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- Geopolymers (3)
- Rheology (3)
- Polycarboxylate ether (2)
- Superplasticizers (2)
- Abwasserinfrastruktur (1)
- Acid resistance (1)
- Acoustic emission (1)
- Admixtures (1)
- Adsorption (1)
- Africa (1)
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Einkomponentige alkaliaktivierte Bindemittel (AAB), d. h. AAB, bei denen der Aktivator als Feststoff im Bindemittel vorliegt, weisen Vorteile hinsichtlich der praktischen Anwendung im Bauwesen gegenüber konventionellen AAB auf. Einkomponentige AAB lassen sich als Gemische aus reaktiver Silica und Natriumaluminat herstellen, wobei die Reaktionsprodukte von der eingesetzten Silica und den Verhältnissen Na2O/Al2O3/SiO2/H2O abhängen: Während Mikrosilica und ähnliche Reststoffe zu zeolithreichen Produkten führen, erhält man aus Reisschalenasche vollständig amorphe Gele (sog. «Geopolymere »). Mörtel auf Basis solcher Bindemittel entsprechen den Anforderungen von DIN 19573:2016-03 hinsichtlich des Widerstands gegen Schwefelsäureangriff (Prüfung bei pH = 1 für 70 d) und sind damit potentiell als Reparatursysteme für Abwasserinfrastruktur geeignet. Untersuchungen mittels 29Si-MAS-NMR und 1H-29Si-Kreuzpolarisations(CP)-MAS-NMR sowie REM-Aufnahmen von erhärteten und dem Schwefelsäureangriff ausgesetzten AAB zeigen, dass der hohe Säurewiderstand v. a. auf die Fällung von Silicagel an der Grenzfläche Mörtel/Säure zurückzuführen ist, wodurch der Korrosionsfortschritt in darunterliegenden Schichten verlangsamt wird.
Mortars for application on concrete, e.g. repair mortars or protective coatings, need to have a durable bond to the substrate. This bond is determined by the adhesion between the two materials and by the differential deformations of the mortar and the substrate. In the present contribution, the hygric deformations (shrinkage/expansion) of novel one-part alkali-activated mortars and their bond to concrete substrates are studied. Shrinkage of the mortars was studied at 50 % r.H., while expansion was studied on mortars stored over an open water surface (> 99% r.H.). The bond behaviour was studied by pull-off tests according to DIN EN 1542 and by optical microscopy.
The alkali-activated mortars exhibit hygric deformations much lower than the deformations of an established, commercial mortar for sewer maintenance that was tested as reference in parallel with the alkali-activated mortars. The bond behaviour of the alkali-activated mortars depends strongly on their mix-design and curing. Optical microscopy showed that in the
mortars with lower bond strength, cracks developed in the mortar during curing. Mortars with appropriate mix-design and curing did not exhibit cracking, and their pull-off strength (up to >3 MPa) conformed to the requirements of relevant standards.
One-part geopolymers, synthesized from three different silica sources and sodium aluminate, were investigated regarding their microstructure and the evolution of their compressive strength on curing at slightly elevated temperatures. The effect of thermal treatment was studied up to 1000 °C and for the investigation of the chemical durability specific samples were treated with sulfuric acid (H2SO4, pH = 1).
Depending on the silica feedstock, different degrees of reaction were observed. One of the silicas had a significantly higher reactivity. For two of the silicas, significant amounts of zeolites occurred as reaction products besides geopolymeric gel, whereas the more reactive silica source lead to the formation of a rather fully condensed geopolymeric network.
The composites indicated promising behavior on heating in so far as no distinct shrinkage step occurred in the temperature range of dehydration. Up to 400 °C the residual strength of those mixes increased. Above 800 °C the samples underwent new phase formation. After exposure to sulfuric acid (pH 1; 70 d) specimens showed a residual compressive strength of about 77 % of the reference, indicating high acid resistance.
During the past 20 years, innovative developments in concrete technology have enabled the production of ultra-high performance fibre reinforced concretes (UHPFC) for the protection and preservation of concrete structures. The main characteristics of UHPFC comprise high compressive and flexural strength as well as high ductility compared to normal strength, normal weight concrete (NSC). This paper focuses on the analysis of crack formation and crack propagation in UHPFC under tensile loading under quasistatic conditions by using acoustic emission (AE) and optical deformation analysis (ODA).
AE is a non-destructive technique to monitor the development of micro structural damage processes caused by external forces. In this context, AE allows for a continuous monitoring of changes in the microstructure (cracks) over time and the corresponding localisation by appropriate algorithms. The AE measurements were performed along with ODA at the surface of the samples in order to analyse the deformation behaviour of the samples and the crack propagation.
For the tests, a UHPFC reference mixture was developed and tested with three different volume percentages of steel fibres (1.0 V.-%, 2.0 V.-%, and 3.0 V.-%). This paper shows that the application of AE is a suitable method to analyse the crack propagation in UHPFC specimens under tensile loading and to localise the crack initiation. Furthermore, insights into the failure mechanism of the fibres are offered by the analysis of the signals of the AE measurements, which allows for a better understanding of UHPFC and its general performance.
Geopolymers are promising alkali activated materials with a wide range of possible applications like heat-resistant coatings and adhesives or concretes with lower C02 emissions than conventional Portland cement-based systems. Conventional ("two-part") geopolymers are synthesized by an activation of aluminosilicate feedstocks like metakaolin or blast furnace slags with highly alkaline solutions like sodium hydroxide solutions and/or water glass solutions.
A more recent approach are so called one-part geopolymers, where the alkaline activator is provided in solid form, so that only water has to be added to initiate the geopolymerisation reaction. The handling of alkaline solutions is avoided, which possesses several advantages compared to conventional geopolymers, in particular regarding safety and health issues, and, thus, it improves economic and social acceptance of these binders. However, the new formulations are less exhaustively studied and the knowledge about conventional geopolymers cannot simply be transferred to this new class of binders.
The production and processing of ordinary Portland cement are responsible for nearly 5% of the world C02 emissions. Due to the world’s increasing requirement for building materials, this air pollution is growing up in the next decades. Alternative cement such as geopolymers can reduce these emissions effectively, Conventional geopolymer binders, produced ffom fly ash or metakaolin as aluminosilicate source and water-glass or alkali hydroxide Solutions as activator, are often used and studied. It is known that under appropriate conditions the starting materials form a three-dimensional aluminosilicate structure and create a gel-like matrix, In our case, a silica source from wastewater treatment of chlorosilane production and solid sodium aluminate as starting materials are initiated to harden by water. This procedure is known as “just add water” or “one-part” geopolymer formulation. These composites have significant advantages for use on construction site, but their Chemical structure is not extensively analyzed. In this work, a well known one-part geopolymer is investigated by 'H and 27A1 magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. Furthermore, it is shown that overlapping NMR lines can resolved with echo experiments and
Todays superplasticizers for self-compacting concrete and high Performance cementitious materials are very versatile. Typically superplasticizers are composed of a polycarboxylic backbone equipped with polyethylene oxide graft chains of variable length and grafting degree. The mode of operation of these admixtures is based on the steric repulsion of the particles upon adsorption. The adsorption is strongly depending upon the charge density of a superplasticizer and the time dependent Adsorption processes control the retention of the flow performance – an important issue particularly for ready-mix and construction site concrete. However, the properties of polycarboxylic superplasticizers typically remain a black box for users. The paper suggests a simple and rapid test method, which can be conducted without sophisticated equipment, to qualitatively distinguish between two types of superplasticizers based on the adsorption mechanism. Interpreting the results in the right way provides a powerful tool to choose the right admixture for individual time dependent flow specifications.
Innovative admixture technology has significantly widened up the ränge of possibilities of concrete engineers. For many decades the water to cement ratio (w/c) was the major influencing factor for the performance of concrete. Due to the need to adjust a consistency, which still allowed reasonable workability, the w/c was typically significantly higher than technologically reasonable. Rheology modifying admixtures Support adjusting the concrete consistency largely independent of the w/c. It was only after the invention of the first superplasticizers that modern concrete technology significantly evolved in terms of flowability, strength, and durability, and only due to the steady evolution of the technology modern innovations, such as Self-Compacting Concrete, Ultra-High-Performance Concrete, or Engineered Cementitious Composites were made possible. Today’s superplasticizers are extremely versatile and can be adjusted to individual technological specifications. However, the other side of the coin of versatility is that cementitious Systems incorporating superplasticizers have become more sensitive against environmental influences, such as the environmental temperature, which may cause unwanted effects or demand for supplementary admixture use such as stabilizing admixtures. Hence, concrete mixture composition with admixtures demands for a high level of expertise and offen there is lack of awareness about the mode of Operation of rheology modifying admixtures among concrete technologists. The paper gives a comprehensive overview about rheology modifying admixtures such as superplasticizers or stabilizing agents, and how they can be used depending upon the application in the most favourable way. Based on experiences with the sub-Saharan African concreting boundary conditions, which exhibit many challenges in terms of environmental boundary conditions and construction site logistics, conclusions are finally drawn, how admixtures can be used in the most beneficial way to improve the concrete casting Situation.
ln order to observe the influence of stabilising agents (STA) based on starch and diutan gum, rheometric experiments and setting tests were conducted on cement pastes with and without PCE superpiasticizers. The results show that with regard to yield stress both STAs show differing behaviours in Systems without PCE. In presence of PCE, yield stress infiuences of the STAs retreat into the background. The Vicat results exhibit that STAs can reduce the retard ing effect of PCE.
Innovative admixture technology has significantly widened up the ränge of possibilities of concrete engineers. For many decades the water to cement ratio (w/c) was the major influencing factor for the performance of concrete. Due to the need to adjust a consistency, which still allowed reasonable workability, the w/c was typically significantly higher than technologically reasonable. Rheology modifying admixtures Support adjusting the concrete consistency largely independent of the w/c. It was only after the invention of the first superplasticizers that modern concrete technology significantly evolved in terms of flowability, strength, and durability, and only due to the steady evolution of the technology modern innovations, such as Self-Compacting Concrete, Ultra-High-Performance Concrete, or Engineered Cementitious Composites were made possible. Today’s superplasticizers are extremely versatile and can be adjusted to individual technologicalspecifications. However, the other side of the coin of versatility is that cementitious Systems incorporating superplasticizers have become more sensitive against environmental influences, such as the environmental temperature, which may cause unwanted effects or demand for supplementary admixture use such as stabilizing admixtures. Hence, concrete mixture composition with admixtures demands for a high level of expertise and offen there is lack of awareness about the mode of Operation of rheology modifying admixtures among concrete technologists. The paper gives a comprehensive overview about rheology modifying admixtures such as superplasticizers or stabilizing agents, and how they can be used depending upon the application in the most favourable way. Based on experiences with the sub-Saharan African concreting boundary conditions, which exhibit many challenges in terms of environmental boundary conditions and construction site logistics, conclusions are finally drawn, how admixtures can be used in the most beneficial way to improve the concrete casting Situation.