Jochen Stark’s research while affiliated with Bauhaus-Universität Weimar and other places

Kurzer historischer Abriss. Chemische und mineralogische Zusammensetzung des Portlandzementklinkers. Zementtechnische Eigenschaften der Klinkermineralien. Rolle des Gipses als Erstarrungsregler. Charakteristik und Eigenschaften der Zementzumahlstoffe Hüttensand, Trass, Flugasche, Ölschiefer und Silicastaub. Zumahlstoffzemente im Beton. Zementmahlung. Hydratation der silicatischen Phasen der Zemente. Vergleich der Hydratationsprodukte. Reaktionskinetik und -geschwindigkeit bei der Hydratation des Portlandzements. Reaktionen während der Frühphase der Hydratation. Der Hydratationsgrad als Maß für den Hydratationsfortschritt. Normalzemente nach DIN EN 197-1 und Zemente mit besonderen Eigenschaften nach DIN EN 197-1, DIN EN 14216 und DIN 1164. Sulfathüttenzement und Feinstzemente.

Kurzer historischer Abriss. Schädigungsmechanismus bei Sulfatangriff. Mikrostrukturelle Veränderungen im Zementsteingefüge bei Sulfatangriff. Physikalischer Widerstand von Beton gegen das Eindringen sulfathaltiger Wässer. Chemischer Angriff durch Sulfate mit Ettringit-, Gips- und Thaumasitbildung. Modellierung der Thaumasitbildung. Schäden an Betonkonstruktionen infolge Sulfatangriff durch Oxidation von Sulfiden. Mauerwerksschäden durch Zementinjektion in sulfathaltiges Mauerwerk. Normative Regelungen bei zu erwartetem Sulfatangriff. Prüfverfahren zur Vorhersage der Sulfatbeständigkeit von Beton bei innerem und äußerem Angriff. Ergebnisse von Untersuchungen zum Einfluss der Sulfationenkonzentration auf den Schädigungsverlauf und Berechnungen zur Ausfällung von Gips in Abhängigkeit vom pH-Wert.

Kurzer historischer Abriss. Mechanismus des Säureangriffs. Angriff durch aggressive Kohlensäure. Biogene Säurebildung in Abwasser- und Biogasanlagen. Kombinierter Säure-Sulfat-Angriff. Säurebildung an Kühltürmen und Wasserbauten. Schutzmaßnahmen gegenüber Säureangriff mittels aktiver und passiver Maßnahmen. Säurewiderstandsfähiger Beton. Performance-Prüfverfahren für Betone mit hohem Säurewiderstand.

Together with the chemical analysis as well as XRD and DTA, effects of steam and autoclave treatments on structure characteristics of basic oxygen furnace slag (BOFS) and electric arc furnace slag (EAFS) are investigated. The results show that steam and autoclave treatments on slag contribute to the conversion of f-CaO and f-MgO to Ca(OH)2 and Mg(OH)2 ,which improve the volume stability of slag by reducing the contents of free lime and periclase, and the autoclave treatment is more effective. Meanwhile, for the utilization of steel slag, the effects on the volume stability by CaO and MgO that were wrapped in eutectic in the crystallite form should be a key factor, besides the contents of f-CaO and coarse periclase crystalline. The thermal reactivity of slag treated by steam and autoclave increased, especially the reactivity of elemental iron and wuestite. The reactivity of EAFS increased more significantly at high temperature due to its higher content of iron.

This paper is a bibliographic tool reviewing experimental and theoretical studies related to cement hydration and microstructure development that have been published within the four years of the interim period between the 12th and 13th International Congress on the Chemistry of Cement.

The aim of the study was to investigate the pozzolanic reaction of silica fume in Ultra-High-Performance Concrete (UHPC). The silica fume content in investigated UHPC is 16 wt.-% based on cement. This is particularly necessary to fill the intergranular space and to achieve a high packing density. Additionally, Calcium-Silicate-Hydrate phases are formed by the pozzolanic reaction. Consumption of silica fume by pozzolanic reaction and degradation of calcium hydroxide were determined by the following three methods: silica fume content was quantified by selective acid solution process in depleted nitric acid solution, calcium hydroxide content was quantified by powder X-ray diffraction and differential thermal analysis. Furthermore, the pozzolanic reaction of silica fume was imaged on fractured surfaces by high resolution Scanning Electron Microscopy by means of different imaging conditions (ESEM WET-mode at 90 % relative humidity, high vacuum, low vacuum conditions using low voltage mode). Results of the present study show that the pozzolanic reaction at low w/c ratio is significantly influenced by the storage conditions. After 56 days of hydration selective acid dissolution experiments reveal in samples with water storage a silica fume consumption of approximately 10 %. In contrast samples with heat treatment (90 °C/48h) show a highly pozzolanic reaction, approximately 49 % of silica fume has reacted. By means of high-resolution Scanning Electron Microscopy (SEM) under low voltage conditions it is the first time possible to image the dissolution structures of silica fume particles caused by pozzolanic reaction in UHPC. Calcium-Silicate-Hydrate phases create a very dense binder matrix close to silica fume particles.

The UHPC microstructure, after different time of hydration, was examined with high resolution SEM imaging techniques. ESEM in WET mode for early hydration process and NanoSEM for extremely dense microstructure. The obtained results have shown that high addition of superplasticizer caused the strong retardation of hydration process. The growth of ettringite is also influenced. At early hydration the clinker particles <= 2 mu m are often partially or completely dissolved, resulting in hollow shells formation in the microstructure. The bond between matrix and aggregate is very strong and the interfacial zone shows no gaps. However, the pozzolanic reaction of silica fume shows low advancement, because of water shortage caused by very small w/b ratio.

In the past few years, the issue of external alkalis and their influence on ASR in concrete has become more important since several concrete airfield pavements have shown ASR-distress related to the use of alkali-containing airfield deicers based on acetates and formates.Experiments with model pore solutions and cement pastes as well as speciation calculations and ASR performance tests were conducted to investigate possible mechanisms. The obtained results indicate that the solubility of portlandite is increased in the presence of acetate-based and formate-based deicers due to the formation of Ca-acetate and Ca-formate complexes. The additional release of OH− ions from portlandite and the supply of alkalis can initiate and highly accelerate ASR in concretes with reactive aggregates. There is also evidence for a reaction of ettringite with such airfield deicers.

Three different methods were applied to study the alkali content of gelpores in cement. In the closed system, the concentration of K+, Na+ and OH− have not reduced with the increase of age. In the open system, the diffusion and transferring of K+ and Na+ towards free space leads to the decrease of total alkali content. In the micro-analysis system, the contents of K+ and Na+ in the first hydrated layer of ground granulated blastfurnace slag (GBFS) are very low, while the contents of calcium and magnesium are relatively high. This phenomenon shows that the mechanism of GBFS preventing alkali aggregate reaction (AAR) is: when GBFS is dissolved by alkali medium, SiO2 and Al2O3 are dissolved into the cement matrix, then around GBFS particles form reaction rings rich in Ca2+ and Mg2+, and the C-S-H gel of positive charges formed in the area repulses K+ and Na+, which are forced to transfer to the mortar’s matrix, pore or mortar sample surface. The transferred K+ and Na+ form alkali gel products with other dissolved ions, then become evenly distributed in the mortar sample and react with Ca(OH)2 in pore solutions to form (Na,K) x−2z ·zCa·(SiO2)y·(OH)x gel products; and thus changes the AAR gel products’ structure. The gel products will not expand, and so they can delay expansion destruction. Keywordsgranulated blastfurnace slag (GBFS)-alkali aggregate reaction (AAR)-alkali content-preventing mechanism