Research On Mechanism And Prevention-Cure Measures Of TSA Towards Cement And Concrete

Thaumasite is formed by sulfate attack on cement and concrete when exposed to sulfate, carbonate and low temperature environment, and this corrosion progress is named TSA(the thaumasite form of sulfate attack). The traditional sulfate attack types(AFt, gypsum and so on) mainly lead to the volume expansion and dehiscence on cement and concrete. While TSA could make CSH disintegrate and translate cement into pulpy mixture with no strength from surface to inside, and its distruction and concealment is more serious than the traditional sulfate attack types'.This paper is funded by National Natural Science Foundation of China (50378075) and 863 High-Tech Research and Development Program of China (No.2005AA332010), effects of both material compositions and environmental conditions on TSA of the cement mortar were studied firstly in China. Though the investigation on TSA examples in Western China and England, the effective appraisal methods of TSA are discussed and determined, the physics and chemistry destruction model of TSA is established, and effective prevention measures are tried. The following conclusions can be drawn:1. Though the investigation on TSA examples in Western China and England, the effective appraisal methods of TSA are discussed and determined. Macroscopical aspects: the surface of corrosive samples fall off and translate into pulpy mixture with no strength from surface to inside, and its inside is surrounded by offwhite pulpy mixture, but its strength remain exist. Microcosmic aspects: as the XRD eigenvalue of thaumasite is so similar to AFt that you can not distinguish between the both by XRD analyse only; SEM-EDS analyse can effectively distinguish the both by testifing the main chemic component of the needle crystal (Ca, Si, S or A1). While the FTIR and Raman spectrum can testfy the existence of[Si(OH)₆] octahedron in thaumasite to distinguish the thaumasite and AFt.2. At the same temperature and humidity environment, the corrosive destruction on the cement mortars will be more serious with the increasing concentration of the same solution type; The attack of Na₂SO₄ solution on cement mortars leads to AFt type corrosion; While immersed in the MgSO₄ solution, the cement mortars will be translated into thaumasite, and it can be testfied that Mg²⁺ ions reinforce the thaumasite formation and the thaumasite form of sulfate attack in the cement mortars.3. The temperature effection on the TSA progression is obvious. At 5℃, 20℃and 5～20℃alternate temperature environments, the cement mortars added with limestone powerder could be all translated into thaumasite corrosion after 1 year time of exposure to the 2%MgSO₄ solution. It breaks out from the traditional viewpoint that TSA only take place below 15℃.4. Thaumasite can also form in special type cements(sulfate resisting Portland cement and sulphoaluminate cement) after a long time exposure to the typical TSA environment. But the proceed of TSA can be delayed to a certain extent. According to the resistance to the thaumasite attack, the order of different cements from best to worst is as follows: sulphoaluminate cement plus ordinary Portland cement, sulphoaluminate cement, sulfate resisting Portland cement, ordinary Portland cement.5. The CH crystal in cement is the corrosive fountainhead, the key to resist TSA lies in the content of CH crystal and early strength development of cement mortars. Under the typical TSA environment at a lower temperature, the effect to resist TSA with 60% GGBS is the best due to its early pozzolana reactivity and reduction on the content of CH crystal; While the effect to resist TSA with fly ash is poor due to its poor early pozzolana reactivity. According to the improved resistance to thaumasite form of sulfate attack, the order of different mineral admixture from best to worst is: 60% GGBS, 5% silica fume and 25% GGBS, 8% silica fume, 30% GGBS, 20% fly ash.6. The chemical progress of TSA is divided into four stages: ion transference, AFt formation, gypsum formation and thaumasite formation. And the physical progress of TSA is divided into three stages: no destruction, expansion and cracking, softening and decomposition.