FG Baustoffe und Bauchemie
Refine
Year of publication
Document Type
Language
- English (29)
- German (21)
- Chinesisch (2)
Keywords
- alkali silica reaction (3)
- Alkali-Kieselsäure-Reaktion (2)
- CO2-reduction (2)
- Fly ash (2)
- Resource conservation (2)
- Waste materials (2)
- aggregate reactivity (2)
- dissolution of silica and alumina (2)
- dissolution rate (2)
- open porosity (2)
Institute
Knowledge's on the dissolution behaviour of SiO2-containing aggregates are the basis for understanding damaging processes regarding ASR. Because of many influences, the solubility of aggregates has still needs to be determined by dissolution experiments. This article is about the reliably modeling of silica dissolution of relatively pure SiO2 containing aggregates in alkaline solutions. Therefore, quartz, quartzite, opal sandstone and flint with different silica modifications were chosen. A kinetic model based on the surface area was derived and numerous thermodynamic data from different authors were evaluated.
The surface area of the aggregate grains were determined by an adapted BET method. For this measurement, the original grain sizes were used as they are also used so in concrete. These aggregates were stored in 0.1 mol and 1.0 mol KOH solution at 40°C and 80°C respectively. The dissolution experiments too were performed with grains in the original particle sizes. The concentration of silica in the liquid phase was measured by ICP-OES.
The model based on a comparison of calculated and experimental determined silica concentrations, because some parameters of the differential equations are still unknown. The model includes the
temperature and molarity dependence of the silica dissolution. The model also takes into account the influences of the SiO2 structures and can vary between crystalline and amorphous SiO2 in a wide range.
Thermodynamic data suitable for modeling are recommended.
In future, these dissolution calculations shall be extended to other non-pure quartzitic aggregates. Instead of time-consuming ASR concrete prism tests the silica dissolution rates may be used then to indicate quickly the ASR sensitivity of aggregates. Of course, it will be possible to distinguish between innocuous, medium and highly reactive grains.
Reactive aggregates, humidity and alkalis are necessary for ASR in concrete structures. If alkalis come additionally from outside, the damaging reaction can be strongly accelerated. However, the reasons therefore are not completely understood. Some researchers discuss a direct attack of alkali ions on the Si-O-Si bonds of quartz structures. This paper provides another point of view to understand this phenomenon.
Different aggregates (grain size 2-8mm) were stored in a highly alkaline KOH solution. Additionally to this solution, sodium chloride with different concentrations was added. The aggregates were selected on the one hand according to their reaction behavior in concrete structures, on the other hand to their dissolution behavior. Long-term dissolution experiments were performed over several months. The silica, the aluminum and of course the sodium concentrations were determined. It can be expected, that under the influence of sodium the silica concentrations in the alkaline solution raise up. The question here is: What happens with the aluminum concentration at the same time? Very surprisingly was, that Al reaches a maximum after some days and then it decreases. Exactly from this time when the Al concentration decreases the silica concentration raises up extremely. The reason therefore can only be a precipitation process. Obviously, there is a correlation between the aluminum content in the solution and the extreme silica release of aggregates, both affected by alkali ions from outside.
Aggregates, which do not release aluminum because of their composition (e.g. quartzite), do not show this behavior. The conclusion is that such aggregates are not so sensitive against alkali attack from outside.
The silica solubility of aggregates is one of the most important components of the alkali-silica reaction.
It is a surface-controlled process that always still requires more detailed studies to better understand the reaction mechanism.
Since strained quartz releases more SiO2 into the pore solution, the properties of grains, crystals and their structure can should be directly quantified. In other work, various possibilities were tested for this purpose in order to obtain analyses of the surface and to correlate these with the mortar bar tests, for example. However, a quantifiable direct measurement of quartz crystal states with satisfactory results has not yet been performed.
In this thesis polarization and reflected light microscopy in combination with Raman and confocal microscopy is used to obtain quantifiable data by direct measurement of the strained crystals.
First measurements show new surprising signals besides the Raman main peak of the quartz. Such signals cannot be found on the whole sample, but only at places where strains are expected, e.g. at contact zones between different quartz crystals or cracks and sometimes inside of quartz grains too.
Thus, a method may have been found to quantify the strained state of different quartz crystals in natural quartzite rocks.
The durability of concrete structures due to alkali-silica-reaction (ASR) is usually been assessed by ASR concrete prism tests (CPT). Therefore, the expansion of concrete specimens indicates an alkalisensitivity.
In Germany these tests are performed at 40°C or accelerated 60°C. Nevertheless, ASR prism tests are expensive, power and time-consuming. For these reasons, an alternative chemical test ("mod. BTU-test") was developed in the past. The "mod. BTU-test" was correlated with German standard CPT.
In this test, the solubility of silica and alumina in the liquid phase is measured by ICP-OES.
For special requirements in concrete road construction, stricter test procedures are necessary. The specimens are repeatedly subjected to cyclic alternating storage in NaCl. On the one hand, the presence of NaCl increases the ASR expansion, on the other hand NaCl also changes the dissolution behaviour of silica and alumina.
For now is not possible to correlate the solubilities of mod. BTU-test and ASR expansions influenced by NaCl. Therefore, four aggregates with different ASR sensitivities were chosen and ASR concrete prism tests (in addition with NaCl) have been performed. According to the "mod. BTU-test" the solubilities of silica and alumina in presence of different NaCl concentrations were measured.
This paper is about the regression analysis of silica and alumina dissolution and ASR expansion tests in presence of NaCl. The regression analysis shows the influence of a quasi-continuous and an interrupted cyclic alternating storage. Additionally the effects of temperature and NaCl concentration are investigated. In the end, a suitable NaCl concentration of the "mod. BTU-test" for dissolution experiments is recommended. The "mod. BTU-Test with NaCl might be a serious and reliable test method for ASR classification for concrete road construction.
While calcined clays in general have been credited with a great potential to mitigate CO2 emissions related to cement production and consumption, calcined brick clays are currently understudied in this
regard. In the present work, two brick clays, a low-grade kaolinitic clay, and a mixed clay composed of 50% brick clay and 50% low-grade kaolinitic clay were studied regarding transformations on calcination,
and strength and durability performance as well as pore structure of mortars made with the blended cements. All calcined clays exhibited pozzolanic reactivity, with the performance of the brick clays inferior to the low-grade kaolinitic clay. However, the mixed clay performed very similar to the low-grade kaolinitic clay, which points to a viable option for optimal use of brick clays in cementitious systems. The carbonation resistance of the blended cement mortars was generally worse than that of the plain Portland cement mortar, as expected, but the former exhibited a significantly improved chloride penetration resistance. The latter improvement was due to pore structure refinement in the blended cement mortars, compared to the Portland cement mortar.
Enrichment of aluminium in the near-surface region of natural quarzite rock after aluminium exposure
(2021)
Alkali–silica reaction (ASR) is an ongoing problem that causes damage to concrete constructions and reduces their durability. Therefore, minimizing this undesired reaction is of great interest for both safety and economic reasons. Additives containing high aluminium content are very effective in reducing the release of silica
and enhancing the durability of concrete; however, the mechanism for this effect is still under discussion. In this study, an enrichment of aluminium in the near-surface region was observed for natural quartzite rock after storage in Al (OH)3 and metakaolin as aluminium sources, from which we conclude that the formation of aluminosilicate sheets of a few nanometres inhibits the silica release; this hypothesis is supported by high-resolution spectra of Al 2p, Si 2p and O 1s.
Loam is a very ecological building material with a great potential. It is found worldwide and completely recyclable. Under dry conditions, loam develops high strength values. However, loam is not moisture-resistant. Permanently acting moisture reduces the strength dramatically. The idea to improve the water resistance of loams is adding materials to the loam with the same basic structure. Therefore, Metakaolin, Calcined Clay, here so called Metaclay and a specially developed Geopolymer were selected. Blends of 4 different loams with different amounts of these additives were produced and tested. Criteria for an evaluation are the dynamical modulus of elasticity and the water resistance. These studies were supplemented by structural investigations using a light and a scanning electron microscope and XRD. The results are very interesting and the effects depends strongly on the kind of loam too. Not all additives lead to an improving of the mechanical properties. Nevertheless, not the samples with the highest mechanical values show the best water resistance behavior. Obviously, a balanced structure between loam and additive particles is necessarily. Such structures are not so dense but enough resistant to water to guarantee the positive property of fast water absorption and delivery of natural loams. The service lives of the loam prisms could be increased from certain minutes to several days. Best results are obtained with geopolymer based materials as an additive. This is not so surprising because both the loam and the geopolymer form alumino silicate structures during hardening.
Calcined Clays can be used in mortar and concretes as a part of a binder (LC3) or as a Supplementary Cementing Material (SCM). The aim here is to sub-stitute fly ash as an additive in concrete recipes. A stable mixture between two regionally available clays, burnt together with a certain ratio of 60 : 40 wt.-% at 650-680°C, could be produced. Such Metaclays produced in larger amounts were investigated in different mortar and concrete mixtures. The production indicates an important problem with the workability of the fresh concrete mixtures. That`s why different superplastizers were tested. It could be found that especially a mixture between a PCE- based material and a special additive, developed for loam sands provides very good results. The workability increases from less than 200 mm slump on a value of about 300 mm. The combination of calcined clay materials and special developed superplastizer mixtures allows producing concrete with very different properties. It can be a closed system for the production of durable concrete structures.
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.