(2020) Quantitative Analysis of Free Mica in Sand and Hardened Concrete Blocks by SEM-EDS
O'Connor L & Goodhue R
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08r: Plenary Hall, Friday 26th June 07:00 - 07:03
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Could tell us exactly the role of mica in the damaging process or in the formation of cracks in concrete?
When mica occurs as discrete (or 'free') flaky grains in fine aggregates, it usually increases the water demand of concrete and also reduces the cohesiveness of the mix, which can adversely affect the final strength and durability of the hardened concrete - this is described in Leas Chemistry of Cement and Concrete. Published technical research on the topics surrounding the implications for concrete made with high levels of muscovite bearing aggregate, provide a plausible concrete block deterioration mechanism: - Aggregate and fines with high levels of mica in it, attracts a disproportionate amount of water compared to other aggregate. - This leads to a reduction in workability and strength of the concrete. - Susceptibility to loss of integrity in freeze thaw conditions, mica rich aggregates are frost susceptible resulting in deterioration and loss of strength during a thaw. - Deleterious effects depend more on the nature of the micaceous particles than on their quantity, muscovite caused profound changes to concrete making properties whereas biotite only caused small effects.
Do you think that the main contribution to the standard deviation (for example in table 6) is the analytical method or the inhomogeneity of the material (concrete in that case)? How many sampling (thus thin sections) could be needed in practical applications to ensure that the result is not affected by local inhomogeneity?
The inhomogeneous nature of concrete blocks is a complicated matrix to analyse and is the main contributing factor to the standard deviation factor. Averaging twelve of more fields of view at 1200x magnification from the same thin section, we were able to quantify the percentage of free mica among both the total fines and the cement matrix in the fields of view tested. The size of the area tested by SEM-EDS is small relative to the concrete block area. A 1SD (standard deviation) graph was prepared for each batch of results and demonstrates averaging twelve fields of view in one thin section ensured the results were not affected by local inhomogeneity.
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