Blast furnace | EPFL Graph Search

A blast furnace is a type of metallurgical furnace used for smelting to produce industrial metals, generally pig iron, but also others such as lead or copper. Blast refers to the combustion air being supplied above atmospheric pressure. In a blast furnace, fuel (coke), ores, and flux (limestone) are continuously supplied through the top of the furnace, while a hot blast of air (sometimes with oxygen enrichment) is blown into the lower section of the furnace through a series of pipes called tuyeres, so that the chemical reactions take place throughout the furnace as the material falls downward. The end products are usually molten metal and slag phases tapped from the bottom, and waste gases (flue gas) exiting from the top of the furnace. The downward flow of the ore along with the flux in contact with an upflow of hot, carbon monoxide-rich combustion gases is a countercurrent exchange and chemical reaction process. In contrast, air furnaces (such as reverberatory furnaces) are natur

Nanoscale modelling of ionic transport in the porous C-S-H network

Khalil Ferjaoui

To reduce the CO2 footprint of construction materials, concrete producers blend their cement with Supplementary Cementitious Materials (SCMs). SCMs such as fly ash or blast furnace slag are mostly the byproducts of other industries. And while SCMs are chosen to match the properties of the common Ordinary Portland cement (OPC), their addition to the cement recipe may alter the chemistry of the system. These changes may lead to different mechanical and transport properties of the cement-based structure which may, in turn, affect its long-term resistance to harmful external agents. Therefore, understanding the relationship between cementâs microstructure and the degradation mechanisms is key for optimizing the design of new cementitious materialsIn this context, chloride attack is the most common reason for steel rebars to corrode especially when exposed to external chloride (seawater, deicer saltsâŠ). At low w/c ratios (typically 13) pore solution, the C-S-H surface develops a negative surface charge density. Electrostatic interactions between the surface and the ions in the pore solution result in a redistribution of the ionic species in two layers of charge at the interface of the solid i.e. the Electrical double layer (EDL). In nanoscopic pores, the EDL is dominated by atomic phenomena which are thought to interfere with the mobility of ions and chloride in particular. In order to understand and quantify the surface effects and their influence on ionic transport, we firstly propose an atomistic model of the EDL formation based on the use of the Metropolis Monte Carlo algorithm. This model is used to compute the ionic distributions and electrochemical potentials of electrolytes at equilibrium in nanoscopic pores. These quantities constitute the main driving forces of ionic transport at the pore scale. The microstructure parameters including the surface charge density of C-S-H, the ionic strength (and pH) of the pore solution, and the pore size are equally investigated and their effect on chlorideâs behavior quantified. Among the other parameters, the model also provides quantitative information on the effect of calcium ions which are thought to play a major role in the binding of chloride on C-S-H. The next step consists in using the calculated atomic-scale properties of the EDL in order to resolve the transport problem at the pore scale and compute microscopic diffusivities of chloride. This is achieved by using the molecular computations from the Monte Carlo (MC) engine in order to implement a modified version of the classical Poisson-Boltzmann system. The method is compared to the classical Finite element analysis of the Poisson-Nernst-Planck (PNP) equations and the data are discussed in the light of established experimental results in the literature.

EPFL2022

Impact of the supplementary cementitious materials on the kinetics and microstructural development of cement hydration

Elise Marie Jeanne Berodier

Supplementary cementitious materials (SCMs) are currently used to replace part of the clinker to reduce the environmental impact of cement and to favor the use of local materials. Two representative SCM have been used in this work: slag and fly ash. Each one corresponds to one of the SCM categories: blast furnace slag is a hydraulic material while siliceous fly ash is a pozzolanic material. A hydraulic material sets and hardens under water while pozzolans need calcium hydroxide (also referred as Portlandite) and water to produce hydrates. As a consequence when slag or fly ash is blended with clinker, its reaction occurs in addition to the cement reaction. This work aims to study the impact of the SCMs on cement hydration to get a generic knowledge on the changes in kinetics and microstructural development when SCMs replace a part of the Portland cement. These changes are important as they can modify the microstructure and change the long term properties. It is interesting to quantify and understand these effects to predict their performance and improve these sustainable cements. A large part of the reaction of clinker phases takes place within two days after the mixing with water. During the same period, slag and fly ash particles are not yet reacting, however they can enhance the hydration of the clinker phases. Using inert quartz powders with different fineness, our results show a clear relation between the inter-particle distance and the kinetics of the acceleration period. This can be explained by the shearing between particles which increases as the inter-particle distance decreases. The micrographs showed that reducing the distance or increasing the mixing speed produces more nucleation sites of C-S-H on the grains surfaces. The study of limestone indicates that limestone powder has an additional effect beyond the effect of increasing the shear produced by other SCMs. The nucleation density is much higher on limestone surface and the morphology of C-S-H is changed to individual needles. The arrangement of the atoms at the surface of limestone seems to explain the increased nucleation. In blended systems, the peak during the deceleration period has sometimes been considered as marking the beginning of the reaction of SCM. In this work, we showed that the peak only corresponds to the second dissolution of C3A although the SCM substitution level changes the kinetics of this reaction. It has been shown that this effect is caused by the faster depletion of sulfate ions, due to the faster reaction of clinker phases and faster formation of C-S-H. A part of the sulfate is adsorbed on the C-S-H structure. This leads to the faster depletion of sulfate in the solution and change the morphology of C-S-H. Our results showed that the adsorption of sulfate promotes the growth of C-S-H as we observed needles diverging from nucleation sites. When the sulfate desorbs the C-S-H needles grow more parallel to each other. This effect was confirmed with the observation of alite where sulfate is not present. The results from 1H NMR showed that the structure itself of C-S-H is affected: few gel pores are formed in presence of sulfate. Over the course of hydration, C-S-H fills the space available between the grains. Once the capillary pores reach a size of 6-8nm, the formation of C-S-H is slowed down and the capillary pores do not reduce further in size. The stabilization of the pore size was explained by the increase of the supersaturation index [...]

EPFL2015

Reactivity tests for supplementary cementitious materials: RILEM TC 267-TRM phase 1

Mathieu Antoni, Mohsen Ben Haha, Shashank Bishnoi, Yuvaraj Dhandapani, Xuerun Li, Anuj Parashar, Karen Scrivener, Ruben Anton Snellings

A primary aim of RILEM TC 267-TRM: Tests for Reactivity of Supplementary Cementitious Materials (SCMs) is to compare and evaluate the performance of conventional and novel SCM reactivity test methods across a wide range of SCMs. To this purpose, a round robin campaign was organized to investigate 10 different tests for reactivity and 11 SCMs covering the main classes of materials in use, such as granulated blast furnace slag, fly ash, natural pozzolan and calcined clays. The methods were evaluated based on the correlation to the 28days relative compressive strength of standard mortar bars containing 30% of SCM as cement replacement and the interlaboratory reproducibility of the test results. It was found that only a few test methods showed acceptable correlation to the 28days relative strength over the whole range of SCMs. The methods that showed the best reproducibility and gave good correlations used the R-3 model system of the SCM and Ca(OH)(2), supplemented with alkali sulfate/carbonate. The use of this simplified model system isolates the reaction of the SCM and the reactivity can be easily quantified from the heat release or bound water content. Later age (90days) strength results also correlated well with the results of the IS 1727 (Indian standard) reactivity test, an accelerated strength test using an SCM/Ca(OH)(2)-based model system. The current standardized tests did not show acceptable correlations across all SCMs, although they performed better when latently hydraulic materials (blast furnace slag) were excluded. However, the Frattini test, Chapelle and modified Chapelle test showed poor interlaboratory reproducibility, demonstrating experimental difficulties. The TC 267-TRM will pursue the development of test protocols based on the R-3 model systems. Acceleration and improvement of the reproducibility of the IS 1727 test will be attempted as well.

SPRINGER2018