Research On Deterioration And Preventive Measures Of Thaumasite Form Of Sulfate Attack (TSA) In Cement-based Mateirals

Thaumasite form of sulfate attack (TSA) directly destroys the main binding phasecalcium-silicate-hydrate (C-S-H) in hardened cement. Cement matrixes are convertedinto mush from the surface inwards. The properities of cement-based materials andconcrete structure durability are strongly influenced by TSA. Although the first caseof TSA was reported in1965, TSA has only received considerable attention in recentyears. There are still many problems remained unsolved by now.Thaumasite is very difficult to distinguish from ettringite due to the close similarityin crystal structure and morphology. Meanwhile, there exist some disputes aboutinfluence factors of TSA, such as whether fly ash can inhibit TSA. The investigationon the possibility of TSA, degradation and deterioration mechanism of cement-basedmaterials under combined actions are also rare. And above all, effective preventivemeasures of TSA have not yet been established. These problems not only influencethe service life of concrete structure seriously but also make great economic loss.A serial of experimental programs were designed to solve the above problems.Accelerating thaumasite formation by internal doping method and quantitativecharacterization of thaumasite were explored. The researches on mechanism andeffects of temperature, fly ash and chloride ions on TSA had also been carried out. Anappropriate setup was designed to exert controllable flexural stress to concretespecimens. The properities and deterioration mechanism of concrete under combinedactions were investigated deeply. TSA inhibitors were developed to prevent TSA inthis paper as well. Main conclusions of this paper are as follows:Thaumasite formation was successfully accelerated by internal doping10%ofmagnesium sulfate in cement-limestone powder pastes at (5±2)℃. Thaumasitecontent was up to23.73%after6-month immersion in water with a typicalcharacteristic of TSA damage. A rapid quantitative identification method ofthaumasite including visual inspection, IR spectrum and XRD/Rietveld refinementwas also established which would improve the accuracy and rapidness of TSA'sresearch.The influences of temperature, fly ash and chloride ions on TSA in cement-basedmaterials were illustrated.â‘´Cement-based materials with limestone powder tookplace TSA and the corrosion products were mainly thaumasite and gypsum at (5±2)℃.But the corrosion products were mainly ettringite and gypsum at (20±1)℃.⑵Effects of fly ash on TSA were closely related to the composition, content andfineness of fly ash. The activity index of fly ash had a maximum gray relevancedegree with quantity of thuamasite formation which could be used as an index forfiltering fly ash to prevent TSA. Fly ash with an activity index>80%could improvethe resistance to TSA in cement-based materials when the replacement reached50%.⑶Chloride ions could slow down the process of TSA by reducing the rate ofthaumasite formation. With the increment of chloride ions concentration in NaCl/Na2SO₄solution, the influence grew gradually. The processes of chloride ions'effects on TSA were divided into three stages: inhibition stage, recession stage andfailure stage.A self-localization detachable stress loading device was invented to exertcontrollable flexural stress to concrete specimens in combined action tests as well.This device consisted of stress loading fixture, self-localization fixture and stressmonitoring fixture which easy to apply regular calibration and maintain constantflexural stress. The new device overcomed inaccurate stress loading and stressrelaxation problems of common setup.The degradation and deterioration mechanism of cement-based materials undercombined actions of low-temperature sulfate attack, wetting-drying cycle and flexuralstress were obtained.â‘´When samples subjected to the combined action of lowtemperature sulfate attack and wetting-drying cycles, the latter greatly weakened thedamage degree of samples with magnesium sulfate by slowing down TSA. Butcrystallization pressure caused by salt crystallization corrosion due to wetting-dryingcycles had destructive effects on samples with salt lake brine.⑵A sudden breakwas the main-type destroys of cement-based materials under the action of both lowtemperature sulfate attack and flexural stress. The latter promoted a major crackforming and propagating rapidly throughout the specimens. And there was nothaumasite in corrosion products.⑶When samples subjected to the combined actionof three-factors, the deterioration was greatly accelerated due to cumulative damagescaused by "wetting-drying cycles+flexural stress". Moreover, corrosion productswere free from of thaumasite.The mechanism for preventing TSA damage in cement-based materials includingphysical solidification and chemical solidification was also presented in this paper. Inview of TSA chemical process and slow release of Ba²⁺of TSA inhibitors, the processof chemical solidification was divided into three stages: barium sulfate formationstage,sulfate attack transitional stage and thaumasite/ettringite decomposition stage. KMtype and K type TSA inhibitors prepared by "multi-coating technique" had along-term multilevel slow release of Ba²⁺. The resistance to TSA of cement-basedmaterials could be significantly improved by these TSA inhibitors. Furthermore, TSAinhibitors had certain improving effects on workability of fresh concrete, strength anddurability of hardened concrete.