Aggregate dissolution in different pore solutions

The occurrence of cracking due to alkali silica reaction (ASR) affects the service life of concrete structures. The dissolution of silica-containing minerals within aggregates causes the formation of ASR products, which fill macroscopic cracks and cause the opening of additional cracks. Three ASR-reactive aggregates from Switzerland were studied to find the chemical composition and crystalline structures of the dissolvable minerals. The ASR reactivity of these Swiss aggregates was assessed using the conventional expansion tests and the dissolution experiments. Quartz (possibly with inclusions of amorphous SiO2), K-feldspar (KAlSi3O8) and Na-feldspar (NaAlSi3O8) were observed to be the most reactive minerals within the studied aggregates. Although similar reactivity trends were observed from the results of expansion and dissolution tests, the results of these approaches cannot be simply correlated. The effect of different factors on dissolution kinetics was studied for the simple model systems. Different approaches were used to measure the extent of silica and feldspars dissolution; directly with the scratch-tracking method, as a new method, with the observation of solid surface and indirectly by measuring the amount of released Si from each solid. The formation of new solids, such as C-S-H and lithium silicates, means that the dissolution rates measured by the released Si may be misleading. An increase in dissolution rates of silica and feldspars was observed in the presence of lithium, calcium and sulfate. However, no noticeable effects on dissolution rates were measured in the presence of iron, magnesium and additional NaCl, KCl or CsCl at high pH. Among all studied elements, aluminium is the only element slowing down dissolution rates of quartz, amorphous silica and Na and K-feldspar at 20, 40 and 60 °C. The effect of paste composition was studied on the pore solution composition over 17 months and on the ASR expansion. Pastes, containing metakaolin, had the lowest amount of alkalis and the highest amount of aluminium in the pore solution. Significant effect of aggregate mineralogy to take up alkalis from the pore solution composition was observed for the feldspar-rich aggre-gate, Bend aggregate. The lowest ASR expansion was measured for the concrete samples made of blended cement pastes mainly due to having low alkali and pH in the pore solutions.