Insights on the molecular structure evolution for tricalcium silicate and slag composite: From 29Si and 27Al NMR to molecular dynamics

Slag, an important supplementary cementitious material, is widely utilized for producing sustainable cement-based materials. In this paper, to study the effect of hydration stage and composition on the microstructure of cement-slag powder composites, the hydrated tricalcium silicate (C3S) curing at 20 degrees C at 28-day and 90-day with slag dosage from 0% to 45% were investigated by Si-29 and Al-27 NMR spectrum and molecular dynamics. Silicate connectivity from Si-29 NMR test showed that with increasing slag substitution content, the mean chain length (MCL) of the C3S-slag hydrated paste at 28-day increased from 2.99 to 4.02 and the AlO4/SiO4 ratio rose from 0 to 0.043. Aluminate coordinates of 27 Al NMR analysis revealed that the incorporation of slag could contribute to transformation from ettringite to Aft and formation of Al [4] in the C-A-S-H gel. Furthermore, reactive force field molecular dynamics was utilized to study the structural, reactivity and mechanical properties of C-A-S-H gel in hydrated composite. The AlO4 species in the C-A-S-H gel played an essential role in healing the defective silicate chains and transforming dimmer-rich silicate chains at low slag powder content to a long aluminate-silicate skeleton with high slag powder content. The structural evolution was attributed to the polymerization reaction between aluminate species and non-bridging silicate tetrahedron, with neighboring water molecule dissociations. In respect of dynamical properties, the aluminate-silicate structure exhibited good stability due to high Al-O-Si connection characterized by time correlated function. Uniaxial tensile modeling revealed that C-AS-H gel with long aluminate-silicate chains exhibits good loading resistance and the cohesive strength of CASH gel is improved to a great extent due to the incorporation of slag powder. Hopefully, this study may provide molecular insights for design of sustainable and durable cement-slag powder composites.