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Effect of different ions on dissolution rates of silica and feldspars at high pH

Mahsa Bagheri, Barbara Lothenbach, Karen Scrivener, Mahdieh Shakoorioskooie

The dissolution kinetics of silica-containing minerals in aggregates influences strongly the process of ASR in concrete. In this paper, the effect of different ions on the dissolution rates of SiO2 (amorphous and quartz) and feldspars at high pH values was studied by following the increase of silicon concentrations in dissolution experiments and with a novel approach of measuring the evolution of scratches of polished surfaces. The second method avoided the problem of precipitation in some systems, such as the formation of C-S-H when calcium was present and lithium silicates in the presence of lithium. At high pH values, lithium, calcium and sulfate increased the dissolution rates of silica and feldspars, while iron, magnesium and additional NaCl, KCl or CsCl showed no significant effect. In contrast, aluminium slowed down significantly the dissolution rates of quartz, amorphous silica and Na and K-feldspar at for all temperatures studies: 20, 40 and 60 degrees C.

PERGAMON-ELSEVIER SCIENCE LTD2022

The effect of paste composition, aggregate mineralogy and temperature on the pore solution composition and the extent of ASR expansion

Mahsa Bagheri, Barbara Lothenbach, Karen Scrivener

The reaction kinetics of the alkali silica reaction depends on the composition of the pore solution. The evolution of the pore solution composition in different cement pastes and concretes was studied. Pastes containing silica fume or metakaolin had the lowest amount of alkalis in the pore solution. In addition, metakaolin increased the aluminium concentrations. The lowest expansion was measured for the concretes made of blended cement pastes with low alkali and hydroxide content in their pore solution, for the duration of the present study, no additional aluminium effect was observed due to the already low pH. Addition of 400 mM of Li slowed down expansion rate of concrete prisms at 40 and 60 degrees C, however, similar expansion was observed for samples with and without Li at 60 degrees C after 1 year. Temperature, alkali concentration and pH of pore solution all have a major effect on ASR expansion.

SPRINGER2022

Aggregate dissolution in different pore solutions

Mahsa Bagheri

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.

EPFL2021