Multi-mode X-ray Study of Sodium and Magnesium Sulfate Attack on Portland Cement Paste

Sulfate attack is potentially one of the most damaging forms of degradation affecting portland cement-based materials. However, despite over six decades of study, considerable uncertainty remains in optimally selecting and proportioning materials for sulfate resistance, and current understanding of the actual mechanisms of degradation in sulfate environments remains incomplete. In this research, x-ray microtomography (μCT) and energy dispersive x-ray diffraction (EDXRD) are used synergistically to produce a time-resolved correlation between progressive physical and chemical damage to sulfate-exposed cement pastes. Companion expansion and strength data has also been obtained for comparison with the x-ray data. For a subset of Type I cement paste samples exposed to 33,800 ppm sulfate in either sodium sulfate (Na2SO4) or magnesium sulfate (MgSO4) solutions, synthesis of results obtained through both traditional testing and x-ray characterization suggests that expansion and cracking are more prevalent forms of damage under sodium sulfate attack, while loss in compressive strength was more prevalent under magnesium sulfate exposure. It is proposed that the expansion and cracking observed in the Na2SO4-exposed samples resulted from the formation of ettringite and/or gypsum in the near-surface region and that the loss of material in the MgSO4-exposed samples resulted, indirectly, from the formation of brucite [Mg(OH)2], which is known to lead to decalcification of the strength-giving C-S-H. INTRODUCTION With worldwide consumption of portland cements at 1.6 billion metric tons per year, the durability of portland cement concrete has a tremendous impact on the economy and the environment [1]. Mehta and Monteiro [2] indicate that in industrialized countries over 40% of the total resources of the building industry are applied to repair and maintenance of existing structures and less than 60% to new installations. Improved durability of infrastructure materials is also ecologically beneficial because of the energy consumption associated with cement and concrete production and because of the CO2 emissions generated during cement manufacture. A durable concrete has far reaching beneficial influences on sustainable development in the construction industry [3]. * Corresponding author: Dr. K.E. Kurtis, 790 Atlantic Dr., Atlanta, GA 30332-0355; phone: 404-385-0825; fax: 404-894-0211; email: [email protected] 63 Copyright ©JCPDS-International Centre for Diffraction Data 2006 ISSN 1097-0002