Alkali Silica Reaction in Mortar at Room Temperature

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Title: Alkali Silica Reaction in Mortar at Room Temperature

Author(s): Qiang Li, R. James Kirkpatrick, Leslie J. Struble

Publication: Special Publication

Volume: 312

Issue:

Appears on pages(s): 1-20

Keywords: Alkali-Silica Reaction, ASTM C1260, Nuclear Magnetic Resonance, C-S-H polymerization, SEM-EDX, X-Ray Diffraction

Date: 10/1/2016

Abstract:
Mortar bars made with silica glass aggregate were tested at 23°C (73°F) to evaluate the applicability of a previously proposed chemical model for the alkali silica reaction (ASR). The model, based on tests at 80°C (176°F), proposes that ASR gel does not form until portlandite (CH) in the hydrated paste is locally depleted and the calcium silicate hydrate (C-S-H) has been locally converted to a more highly polymerized and lower Ca/Si form. SEM-EDX, XRD, and 29Si NMR spectroscopy of the 23°C (73°F) mortars show that the same chemical processes operate at both temperatures. At 23°C (73°F) and up to 60 days, only a small amount (~1%) of ASR gel forms and is confined to cracks entirely within the aggregate grains, but this small amount of gel containing Na, K, and Ca is sufficient to cause substantial expansion. There is no large-scale depletion of CH or increase in the C-S-H polymerization in the paste due to the small amount of gel formed and its confinement in the aggregate grains. Local reduction in both the amount of CH and the Ca/Si ratio of C-S-H in the paste is observed near places where gel-filled cracks in the aggregate contact paste, consistent with the proposed chemical model.