Effect of curing regime on degree of Al³⁺ substituting for Si⁴⁺ in C-S-H gels of hardened Portland cement pastes

Abstract

The effect of curing regime on degree of Al³⁺ substituting for Si⁴⁺ (Al/Si ratio) in C-S-H gels of hardened Portland cement pastes was investigated by ²⁹Si magic angel spinning (MAS) nuclear magnetic resonance (NMR) with deconvolution technique. The curing regimes included the constant temperature (20, 40, 60 and 80 °C) and variable temperature (simulated internal temperature of mass concrete with 60 °C peak). The results indicate that constant temperature of 20 °C is beneficial to substitution of Al³⁺ for Si⁴⁺, and Al/Si ratio changes to be steady after 180 d. The increase of Al/Si ratio at 40 °C is less than that at 20 °C for 28 d. The other three regimes of high temperature increase Al/Si ratio only before 3 d, on the contrary to that from 3 to 28 d. However, the 20 °C curing stage from 28 to 180 d at variable temperature regime, is beneficial to the increase of Al/Si ratio which is still lower than that at constant temperature regime of 20 °C for the same age. A nonlinear relation exists between the Al/Si ratio and temperature variation or mean chain length (MCL) of C-S-H gels, furthermore, the amount of Al³⁺ which can occupy the bridging tetrahedra sites in C-S-H structure is insufficient in hardened Portland cement pastes.

Keywords

constant temperature / variable temperature / degree of Al³⁺ substituting for Si⁴⁺ / C-S-H / hardened cement pastes

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