Comprehensive study of cement paste degradation under accelerated leaching: Insights from nanostructure to microscopic behavior via experimental and simulation approaches

This study systematically examines the degradation of cement paste under accelerated leaching conditions using a combined multi-scale experimental and numerical simulation approach, aiming to connect nanoscale structural alterations to macroscale performance decline. Cement paste sheets were leached in 5 M NH₄Cl solution (pH ≈ 4.5) for 7 d, 14 d, 28 d, and 56 d. The findings indicate a hierarchy of leaching resistance of cement hydration products: hydrotalcite > C-(A)-S-H > hydrogarnet > ettringite > portlandite, whereas clinker phases (C₃S, C₂S) are easier to decalcify than C-(A)-S-H. Comprehensive analyses elucidate the mechanisms of Al migration from C-(A)-S-H into Al-Si gel, emphasising the preferential transformation of Al-containing segments and the stabilization of Al primarily in tetrahedral coordination (Al4). Analysis of pore structure shows that initial leaching mainly enlarges large pores (200–1000 nm) due to the dissolution of portlandite, while subsequent stages primarily increase gel pores (∼10 nm) as a consequence of C-(A)-S-H decalcification. Molecular simulations further illustrate that the deterioration of mechanical properties at the nanoscale is primarily influenced by the decrease in packing density rather than the reduction in the Ca/Si ratio. Furthermore, the increase in pore connectivity markedly increases permeability during leaching. This research offers integrated multi-scale insights that enhance the fundamental comprehension of cement paste durability under leaching conditions.