The alkali-silica reaction (ASR) poses a serious threat to the durability of concrete structures. Although the mechanism of this reaction was described as early as the 1940s, assessing the potential alkali reactivity of aggregates is still a research and engineering challenge. This article discusses the classification of the alkali reactivity of fine natural aggregates based on the results obtained by methods PB/1/18 (accelerated) and PB/2/18 (long-term), developed in accordance with the guidelines of the General Directorate for National Roads and Motorways. In the experimental part, the authors analysed fine natural aggregates previously used in concrete structures that showed no signs of damage due to ASR. The study confirmed the presence of reactive forms of silica, including deformed and polycrystalline quartz. Significant differences in alkali reactivity classification results were shown depending on the method used, confirming the limitations of the accelerated method and the possibility of erroneous results. It emphasised the need for careful interpretation of short-term test results and the need for further refinement of aggregate reactivity assessment methods to achieve better correlation with long-term methods, which more accurately reflect the service conditions of concrete.