Comparison of Accelerated Mortar Bar Tests for Evaluating Alkali–Silica Reactivity of Reactive vs. Non-Reactive Siltstone Aggregates: Case Study from the Qinghai–Tibet Plateau

The accurate identification of the degree of alkali reactivity of aggregates is crucial for preventing alkali–silica reaction (ASR) damage in concrete. Two siltstones were collected from the main stream and a tributary of the Yarlung Tsangpo River. The alkali reactivity of these siltstones was examined through a combination of the Petrographic Method, the Accelerated Mortar Bar Test (AMBT), and the Chinese Universal Accelerated Mortar Bar Test (CAMBT). The influence of the petrographic characteristics of siltstones on the applicability of two accelerated expansion tests was also evaluated. Results show that both siltstones exhibit blocky textures and contain cryptocrystalline–microcrystalline quartz. However, the major alkali reactive components in the two siltstones show significant differences in morphology, distribution and content, with cryptocrystalline–microcrystalline quartz contents of approximately 20% in FS-1 and 10% in FS-2. This study reveals that the CAMBT identified two siltstones as alkali-reactive, whereas the standard AMBT classified them as non-reactive. The petrographic structures of the siltstones and the microstructural distribution of reactive quartz constituents are the key factors governing their expansion behavior and the applicability of different testing methods. By employing a single-graded aggregate that preserves the original rock fabric, the CAMBT improves the reliability of the detection of alkali reactivity in siltstones.