Release of Alkalies from Feldspar in Concrete and Mortar


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Title: Release of Alkalies from Feldspar in Concrete and Mortar

Author(s): E. Poulsen, T. S. Hansen, and H. E. Sørensen

Publication: Special Publication

Volume: 192


Appears on pages(s): 807-824

Keywords: alkali-aggregate reactions; chert; expansion; mortars; quartz; silica fume; thickness

Date: 4/1/2000

A necessary condition for crack development in concrete due to alkali-aggregate reaction is that the concrete contains: *Reactive rock types or minerals, e.g. porous, opaline chert. *Alkali metal ions, e.g. sodium ions Na+ and potassium ions K+ *Water, e.g. concrete humidity more than 80% RH. Aggregates with alkali minerals may release alkalies in concrete, since the pH values are often 12-13 in most concrete. If alkalies are released from the aggregates it might increase the risk of getting 'deleterious' alkali relations, when alkali reactive aggregates are resent in the concrete in a sufficient intensity. So far, this question has not been given much attention, when specifications for the maximum alkali content in fresh concert has been made. A test series was carried out in order to subject this question to a critical examination. It was decided to perform two main series o mortar bar test with: *Water/cement-ratio=.40 by mass. *Binder/aggregate-ratio=.42 by mass *Flow of mortar+130 mm and variable parameters: *Chert/aggregate-ratio, which varied from 0 to 10 percentage by volume *As inert aggregate either quartz or feldspar were chosen *Concentration of alkali metal ions in the mortar, which was given the values 3.0 and 6.0 kg of equivalent Na 2O per m3 mortar *In order to study the influence of added silica fume systematically, the ratio of silica fume to binder was chosen as 0,4 and 8 percentage by mass. The following conclusions were drawn from the test series performed: * The maximum expansion increases (at pessimum proportion) when feldspar is used as inert aggregate instead of quartz, other parameters being equal *A delay of expansion is observed when silica fume is added to the mortar *The delay increases with the amount of silica fume added.