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Dolomite calcined clay composite cement - hydration and durability
No full textConsiderable amounts of CO2 are emitted during the production of cement. One way to reduce these CO2 emissions is to replace some of the cement clinker with supplementary cementitious materials (SCMs). A cement containing SCMs is called a composite cement. The use of composite cements has considerably increased over the last few decades and is predicted to continue to do so as the global demand for concrete increases in the future. This will lead to a high demand for SCMs, which in the case of some of the traditionally used SCMs could exceed supply. There is therefore a need to identify potential new SCMs.
The PhD project investigated the possibility of using dolomite in combination with calcined clays as SCMs in Portland composite cements. In this project, metakaolin was used as a model material for industrially available calcined clays. The main objectives of the PhD project were to understand the hydration of Portland composite cements containing dolomite and metakaolin, the resulting phase assemblage, and its stability during carbonation, leaching and chloride exposure. Special focus was put on the hydrate hydrotalcite that was shown to form in composite cements containing dolomite. The compressive strength, phase assemblage, and microstructure of samples, in which various amounts of Portland cement clinker were replaced with either dolomite or with a combination of dolomite and metakaolin, were investigated.
In the first part of the project, the hydration study, two experimental approaches were applied. In the first one, dolomite or limestone were used to replace 0-20%wt of a Portland metakaolin cement. The Portland metakaolin cement represented a mix of Portland cement clinker and metakaolin in the ratio 6:1. It was shown that approximately 10%wt of a Portland metakaolin cement could be replaced with either dolomite or limestone without impairing its compressive strength after curing at temperatures of 20 °C and 38 °C for 28 days and 90 days. The phase assemblages in samples containing limestone or dolomite were generally quite similar, though dolomite was shown to be slightly less reactive than limestone, especially at lower curing temperatures. This lower reactivity resulted in less carbonates delivered from dolomite to form carbonate AFm phases.
In the second experimental approach, various combinations of dolomite and metakaolin were used to replace 40%wt of a Portland cement clinker. After curing for 360 days, especially at elevated temperatures (38 °C and 60 °C), the dolomite in samples with little or no metakaolin content had reacted significantly. The dolomite reaction was shown to consume portlandite and resulted in the formation ofhydrotalcite (Mg6Al2(OH)18·3(H2O)) and calcite:
6CaMg(CO3)2 + 2Al(OH)3 + 6Ca(OH)2 + 3H2O • Mg6Al2(OH)18·3(H2O) + 12CaCO3
This reaction was shown to depend on the availability of portlandite in the system. In samples with high metakaolin content, the reactive metakaolin consumed the portlandite before the less reactive dolomite started to react, which limited the dolomite reaction. The pore space and the availability of aluminium were found to play minor roles in limiting the dolomite reaction.
The durability part of the PhD project investigated the stability of the hydrotalcite formed in composite cements containing dolomite. Samples with little or no metakaolin content were selected because they formed the greatest amount of hydrotalcite in the hydration study. It was shown that hydrotalcite can withstand high degrees of leaching and carbonation. After chloride exposure, the formation of a chloride-containing hydrotalcite was observed, which increased the chloride binding of the cement paste samples investigated. With mass balance calculations, it was shown that the chloride-containing hydrotalcite in samples containing dolomite can contribute to the chloride binding of the cement paste to an extent comparable to Friedel's salt in samples containing limestone.
It could be concluded that dolomite performs in a similar way to limestone in cementitious systems with regard to compressive strength and phase stability. The phase assemblages in composite cements containing dolomite and limestone are similar. The reaction of dolomite in systems containing little or no metakaolin was shown to result in the additional formation of hydrotalcite, a stable reaction product in the aggressive environments tested, which is able to take up a significant amount of chloride.Digital full text not availabl
Impact of leaching on chloride ingress profiles in concrete
Get PDFKloridinitiert armeringskorrosjon er en av hovednedbrytningsmekanismene for armerte betongkonstruksjoner. Klorider trenger igjennom betongoverdekningen over tid og forårsaker korrosjon når de når et kritisk kloridinnhold ved armerings overflaten. Betongens motstand mot kloridinntrenging er derfor avgjørende for levetiden. I denne studien ønsket vi å undersøke effekten utlekking har på kloridinntrengingsprofiler i betong. Betong- og mørtelprøver ble eksponert for to ulike eksponeringsløsninger i 90 dager: 3 % NaCl, og 3 % NaCl tilsatt KOH for å begrense utlekking. Løsningene ble byttet ut ukentlig. Etter eksponering ble kloridprofiler i prøvene bestemt, i tillegg til portlanditt- og kaliumprofiler for å bestemme graden av utlekking. Vi observerte at utlekking fører til betydelig høyere maksimalt kloridinnhold, sammenliknet med prøvene med begrenset utlekking (150 % økning). Kloridinntrengingsdybden var derimot mindre påvirket av utlekking, noe som betyr at levetiden ikke nødvendigvis blir redusert på grunn av utlekking. De totale kloridprofilene er i hovedsak styrt av kloridbinding i prøvene. Det ble demonstrert at dette kan føre til feilaktig estimering av levetid ved bruk av empiriske modeller, derfor burde mer mekanistiske levetidsmodeller tas i bruk.Chloride induced reinforcement corrosion is one of the main deterioration mechanisms for reinforced concrete structures. Chlorides penetrate the concrete cover with time, and initiate corrosion as they reach a critical chloride content at the reinforcement surface. In this study we want to investigate the effect of leaching on chloride ingress profiles in concrete. Concrete and mortar samples were exposed for 90 days to two different exposure solutions: 3% NaCl, and 3% NaCl with additional KOH to limit leaching. The solutions were exchanged weekly. After exposure, total chloride profiles were determined to investigate the chloride ingress, as well as portlandite and potassium profiles to investigate the extent of leaching. We observed that leaching leads to considerably higher maximum total chloride contents, compared to the samples where leaching was limited (150% increase). The chloride ingress depth, however, was not greatly affected by leaching, meaning that the service life is not necessarily reduced due to leaching. Thus, total chloride profiles are mainly governed by the interaction between the solids and the chlorides. This can lead to erroneous service life prediction when using empirical models, hence mechanistic service life models should be used
Frühe Hydratphasenentwicklung und rheologische Eigenschaften in portlandzementbasierten Systemen: Ettringit und seine Bedeutung für die Rheologie in Hinblick auf neue Technologien wie den 3D-Druck
No full textThe topic of this dissertation is to unravel the contribution of different chemical reactions to the early rheological development of hydrating cementitious pastes. The binder systems investigated in the experiments described here were pure Portland cement and ternary OPC-CSA-C\$ blends. An investigation of the early time period of hydration and the resulting rheological behavior is of critical importance for the workability of mortar or concrete especially in applications such as self-compacting concrete or 3D printing. Specific applications pose increasingly complex requirements for cementitious systems, especially regarding the key aspects of setting behavior and strength development. It is therefore essential to acquire a deeper understanding of the fundamental mechanisms governing their rheology to be able to control the properties of these systems precisely at any given time.
In this work, the hydration kinetics and phase development of various OPC-based systems were investigated with the help of QXRD, isothermal calorimetry, and pore water analysis. The results were correlated with microstructural and rheological developments obtained through NMR, penetration tests, rheological measurements, ultrasound velocity tests, and compressive strength measurements. The aluminate reaction, and particularly the formation of ettringite, was specifically controlled by adding chemical admixtures to evaluate the impact of ettringite formation on the early-stage rheological properties of such systems.
It can be shown that the hydration process can be effectively tailored for specific requirements with the addition of organic and inorganic admixtures. Highly-charged PCEs, aluminum sulfate-based shotcrete accelerators, and a hydration control agent have proven to be highly effective in modifying the aluminate reaction to meet the specific requirements of different applications. The investigated systems show great potential for use in extrusion-based 3D printing, both as one-component and two-component systems. Additionally, ternary OPC-CSA-C\$ mixtures as two-component systems offer flexible mix designs that can be adapted to various performance requirements, making them suitable for 3D printing applications as well.
In all studied systems, a strong correlation between ettringite formation, heat of hydration, and rheological properties was observed before the onset of the silicate reaction. In systems with overdosed highly-charged PCE, hemicarbonate was also detected alongside ettringite, further contributing to the rapid initial stiffening of the paste. However, significant amounts of other hydrate phases such as C-S-H or AH\textsubscript{3} weren't observed in any investigated system during this early time period. This supports the hypothesis that ettringite is the decisive phase governing the rheological properties at this early hydration stage. One of the important factors seems to be the influence of solid volume fraction due to the high water content of this phase. Another decisive element is probably interparticle connections but it remains unclear whether these connections result from an intergrowth of ettringite crystals forming bridges between particles or from altered surface charges of the particles in regions of high ettringite crystal density
Carbonation resistance of concrete and mortar containing novel low clinker cement
Get PDFCorrosion of reinforcement steel is a major deterioration mechanism for reinforced concrete structures. Corrosion can be caused by the carbonation of concrete. In this report we test the carbonation resistance of a novel low clinker cement with ground granulated blast-furnace slag (GGBFS) and limestone developed in the EnDurCrete project (CEMII/CM(S-LL)), by comparing its performance to an already commercially available blended cement (CEMII/A-S). We determine and compare the ingress of the carbonation front under accelerated laboratory conditions (1% CO2, 60% RH) on both mortar and concrete, for both cements, after 14, 28 and 90 days. We estimate the carbonation depth using two different techniques: thymolphtalein pH indicator and portlandite profiles determined by thermogravimetric analysis (TGA). In addition, we determine the relative humidity in the samples prior to exposure. The average RH for the mortar and concrete samples prior to exposure was similar. This suggests that mortar samples can be used to describe the moisture state in concrete. The RH ranged between 88 and 92%, which agrees with the level one would obtain by self-desiccation in sealed samples. The portlandite content in the non-carbonated EnDurCrete mortar is lower compared to the reference mortar, as can be expected due to the higher replacement levels with GGBFS. A lower carbonation resistance could be expected because of this, however this was not the case for the concrete samples. The carbonation depth determined by the pH indicator and the portlandite profiles agree well, taking into account the different ways of sampling for the two methods. The carbonation ingress results for mortar and concrete agree well, except for the reference mortar samples which showed a lower carbonation ingress. The reason for this is unknown. Based on the carbonation ingress results obtained on concrete, the EnDurCrete concrete exhibits a similar carbonation resistance as the reference concrete with the commercially available cement. This indicates the possibility to introduce the EnDurCrete cement as a valuable alternative to commercially available cements, providing similar properties and substantial reduction in greenhouse gases during production
Carbonation resistance of concrete and mortar containing novel low clinker cement
No full textCorrosion of reinforcement steel is a major deterioration mechanism for reinforced concrete structures. Corrosion can be caused by the carbonation of concrete. In this report we test the carbonation resistance of a novel low clinker cement with ground granulated blast-furnace slag (GGBFS) and limestone developed in the EnDurCrete project (CEMII/CM(S-LL)), by comparing its performance to an already commercially available blended cement (CEMII/A-S). We determine and compare the ingress of the carbonation front under accelerated laboratory conditions (1% CO2, 60% RH) on both mortar and concrete, for both cements, after 14, 28 and 90 days. We estimate the carbonation depth using two different techniques: thymolphtalein pH indicator and portlandite profiles determined by thermogravimetric analysis (TGA). In addition, we determine the relative humidity in the samples prior to exposure. The average RH for the mortar and concrete samples prior to exposure was similar. This suggests that mortar samples can be used to describe the moisture state in concrete. The RH ranged between 88 and 92%, which agrees with the level one would obtain by self-desiccation in sealed samples. The portlandite content in the non-carbonated EnDurCrete mortar is lower compared to the reference mortar, as can be expected due to the higher replacement levels with GGBFS. A lower carbonation resistance could be expected because of this, however this was not the case for the concrete samples. The carbonation depth determined by the pH indicator and the portlandite profiles agree well, taking into account the different ways of sampling for the two methods. The carbonation ingress results for mortar and concrete agree well, except for the reference mortar samples which showed a lower carbonation ingress. The reason for this is unknown. Based on the carbonation ingress results obtained on concrete, the EnDurCrete concrete exhibits a similar carbonation resistance as the reference concrete with the commercially available cement. This indicates the possibility to introduce the EnDurCrete cement as a valuable alternative to commercially available cements, providing similar properties and substantial reduction in greenhouse gases during production
Impact of leaching on chloride ingress profiles in concrete
No full textKloridinitiert armeringskorrosjon er en av hovednedbrytningsmekanismene for armerte betongkonstruksjoner. Klorider trenger igjennom betongoverdekningen over tid og forårsaker korrosjon når de når et kritisk kloridinnhold ved armerings overflaten. Betongens motstand mot kloridinntrenging er derfor avgjørende for levetiden. I denne studien ønsket vi å undersøke effekten utlekking har på kloridinntrengingsprofiler i betong. Betong- og mørtelprøver ble eksponert for to ulike eksponeringsløsninger i 90 dager: 3 % NaCl, og 3 % NaCl tilsatt KOH for å begrense utlekking. Løsningene ble byttet ut ukentlig. Etter eksponering ble kloridprofiler i prøvene bestemt, i tillegg til portlanditt- og kaliumprofiler for å bestemme graden av utlekking. Vi observerte at utlekking fører til betydelig høyere maksimalt kloridinnhold, sammenliknet med prøvene med begrenset utlekking (150 % økning). Kloridinntrengingsdybden var derimot mindre påvirket av utlekking, noe som betyr at levetiden ikke nødvendigvis blir redusert på grunn av utlekking. De totale kloridprofilene er i hovedsak styrt av kloridbinding i prøvene. Det ble demonstrert at dette kan føre til feilaktig estimering av levetid ved bruk av empiriske modeller, derfor burde mer mekanistiske levetidsmodeller tas i bruk
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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