Calcium Transfer In The Formation Of Sulfate Corrosion Products For Cement-based Materials

The mechanism of sulfate attack is complicated due to a coupling of physical（ions diffusion）and chemical（chemical reaction）factors in the process of sulfate attack.In general,environmental conditions(such as p H value,temperature,concentration of SO₄^2-and CO₃^2-,etc.)and the characteristics of cement-based materials（chemical composition,pore structure）may play a crucial role in sulfate attack.However,due to the lack of comprehensive insight into these various factors in the current research,the mechanism of sulfate attack remains an enigima.The principal controversies over sulfate attack include the formation sequence of corrosion products,the inhibition effect of admixtures on sulfate attack,and the formation mechanism of thaumasite.The external sulfate environment can only provide SO₄^2-,and the formation of corrosion products still requires cement hydration products to provide calcium.Therefore,research on the source of calcium required for the formation of corrosion products and the corresponding relationship of calcium provided by hydration products（calcium migration）will be beneficial to further understand the mechanism of sulfate attack.The cement paste slices with a thickness of 3±1 mm（avoided the influence of penetration speed）were immersed in Na₂SO₄ solutions with different SO₄^2-concentrations（1000 mg/L,10000 mg/L,35000 mg/L）and p H values（10.8,11.8,12.6）.The effects of Ca/Si of C-S-H gel,Ca（OH）₂contents and the Al phase in the supplementary cementitious materials（SCMs）on calcium transformation process during the formation of sulfate corrosion products were analyzed by the XRD,BSE-EDS,²⁹Si NMR and FTIR and the formation mechanism of sulfate attack products was further clarified.The following conclusions are obtained.（1）The decalcification sequence of hydration products is independent of the SO₄^2-concentration and p H value of the solution.The calcium required for the formation of ettringite,gypsum and thaumasite is firstly provided by Ca（OH）₂,and the decalcification of the interlayer calcium of C-S-H gel will not take place until the Ca（OH）₂ are fully consumed.（1）Under the condition of high p H value（≥12.6）,Al content in the systemis the main influencing factor of sulfate attack.A larger amount of Al phase can generate more ettringite,which reduces the sulfate attack resistance of cement pastes.（2）Under the condition of low p H values（≤11.8）,ettringite and gypsum will formed simultaneously in cement-based materials,sulfate attack mainly depends on the content of available calcium in the system.That is to say,the higher the Ca（OH）₂ content or the greater Ca/Si of the C-S-H gel,the more calcium is available in the system,and the more sulfate corrosion products will be formed.（2）During the sulfate attack process,the ettringite precipitates faster than gypsum,and thus ettringite mainly forms in the early stage of sulfate attack（28d～56d）.After 56d of corrosion,the amount of ettringite tends to be stable,while the content of gypsum increases with the sulfate attack duration.（3）The formation of thaumasite under low temperature condition can be divided into three stages:（1）Ettringite（gypsum）formation:Within the first 56 days of corrosion,Ca（OH）₂ in hydration products provides calcium to form ettringite（gypsum）.（2）Woodfordite solid solution nucleation formation:Ca（OH）₂ continues to provide calcium in the following 56-90d,and ettringite combines with C-S-H gel to form woodfordite solid solution following the"ettringite conversion mechanism".（3）Crystal growth of thaumasite:After 90 days of corrosion,woodfordite solid solution provides nucleation site for thaumasite,which promotes C-S-H gel and gypsum to participate in the"direct formation mechanism of C-S-H gel"to form thaumasite.If the gypsum cannot be formed in the first stage,the process of thaumasite-type corrosion can only proceed to this stage.（4）The method of inhibiting sulfate attack by reducing the content of available calcium is proposed in this research.The corresponding suggestions to the concrete sulfate attack test methods and durability design standards are proposed.（1）The standard states that the SCMs were recommended to improve sulfate attack resistance of concrete by reducing the permeablity.However,the aluminum-containing SCMs can generate more ettringite under sulfate environment.Therefore,high-silica SCMs without aluminum phase should be recommended to reduce the content of available calcium in cementitious materials when designing concrete for sulfate resistance.（2）Considering the high concentration of sulfate ions in some salt lake areas in the field environment,a higher sulfate concentration level of 10000mg/L～35000 mg/L in the sulfate attack test should be supplemented.（3）Since the p H value has a large influence on the type of sulfate corrosion products,the p H value of the sulfate solution should be strictly controlled instead of regular replacement of the solution in laboratory,so as to ensure that the sulfate corrosion type is more consistent with the field environment.