Microstructural and chemical characterization of glass powder and silica fume ternary concrete after seven years of alkali-silica reaction expansion exposure

It is recognized that the combination of glass powder (GP) with other supplementary cementitious materials (SCMs) has a synergistic effect in controlling ASR expansion, as GP alone at OPC replacement levels of 20 and 30 % is not sufficient to limit expansion below the normative threshold of 0.040 % in two years. However, the mechanisms behind this synergy are not well understood, particularly concerning the prevention of late alkali release from GP. To investigate this, a protocol designed for the analysis of alkali-rich materials was applied to GP/silica fume ternary concrete samples stored for over seven years under concrete prism expansion test (CPT) conditions, using microprobe microstructural and chemical characterization, together with elemental mapping of solid phases (C-S-H, ASR products and GP particles). The results indicate that a significant proportion of the GP does not fully react but does alter the Na and Si content in the hydrates. Furthermore, doubling the SF content in the mixture tripled the Na/Si ratio in the C-S-H for a similar GP content, while tripling the GP content had minimal effect when keeping the SF content constant. A portlandite barrier limits GP dissolution at lower dosages. The GP reactivity is critical; faster dissolution promotes early alkali uptake in C-S-H with a Na/K incorporation ratio of 3:1. The results confirm the importance of combining GP with a highly reactive SCM, such as SF, to optimize alkali binding in the C-S-H structure. • GP particles of varying sizes remained unreacted in concretes aged over seven years. • High GP dosages and NaOH required addition in CPT specimens reduce GP dissolution. • Na is mainly uptake in C-S-H over K but both are equally present in ASR products. • Doubling SF raised Na/Si in C-S-H; tripling GP had minimal proportional effect. • Balanced GP/SF ratios optimized alkali binding in C-S-H structures.