The role of magnesium in concrete deterioration

This dissertation has been written in two parts: the chemical characteristics of Iowa highway concrete containing coarse dolomite aggregate, and an experimental study of deterioration of Iowa highway concrete;Two groups of Iowa highway concretes studied here can be differentiated: durable concretes with extended service lives of \u3e40 years and non-durable concretes with short service lives of 8-12 years. Compared to durable concretes, non-durable concretes are far more deteriorated, contain more cracks or micro-cracks, and exhibit more abundant and better developed reaction rims at dolomite aggregate - cement paste interfaces. Both petrographic and SEM studies reveal that the reactions responsible for reaction rim development are dolomite-specific. Reactive dolomite aggregates exhibit higher porosity and consist of fine-grained dolomite crystals with a diameter of 0.005-0.05 mm, whereas non-reactive dolomite aggregates show lower porosity and consist of coarse, extremely well-crystallized, and tightly intergrown dolomite crystals with a diameter of 0.05-0.45 mm;The best-developed interface pattern is: unaltered dolomite aggregate interior (zone A) → inner dark dolomite reaction rim (zone B) → outer light-colored dolomite reaction rim (zone C) → light-colored paste reaction rim (zone D) → dark, presumably unaltered, cement paste (zone E). Regardless of the presence or absence of zone C, there is an increase in Ca and a concomitant decrease in Mg near the edge of dolomite aggregate close to the aggregate-paste interface, along with the formation of calcite and brucite near the interface. Zone D is calcite-rich. The compositional variations and mineralogy strongly suggest that these reaction rims were formed as a result of dedolomitization reactions between reactive dolomite aggregates and alkaline cement paste, and that dedolomitization is the primary cause of Iowa concrete deterioration;Both durable and non-durable Iowa concretes were subject to wet/dry, freeze/thaw, and continuous soaking treatment with magnesium chloride, calcium chloride, sodium chloride solutions, and water. No deterioration was evident in concrete samples treated with water, and sodium chloride solution was found to be relatively benign in terms of concrete deterioration. However, magnesium and calcium chloride solutions caused severe deterioration to durable and non-durable concretes;Magnesium chloride solution typically caused severe paste decomposition, disintegration and dark brownish paste discoloration, and showed similar deleterious effects on concrete to magnesium sulphate, magnesium acetate, and magnesium nitrate. The reactions between magnesium chloride and cement paste may involve leaching out Ca(OH)2 from cement paste, replacement of Ca2+ in calcium silicate hydrate by Mg2+, and decomposition of calcium aluminate hydrate, to form non-cementitious magnesium silicate hydrate, brucite, and calcite, causing paste disintegration and discoloration. Magnesium from any source, either from magnesium road deicers or from reactive dolomite aggregate, is thought to play a major role in highway concrete deterioration;Calcium chloride attack is characterized by crack development and brownish paste discoloration. The precipitation of neo-formed materials (calcite, Freidel\u27s salt, ettringite, gypsum, etc.), due to the reaction between CaCl2 solution and cement paste, and the stress associated with crystal growth are thought to cause concrete expansion and lead to concrete cracking