Study On The Structure Identification And Properties Of Thaumasite

Thaumasite is one of the hydrates from sulfate attack in concrete at low temperature.Once thaumasite is generated, eventually, concrete strength will be completely lost.Therefore, it brings a serious destroy to construction applications and causes hugeeconomic losses. The investigation on thaumasite has not attracted widespread attentionuntil rencently. The research on identification methods and properties of thaumasite can behelpful to understand thaumasite fully, and it will provide references for improving thedurability of concrete.First, ettringite and thaumasite were synthesized by chemical methods, andcharacterized by XRD (X-ray diffraction), IR (Infrared radiation), Raman spectroscopy,SEM (Scanning Electron Microscope) and EDS (Energy Dispersive Spectrometer),followed by the exploration of the discrimination methods between ettringite andthaumasite. Second, the effect of exogenous sulfate on the formation of thaumasite wasthen studied. Third, the three properties of thaumasite including solubility property,thermal decomposition property and growth performance were considered by contrast withthose of ettringite. Last, the restrain method of thaumasite formation was discussed.The main results and conclusions are described as follows.(1) Due to the presence of [SiO6] structure in thaumasite, IR and Raman spectroscopyare two feasible ways to identify thaumasite structure. Thaumasite has characteristic peaksat500cm-1in IR spectrum and at658cm-1in Raman spectrum respectively. However, theformer is significantly weaker than that of the latter, so Raman spectrum is better toidentify thaumasite.(2) The microscopic analyses, strength tests and specimen surface indicate that thecement pastes are most susceptible to be corroded in MgSO4solution.(3) The minerals of cement clinker were sololy synthesized by sol-gel method. ThePortland cement was prepared by the minerals and dihydrate gypsum according to acertain percentage. The resulting cement has desirabe performances, and can reach33MPa at28days, so it can be used to simulate P.O325cement.(4) The results of the properties of thaumasite are summarized below by comparing those of ettringite.①Thaumasite can be dissolved fully in methanol-ethylene glycolsolvent at room temperature. However, the solubility is slight in ethanol, n-propanol andacetone solvent. Thaumasite can be decomposed in dilute acids including hydrochloricacid, nitric acid and sulfuric acid. Barium nitrate other than sodium carbonate can reactwith thaumasite in solution.②Thaumasite can be exsitent under a certain time at70℃,but be decomposed quickly at90℃. Thaumasite has better thermal stability relative toettringite.③Thaumasite can be directly generated in cement system without conversionfrom ettringite, but it needs plenty of time. Thaumasite formation decreases with the pHvalue of the pastes. The production rate of thaumasite is very slow even if it can begenerated at a higher pH value.(5) The cement paste with30%fly ash has satisfied inhibition effect on thaumasiteformation.