| The thaumasite form of sulfate attack(TSA) has been recognized as a new subspecies of sulfate attack by many domestic and foreign scholars. TSA will directly lead to the decomposition of the main intensity component of concrete, that is, the decomposition of C-S-H gel. Due to the similarity of the crystal structure between thaumasite and ettringite, TSA is often ignored and confused with other traditional kinds of sulfate attack, thus, it is more destructive and elusive to concrete. The 1st International Symposium on TSA was held in England in 2002, which marked a new trend and hotspot of concrete research and industry. Nowadays, China is greatly developing the west regions which involve many cross-regional significant engineering projects. As the aggressive geologic and climate conditions in these areas is somewhat susceptible to the formation of TSA, the study on the TSA behavior and mechanism is of great importance to the durability of concrete construction in cold areas, such as the west and the north of China.At present, the behavior and mechanism of TSA is not well realized and is suffering from many uncertainties. First, the current research on thaumasite lacks thermodynamic analysis to determine the critical temperatures of its formation under different curing conditions, and the thermodynamic model is also unclear. Second, ettringite rich in alumina/ aluminides phases may act as the core of thaumasite, but the effect of its configuration and amounts on the formation of thaumasite and the mechanism is not thoroughly understood. Third, the absence of the effective restriction measures against TSA. This paper focuses on the above target issues and is supported by National Science Foundation of China(NSFC 51178363). The main achievements are as follows:1. According to the thermodynamic analysis results, the critical temperatures of thaumasite formation and the critical ratios of Ca O and Si O2 of hydrated calcium silicate were found out. The role of Ca(OH)2 in TSA was also well explained. Furthermore, the thermodynamic models related to the p H-SO42--temperature and the CO2-p H- SO42- ternary systems were constructed to reveal the causes and principles of TSA, thus the theoretical basis in this field was extended.(1) The thermodynamic calculation was performed under the standard conditons with the variation of the ratios of Ca/Si. The results manifested that the ΔrGm of thaumasite formation reaction was negative when the ratios of Ca/Si ranging from 0.83-1.67 in case of Ca(OH)2, which means that such a reaction is favoured and can proceed spontaneously. While without Ca(OH)2 the reaction will not occured until the ratios of Ca/Si is above 1.58. The ΔrGm shows a clear reduction tendency with the increasing of the ratios of Ca/Si that favors the formation of thaumasite.(2) The critical temperatures of thaumasite formation was determined based on the thermodynamic calculation under other non-standard states. The maximum reaction temperatures of TSA were 52 for C℃ a/Si=1.67 and 31 for C℃ a/Si=0.83 in case of Ca(OH)2. However, the temperatures decreased to 36 ℃for Ca/Si=1.67 and 0 for C℃ a/Si=0.83 without Ca(OH)2. In other words, the ambient temperature above these values will not lead to TSA. These results manifested that a higher temperature can also cause TSA, and the previously reported “low temperature” range was extended.(3) The thermodynamic models related to the p H-SO42--temperature and the CO2-p H-SO42- ternary systems was built up for the first time. The minimum concentration of SO42- needed in the progress of TSA increased with the increasing of temperature and p H in the p H-SO42--temperature ternary system. While in the the CO2-p H-SO42- ternary system the partial pressure of CO2 increased with p H but decreased with concentration of SO42-.2. The effects of mineral components of cement and surrounding soluble aluminides on TSA in mono-minal clinker were commented, including the aluminides effect on the formation of thaumasite and thaumasite sosoloid. The mechanism of the above progress was identified by means of XPS, Solid-state NMR and so on.(1) The presence of C3 A in the initial system can accelerate the decomposition of C-S-H gel(the hydrate of C3S) and the transformation to thaumasite in TSA conditions. Different sulfate led to various extents of concrete corrosion.(2) The transformation time of C-S-H with low alkali(the hydrate of pure C2S) to thaumasite was longer than that of C3 S in TSA conditions. After curing for 24 months, no thaumasite that only derived from the hydrate of pure C2 S was detected. However, with adding 2% C3 A in the initial C2 S only system the thaumasite appeared in 18 months.(3) Two kinds of solid solution with the continuous variation of crystal lattice size were discovered when Al/Si ranging from 1:9 to 9:1. The thaumasite form solid solution has a size 11.05-11.101? in a axis and 10.475-10.423? in c axis. The ettringite form solid solution center between 11.191-11.235? in a axis and 21.370-21.466? in c axis. The interspace of solid solution was 11.101-11.191?. The maximum percentage was 48% for Al substituted by Si, and 36% for Si substituted by Al according to SEM-EDS and IR analysis under this test conditions.(4) This paper presented the mechanism and a hypothesis of ettringite on the formation of thaumasite. The octahedral geometry of thaumasite was built up according to ettringite, and then the Si atom replaced the Al in the ettringite octahedron to form the thaumasite octahedron. We considered that thaumasite can grow along with other homogeneous crystal, but can’t develop according to the similar but heterogeneous crystals.3. The effects of some important elements on TSA were discussed, such as Ca/Si, the solubility of Ca(OH)2, temperature and so on.(1) The increase of Ca(OH)2 solubility enhanced the formation of thaumasite. The solubility and the thaumasite amount of 16 th week showed a linear relationship of y=24.788x+9.72.(2) The extent of TSA for various Ca/Si was characterized after 12 months curing. The hydrated calcium silicate with low Ca/Si can contain and defer TSA due to the differences of polymerization degree and structure of hydrated calcium silicate with various Ca/Si. The mechanism of low Ca/Si on resisting TSA complied with the thermodynamic calculation results.(3) The sorbitol additive can effectively restrain the straight formation of thaumasite and the transformation from ettringite. The other additives, sodium lignosulfonate, sodium citrate, ammonium tartrate showed some resistant function, but not as effective as sorbitol.(4) The substitution of Al by Si decreased with the increase of temperature. The maximum percentage was 45% for Al substituted by Si, and 25% for Si substituted by Al at 20℃. When the temperature increased to 40℃, the maximum percentage decreased to 40% for Al substituted by Si, and 24% for Si substituted by Al.4. Based on the above analysis a novel resistant method against TSA was created.We used silica fume as the admixture to increase the impermeability and Ca/Si of cement-based materials, and then added the diatomite contained Ba(OH)2, Ba CO3 and sorbitol to consist of the inhibitor. The cement-based materials without using the above resistant method lost 94% strength, and a large amount of thaumasite generated. In contrast, no strength loss and no thaumasite was observed in the cement materials contained the inhibitor. |
PDF Full Text Request