Retardation effect of the pozzolanic reaction of low-calcium supplementary cementitious materials on clinker hydration at later age: Effects of pore solution, foreign ions, and pH

Clinker hydration at later age in blended cement pastes was found to be retarded by the pozzolanic reaction of low-calcium supplementary cementitious materials (SCMs). Various mechanisms were explored, including the consumption of pore solution, the presence of foreign ions, and pH-related effects. Experimental results indicated that the dissolution of the foreign ions from SCMs was a secondary effect. The primary cause of the retardation effect was the reduction in pH caused by the pozzolanic reaction. Aqueous precipitation tests yielded the following findings related to pH: (1) At lower pH levels, the precipitation of aluminium-incorporated calcium silicate hydrate(C–A–S–H) gel was reduced due to the decreased nano-adhesion force. (2) The morphology of C–A–S–H gel consisted of small spherical particles at lower pH levels, resulting in a denser capillary pore structure. The latter led to increased water retention within the fine gel pores but a reduced amount of pore solution available in the capillary pores for cement hydration.