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Shrinkage and Related Properties of Alkali-Activated Binders Based on High MgO Blast Furnace Slag
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Fire Engineering.ORCID iD: 0000-0002-5328-4073
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Concrete is the second most used material in the world just after water. A drawback is that it is mostly based on Portland cement, which has an extremely high carbon footprint reaching a staggering 900 kg/tonne. The carbon dioxide emissions related to the production of the Portland cement accounts for nearly 8 % of the global total. Consequently, the construction sector is engaged in an active search for sustainable alternatives. Over the past few decades, alkali-activated materials (AAMs) emerged as one alternative and attracted strong scientific and commercial interests. Many industrial by-products produced in large volumes can be used as precursors for the AAMs system. The most common include blast furnace slag, fly ash, mine tailings, metallurgical slags, and bauxite residues. So far, products based on ground granulated blast furnace slag (GGBFS) showed the best price/performance ratio. Still, there are a number of unresolved issues, which must be addressed to ensure the economical and safe full-scale utilisation of that material. The research work presented in this thesis focuses on alkali-activated concretes based on Swedish water-cooled high-MgO ground granulated blast furnace slag. The objective of this work was to identify experimentally factors that are controlling the shrinkage and the creep of concretes made with this type of GGBFS and to understand their influence on various physical and chemical properties of fresh and solidified systems. Liquid sodium silicate, powder sodium carbonate and a combination of both were used to activate the binder chemically. Two curing procedures were followed; laboratory curing and heat curing at 65°C applied for 24 hours. Various properties were determined including workability, setting time, hydration heat development, shrinkage, creep, efflorescence, carbonation, compressive strength, microstructure and phase composition. Additionally, the effects of the activator type, dose, binder fines, binder composition and curing regime were investigated. The results revealed that the particle size distribution of the binder as well as the activator type and its dosage have strong effects on the produced materials. Increasing the activator amount or decreasing the alkali modulus of the used sodium silicate activator improved the early-age compressive strength and accelerated the hydration reaction. Alkali-activated high-MgO slag concrete showed higher autogenous and drying shrinkage, as well as higher creep in comparison to the Portland cement-based reference concrete. The sodium silicate increased the slump, shortened the setting time, increased the compressive strength and shrinkage but lowered the creep in comparison with the sodium carbonate-activated mixes. Replacing 20% of the slag with fly ash and decreasing the alkali modulus of the sodium silicate activator increased the autogenous shrinkage but decreased the ultimate drying shrinkage. Application of a heat treatment produced in general a higher early age compressive strength, a lower VI later strength development, a more porous microstructure and a decreased ultimate measured shrinkage. Sealed curing decreased the ultimate shrinkage by up to 50%. Some of the produced mixes showed strong efflorescence. Two years of curing in laboratory conditions resulted in an extensive carbonation of some of the mixes. This weakened the silicate binding of the gel and produced a coarser porosity due to the decalcification of C-(A)-S-H. The heat-cured samples activated with sodium silicate were the most affected. Many mixes showed an extensive microcracking of the binder matrix. However, the within this study newly developed mixes were substantially less effected. These optimised mixes were based on a combination of sodium silicate and sodium carbonate activators, combined with a heat treatment and partial replacement of the slag with fly ash. The main hydration phase that formed was C-(A)-S-H, with gaylussite, calcite, nahcolite and hydrotalcite as secondary phases. The partial replacement of slag with fly ash resulted in a dominant formation of N-(A)-S-H and C-(A)-S-H.

Place, publisher, year, edition, pages
Luleå University of Technology, 2019. , p. 90
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
Keywords [en]
Alkali-activated slag, alkali-activated materials, high-MgO blast furnace slag, shrinkage, creep, carbonation, strength, heat curing
National Category
Engineering and Technology Other Materials Engineering
Research subject
Building Materials
Identifiers
URN: urn:nbn:se:ltu:diva-76356ISBN: 978-91-7790-469-4 (print)ISBN: 978-91-7790-470-0 (electronic)OAI: oai:DiVA.org:ltu-76356DiVA, id: diva2:1360129
Public defence
2019-12-10, C 305, LTU / C-Building, Luleå, 08:30 (English)
Opponent
Supervisors
Available from: 2019-10-11 Created: 2019-10-11 Last updated: 2019-11-27Bibliographically approved
List of papers
1. Alkali activation of a high MgO GGBS: fresh and hardened properties
Open this publication in new window or tab >>Alkali activation of a high MgO GGBS: fresh and hardened properties
2018 (English)In: Magazine of Concrete Research, ISSN 0024-9831, E-ISSN 1751-763X, Vol. 70, no 24, p. 1256-1264Article in journal (Refereed) Published
Abstract [en]

In this study, concretes and pastes were produced from a high magnesium oxide (MgO) ground granulated blast furnace slag (magnesium oxide content 16.1 wt%) by alkali activation with various amounts and combinations of sodium carbonate and sodium silicate. Sodium carbonate activators tended to reduce slump compared to sodium silicate at the same dose, and, in contrast to the literature for many blast furnace slags with more moderate magnesium oxide, to shorten the initial and final setting times in comparison with concretes activated by sodium silicate for dosages less than 10 wt%. Higher heat curing temperatures and the use of larger dosages of alkali activators resulted in higher early-age compressive cube strength values. The X-ray diffraction analysis of 7 and 28 d old pastes activated with sodium carbonates revealed formation of gaylussite, calcite, nahcolite and calcium-aluminium-silicate-hydrate (C-A-S-H) gel. Curing at 20 degrees C appeared to promote dissolution of gaylussite and calcite, while heat curing promoted their replacement with C-A-S-H, which also resulted in higher ultimate cube compressive strength values. Conversely, mixes activated with sodium silicate contained less crystalline phases and more disordered gel, which strengthened the binder matrix.

Place, publisher, year, edition, pages
Westminster England: ICE Publishing, 2018
Keywords
compressive strength, curing, setting properties
National Category
Infrastructure Engineering
Research subject
Structural Engineering
Identifiers
urn:nbn:se:ltu:diva-71837 (URN)10.1680/jmacr.17.00436 (DOI)000450222300002 ()2-s2.0-85049372488 (Scopus ID)
Note

Validerad;2018;Nivå 2;2018-11-30 (inah)

Available from: 2018-11-30 Created: 2018-11-30 Last updated: 2019-10-11Bibliographically approved
2. The Effect of Blast Furnace Slag/Fly Ash Ratio on Setting, Strength, and Shrinkage of Alkali-Activated Pastes and Concretes
Open this publication in new window or tab >>The Effect of Blast Furnace Slag/Fly Ash Ratio on Setting, Strength, and Shrinkage of Alkali-Activated Pastes and Concretes
2019 (English)In: Frontiers in Materials, ISSN 2296-8016, Vol. 6, no 9Article in journal (Refereed) Published
Abstract [en]

The aim of this study was to determine the effects of partial fly ash substitution in to a series of alkali-activated concrete based on a high-MgO blast furnace slag BFS. Mixes were activated with various amounts of sodium silicate at alkali modulus (mass ratio SiO2/Na2O) values of 1.0, 0.5, and 0.25. The results showed that, an increase in the fly ash content extended the initial setting time but had very little effect on the final setting time, although the early age compressive strength was decreased. The fly ash addition had no effect on the drying shrinkage but lowered the autogenous shrinkage. The mixes activated with sodium silicate at a lower alkali modulus showed a significantly higher autogenous shrinkage but lower drying shrinkage values. Severe micro cracking of the binder matrix was observed only for mixes without fly ash, activated with sodium silicate solution at higher alkali modulus. Decreasing the alkali modulus resulted in a higher autogenous shrinkage, less micro cracking and a more homogenous structure due to more extensive formation of sodium-aluminate-silicate-hydrate gel (N-A-S-H), promoted by the addition, and more extensive reaction of the fly ash.

Place, publisher, year, edition, pages
Frontiers Media S.A., 2019
Keywords
fly ash FA, blast furnace slag BFS, initial and final setting time, shrinkage, alkali-activated binders
National Category
Other Materials Engineering
Research subject
Building Materials
Identifiers
urn:nbn:se:ltu:diva-73145 (URN)10.3389/fmats.2019.00009 (DOI)000458669200001 ()2-s2.0-85062450166 (Scopus ID)
Note

Validerad;2019;Nivå 2;2019-03-08 (johcin)

Available from: 2019-03-08 Created: 2019-03-08 Last updated: 2019-10-11Bibliographically approved
3. Effects of fineness and chemical composition of blast furnace slag on properties of alkali-activated binder
Open this publication in new window or tab >>Effects of fineness and chemical composition of blast furnace slag on properties of alkali-activated binder
2019 (English)In: Materials, ISSN 1996-1944, E-ISSN 1996-1944, Vol. 12, no 20, article id 3447Article in journal (Refereed) Published
Abstract [en]

Abstract: The effects of fines and chemical composition of three types of ground granulated blast furnace slag (GGBFS) on various concrete properties were studied. Those studied were alkali activated by liquid sodium silicate (SS) and sodium carbonate (SC). Flowability, setting times, compressive strength, efflorescence, and carbonation resistance and shrinkage were tested. The chemical composition and microstructure of the solidified matrixes were studied by X-ray diffraction (XRD), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM) coupled with EDX analyser. The results showed that the particle size distribution of the slags and the activator type had significantly stronger effects on all measured properties than their chemical composition. The highest compressive strength values were obtained for the finest slag, which having also the lowest MgO content. SC-activated mortar produced nearly the same compressive strength values independently of the used slag. The most intensive efflorescence and the lowest carbonation resistance developed on mortars based on slag containing 12% of MgO and the lowest fineness. The slag with the highest specific surface area and the lowest MgO content developed a homogenous microstructure, highest reaction temperature and lowest drying shrinkage. Thermogravimetric analysis indicated the presence of C-(A)-S-H, hydrotalcite HT, and carbonate like-phases in all studied mortars.

Place, publisher, year, edition, pages
Basel, Switzerland: MDPI, 2019
Keywords
alkali-activated slag GBFS, strength, microstructure of AAS, hydration products, shrinkage
National Category
Engineering and Technology Other Materials Engineering
Research subject
Building Materials
Identifiers
urn:nbn:se:ltu:diva-76497 (URN)10.3390/ma12203447 (DOI)31640292 (PubMedID)2-s2.0-85074230932 (Scopus ID)
Funder
Luleå University of Technology, 143103
Note

Validerad;2019;Nivå 2;2019-10-28 (johcin)

Available from: 2019-10-24 Created: 2019-10-24 Last updated: 2019-11-20Bibliographically approved

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