FG Baustoffe und Bauchemie
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- alkali silica reaction (3)
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- Fly ash (2)
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- aggregate reactivity (2)
- dissolution of silica and alumina (2)
- dissolution rate (2)
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Fly ash is a typical mortar constituent in Germany, other European countries and worldwide. If cement alone is used as a binder, then the mortar is too expensive and cannot provide all properties. That`s why many companies have special recipes with fly ash, especially hard coal fly ash components, in the mortar. The aim of this project is to substitute such fly ashes by lignite coal fly ashes, which are cheaper, available at every times or have better properties. However this means, that the calcium free or calcium poor ash should be substituted by calcium rich ashes, which are not certificated because of fluctuations.
To compensate the fluctuations in the ash compositions, several ashes are mixed. The European standard EN 450, which contains requirements, for example for the chemical composition of ashes, has to be taken into account. Fly ash mortar compound prisms have been produced and the different mechanical and deformation values were determined in comparison to a reference material.
The basis for all experiments was the recipe of the company for a special mortar. An ash compound could be developed with the same workability by considering the economic reasons of the company. An important result too is that the fluctuations of the fly ash batches can be controlled by special developed chemical (color test) and physical (Ultrasonic velocity) test methods.
Im Rahmen eines AiF-geförderten Forschungsprojektes wurde ein alumosilicatisches Bindersystem entwickelt, welches nur durch die Zugabe von Wasser zu einem Feststoff mit definierten Eigenschaften erhärtet. Ein wesentlicher Grundgedanke bei
der Auswahl der Ausgangsstoffe war, industrielle Reststoffe zu verwenden, die verfügbar sind und beispielweise nicht über
einen aufwändigen C alcinierprozess hergestellt werden müssen. Diese sollten andererseits aber eine sehr gute Löslichkeit
besitzen, damit sich das hoch alkalische Milieu sofort nach Wasserzugabe einstellt und der alumosilicatische
Strukturbildungsprozess unmittelbar in Gang gesetzt wird.
Unterschiedliche silicatische und aluminatische Ausgangsmaterialien wurden getestet. Es wurde ein Stoffsystem unter Verwendung eines silicatischen Reststoffes aus der Glasproduktion in Kombination mit einer Natriumaluminatkomponente aus der Waschmittelproduktion entwickelt, welches die gestellten Anforderungen gut erfüllt. Dieses Stoffsystem dient gleichzeitig als Modell, an dem der Strukturbildungsprozess, der zur Bildung eines alumosilicatischen Netzwerkes (ASN) führt, z.B. mittels
NMR-Untersuchungen untersucht werden kann. Im Unterschied zu den „klassischen“ Geopolymermaterialien Metakaolin,
Hüttensand bzw. Flugaschen, ist das für den Strukturbildungsprozess entscheidende Verhältnis zwischen Si und Al in einem breiten Spektrum frei einstellbar und damit auch die mit dem Binder erzielbaren Eigenschaften variierbar. An Mörtel- und Betonprismen wurden zerstörungsfrei und zerstörend Festigkeiten und Formänderungen untersucht und
Dauerhaftigkeitskennwerte bestimmt. Hohe mechanische Festigkeiten bis 60 N/mm2 Druckfestigkeit, aber insbesondere der sehr gute Widerstand gegen Säureangriffe machen Mörtel und Betone unter Verwendung des ASN-Binders interessant für
Anwendungen im Bereich der Einwirkungen von aggressiven chemischen Reagenzien. Entsprechende Untersuchungen zum
Säureangriff wurden durchgeführt und die Veränderungen mikroskopisch und mittels Eigenfrenquenzanalyse (Grindosonic)
verfolgt.
Supplementary cementing materials (SCMs) are an important part of modern concrete structures. Their use has many advantages (reduction of the cement amount, improvement of the durability and therefore CO2 reduction). For example certain SCMs added to concrete mixtures can avoid the alkali-silica reaction (ASR). But such materials have a wide range of composition and therefore the mechanisms can be very differently. The knowledge of dissolution processes of aggregates and SCMs separate and together in alkaline solutions can help to understand the damage of concretes caused by ASR and the effects to avoid ASR in more details. Many dissolution experiments were performed in highly alkaline solutions (KOH) under CPT conditions (pH 14, 40°C) using aggregates in original grain sizes and Si and Al containing SCMs in different ratios. Also Calcium hydroxide was added to investigate the influence of calcium ions as a main part of the pore solution. The concentrations of soluble silica and alumina were determined by ICP-OES. Such dissolution experiments, with which an interaction was settled between the aggregate and the SCM via the alkaline solution, arose that under specific conditions the dissolution of aggregates was strongly influenced. Obviously both main constituents of concretes interact with each other. Additionally effects of the dissolved calcium ions must be considered. It was found that the efficiency depends on the amount and chemical composition of the SCMs used and on the reactivity of aggregates.
The basis of the primary energy supply in the Czech Republic is mainly lignite combustion. Power plants are for example Chvaletice, Melnik, Pocerady, Porici, Prunerov, Ledvice, Tusimice, Tisova and Hodonin [1,2]. Accordingly, there is a large amount of fly ash with its latent hydraulic and puzzolanic properties, depending on the composition. In terms of sustainability, such industrial residues offer huge potential for conversation of resources and often connected with it, a CO2-reduction. Especially lime-rich fly ashes offer a variety of possibilities in the high quality production of self-hardening loam-bricks, having an improved durability. Different clays/loams were mixed with fly ashes (10 to 30 wt-%) and homogenized by addition of water. Out of these mixtures, samples were moulded in different dimensions by using an extruding press. After curing under different humidity conditions at room temperature, the samples were tested for their strength and their durability. The investigations showed, that the choice of the starting materials has significant impact to the workability and the development of the strength. Furthermore, there is an optimal amount for the ash in the recipes to prevent a rapid destruction by capillary water absorption. The modified clay bricks could make a decisive contribution to the improvement of weather resistance e.g. in tropical areas with sudden rain showers.
Alumino silicate compounds are important for alternative binders for concretes because of the reduction of CO2 and resource conservation. Such multi-phase systems consist of a solid component, such as e.g. slag, ash, calcined clays and others, and a liquid component, in generally highly concentrated alkaline solutions (water glass or NaOH/KOH). Because of safety and other reasons, such systems exclude large-scale practical applications.
The newly developed alternative binder consists of a highly soluble silicate waste material, which is not common in geopolymere production, and a powdered Al-provider. It is called alumino silicate network (ASN) binder. Both components can be mixed dry in different ratios. Only after addition of water, a polymerization process is induced and the formation of amorphous phases, and sometimes zeolites can be observed. Mortar bars were produced and tested for different properties. The best results regarding to workability, strength development and sustainability were obtained with a molar Si/Al ratio of 2:1 in combination with fine-grained rock filler components. Investigations on the hardening process showed, that the mechanical properties varied over a wide range. Strength reserves, a ratio between compressive and tensile strength of approx. 3 and so far good resistance against acid solutions allow the use as a binder in high performance mortar and concrete structures.
Different aggregates stored in alkaline solutions have different solubility of silica and alumina. The dissolved silica reacts with water and alkalis to form an alkali silica gel, which can swell. ASR swelling can be reduced or prevented, if enough space is available for the alkali silica gel formation. This “space” is the open porosity, which can also be determined directly at the aggregate grains.
Concrete specimens made with different aggregates were produced. They all were stored in a fog chamber at 104°F (40°C) and the ASR swelling was measured for one year. At the same time, the aggregates were stored in 0.1 mol KOH solution at 176°F (80°C) for 56 d only. The solubility of silica and alumina was determined continuously by ICP-OES.
The obtained dissolution rates, open porosity and expansion data were compared and a reliable model for predicting the ASR sensitivity of concrete aggregates was derived.
Changes of pore solution composition under accelerated mortar bar and concrete prism test conditions
(2012)
Accelerated tests of mortar bars and concrete prism were carried out at temperatures ≥ 40°C in order to reduce the test duration. Because the OH-/SO42- equilibrium is shifted to lower values with increasing
temperature and alkali content of the cement, there exists the possibility of side reactions between sulphate ions from the cement and aluminate ions which can come from mineral admixtures and aggregates. Solubility measurements of two types of aggregates in sulphate free KOH solutions indicate that the temperature effect on the silica releasing rate is bigger than the inhibiting effect of the decreasing OH- concentration. To avoid side reactions during aggregate test procedure, expansion measurements should be performed at 40°C in combination with adapted solubility measurements.
Mortar bar and concrete prism tests are the most commonly used methods for the evaluation of alkali-silica reactivity (ASR). Despite their widespread use, both methods have significant drawbacks. The largest issue is that neither test is a direct measurement of the aggregate. Matrix effects from the mortar or
concrete, or variations in the individual non-aggregate materials can influence the outcome. This paper describes fundamental results as the basis for the chemical and physical test method (BTU-SP test) directly carried out at aggregate grains. Additional methods (BET, microscopy) confirm that not only the aluminate from the aggregate controls the silica release but also the pore structure inside of rocks (i.e. voids, cracks) buffer the swelling potential of the formed alkali-silica gel. As a result a reaction model describing the interactions of chemical and physical parameters in 4 phases has been developed for an alkalisilica reaction.