Browsing by Author "Lesage, Karel"

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Citation Count: 18
Early age reaction, rheological properties and pore solution chemistry of NaOH-activated slag mixtures
(Elsevier Sci Ltd, 2022) Yardımcı, Mert Yücel; Aydin, Serdar; Yardimci, Mert Yuecel; Lesage, Karel; De Schutter, Geert; İnşaat Mühendisliği / Civil Engineering
In this study, the rheology and early reaction process of sodium hydroxide-activated ground granulated blast furnace slag (GGBFS) were investigated. Test results showed a strong relationship between the Vicat initial setting time and ultrasonic pulse wave for NaOH activated slag mixtures identifying the characteristic points or inflection points in the ultrasonic curves and reaching a specific value in the ultrasonic velocity. The early re-action process of the sodium hydroxide-activated slag pastes determined by ultrasonic pulse wave and calorimeter consisted of three stages: dissolution, acceleration/condensation and deceleration stages. The yield stress and apparent viscosity of the pastes increased by the addition of NaOH, and the pastes started to show shear -thickening behavior when NaOH concentration reached 8 M. It was also revealed that Si, Al and Ca ion concentrations in the pore solution increased with an increase in NaOH concentration, and the pore solution of the pastes was dominated by Na and OH-. No significant influence of NaOH concentrations upon 4 M on the compressive strength of the mortar samples was observed.
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: 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: 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.