Itsn't it contrary that addition of silica fume reduces the ASR?
eg. of test report is linked here https://ctr.utexas.edu/wp-content/uploads/pubs/0_4085_1.pdf
Hi
Thanks for asking. This is a valid question, because silica rich aggregates can cause ASR, depending on the cement type. In normal concrete without any solid additives, like silica fumes, will have very high pH. At high pH these silica rich aggregates will partially dissolve into expansive alkali-silica gel. With the fine grained silica fume intermixed with the cement the reaction will differ since silica fume will dissolve and control the pH and lower it beyond the solubility of relatively large aggregates. This in nutshell the main process which makes silica fume so effective against ASR. In this process, other reactions take place like the pozzolainc reactions and binding of alkalis. In my opinion the lowering of pH is the most important one. I hope that this answer your question. Best regard Gísli.
Page 12 of the report you posted gives the explanation. Silica fume will reduce the pH of the pore solution. This will reduce the susceptibility of silica-rich aggregates to undergo ASR. The C-S-H formed by the pozzolanic reaction of silica fume with portlandite (from the cement hydration) will help to immobilize alkalis. The consumption of portlandite will also lower the pH.
In addition to above mentioned comments,
Reduction in permeability of water by the introduction of SF filling pores will increase the resistance against to ASR, especially in alkaline solutions saturated to alkali hydroxides at high temperatures.
Moreover, pessimum reactive aggregate ratio is known in terms of size fraction and amount of reactive aggregate. Thus, SF introduction may result in being out of the pessimum rate of the mixtures.
Basicaly the pozzolanic reaction of silica fume is the same as for other pozzolana. The pozzolanas react with the Ca ( OH )2 from the cement/water reaction and produces more CSH phase. That lower the Ca( OH )2 concentration and at the same time the PH in the pore solution of the concrete. Because of the huge surface area of the silica fume its reaction is much faster and more complete than with other coarser pozzolanas. Then the pozzolana have the property to build in aluminium in the silicium thedraether construction, increasing the Al/Ca ratio in CSH. That increases the CSH-adsorption of alkalis out of the pore solution. All this decreases the PH of the pore solution and also the probability of alkali silica reaction.
Referance:
https://www.impbautest.ch/upload/cms/user/DissertationBuergisser-EinflussvonpuzzolanischenZusatzstoffenvulkansichenUrsprungsaufdieAKRimBeton.pdf
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