Composition and microstructure of 20-year-old ordinary Portland cement–ground granulated blast-furnace slag blends containing 0 to 100% slag

The morphology of outer-product (Op) C–S–H in 20-year-old slag–cement pastes appeared in most blends to be finer than at younger ages. The Ca/Si and Ca/(Si + Al) ratios of the Op C–S–H decreased with increasing slag content, and the Al/Si ratio increased. The Ca/Si ratio of C–S–H in the slag-containing pastes was lower at 20 years than at 14 months and the amount of Ca(OH)2 was reduced indicating that additional slag must have reacted. The mean aluminosilicate chain length of the C–S–H was very long in all the samples and would be expected to have increased with age. The TEM-EDX and NMR data are consistent with nanostructural models for C–S–H. The Mg/Al ratio of the Mg–Al layered double hydroxide phase (LDH) was lower at 20 years than at 14 months in all cases except for the neat slag paste; aluminium hydroxide-based structure might be interstratified with those of the Mg–Al LDH.