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A serendipitous discovery leads to new concrete for dam construction
MgO concrete was developed by Chinese dam engineers and has been applied predominantly in the construction of concrete dams in Chinese-speaking regions. It has been successfully used in about 30 dams over the past three decades and remarkable results and experiences have been collected. However, knowledge of this novel material and technology is not widespread in other countries.
Magnesium oxide-based shrinkage-compensating concrete (MgO concrete) is concrete with an additive of lightly burnt MgO powder. When MgO hydration takes place, the final product is magnesium hydroxide, Mg(OH)2. Because Mg(OH)2 has a larger volume than its constituents, MgO concrete is expansive. Concrete made with more typical expansive cements like ASTM C 8451 Types M, K, and S expand at early ages. However, most of the volume expansion of MgO concrete occurs at late ages (after 7 days). The expansion of MgO concrete therefore closely matches the shrinkage of mass concrete as it cools, and the concrete has been of particular interest in construction of dams to minimize crack development, simplify temperature-control measures, and speed up construction.
A BRIEF HISTORY
The shrinkage-compensating effect of MgO concrete was discovered by chance. The Baishan concrete arch gravity dam in Northeast China was constructed during China's "Cultural Revolution" (1966-1976), when technical regulations and standard engineering practice could not be implemented for dam construction. As a consequence, temperature-control measures for mass concrete were not taken seriously during construction of the dam.
Most of the base concrete was placed during the summer, and the differential between the maximum temperature rise and the stable temperature of the concrete was over 40 °C (72 °F). However, no significant cracking was detected in the dam concrete, even though it later experienced severe cold weather conditions. This astonished the dam builders. Engineers traced the phenomenon and found that the main and only possible factor was that the content of MgO in the cement used was high (up to 4.5%), and the burning temperature of the cement was relatively low. The conclusion was that lightly burnt MgO is reactive, provides a long-term expansive effect that occurs from 7 to 1000 days after the concrete hardens, and does not lead to unsound concrete. The study and intentional application...