178 research outputs found
The interaction of pH, pore solution composition and solid phase composition of carbonated blast furnace slag cement paste activated with aqueous sodium monofluorophosphate
No full textBlast Furnace Slag (BFS) is a waste product of industrial steel production and a common additive in the cement industry in Northern European countries. However, cementitious materials made from slag-rich cement, particularly CEM III /B, are very susceptible to carbonation. Recent investigations have shown that the surface application of aqueous sodium monofluorophosphate (Na-MFP) as pre- and post-carbonation treatment can improve the surface durability of cementitious materials with a high BFS content. Significant improvements have been observed in the micro-mechanical characteristics of concrete surface and frost salt scaling resistance. On the basis of previous studies we are investigating self-healing of blast furnace slag cement (BFSC) products treated with the inorganic self-healing agent Na-MFP from a mineralogical point of view. In this study we combine results of pore solution pH analyses and main element composition under the influence of Na-MFP with the presence of crystalline phases found in the solid aliquot of the samples. Pore solutions were investigated by inductively coupled optical emission spectrometry (ICP-OES). Solid-material investigation was performed by X-ray powder diffractometry, including Rietveld quantitative phase analyses. Our results show that the element concentration and the pH of the paste pore solutions have direct influence on the formation of crystalline and amorphous phases forming in the solid sample aliquot during carbonation and self-healing by Na-MFP. In this work we focus especially on the influence of sulfur in solution and the formation of ettringite. In addition we discuss, why the formation of the crystalline phosphate apatite does not occur in cementitious products after Na-MFP treatment.Structural EngineeringCivil Engineering and Geoscience
The characterisation, improvement and modelling aspects of Frost Salt Scaling of Cement-Based Materials with a High Slag Content
No full textBlast furnace slag cement concrete is used extensively in a number of countries. In comparison with OPC, it is particularly well known for its excellent performance in marine environments. One dis-advantage of slag cement is its vulnerability to scaling under the combined load of freezing-thawing and de-icing salts. The current investigation was triggered by positive observations regard-ing certain grinding agents used in slag cement production to improve frost salt scaling resistance. The investigation was aimed at explaining the cause of this improvement, at finding alternative methods to improve scaling resistance and at developing a model that would be suitable for the simulation of frost salt scaling behaviour. The investigation conclusions are essentially confined to high slag cement, particularly type CEM III 42,5/B which has a 67% granulated slag content. The w/c ratio of the paste, mortar and concrete specimens is generally maintained at 0,45. Carbonation, known as the critical parameter in frost salt scaling, constituted the key area of inter-est. From previous investigations it is known that carbonation increases porosity and coarsens the pore system in slag cement paste while it actually does the reverse in OPC paste. In the light of lit-erature a new hypothesis has been suggested that the transition zones, which are the weakest points in normal-performance cement-based materials, critically determine frost salt scaling resis-tance. These zones are even more indicative in the case of slag cement pastes because of the sig-nificant amount of transition zones that can be weakened by carbonation unlike with low-slag ce-ment or OPC pastes. In the present investigation it was observed that carbonation causes significant slag cement paste shrinkage. It was especially the transition zones between non-reacted slag particles and hydration products that were found to be affected. Consequently this process leads to the paste having a coarser pore structure thus making it prone to greater water uptake when compared to non-carbonated slag cement paste or OPC paste. The new hypothesis was supported by findings emerging from the ESEM study. It was observed that frost salt scaling attack generates cracks in the microstructure which adhere to slag-matrix interfacial zones. This was confirmed by nano-indentation tests which demonstrated that carbonation creates a significant number of weak zones in the slag cement paste. In the case of OPC paste the picture that emerged was quite different. Natural air carbonation influences the mineral characteristics of cement pastes. The XRD study re-vealed that both slag cement paste and OPC paste possess various types of carbonate minerals, namely: calcite, aragonite and vaterite. However, accelerated carbonation creates overwhelmingly stable calcite phases in both types of cement pastes which are subsequently transformed from me-tastable carbonates. This observation draws attention to the role played by Ca(OH)2 in the good scaling resistance of OPC or low-slag cement systems. A curing regime, especially curing in lime water, appears to be favourable for slag cement materials. However, when compared to the effect of carbonation, the influence that the curing water quality has on scaling resistance is minimal. The contribution made by prolonged water curing to scaling resistance could have been greater but, as it was, the curing periods were limited to 5 weeks in the interests of remaining realistic and practi-cal. The main goal of the project was to investigate the improvements in frost salt scaling resistance in-stigated by chemical grinding agents on the basis of the various positive results gained from the preliminary tests. The intention was to study the effects that the chemicals had on the cement paste microstructure in order to understand frost salt scaling resistance in slag cement concrete and so as to contribute to structural improvements in that area. A microstructural comparative study was carried out on slag cement pastes that contain alkanola-mines/hydrocarboxylates (the best performing ones) and diethylene glycolâbased (the worst per-forming example) grinding agents. The most notable difference was in the pore structure of the paste samples. Alkanolamines/hydrocarboxylates-based grinding agents were found to produce smaller pore sizes when compared to the ones containing diethylene glycol. This is consequently likely to give rise to higher carbonation resistance, lower water uptake and, eventually, to higher frost salt scaling resistance. However, the improvement achieved by alkanolamines / hydrocarboxy-lates is not sufficient to enhance the scaling resistance of the slag cement materials investigated in similar detail to OPC pastes. Another technique that was investigated was sodium monofluorophosphate (Na-MFP) surface treatment. Remarkable improvements in frost salt scaling resistance were achieved by applying a 10% Na-MFP solution to the surface of the carbonated slag cement paste and concrete. The scal-ing resistance improved by about 95% after 7 freeze-thawing cycles. Evidence was found pertain-ing to the reaction between Na-MFP and metastable carbonates in the carbonated slag pastes. The application appears to significantly increase the tensile strength of the carbonated slag cement paste which is extremely favourable in terms of scaling resistance. The study finally resulted in the development of a new integrated model. The model mainly takes into account the glue-spall theory and the hypothesis developed in this thesis and it runs on the Delft Lattice Model platform. The model successfully demonstrates the experimental observations and the crack patterns created by the scaling action. The glue-spall theory suggests that cement-based material surface scaling derives from external ice layer cracking due to further cooling. Cooling consequently generates tensile stress due to the shrinkage of ice and causes cracking when the stress exceeds the tensile strength of the ice. This theory can explain many phenomena including the pessimum effect. On the basis of this theory, the new integrated model proved to be capable of simulating two important experimental observa-tions. Under identical conditions the model can predict higher surface scaling at a 3% salt concen-tration level in relation to higher and lower values. The effect of ice layer thickness is furthermore found to be crucial with respect to frost salt scaling. Under identical material and environmental conditions the thicker external ice layer creates more damage than thinner ice layers. This observa-tion was also successfully demonstrated with the new integrated model.Civil Engineering and Geoscience
Electric curing of cement mortar
No full textIn this research, the effect of curing with different types of current, current densities/voltage intensity on mortar is studied. The main objective is to explore the outcomes of electric curing on the setting time of CEM-I mortar. For achieving the main objective, an electrical curing setup is designed to accommodate curing with direct current, alternating current and pulsed direct current and the conventional vicat apparatus and its test procedure are modified in order to carry out the testing while a sample is being cured with current. Additionally, the effect of passing current on material properties such as compressive strength and resistivity is also studied. The temperature development during the curing process has also been monitored in the research. The electric charge and power calculation are done, for possible current/voltage regimes. The experiments revealed that the setting time can be altered by electrical curing of CEM-I mortar. The influence of electric curing for the chosen curing period is not significant as revealed from the compressive strength testing while the development of resistivity and temperature reveals that there is a correlation between these two properties. The effect of electric power applied to a sample is reflected in the results of the setting times. In the last chapter the findings of the study are summarised and recommendations, keeping in mind the possibility of expanding research in this field of study, have been made.Civil Engineerin
Chloride Ingress of Carbonated Blast Furnace Slag Cement Mortars
No full textIn the Netherlands civil engineering structures, such as overpasses, bridges and tunnels are generally built using blast furnace slag cement (BFSC, CEM III/B) concrete, because of its high resistance against chloride penetration. Although the Dutch experience regarding durability performance of BFSC concrete has been remarkably good, its resistance to carbonation is known to be sensitive, especially when the used slag percentage is high. In a field investigation on a highway overpass damage was found in sheltered elements such as abutments and intermediate supports, which was attributed to chloride induced corrosion enhanced by carbonation that occurred prior to the chloride exposure.Many structures built using BFSC could be prone to this mechanism, i.e. carbonation enhanced chloride induced corrosion, negatively affecting their durability. Focus of the research was given on the influence of carbonation on the chloride penetration resistance of BFSC mortars with varying slag content. In light of the characteristics from the overpass case, it was assumed that first there is a period of carbonation during sheltered exposure, and subsequently joint leakage causes exposure to chlorides. In order to identify the influence of slag content on carbonation, chloride penetration resistance and their coupled effect, mortars with twelve cement blends in a range of 0–70% slag were evaluated based on chloride migration coefficient, accelerated carbonation and electrical resistivity.This study shows that carbonation of BFSC mortars increases the porosity, consequently decreasing the chloride penetration resistance. Binders with 50% or more slag were found to have a significantly lower resistance after carbonation. Consequently, the chloride penetration resistance of a given concrete cover strongly depends on the duration of carbonation and the resulting carbonation depth, hence influencing its lifespan. The service life was estimated using a simplified model for the chloride penetration time of a combined carbonated and uncarbonated layer. It was found that mortar with a slag content between 35 and 50% that was carbonated before chloride exposure show the lowest influence of carbonation on the chloride penetration resistance.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Materials and Environmen
3D imaging and micro-structural characterization of reinforced concrete structures naturally-deteriorated due to corrosion
No full textIn this study, a three-scale characterization of eight 20-years-old reinforced concrete specimens naturally deteriorated due to chloride-induced corrosion was carried out. The main aim of this research was to investigate some relevant parameters to accurately model corrosion of reinforced concrete structures, such as which are the locations where corrosion pits are more likely to initiate as well as if the Elastic modulus of corrosion product depends on the concrete mix that the reinforcement was embedded in. Each scale of characterization had a specific goal: the macro-scale characterization aimed to analyze the state of the specimens after 20 years of outside (unsheltered) exposure through Non-destructive techniques (i.e. potential measurements, corrosion rate, resistivity of concrete), visual inspections and analysis of historical records. Also, since specimens were mis-labelled during the years, the cement type that each specimen was cast with was identified through polished sections of concrete portions analyzed under the Scanning Electron Microscope (SEM). The meso-scale characterization of the reinforcement aimed to quantify the volume loss of the steel as well as to investigate which were the locations where corrosion pits occurred the most. Out of each reinforced concrete specimen, a core of 20 mm diameter and 100 mm height has been drilled, embedding a steel bar of 8 mm diameter. Each core has been then analyzed through X-ray Computed Tomography (CT-scan) and image analysis was performed. Apart from revealing different volume loss for bars embedded in different mixes (i.e. CEM I was the most corroded, CEM III/B the least), this characterization suggested that the most influencing factor for corrosion pits to form is the presence of defects (i.e. air voids) at the steel/concrete interface. Finally, the micro-scale characterization of corrosion product aimed to investigate the relation between Elastic modulus of corrosion product (퐸cp), its micro-structure and its chemical composition, with regard to the different concrete mix that steel was embedded into. 퐸cp was measured through Nano-indentation, while chemical composition was measured through SEM/EDS (Energy Dispersive Spectrometry) elemental mapping and spot analysis. This phase of the research revealed that differences between Elastic modulus of corrosion product generated in different mixes are not significant, and that the relation between 퐸cp and Fe/O seems to be (linearly) proportional. In the last chapter, the findings of the research are summarized and some recommendations are made regarding future endeavors.Civil Engineering | Building Engineerin
EMABM 2015: Proceedings of the 15th Euroseminar on Microscopy Applied to Building Materials, Delft, The Netherlands, 17-19 June 2015
No full textStructural EngineeringCivil Engineering and Geoscience
The effect of blast furnace slag chemistry on carbonation characteristics of cement-slag systems
No full textIn order to compensate the limited availability of raw material resources and meet the growing demand for decreasing CO2 emissions during cement and concrete productions, a practical method is to decrease the clinker content in cement. This strategy mainly consists of substituting a part of the clinker with supplementary cementitious materials (SCMs) at the cement and concrete production levels. As a mature addition in cement industry, blast furnace slag is a high-performance alternative that has been widely used in Europe and North America, as a SCM. Cement blended with slag is known to exhibit a high resistance to many chemical deteriorations such as alkali silica reaction, sulfate attack, and chloride ingress. An exception is carbonation, which renders a poor microstructure at the skin area of slag-rich concrete. The main aim of the thesis was to investigate the connection among slag chemistry, reactivity, and carbonation resistance of slag-rich cement paste. For this reason, the variation of slag composition was firstly identified through (1) literature review (Chapter 2) and (2) examining unhydrated slag grains existing in old slag concrete structures with different service life. Therefore, Chapter 3 studied the feasibility of using EDS microanalysis as a tool for quantitative measurement of the compositions of unhydrated slags in existing field concretes. The results revealed the variation trend of slag composition with time in the Netherlands. Then, synthetic slags covering the mentioned composition variation were produced in the laboratory, to eliminate the potential interferences and focus on slag chemistry only. The effect of slag composition on reactivity and carbonation resistance of slag-rich cement paste were investigated systematically. In Chapter 4, synthetic slags based on CaO-SiO2-Al2O3-MgOsystem and commercial slags were considered to estimate the correlation between slag chemistry and reactivity. It was found that higher MgO and/or Al2O3 contents of slag led to a higher reactivity. Chapter 5 observed the carbonation products in the slag-rich cementitious systems (mainly CEM III/B) upon three different exposure conditions, namely, long term exposure in the field, indoor natural exposure, and accelerated carbonation testing. Emphasis was laid on the carbonation of monosulfate and hydrotal cite-like phase in particular. The author believed that there was enough evidence indicating these two phases being the key components towards formulating blast furnace slag systems resistant to carbonation. Chapter 6 revealed the correlation between slag chemistry and CO2 binding capacity of the blended system. To simplify the composition of mixture, model paste containing only C3S, synthetic slags and gypsum was employed. In Chapter 7 and 8, the effect of MgO and Al2O3 contents of slag on the carbonation characteristics of cement-slag system was explored, respectively. Accelerated carbonation testing was performed on slag cement paste. The evolution of phase assemblage, microstructure, and micro-mechanical properties of each mixture before and after carbonation testing was evaluated. Finally, the connection among slag chemistry, reactivity, and carbonation resistance was discussed comprehensively. It was noted that the classification employed for slag reactivity cannot be extended to characterize carbonation resistance of cement-slag system directly. The main challenge occurred for slag with high alumina content. Al2O3-rich slag was reactive as a blended cement component but it did not contribute to carbonation resistance. Considering the effect of slag chemistry on reactivity and carbonation resistance together, slag, with a CaO/SiO2 ratio ≈ 1 and presenting high MgO (> 10 wt.%) and moderate Al2O3 (10-15 wt.%) contents, was recommended to design slag rich concrete structure with improved hydration performance and carbonation resistance.Materials and Environmen
Revealing the dark side of Portlandite Clusters in cement paste by circular polarization microscopy
No full textPlane and crossed polarization are the two standard light modes in polarized light microscopy that are widely used to characterize crystalline and amorphous phases in cement-based materials. However, the use of the crossed polarized light mode has been found to be restrictive for studying birefringent phases quantitatively due to the extinction phenomenon that arises depending on the crystal orientation. This paper introduces circular polarization microscopy as an alternative technique to overcome the extinction problem during the examination of cementitious materials’ microstructure with optical microscopy. In order to evaluate the feasibility of this technique, selected optical and micromorphological features of portlandite clusters were investigated in cement paste. Image analysis results showed that compared to the conventional crossed polarization technique, circular polarization offers significant advantages when portlandite quantification is of interest, and it stands out as a promising low-cost alternative to backscattered electron microscopy.Structural EngineeringCivil Engineering and Geoscience
The effects of intravenous and local tranexamic acid on bone healing: An experimental study in the rat tibia fracture model
No full textBackground: Tranexamic acid (TXA) is an antifibrinolytic agent. It has long been used to reduce the need for perioperative blood loss in various surgeries. Few studies have investigated the effects of local and intravenous administration of TXA on fracture healing. Thus, we aimed to evaluate if TXA influences hematoma volume and fracture healing in the rat tibia fracture model. Materials and methods: A tibia fracture with intramedullary Kirschner wire fixation was created in all animals. Rats were randomly divided into three groups as local TXA, intravenous TXA, and control. A dose of 50 mg/kg local and intravenous TXA was administered to the study groups. Hematoma volume was measured on the first and third days of the study. The animals were sacrificed on the 14th and 21st days for radiological and histopathological examinations. Results: There was no significant difference between the groups in terms of hematoma volume measured on Day 1 and the mean decrease of hematoma volume from Day 1 to Day 3 (p = 0.158 and p = 0.239, respectively). The total radiological scores of Day 14 and Day 21 were similar in all groups (p > 0.05 for all). There was also no significant difference between the histological staging of the fracture repair on Day 14 and Day 21 for all groups (p > 0.05 for all). Conclusion: Our findings suggest that TXA's local and intravenous application makes no significant difference in fracture healing. (C) 2020 Elsevier Ltd. All rights reserved
3-Dimensional insight into zonation within slag rims of aged blended cement
No full textThrough the integration of SEM-BSE and TEM, we gained a comprehensive 3-dimensional understanding of different distribution patterns of inner hydration products of slag. For fully hydrated small slag grains, two distinct sub-zones were formed in the rims. Lath-like, well-crystalline hydrotalcite-like crystals were found to precipitate, grow, and accumulate near the boundary, forming a layer with a thickness slightly exceeding 0.5 μm. In the center, entrapped calcium and silicon played roles in the formation of a homogeneous and fibrous C−(A)–S–H gel phase. The concentration equilibrium between cement matrix and grain core led to the establishment of a similar grey pixel value and Ca/Si atomic ratio of gel phase at ~1.10. As the size of slag grains increased, three sub-zones became visible. Hydrotalcite-like phase was enriched near the boundary, followed by a sandwiched area abundant in C–(A)–S–H gel phase. Due to the low mobility and increased migration distance, newly released magnesium from reaction front accumulated locally to form a new Mg-rich region.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Materials and Environmen
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