Thaumasite Sulfate Attack, Popcorn Calcite Deposition and acid attack in concrete stored at the «Blindtarmen» test site Oslo, from 1952 to 1982

In past decades, concrete in contact with Oslo Alum Shale suffered severe deterioration, frequently involving quick and complete degradation of the cement paste matrix. The Norwegian Alum Shale Committee (1947–1972) concluded that this was due to secondary ettringite deposition and acid attack. The most resistant concrete mixes were based on Sulfate Resisting Portland Cements or those containing a pozzolanic supplementary cementitious mineral. However, the present work concludes that this “classic” sulfate attack was triggered by deleterious Thaumasite Sulfate Attack and Popcorn Calcite Deposition in close association with effects of externally derived acid attack. This paper provides an outline of the previous work and the first petrographic documentation of concrete prisms previously exposed at the “Blindtarmen” test site. Two important novel reaction mechanisms were also found: 1) the development of high crystallisation pressure associated with Thaumasite Formation, and 2) full scale Thaumasite Sulfate Attack was most efficient after partial or complete decalcification of calcium silicate hydrate: thaumasite had formed at the expense of remaining amorphous silica and associated secondary popcorn calcite. Comparison with modern sprayed concrete on Alum Shale shows that influence of Thaumasite Sulfate Attack and internal carbonation was faster under a hydraulic gradient than under static conditions.