Filtern
Erscheinungsjahr
Dokumenttyp
- Zeitschriftenartikel (48)
- Beitrag zu einem Tagungsband (39)
- Vortrag (33)
- Beitrag zu einem Sammelband (5)
- Posterpräsentation (1)
- Forschungsbericht (1)
Sprache
- Englisch (95)
- Deutsch (31)
- Mehrsprachig (1)
Schlagworte
- Alkali-activated materials (41)
- Geopolymers (26)
- Concrete (15)
- Carbonation (11)
- Acid resistance (8)
- Cement (6)
- Corrosion (6)
- Alkali-activation (5)
- Durability (5)
- Steel corrosion (5)
Organisationseinheit der BAM
- 7 Bauwerkssicherheit (70)
- 7.4 Baustofftechnologie (70)
- 6 Materialchemie (11)
- 6.3 Strukturanalytik (11)
- 7.1 Baustoffe (7)
- 7.6 Korrosion und Korrosionsschutz (5)
- 7.5 Technische Eigenschaften von Polymerwerkstoffen (4)
- 1 Analytische Chemie; Referenzmaterialien (2)
- 1.1 Anorganische Spurenanalytik (2)
- 5 Werkstofftechnik (2)
Paper des Monats
- ja (3)
Eingeladener Vortrag
- nein (32)
Die Oberflächenrauheit eines Bauteils ist eine wichtige Kenngröße für zahlreiche seiner Eigenschaften. Der im Bauwesen übliche Rauheitsparameter ist die Rautiefe Rt bzw. MTD, die durch volumetrische Verfahren (Sandflächenverfahren) ermittelt wird und in zahlreichen Normen Verwendung findet. Laserbasierte Verfahren müssen deshalb eine Umrechnung der aus Oberflächenprofilen gewonnenen Informationen in Rt bzw. MTD ermöglichen. Hierzu wird i.A. ein empirischer Zusammenhang zwischen Parametern zur Beschreibung des Profils und der Rautiefe der entsprechenden Oberfläche verwendet. Der Aufsatz stellt verschiedene Rauheitsdefinitionen vor und vergleicht sie miteinander. Es wird ferner gezeigt, dass die nach verschiedenen Prüfvorschriften ermittelten Rautiefen voneinander abweichen. Das Vorgehen bei der Ermittlung des Zusammenhangs zwischen Profilparametern und Rautiefen nach einem volumetrischen Verfahren wird demonstriert und der Stand der laufenden Entwicklung eines Prüfgeräts auf Basis eines Linienlasers vorgestellt.
Fly ash-based, ambient-cured geopolymer concretes for a round-robin durability testing program were designed and characterized. Optimum activator composition was determined based on the Chemical composition of the amorphous part of the fly ash. Fly ash content, water content, and grading curves of the aggregates were varied for the concretes. Characterization of the concretes involved flow diameter, air void content, density, compressive strength, and permeability. The workability of the concretes was notably sensitive to changes in the mix-design. The majority of strength development in the concretes occurred within the first 28 days of curing; concrete strengths reached up to 86 MPa after 56 days. Two concretes, both with sufficient workability, but significantly different grading curve, water content, strength and permeability were identified to be suitable for the round-robin testing program.
Die Funktionalität und Dauerhaftigkeit von Instandsetzungs- und Verstärkungssystemen an Bauwerken hängt in einem hohen Maß von der Beschaffenheit der Bauteiloberfläche ab. Als aussagekräftiger Parameter bei der Instandsetzung dient hierbei die Rauigkeit oder Rautiefe. Die Bestimmung der Rautiefe wird im Baubereich seit vielen Jahrzehnten unter Verwendung des Sandflächenverfahrens durchgeführt. Dieses volumetrische Messverfahren hat sich als robust und sehr praktikabel erwiesen. Es ist vergleichsweise einfach und ohne größeren Kostenaufwand umsetzbar. Dennoch weist es einige Schwächen in der Anwendung auf. Der größte Mangel ist die Beschränkung auf horizontale, trockene Oberflächen. Sowohl auf vertikalen, geneigten oder gekrümmten Oberflächen als auch „über Kopf1 lässt sich dieses Verfahren nicht anwenden. Bedingt durch verschiedene nationale Normen und unterschiedliche Anwendungsbereiche, weichen sowohl die zu verwendenden Prüfmittel als auch die zu ermittelnden Kenngrößen stark voneinander ab. Dies macht eine genaue Kenntnis über die verschiedenen Vorschriften und deren Inhalte notwendig. Ein weiterer Nachteil ist die, durch das Verfahren bedingte, starke Streuung über den „Prüfereinfluss“ in den zu erzielenden Ergebnissen. Untersuchungen zur Wiederholgenauigkeit ergaben Schwankungen von bis zu 20 Prozent in Abhängigkeit von Prüfer und Prüfvorschrift. An der BAM wurde deshalb ein automatisiertes Messsystem entwickelt,
Geopolymer-zeolite composites were synthesized using a silica-rich industrial byproduct from chlorosilane production and sodium aluminate. Pastes were cured at 80 °C and 80% RH, and subsequently dried in two different climates: at 23 °C and 50% RH, and under vacuum. 1H MAS, 23Na MAS and 29Si MAS NMR and XRD measurements were performed after the drying procedures as well as after subsequent aging. Zeolite Na-A was found beside traces of faujasite-type zeolite and zeolite EMT as major crystalline phases in the cured composites; the fraction of geopolymeric gel in the reaction products was determined to be ~18% on a molar basis. Various water species could be distinguished using 1H MAS and 1H-29Si CP MAS NMR, applying rotorsynchronized echo experiments. The largest fraction of the pore water resides in the α-cages of the zeolite Na-A and in the geopolymeric gel; in addition, water exists in the β-cages of the zeolites and adsorbed at sodium ions. The water species in α-cages and in the pores of the geopolymeric gel exhibit slightly different chemical shifts of 4.7 ppm and 4.9 ppm, respectively, in the 1H MAS NMR spectra. Changes of the water content in the geopolymer pores of differently dried samples were observed and led to slightly varied chemical shifts in the 29Si MAS NMR spectra too. Measurements after more than 500 days revealed no significant aging effects of the composites, which confirm their chemical stability.
Die Bestimmung der Rauheit von Bauteiloberflächen stellt eine wichtige Messaufgabe im Bauwesen dar. Vor allem bei der Betoninstandsetzung, im Straßenbau und bei der Herstellung von Stahlbeton-Halbfertigteilen ist die Rauheit – gemessen als Rautiefe – eine wichtige Kenngröße. Zurzeit werden Messungen der Rautiefe mittels volumetrischer Verfahren (Sandflächenverfahren) durchgeführt. Diese bereits seit mehreren Jahrzehnten angewandten Verfahren sind jedoch auf horizontale bzw. wenig geneigte und trockene Oberflächen beschränkt. Darüber hinaus hängen die erzielten Ergebnisse sehr stark von der angewandten Prüfvorschrift, den darin vorgeschriebenen Prüfmitteln und Geräten und dem jeweiligen Anwender ab.
Als Alternative zu den volumetrischen Verfahren wurde an der Bundesanstalt für Materialforschung und -prüfung (BAM) ein automatisiertes, laserbasiertes Messverfahren entwickelt, mit dem Rautiefen von Bauteiloberflächen in Anlehnung an bereits standardisierte Verfahren bestimmt werden können. Der vorliegende Beitrag befasst sich zunächst mit den konventionellen Prüfverfahren, beschreibt die wichtigsten Zusammenhänge und zeigt die Grenzen dieser Methoden auf. Im zweiten Teil wird das an der BAM entwickeltes Messsystem, als zielführendes Alternativverfahren für die konventionellen Methoden, vorgestellt und dessen Leistungsfähigkeit näher beschrieben.
The main objective of this work is to determine chloride migration coefficients for alkali activated fly ash-based mortars. The effect of various mixture composition is studied. The identified values will be used in simulations of chloride transport in the alkali-activated composites, mainly to determine a critical time span of exposition to chlorides causing corrosion of reinforcement. Rapid chloride migration experiments (RCM) are performed on cylindrical specimens, 100 mm diameter, 50 mm height. Low voltage electric field (10–30 V DC) is applied as a driving force for the accelerated chloride penetration. The chloride front in the sample is identified according to NT Build 492 standard after given time of penetration (depending on the electrical current ~ 3 hours) on a fracture surface using silver nitrate solution.
Further, accelerated diffusion experiments are performed in order to verify the migration coefficients obtained using RCM. The total chloride content is determined, and chloride profiles obtained on powder ground from separate layers of the specimen.
The development of alkali‐activated materials (AAMs) as an alternative to Portland cement (PC) has seen significant progress in the past decades. However, there still remains significant uncertainty regarding their long term performance when used in steel‐reinforced structures. The durability of AAMs in such applications depends strongly on the corrosion behaviour of the embedded steel reinforcement, and the experimental data in the literature are limited and in some cases inconsistent. This letter elucidates the role of the chemistry of AAMs on the mechanisms governing passivation and chloride‐induced corrosion of the steel reinforcement, to bring a better understanding of the durability of AAM structures exposed to chloride. The corrosion of the steel reinforcement in AAMs differs significantly from observations in PC; the onset of pitting (or the chloride ‘threshold’ value) depends strongly on the alkalinity, and the redox environment, of these binders. Classifications or standards used to assess the severity of steel corrosion in PC appear not to be directly applicable to AAMs due to important differences in pore solution chemistry and phase assemblage.
The passive fire protection of steel structures and other load-bearing components will continue to gain importance in future years. In the present contribution, novel intumescent aluminosilicate (geopolymer-bound) composites are proposed as fire-protective coatings on steel. Steel plates coated with these materials were exposed to the standard temperature-time curve as defined in ISO 834 – 1:1999. The coatings partially foamed during curing and expanded further during thermal exposure, demonstrating their intumescent characteristic.Thermogravimetryandoscillatory rheometry determined that the intumescent behavior is attributed to a transition to a viscous state (loss factor > 1) in the temperature range of major water release, differing from conventional geopolymers. XRD and SEM images showed that the coatings had characteristics of ceramic or glass-ceramic foams after fire resistance testing, suggesting superior performance under challenging conditions. The thickness of the coatings influenced their foaming and intumescent behavior and thus the time for the coated steel plates to reach 500 °C. A number of additives were also studied with the best performance obtained from samples containing sodium tetraborate.Acoating of just 6mmwas able to delay the time it takes for a steel substrate to reach 500 °C to more than 30 minutes.
Alkali-activated materials such as geopolymers are currently receiving a lot of attention because of their potential to be used as binders for concrete with advantageous engineering properties and reduced manufacturing CO2 emissions. Knowledge of the durability of steel in these concretes and related properties of alkali-activated materials is a prerequisite for their application as building materials, if they are to be used for steel reinforced elements. However, to date only limited data exists on this topic. The present contribution focuses on durability-related transport properties of geopolymer-based mortars (as model systems for concrete). We report results of accelerated carbonation, rapid chloride migration (RCM) and air permeability measurements as well as porosity data for fly ash-based geopolymer mortars, including mixes containing ground granulated blast furnace slag (GGBFS). In addition, we report polarization resistance data and corrosion potential vs. time curves for carbon steel bars embedded in two of the fly ash-based geopolymer mortars and a CEM I-based mortar (as reference).
Despite comparable total porosities, the carbonation depths, the chloride migration coefficients and the air permeabilities of the mortars differed significantly. In general, the addition of GGBFS to the binders improved the performance (decreased transport coefficients); however, this was not found to be true for the air permeability in all cases. This latter effect can be explained by drying damage of the C-(A-)S-H gel in GGBFS-containing binders. On the other hand, low transport coefficients can also be achieved by optimization of the binder formulation without the addition of GGBFS, which is also reflected in the material’s air permeability. Thus, there exists no simple correlation between air permeability (of harshly dried mortars) and durability-related transport coefficients for the studied alkali-activated materials.
The corrosion potential vs. time curves in combination with polarization resistance values reveal that the steel reinforcement in geopolymer mortars assumed a passive state. However, this happened considerably later than for steel in CEM I-based mortars. The free corrosion potential of carbon steel reinforcement in the geopolymer mortars had different values than the free corrosion potential values for the CEM I-based mortar for both the active and the passive state; possible reasons for this behavior are discussed.
Alkali-activated materials such as geopolymers are currently receiving a lot of attention because of their potential to be used as binders for concrete with advantageous engineering properties and reduced manufacturing CO2 emissions. Knowledge of the durability of steel in these concretes and related properties of alkali-activated materials is a prerequisite for their application as building materials, if they are to be used for steel reinforced elements. However, to date only limited data exists on this topic. The present contribution focuses on durability-related transport properties of geopolymer-based mortars (as model systems for concrete). We report results of accelerated carbonation, rapid chloride migration (RCM) and air permeability measurements as well as porosity data for fly ash-based geopolymer mortars, including mixes containing ground granulated blast furnace slag (GGBFS). In addition, we report polarization resistance data and corrosion potential vs. time curves for carbon steel bars embedded in two of the fly ash-based geopolymer mortars and a CEM I-based mortar (as reference).
Despite comparable total porosities, the carbonation depths, the chloride migration coefficients and the air permeabilities of the mortars differed significantly. In general, the addition of GGBFS to the binders improved the performance (decreased transport coefficients); however, this was not found to be true for the air permeability in all cases. This latter effect can be explained by drying damage of the C-(A-)S-H gel in GGBFS-containing binders. On the other hand, low transport coefficients can also be achieved by optimization of the binder formulation without the addition of GGBFS, which is also reflected in the material’s air permeability. Thus, there exists no simple correlation between air permeability (of harshly dried mortars) and durability-related transport coefficients for the studied alkali-activated materials.
The corrosion potential vs. time curves in combination with polarization resistance values reveal that the steel reinforcement in geopolymer mortars assumed a passive state. However, this happened considerably later than for steel in CEM I-based mortars. The free corrosion potential of carbon steel reinforcement in the geopolymer mortars had different values than the free corrosion potential values for the CEM I-based mortar for both the active and the passive state; possible reasons for this behavior are discussed.
Mixing and curing of geopolymers and related alkali-activated materials without storage and handling of highly alkaline solutions possesses advantages regarding safety and economic viability; one possible approach is to produce these materials from solid silica feedstocks and solid sodium aluminate, and subsequent mixing with water. We present a comparison between geopolymers and geopolymer-zeolite composites synthesized by this route from different silica feedstocks (by-product silica from chlorosilane production, microsilica, rice husk ash) and with different SiO2/Al2O3 ratios, using results from XRD, NMR, SEM, thermal analysis, mechanical and acid resistance testing. The use of rice husk ash favors formation of a fully amorphous geopolymer with high strength. Utilization of the other silica feedstocks leads to formation of geopolymer-zeolite composites, the amount and kind of zeolites depending on the feedstock and the SiO2/Al2O3 ratio. These composites show beneficial dehydration behavior, viz. no distinct dehydration step of thermal strain, with the phase assemblage after heating to 1000 °C controlled by the starting composition. Mortars produced from both, the geopolymers as well as the compo¬sites, exhibit high resistance to sulfuric acid attack, making them promising materials for the construction and the repair of industrial and sewer structures.
One-part alkali-activated (geopolymer) mortars based on three different silica-rich starting materials and sodium aluminate, with and without ground granulated blast furnace slag (GGBFS) addition, were tested regarding sulfuric acid resistance according to DIN 19573:2016-03 (70 days at pH = 1). Corresponding pastes were characterized by XRD, SEM, chemical analysis, 29Si MAS NMR and 1H-29Si CPMAS NMR after water storage and after acid exposure. The mortars exhibited a high resistance against sulfuric acid attack, with the best ones conforming to the requirements of DIN 19573:2016-03. The analytical results showed that this was due to precipitation of silica gel at the acid-mortar interface, which formed a mechanically stable layer that protected the subjacent mortar and thus inhibited further degradation. The addition of GGBFS decreased the acid resistance via formation of expansive calcium sulfate phases.
Slags from the nonferrous metals industry have great potential to be used as feedstocks for the production of alkali-activated materials. Until now, however, only very limited information has been available about the structural characteristics of these materials. In the work presented herein, synthetic slags in the CaO–FeOx–SiO2 system, representing typical compositions of Fe-rich slags, and inorganic polymers (IPs) produced from the synthetic slags by activation with alkali Silicate solutions have been studied by means of X-ray absorption near-edge structure (XANES) spectroscopy at the Fe K-edge. The iron in the slags was largely Fe2+, with an average coordination number of approximately 5 for the iron in the amorphous fraction. The increase in average oxidation number after alkali-activation was conceptualized as the consequence of slag dissolution and IP precipitation, and employed to calculate the degrees of reaction of the slags. The degree of reaction of the slags increased with increasing amorphous fraction. The iron in the IPs had an average coordination number of approximately 5; thus, IPs produced from the Fe-rich slags studied here are not Fe-analogs of aluminosilicate geopolymers, but differ significantly in terms of structure from the latter.