Expansive mineral growth and concrete deterioration: a microstructural and microanalytical study

Abstract

<p>In order to evaluate the role of newly-formed minerals in premature deterioration of highway concrete, a three-phase study was undertaken. In the first phase, petrographic and SEM/EDAX analyses were performed to determine chemical and mineralogical changes in the aggregate and cement paste of samples taken from Iowa highways that showed premature deterioration. In the second phase, experimental simulations of environmental changes in highway concrete after applying different deicer chemicals were conducted to evaluate the role of deicers in premature deterioration. In the third phase, experiments were done to evaluate whether crystallization inhibitors can reduce damage and the growth of secondary minerals in concrete and to help understand the mechanism of deterioration by secondary mineral growth in concrete;In the first phase of study, it was evidenced that two major expansive minerals, ettringite and brucite, were responsible for premature deterioration. Severe expansion cracking of cement paste was often associated with ettringite locations, and strongly suggests that secondary ettringite was a major cause. Brucite forms in cement paste of concretes containing reactive dolomite aggregate via dedolomitization reactions. No cracking was observed to be spatially associated with brucite, but expansion stresses associated with its growth at innumerable microlocations might be relieved by cracking at weaker locations in the concrete;Deicer salts cause characteristic concrete deterioration by altering dedolomitization rims at the coarse-aggregate paste interface, altering cement paste and/or forming new secondary minerals. Magnesium in deicer solutions caused the most severe paste deterioration by forming non-cementitious magnesium silicate hydrate and brucite. Chloride in deicer solutions promotes decalcification of paste. CMA and Mg-acetate produced the most deleterious effects on concrete, with Ca-acetate being much less aggressive. In order to use CMA as an alternative deicer and to prevent premature deterioration, it is recommended that it possess a high Ca/Mg ratio;Three types of commercially inhibitor chemicals, polyphosphonate, polyacrylate, and phosphate ester, were effective in reducing the formation of ettringite and also in reducing concrete expansion due to ettringite. Phosphonate inhibitors are the most effective among those inhibitors. These inhibitors are not effective in preventing formation of brucite and MSH from CMA and magnesium acetate solution.</p>