Undispersed Granulated Silica Fume in Concrete - Chemical System and Durability Problems
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Silica fume is a by-product from the production of silicon or silicon alloys. When it comes from the electric furnace it is a gaseous suboxide which in contact with air oxidizes and condenses to very small particles of amorphous silica (SiO2). Originally, silica fume is very light and thus difficult to handle. While the compact 3 density of CSF is around 2.2 g/cm 3 the bulk density of powder is only around 0.2 g/cm . This makes it difficult to handle the material. Thus, the CSF is sold either in a water-based slurry or in a granulated form as small pellets. The slurry is difficult and expensive to handle. Thus financial and technological considerations often make the granulate form preferable. The CSF can be granulated to different density and sizes. The granules also show a tendency to become denser with time. The different granulated CSF available in Sweden that we have tested have a bulk density from around 0.4 to 0.8 g/cm 3 and a maximum size of around 2 millimetre. Larger density and size makes the granulated CSF more difficult to disperse. Granulated condensed silica fume (CSF) is not easily dispersed. The granules do not disperse themselves. A series of tests with different mixing sequences show that a certain mixing order is needed. Superplasticizers may help but are not essential. The data shows that mechanical crushing is the most important factor. The fine fraction of the aggregate must be larger than the size of the granule to achieve a good crushing. The tests indicated that the smallest aggregate grain size should be twice that of the CSF granulates. The cement and water should always be added late. The conditions for dispersion is discussed in Lagerblad and Utkin[1 ]• 89 Mat. Res. Soc. Symp. Proc. Vol. 370 @1995 Materials Research Society
DURABILITY INVESTIGATIONS To find out if the undispersed CSF granules may cause durability problems, concrete samples containing different amounts of CSF and different degrees of dispersion were tested by accelerated alkali-silica reaction (ASR) and freeze/thaw (F/T) tests. In the experiments a whole series of different concretes (Tab I) were prepared. Concrete mixes 1 to 10 (series 1)were prepared in a 250 litre paddle mixer while samples 15 to 20 (series 2) were prepared in a smaller 50 litre paddle mixer. The mixer was kept running while the ingredients were put into it during a short time period. Samples 15 to 20 were deliberately badly mixed. Before testing all the concrete samples were water cured for at least 28 days. All concretes contain 360 kg of binder (cement + CSF), 1000 kg coarse aggregate (8-16 mm) and 920 kg fine aggregate (0-8 mm). A high alkali (Na20 eqv = 1,1 wt. %) ASTM type I cement was used in all mixes except nr 18 in which a low alkali (Na20 eqv < 0.6 wt. %) ASTM type V (sulphate resistant) cement was used. The aggregates were granitoid of glaciofluvial origin. They are not alkali-reactive. An air-entraining agent was used in all mixes. In mixes 9 and 10 a slurry (50 % water) was used. Mixes 5, 6, 15, 16, 19 and 20 contained a lignosulphona
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