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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
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
The impact of curing temperatures on Portland composite cements – hydrate assemblage, porosity, and compressive strength
Get PDFCement production stands for approx. 8% of the man-made CO2 emissions. One of the most common ways to reduce CO2 emissions in cement production is by replacing part of the clinker with supplementary cementitious materials (SCMs) to produce Portland composite cements.
Todays’ Portland composite cement produced at the Norcem cement plant in Brevik is a CEM II/B-M containing 18wt.% fly ash (FA) and 4wt.% limestone (L). However, the availability of FA is decreasing in Europe leading to the unavoidable need for other SCMs with properties similar to FA.
This PhD thesis investigated the impact of a selection of SCMs and curing temperatures on the phase assemblage and porosity of hydrated composite cement pastes and how that in turn affect the compressive strength of the respective mortar samples.
Cement pastes with a w/b-ratio of 0.50 and mortar samples with a water-cement-sand ratio of 0.50:1:3 were prepared and cured sealed at 5, 20, and 38 °C for at least 180 days to ensure a sufficient reaction of the SCM. Composite cements with a composition of 78wt.% Portland cement (PC), 18wt.% supplementary cementitious material (SCM), and 4wt.% limestone (L), as well as two reference cements, one containing 78wt.% PC and 22wt.% limestone (PC-L), and the other being a pure PC were investigated. Three SCMs were used in this study, i.e., a siliceous fly ash (FA), a silicomanganese (SiMn) slag (S), and volcanic pozzolan (VP).
The phase assemblage was investigated in-depth by determining the C(-A)-S-H composition, the degree of reaction of the components, the mass of hydration phases, and the porosity characteristics. The compressive strength was determined on the mortar samples. The investigations applied a multi-technique approach including measuring techniques, i.e., XRD, TGA, SEM-EDS, water suction, MIP, DVS, and compressive strength testing, and thermodynamic modelling using GEMS and mass balance calculations.
The study of the C(-A)-S-H composition for the SCM-containing cements showed that the C(-A)-S-H has a lower Ca/Si and S/Si, but higher Al/Si ratio than the C(-A)-S-H of the PC as expected, because the SCMs contribute with silicates and aluminates. With increased curing temperatures, the SCMs show an enhanced reaction and a further decrease in the Ca/Si and S/Si ratios, and a further increase in the Al/Si ratios. The quantification of the changes in the C(-A)-S-H composition were crucial, as it had a considerable impact of the mass balance calcualtions of these systems.
The degree of reaction of the SCMs was determined both using SEM combined with image analysis and using mass balance calculations combined with portlandite quantification using TGA. Similar reaction degrees were obtained using the two different methods. In general, similar reaction degrees weredetermined for the three SCMs despite their different origins and chemical compositions. The reaction degrees were enhanced with increased curing temperatures, i.e., 20, 36, and 47% for FA, 34, 43, and 60% for S, and 35, 60, and 70% for VP when cured at 5, 20, and 38 °C, respectively. The degree of reaction of the SCMs are included in the mass balance, which enabled the calculation of the mass of the hydration phases formed.
For the PC and PC-L reference mortars, the compressive strength gradually decreased with increasing curing temperature. However, a different temperature dependency of the compressive strength was measured for the SCM-containing mortar samples, and the highest compressive strength was measured after curing at 20 °C, and lower strengths were observed for both 5 and 38 °C. The deviating temperature dependencies of the compressive strength between the reference and the SCM-containing mortars was investigated by relating the compressive strength to the degree of reaction of the components, the amount of C(-A)-S-H, and the total volume of the solids (unreacted solids and hydration phases) both with and without the fine porosity associated with C(-A)-S-H. The temperature dependency of the compressive strength was reflected in all these parameters for the PC and PC-L references. However, in the case of the SCM-containing composite cements, none of these characteristics could explain the curing temperature dependency of the compressive strength. One of the main differences between the reference (PC-L) and the SCM-containing cements was an enhanced reaction degree at higher curing temperatures.
Correlating the porosity characteristics determined using a range of porosity techniques to compressive strength showed a clear link for the PC and PC-L references. However, this was not observed for the SCM-containing cements. There seemed to be a need to differentiate the pore size range when investigating the porosity characteristics of SCM-containing cements because the gel porosity may have a lower impact on the compressive strength than capillary and macro porosity. The differentiation of the pore sizes enabled to show the contribution of the SCMs to the compressive strength, but did not explain the temperature dependency of the composite cements.
In a last step, the impact of the curing temperatures and SCMs on the C(-A)-S-H density was calculated. Neither for the PC and PC-L references nor for the SCM-containing cements was a clear temperature impact on the C(-A)-S-H density observed. This might be due to the limited range of curing temperatures used. In the presence of the SCMs, the C(-A)-S-H densities were slightly higher than in PC-L meaning that the pozzolanic reaction of the SCMs led to a densification of the C(-A)-S-H. Though, the C(-A)-S-H densities in the composite cements were considerable lower compared to the PC reference indicating that the space available for the C(-A)-S-H to form, e.g. effected by the w/b-ratio, had a considerable impact on the C(-A)-S-H density.
The variations in the hydrate assemblage and the porosity did not explain the curing temperature dependency of the compressive strength of SCM-containing composite cements, as they did for the PC and PC-L references. Further research on the dependency of the C(-A)-S-H intrinsic strength on its composition and microstructure is needed to understand how the curing temperature impacts the compressive strength of SCM-containing composite cements.
The present thesis showed that the composite cements containing 18wt.% of the novel SCMs, i.e., iMn slag and volcanic pozzolan, had a similar response to variations in the curing temperature and a similar impact on the hydrate assemblage and porosity as the conventional composite cements containing fly ash. Thus, from the perspective of the hydration at later age, the composite cements containing the novel SCMs are valid alternatives for the currently used composite cements containing fly ash.Fulltext not availabl
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
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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