been obtained for 5D hook geometry as compared to conventional 3D fibres, while 4D fibres did not improve these engineering properties probably due to their relatively insufficient fibre strength for this aspect ratio. It has been revealed that the effect of the 4D steel fibre on the mechanical properties of high strength concrete was more pronounced in higher aspect ratio fibres. The increase of fibre aspect ratio for 3D steel fibres negatively affected the fracture energy of concrete. As compared to static loading, 1.4 to 2.6 times higher flexural strength and 1.2 to 3.0 times higher fracture energy values were obtained under impact loading. The best performance under flexural impact loading has been obtained from steel fibre reinforced high strength concretes with 5D fibres having aspect ratio of 65 and 4D fibres having aspect ratio of 80.
Citation Count: 79
Effects of activator properties and GGBFS/FA ratio on the structural build-up and rheology of AAC
(Pergamon-elsevier Science Ltd, 2020) Yardımcı, Mert Yücel; Aydin, Serdar; Yardimci, Mert Yucel; Lesage, Karel; De Schutter, Geert; İnşaat Mühendisliği / Civil Engineering
The effects of ground granulated blast furnace slag/fly ash (GGBFS/FA) ratio, mass ratio of SiO2 to Na2O (M s ) of activator solution and sodium silicate dosage on structural build-up, flow properties and setting characteristics of alkali-activated cement (AAC) mixtures were investigated. The solid-like behavior becomes more dominant with increasing GGBFS/FA ratio. M, value had significant effect on the structural build-up. Significantly higher initial storage modulus with a low increasing rate was observed for the M, values lower than 0.8. However, for higher M, values, a sudden increase in storage modulus was observed after negligible initial structuration. An increase in sodium silicate dosage caused a considerable delay in the abrupt increase in the structural formation. It was observed that flow curves of AAC fit the Herschel-Bulkley model with shear-thickening behavior. ICP-OES tests revealed the lower release of aluminum and calcium into the pore solution of GGBFS/FA mixtures with low M, values.
Citation Count: 25
Enhancing thixotropy and structural build-up of alkali-activated slag/fly ash pastes with nano clay
(Springer, 2021) Yardımcı, Mert Yücel; Ren, Qiang; Aydin, Serdar; Yardimci, Mert Yucel; Lesage, Karel; De Schutter, Geert; İnşaat Mühendisliği / Civil Engineering
This study investigates the influence of nano clay on the rheological properties of alkali-activated cement pastes having different GGBFS/FA ratios. The thixotropic index, structural build-up, dynamic yield stress and heat evolution of fresh AAC pastes with addition of nano clay are studied. Test results showed that nano clay had a strong influence on the thixotropy/structural build-up and dynamic yield stress of AAC pastes depending on the GGBFS/FA ratio of the mixture. It was found that the pastes with lower GGBFS/FA ratio exhibited higher thixotropic index, but lower dynamic yield stress in the presence of nano clay. This study reveals the importance of GGBFS/FA ratio in the presence of nano clay for obtaining AAC mixtures with low dynamic yield stress for a better flowability and high thixotropy/structural build-up for stability.
Citation Count: 8
Experimental investigation for modeling the hardening of thermosetting polymers during curing
(Elsevier Sci Ltd, 2021) Yardımcı, Mert Yücel; Yardimci, Mert Yucel; Zecchini, Michele; Daisse, Gilda; Marchesini, Flavio H.; De Schutter, Geert; Wan-Wendner, Roman; İnşaat Mühendisliği / Civil Engineering
During curing of thermosetting polymers, crosslinking results in hardening or stiffening of the material. In electronics, for example in encapsulating integrated circuits (die bonding), thermosets are fully cured in a controlled environment (under UV-light or within a thermal oven) such that the highest stiffness possible has been achieved. In building materials, specifically in thermosets used in fastening systems (adhesive anchoring), hardening occurs at environmental temperature. Daily temperature variations alter the curing process and possibly lead to a lower stiffness. We demonstrate a modeling approach for the mechanical response dependency on the degree of cure by means of rheometer measurements under a specific temperature profile. Precisely, we perform oscillatory rheometric tests and convert the storage and loss moduli to material parameters depending on the degree of cure. Moreover, the temperature dependency as well as chemical shrinkage have been determined by the same experimental protocol. The presented approach has been applied to a commercially available (epoxy) thermoset used as an adhesive. We have observed a hardening after a gelation point of 0.7 and an adequate fit for mechanical response by polynomial functions of degree four.
Citation Count: 2
Fresh and hardened state properties, reaction kinetics and microstructure of sodium sulfate/sodium hydroxide - Activated slag mixtures
(Elsevier Sci Ltd, 2023) Dai, Xiaodi; Yardımcı, Mert Yücel; Yardimci, Mert Yucel; De Schutter, Geert; İnşaat Mühendisliği / Civil Engineering
This study investigated the impact of incorporating sodium hydroxide (NaOH) on the properties of ground granulated blast furnace slag (GGBFS) activated by sodium sulfate (Na2SO4). Specifically, the rheological properties, setting behavior, reaction kinetics, mechanical performance, and microstructure of the mixtures were examined. Results indicated that using only Na2SO4 as the activator resulted in mixtures with long setting times and no compressive strength at early ages. However, the addition of NaOH to the Na2SO4 activator significantly improved the reaction kinetics of the GGBFS. By optimizing ratio between NaOH and Na2SO4, mixtures with improved workability, faster structuration, reasonable setting times, and desirable mechanical properties were achieved. The superior mechanical properties were attributed to a well-packed microstructure consisting of C -(A)-S-H, ettringite, and hydrotalcite in the binary NaOH-Na2SO4 activator systems.
Citation Count: 69
Influence of water to binder ratio on the rheology and structural Build-up of Alkali-Activated Slag/Fly ash mixtures
(Elsevier Sci Ltd, 2020) Yardımcı, Mert Yücel; Aydin, Serdar; Yardimci, Mert Yucel; Lesage, Karel; de Schutter, Geert; İnşaat Mühendisliği / Civil Engineering
In this study, the effects of water to binder (w/b) ratio on the rheological and fresh state properties of alkali-activated cement (AAC) pastes have been investigated. A mixture of 50% type F fly ash (FA) and 50% ground granulated blast furnace slag (GGBFS) was activated by a mixture of sodium hydroxide and sodium silicate, in three different w/b ratios of 0.32, 0.37 and 0.42. Setting time, ultrasonic pulse velocity (UPV), heat evolution by isothermal calorimetry, flow spread by mini-slump test, rheological properties by means of flow curve and viscoelastic properties such as storage modulus (G') and loss factor (G '') were determined on the paste mixtures. The results showed that the higher w/b ratio led to structural build-up with higher rate compared to lower w/b ratios. GGBFS provided the main contribution to the increase of storage modulus in the early stage of hybrid mixture of GGBFS and FA. The dynamic yield stress, plastic viscosity and thixotropic index values decrease with an increase in w/b ratio of AAC mixture. The setting times of AAC mixtures were found to be less dependent on the w/b ratio as compared to ordinary Portland cement mixtures. (C) 2020 Elsevier Ltd. All rights reserved.
Citation Count: 38
Rheology and microstructure of alkali-activated slag cements produced with silica fume activator
(Elsevier Sci Ltd, 2022) Yardımcı, Mert Yücel; Aydin, Serdar; Yardimci, Mert Yucel; Lesage, Karel; De Schutter, Geert; İnşaat Mühendisliği / Civil Engineering
The effects of silica fume and sodium silicate-based activators (SFA and SSA, respectively) with different Ms (SiO2/Na2O) values on the setting behavior, rheological, mechanical, and microstructural properties of alkaliactivated slag cement (AASC) were investigated. Setting time test results showed that the setting time of AASCs activated by SFA prolonged significantly with an increase of Ms value opposite to SSA activation case. From the rheological point of view, SFA-activated mixtures exhibited a slower structural build-up in the early stage and better workability retention than SSA-activated mixtures. In addition, SFA mixtures showed lower drying shrinkage and slightly higher mechanical properties as compared to SSA mixtures. Microstructure analysis revealed that the mixture produced by SFA with Ms value of 1.2 had less micro-cracks and a well-packed microstructure as compared to the mixtures produced by SSA. The overall evaluation of the test results revealed that SFA could be more economical and sustainable alternative to SSA with its lower cost, much lower CO2 emissions, and more favorable engineering properties.
Citation Count: 47
Rheology, early-age hydration and microstructure of alkali-activated GGBFS-Fly ash-limestone mixtures
(Elsevier Sci Ltd, 2021) Dai, Xiaodi; Yardımcı, Mert Yücel; Yardimci, Mert Yucel; Qiang, R. E. N.; Lesage, Karel; De Schutter, Geert; İnşaat Mühendisliği / Civil Engineering
In this study, the effects of limestone powder on the rheological behavior, pore solution chemistry, mechanical properties and microstructure of alkali-activated cements have been investigated. The results exhibit that, with the increasing content of limestone powder in the ternary alkali-activated system, the structural build-up of the mixture increases earlier. It was observed that flow curves of pastes fit the Bingham model well. With the increasing content of fly ash in the ternary mixtures, the plastic viscosity decreased as expected by the particle packing effect and the increased water film thickness as well as the spherical shape of fly ash particles. As a result of the higher specific surface and improved nucleation provided by the limestone powder, the reaction process was enhanced and accelerated for the mixtures with higher limestone powder contents. The calcium and alumina concentrations in the pore solution rapidly evolved at first for a certain time, but decreased afterwards. The significant influence of the Ms value of the activator was observed on the evolution of the elemental concentrations. Microstructure analysis revealed that the early age reaction product is C-A-S-H for the slag mixtures incorporating limestone or fly ash. The compressive strength of the ternary mixtures decreased with the incorporation of limestone powder due to the inert character of the limestone powder.