Influences Of Different Aluminates On Thaumasite Sulfate Attack In Cement Materials

Sulfate attack is one of the deteriorations commonly observed.It results from the formation of ettringite or gypsum by the reactions between the foreign sulfates and aluminates and sodium hydroxide in hydrated cement,leading to spalling of concrete or mortar.In contrary the traditional sulfate attacks,the thaumasite sulfate attack（TSA）transforms the C-S-H gel into a non-cementitious silt and typically no significant expansion cracks are observed during the process,all of which suggests the difficulties to detect and prevent the occurrence of TSA.The influences of the aluminate on the thaumasite formation and the deteriorations caused by TSA remain unclear so far.In view of this issue,this study explores the roles of different aluminate phases in the TSA using cement minerals and clinkers.This study explores the effects of different aluminum phases on the formation of thaumasite through the X-ray diffraction,Fourier transform infrared spectroscopy and Raman spectroscopy.During the experiments,the total amount of Al₂O₃ remain constant.The influences of crystalline aluminate phase（C₃A）and the aluminate phases in the mineral admixtures on the TSA in C₃S or clinker are discussed.Results are summarized as follows:（1）In pure C₃S,no ettringite was formed due to the absence of aluminates.There was no thaumasite formation,either,even though abundant carbonate,sulfate and water were present.Such results suggest that the thaumasite is difficult to form through the ionic reactions.（2）With the addition of crystalline C₃A in C₃S,the thaumasite was formed even though the addition was few（6% C₃A）.The degree of degradation by TSA was increased by C₃A in the range of addition between 0% and 30%.The thaumasite formation was formed the most when the C₃A addition was 10-15%（or 6.05%-7.79% Al₂O₃eq）.（3）With the addition of glassy slag,the formation of thaumasite was inhibited due to the slower releases of aluminum ions when compared with other additives.There was an optium amount of addition.The thaumasite formed the most when the Al₂O₃ was 7.79%.（4）With the addition of glassy metakaolin,the formation of thaumasite was favored and formed the most when the Al₂O₃ was 7.79%.（5）Such results suggest that the thaumasite formation in the cement-based materials was likely due to the ionic exchanges,in which the Al³⁺,3SO₄^2-+2H₂O in the ettringite were replaced by Si₄₊,2SO₄^2-+2CO₃^2-.