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Does a High Amount of Unhydrated Portland Cement Ensure an Effective Autogenous Self-Healing of Mortar?
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Fire Engineering.
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering.
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Fire Engineering.ORCID iD: 0000-0001-7279-6528
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Fire Engineering. Skanska, Stockholm, Sweden.
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2019 (English)In: Materials, ISSN 1996-1944, E-ISSN 1996-1944, Vol. 12, no 20, article id 3298Article in journal (Refereed) Published
Abstract [en]

It is commonly accepted that the autogenous self-healing of concrete is mainly controlled by the hydration of Portland cement and its extent depends on the availability of anhydrous particles. High-performance (HPCs) and ultra-high performance concretes (UHPCs) incorporating very high amounts of cement and having a low water-to-cement ratio reach the hydration degree of only 70–50%. Consequently, the presence of a large amount of unhydrated cement should result in excellent autogenous self-healing. The main aim of this study was to examine whether this commonly accepted hypothesis was correct. The study included tests performed on UHPC and mortars with a low water-to-cement ratio and high cement content. Additionally, aging effects were verified on 12-month-old UHPC samples. Analysis was conducted on the crack surfaces and inside of the cracks. The results strongly indicated that the formation of a dense microstructure and rapidly hydrating, freshly exposed anhydrous cement particles could significantly limit or even hinder the self-healing process. The availability of anhydrous cement appeared not to guarantee development of a highly effective healing process.

Place, publisher, year, edition, pages
MDPI, 2019. Vol. 12, no 20, article id 3298
Keywords [en]
continued hydration, ultra-high performance concrete, cracking, microstructure, calcite
National Category
Other Materials Engineering
Research subject
Building Materials
Identifiers
URN: urn:nbn:se:ltu:diva-76510DOI: 10.3390/ma12203298OAI: oai:DiVA.org:ltu-76510DiVA, id: diva2:1365824
Note

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

Available from: 2019-10-25 Created: 2019-10-25 Last updated: 2019-11-20Bibliographically approved
In thesis
1. Self-Healing Concrete
Open this publication in new window or tab >>Self-Healing Concrete
2019 (English)Licentiate thesis, comprehensive summary (Other academic)
Alternative title[sv]
Självläkande Betong
Abstract [en]

Concrete is a brittle material prone to cracking due to its low tensile strength. Crack repairs are not only expensive and time-consuming but also increase the carbon footprint. Designing a novel concrete material possessing the ability to self-repair cracks would enhance its sustainability. Self-healing can be defined as a material’s ability to repair inner damage without any external intervention. In the case of concrete, the process can be autogenous, based on an optimized mix composition, or autonomous, when additional capsules containing some healing agent and/or bacteria spores are incorporated into the binder matrix. The first process uses unhydrated cement particles as the healing material while the other utilizes a synthetic material or bacteria precipitating calcite which are released into the crack from a broken capsule or activated by access to water and oxygen. The main disadvantages of the autonomous method are the loss of the fresh concrete workability, worsened mechanical properties, low efficiency, low survivability of the capsules and bacteria during mixing and the very high price. On the other hand, the autogenous self-healing was found to be more efficient, more cost effective, safer, and easier to implement in full-scale applications. Knowledge related to mechanisms and key factors controlling the autogenous self-healing is rather limited. Therefore, the aim of this research work was to better understand the autogenous self-healing process of concrete and to optimize the mix design and exposure conditions to maximize its efficiency. This licentiate thesis summarizes the main findings of the first 2.5 years of the PhD project. Several factors affecting autogenous self-healing were studied, including the amount of unhydrated cement, mix composition, age of material, self-healing duration and exposure conditions. The process was investigated both externally, at the surface, and deeper inside of the crack, by evaluating the crack closure and chemical composition of formed self-healing products. In addition, the flexural strength recovery was also studied. It was observed that a large amount of cement in the concrete mix does not ensure an efficient autogenous self-healing of cracks. A very dense and impermeable binder microstructure limited the transport of calcium and silicone ions to the crack and diminished the precipitation of the healing products. Addition fly ash increased the crack closure ratio close to the crack mouth, but its presence did not support the recovery of the flexural strength, presumably due to a very limited formation of load bearing phases inside the crack. Calcium carbonate was detected mainly at the crack mouth, whereas calcium silicate hydrate (C-S-H) and ettringite were found deeper inside the crack. The formation of C-S-H and ettringite presumably resulted in a regain of the flexural strength. On the other hand, calcite crystals formed close to the surface of the specimen controlled conditions inside the crack through its external closure. Healing exposure based on pure water appeared to be inefficient even despite the application of different temperature cycles and water volumes. The application of a phosphate-based retarding admixture in the curing water resulted in the highest self-healing efficiency. The admixture presumably inhibited the formation of a dense hydration shell on the surface of the unhydrated cement grains and promoted the precipitation of calcium phosphate compounds inside the crack. In addition, water mixed with microsilica particles caused a regain of the flexural strength through formation of C-S-H in the crack.

Place, publisher, year, edition, pages
Luleå tekniska universitet, 2019
Series
Licentiate thesis / Luleå University of Technology, ISSN 1402-1757
Keywords
cementitious materials, self-healing, exposure, fly ash, calcite, C-S-H, cracking
National Category
Engineering and Technology Other Materials Engineering
Research subject
Building Materials
Identifiers
urn:nbn:se:ltu:diva-76527 (URN)978-91-7790-490-8 (ISBN)978-91-7790-491-5 (ISBN)
Presentation
2019-12-12, C305, University of Technology, Luleå, 08:00 (English)
Opponent
Supervisors
Funder
Svenska Byggbranschens Utvecklingsfond (SBUF)Swedish Transport Administration
Available from: 2019-10-28 Created: 2019-10-28 Last updated: 2019-11-27Bibliographically approved

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Rajczakowska, MagdalenaHabermehl-Cwirzen, KarinHedlund, HansCwirzen, Andrzej

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