Browsing by Subject "kvartsi"

The weathering of concrete by alkali-aggregate reaction is a process of deterioration recognized also in concrete manufactured in Finland. Alkali-aggregate reactions (AAR) are deleterious reactions between the alkali hydroxides (NaOH and KOH) in cement paste and silicon dioxide in aggregate used in concrete. The alkali hydroxide in cement paste will dissolve the crystal structure of silicon dioxide, and the dissolved silicon dioxide hydroxylates to orthosilicic acid. In the presence of impurities, the orthosilicic acid can form covalent bonds and coagulate to form silica gel. The expansion caused by swelling of highly hygroscopic silica gel will lead to excess of yield stress and thus deleterious cracking of concrete. The atomic features of fracture surfaces in natural aggregates will determine their reactivity in concrete. The surface in reactive mineral has unstable equilibrium and has crystallographic defects or insufficient crystal structure. Practically all natural aggregates used in Finnish concrete are heterogeneous mixtures of silicate minerals. The surface features of silicate minerals are composed in various rock forming geological processes. Rocks that are crystallized by slow cooling have more stable equilibrium than rocks that have undergone a low metamorphosis. The geological features and strength measurements are the base of good quality natural aggregate ratings. On the grounds of observations, even the high quality aggregates can cause deleterious alkali-aggregate reactions. It is recommendable to use aggregates that have stable equilibrium state, a low alkalinity cement and additives such as fly ash, blast-furnace slag or silica fume to concrete structures exposed to moisture.