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Abstract. Damage and defects observed in concrete elements, such as a network of microcracks, popouts and eflorrescence can be caused by a variety of deleterious processes. The causes can include mechanical (overloading), physical (freeze-thaw cycle) or chemical exposure (sulphate corrosion, alkali-aggregate reaction). This paper analyses distress due to alkali-silica reaction, detected in selected concrete structures. The analysed concrete elements exhibited cracking, exudations and surface popouts. Identification of the presence of hydrated sodium-potassiumcalcium silicate gel can be considered the primary symptom suggestive of an alkali-silica reaction attack. Other damage-causing mechanisms can occur simultaneously.

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Abstract. Alkali-silica reaction (ASR) is a reaction between amorphous or poorly crystallized siliceous phase, present in aggregates, and sodium and potassium hydroxides in the pore solution of concrete. Chemical admixtures such as lithium compounds are known to have high potential of inhibiting ASR. The aim of this study was to determine the effect of lithium nitrate on ASR in mortars containing high reactive opal aggregate over a long period of time. Mortar bar expansion tests were performed and microstructures of mortar bars were observed by scanning electron microscopy coupled with an energy dispersive X-ray microanalyser. Results from this study showed that effectiveness of lithium nitrate in mitigating ASR was limited over a long period of time. A larger amount of ASR gel which was formed in the presence of lithium nitrate indicated that the deterioration processes intensify within longer periods of time, which so far has not been observed in literature. Microscopic observation confirmed the presence of alkali-silica gel and delayed ettringite in mortars with lithium nitrate.Key words: alkali-silica reaction, lithium nitrate, ettringite formation, expansion, SEM/EDS. Effect of lithium nitrate on the reaction between opal aggregateand sodium and potassium hydroxides in concrete over a long period of time J. ZAPAŁA-SŁAWETA * and Z. OWSIAK Kielce University of Technology eses have been put forward to explain the role of lithium ions in the reaction. These include, but are not limited to, a hypothesis of reduced silica reactivity caused by the formation of a layer of lithium silicate crystallites or lithium ion-rich amorphous products on the silica surface. This layer on the surface of the aggregate prevents the reaction of silica with sodium and potassium hydroxides [12,13]. Recent years have brought more reports on the modified composition and properties of sodium-potassium-calcium silicate gels with lithium ions. Sodium and potassium ions in gels are exchanged for lithium ions, which makes them less swellable [14,15]. Gels with lithium ions are more compact, which limits the diffusion of hydroxyl, sodium and potassium ions into reactive grains [16]. Studies of the alkali-aggregate reaction have often indicated the presence of delayed ettringite [17,18]. The coexistence of aggregate reaction products with ettringite has been widely observed, especially in concrete railway sleepers in Finland, Sweden, Australia, Germany, South Africa, United States and China [19][20][21][22][23]. According to the current state of knowledge, the delayed ettringite crystallizes in the microcracks generated by the aggregate reaction with sodium and potassium hydroxides, but is not the primary source of the cracks [24,25]. Ettringite is not produced in strongly alkaline environments. However, when the concentration of sodium and potassium hydroxides decreases as a result of reacting with reactive aggregates, the conditions for its crystallization become favourable [26,27].Data on the simultaneous effects of lithium ions on alkali...