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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.
