This paper investigates the microstructural development of limestone calcined clay cements (LC3) using microstructural modelling. The microstructure of ternary blended cements containing 15, 30 and 45% limestone and calcined clay is simulated using microstructural modelling platform μic. It is found that replacing cement with limestone and calcined clay significantly increases the porosity of pastes. The increase in the porosity is observed to depend on the percentage replacement of cement with limestone and calcined clay. It is also observed that just the addition of fine limestone and calcined clays is not sufficient to explain the reduction in experimentally observed threshold pore diameter in pastes containing limestone and calcined clay. The results suggest that it is important to consider the modification in the hydration product (CASH) in order to explain the reduced threshold pore diameter of LC3 systems. © RILEM 2020.