Microstructure and composition characterisation of three 20-year-old GGBS-OPC blended pastes

• After 20 years, C–S–H, portlandite, AFm, HTlp, ettringite and carbonate existed.
• Increasing GGBS enhances incorporation of H+ and AlO45− into C–S–H structures.
• Chain growth is proportional to GGBS contents, enhanced by high temperatures.
• CH functions as “a catalyst” in the process of pozzolanic reaction of GGBS.

It has become a common practice to use composite cement instead of pure Portland cement in many construction applications, then acknowledging the long-term microstructures and compositions of the composites, to ensure a long term stable performance and durability, becomes essential. Thus, this paper is presenting the characterisation of three 20-year-old cement-GGBS-blended pastes which experienced a curing history of at 40 °C for the first five years and then room temperature for the rest years. The main existed phases included C–S–H gel, portlandite, AFm, and hydrotalcite-like phase as hydration products, as well as the products from the carbonation occurring during storage, including ettringite (this ettringite was from the carbonation of AFm.), calcite, vaterite, hemicarbonate and monocarbonate. The comparison among the results obtained for three pastes indicated that increasing GGBS contents enhanced the substitution of bridging SiO44− tetrahedra by AlO45− tetrahedra in the aluminosilicate structure of C–S–H gel, the incorporation of proton ions into the chains to balance the charge differences caused by the substitution, as well as the chain growth. The chain length was found to be direct proportional to GGBS contents, which was enhanced by higher curing temperatures. Through comparing to the results obtained for parallel pastes but hydrated at room temperature for the whole 20 years, a similar hydration degree was observed for OPC in each paste, but higher hydration degree for GGBS in the paste with the experience of hydration at 40 °C, as well as morphological and compositional differences were caused by the different hydration histories. In addition, calcium hydroxide functioning as “a catalyst” in the process of pozzolanic reaction of GGBS was suggested by similarity between calcium contents but different hydration degrees of GGBS in both parallel pastes with and without hydration experience at 40 °C.