Investigation On Thaumasite Formation In Cementitious Materials

Thaumasite form of sulfate attack is a serious erosion of cement concrete in sulfate environment at low temperature, which can convert the C-S-H gel in the cement and concrete into mud-like substance with little strength, resulting in a significant decrease in the strength of cement-based materials, and weakening the durability of cement and concrete. How to strengthen protective measures in the real project to improve the performance of concrete against TSA becomes a fundamental problem.Under the support of the National Natural Science Foundation of China (51278497), aiming at conditions and environment of thaumasite form of sulfate attack in cement-based materials, mechanisms and factors of thaumasite formation were studied. Thaumasite form of sulfate attack of cement-based materials in various sulfate erosion solutions as well as chemical admixtures and mineral admixtures on inhibition of thaumasite formation were studied.The main experiments and results in this study are as follows.1. The thermodynamic model for mechanism of thaumasite formation reactions were calculated and analyzed. Gibbs free energy change of the direct route indicates that thaumasite can be synthesized by calcium carbonate, gypsum, water and C-S-H gel directly between0and25℃. Gibbs free energy change of the woodfordite route shows that between0and25℃C-S-H gel, calcium carbonate, ettringite and water can not form thaumasite, and at5℃thaumasite tends to transform to woodfordite with ettringite, rather than exist as pure thaumasite.2. The heterogeneous nucleation mechanism of thaumasite formation was studied by Struble approach. Structures of ettringite and thaumasite are very similar, and many scholars hold that ettringite can act as heterogeneous nucleation of thaumasite to promote its growth. Compared to ettringite (Ca6[Al(OH)6]2·24H2O·[(SO4)3·2H2O]), Sn-thaumasite(Ca6[Sn(OH)6]2·24H2O·[(SO4)2·(CO3)2]) possesses more similar structure with thaumasite (Ca6[Si(OH)6]2·24H2O·[(SO4)2·(CO3)2]). Therefore Sn-thaumasite is more likely to provide thaumasite with nucleation. The addition of stannate sodium in the mixtures by struble approach leads to formation of Sn-thaumasite. However, Sn-thaumasite, as heterogeneous nucleation of thaumasite, has little influence on thaumasite formation.3. The direct route of thaumasite formation was studied by Struble approach. The increase of concentration of sulfate and carbonate pose little effect on formation of thaumasite. The formation of Ca3[Si(OH)6·12H2O]4+is relevant to the transition of silicon-oxygen tetrahedral in silicate to silicon-oxygen octahedra in thaumasite, which is the rate-determined step in the reaction of thaumasite formation. There is a great likelihood that sucrose combine silicate to form complexes with five-or six-coordinated silicon, which may promote the transition of silicon-oxygen tetrahedral to silicon-oxygen octahedra, and accelerate thaumasite formation. Increasing concentration of calcium silicate can facilitate the transition of silicon-oxygen tetrahedral to silicon-oxygen octahedra. As a result, calcium ions play a significant role in thaumasite formation.4. Factors, such as ionic strength and pH levels of solution, calcium phosphate, of thaumasite formation were studied by Struble approach. Increasing the ionic strength of solution can accelerate thaumasite formation at the initial period, but the amount of thaumasite did not increase in7th month. Thaumasite formed more easily at pH levels at13than at14. Calcium phosphate furthered thaumasite formation initially. Nevertheless, the amount of thaumasite decreased as time goes in the sample with calcium phosphate.5. Sulfate attack of cement-based materials with ground limestone at low temperature (5℃) was studied. Under sulfate attack for16months in a5%sodium sulfate solution, the surface of cement pastes fell off, and converted to mud-like substance, and corrosion products of cement pastes mainly include thaumasite, which were characteristics of TSA. TSA of cement-based materials, in5%sodium sulfate and sodium chloride composite solution, were weakened. Loss of the surface of cement pastes and the amount of thaumasite reduced. Moreover, the higher the concentration of sodium chloride are, the fewer TSA of cement pastes are. Cement pastes soaked in a solution of5%sodium sulfate and sodium carbonate for16months has intact surface with calcium carbonate and pissonite, indicating that carbonate ions can produce dense protective film on the surface of cement pastes to reduce TSA.6. Effects of chemical admixtures on sulfate attack of cementitious materials were studied. Glycerol can reduce sulfate attack of cementitious materials, and the corrosion products were mainly ettringite. The amount of thaumasite decreases with the increase of glycerol content. Glycol accelerated sulfate attack of cementitious materials, and the amount of gypsum increased. Ettringite and thaumasite coexisted in the corrosion products. Calcium phosphate can reduce sulfate attack of cement pastes. The addition of sodium silicate in cement pastes resulted in more obvious cracking, but the amount of corrosion products thaumasite decreased with the addition of sodium silicate.7. Effects of mineral admixtures on sulfate attack of cementitious materials were studied. Na-bentonite, basalt and granite can reduce sulfate attack of cement pastes and erosion mitigates with increased sodium bentonite, basalt and granite. The main corrosion products were ettringite. Ca-bentonite and quartz powder had little impact on sulfate attack of cementitious materials.