Petrographic Examination of Carbonate Aggregate Prism Specimens Treated with Chemical Solutions Normally Occurring in Portland Cement Concrete

About the report:

The original rock prism specimens were prepared and the early length change data were taken as part of a Federal Highway Administration project on the behavior of carbonate aggregates in concrete. It had been shown that the length changes of the aggregates bore a direct relationship to the pH of the solution in which they were tested. The present study shows that the chemical reactions causing the length changes can be detected in the prisms by quantitative measurements of the mineral composition by means of X-ray diffraction and by examination of thin sections under a petrographic microscope. It is concluded that chemical solutions of pH over 12 can cause expansion and dedolomitization in carbonate rocks susceptible to alkali reactions and that chemical solutions of low pH may cause dissolution of carbonate rocks and shrinkage. It is noted that detailed, statistically defensible conclusions regarding the exact nature of the reactions occurring between the chemicals of concrete paste and the carbonate aggregates therein will probably require that data be obtained from replicate reactions and that weight changes as well as length changes be measured.

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