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Title: Reactive Phases in Alkali Carbonate Reaction Reference Aggregate

Author(s): Mengesha Beyene, Jose Munoz, Richard Meininger, and Anant Shastry

Publication: Materials Journal

Volume: 117

Issue: 6

Appears on pages(s): 281-292

Keywords: acid-insoluble residues (AIR); alkali-carbonate reaction (ACR); alkali-silica reaction (ASR); ASR gel; cryptocrystalline quartz; dolomitic limestone; microcrystalline quartz; scanning electron microscopyenergy- dispersive spectroscopy; quantitative image

Date: 11/1/2020

Abstract:
The reference Pittsburg Ontario alkali carbonate-reactive (ACR) aggregate source was characterized using a holistic approach to identify and quantify mineral phases, particularly reactive forms of silica and expansive types of clays in the aggregate which may have a role in the controversial ACR mechanism and resulting expansion and cracking of concrete. This research was performed using state-of-the-art analytical techniques and methods that included polarized light microscopy (PLM), quantitative image analysis (QIA) of backscattered electron (BSE) scanning electron microscope (SEM) images, acid-insoluble residue (AIR) tests, X-ray diffraction (XRD), X-ray fluorescence (XRF), and a chemical ratio method of identifying alkali-reactive carbonate rocks. Three types of silica phases were identified through PLM examination: upper silt-sized quartz grains both in the virgin aggregate and acid insoluble residue (AIR); cryptocrystalline silica dispersed and hidden in the fine-grained rock matrix and identified only in the AIR; and cryptocrystalline-to-microcrystalline silica occupying interstitial spaces of dolomitic limestone particles which lacked clay in their matrix. PLM findings were confirmed through QIA of the AIR. Particle size distribution of silica phases through QIA showed that silica phases in sizes of 0.5 to 2 μm (0.00002 to 0.00008 in.) occur in high abundance. QIA of AIR identified illite as the major clay mineral in the aggregate. While this clay type is not known to be expansive, microcrystalline to cryptocrystalline silica phases are potentially alkali-silica reactive (ASR) in concrete as opposed to ACR.


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