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- Alkali-activated materials (2)
- Abwasserinfrastruktur (1)
- Acid resistance (1)
- Alkaliaktivierte Bindemittel (1)
- Biogene Schwefelsäurekorrosion (1)
- Blended cements (1)
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- Brick clays (1)
- Calcined clays (1)
- Calcinierte Tone (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.
Materials with a high acid resistance are required in different important infrastructures. Examples include repair systems for sewer structures, where biogenic sulfuric acid corrosion is the major degradation mechanism. Low-calcium alkali-activated materials (AAMs) have been repeatedly observed to exhibit high acid resistance. However, the reasons for the high acid resistance of these materials were not fully under¬stood until recently, and the use of highly alkaline activator solutions to produce AAMs appears to hamper their commercial uptake. These issues have been tackled by characterising one-part AAMs and studying their alteration when exposed to sulfuric acid.
One-part AAMs were synthesized by mixing blends of solid silica and sodium aluminate with water, and subsequent curing at 60–80 °C. Acid resistance testing was performed according to DIN 19573, i.e. exposure to sulfuric acid at pH = 1 for 70 days. Characterisation of the cured and the acid-exposed materials was done by XRD, ATR-FTIR, SEM as well as 29Si, 27Al and 1H MAS NMR spectroscopy, including cross-polarisation and double-resonance methods.
Materials synthesized from industrial silicas were gel-zeolite composites, containing a substantial amount of unreacted ‘excess’ silica, while materials synthesized from rice husk ash were fully amorphous, containing ‘excess’ hydrous alumina. The sulfuric acid resistance of mortars based on these binders conformed to the requirements of DIN 19573 for sewer repair applications. The high acid resistance was caused by precipitation of silica gel at the mortar–solution interface, inhibiting further degradation. The presence of alumina gel may inhibit bacterial activity, potentially further improving performance in sewer environments.
The phase assemblage of silica/sodium aluminate-based one-part AAMs can be adjusted via choice of the silica starting material. Properly designed materials exhibit excellent acid resistance, caused by precipi¬tation of silica gel which protects subjacent regions, and they may also inhibit bacterial activity.
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.
The production of Portland cement causes a substantial environmental impact, since the calcination of limestone results in high emissions of carbon dioxide. The use of supplementary cementitious materials such as calcined clays as partial replacement for Portland cement offers a solution to limit this environmental impact. This paper investigates four clays from deposits in central Germany with the aim of obtaining pozzolans for the production of Portland-pozzolana cement. The results obtained show that the calcined clays possess pozzolanic proper¬ties, which differ depending on calcination temperature and the relative amounts of kaolinite and 2:1 clay minerals.
Der Einsatz von calcinierten Tonen als Zementhauptbestandteil ist ein vielversprechender Weg, die mit der Zementherstellung und -verwendung verbundenen CO2-Emissionen zu senken. Im Vortrag werden die Randbedingungen hierfür sowie Ergebnisse eines aktuellen Vorhabens zur Calcinierung von illitreichen Tonen (Ziegeltonen) und anderen Tonen vorgestellt.