Maximizing the use of supplementary cementitious materials (SCMs) in blended cements

Partial replacement of clinker with supplementary cementitious materials (SCMs) is an effective approach to reduce CO2 emissions related to cement production. However, there is a maximum substitution level with SCMs beyond which strength decreases. The understanding of the factors controlling the reactivity of SCMs is the main objective of this thesis. This study fo-cused on three main SCMs: limestone, calcined clay and slag. A decrease of clinker content down to 80% and replacement with fine limestone permits to main-tain similar strength to PC up to 7 days. The filler effect of fine limestone enhanced the hydration of C3S which compensates the dilution of the clinker up to 7 days. The combination of fine lime-stone (15%) and coarse limestone (5%) showed similar strength to the system containing 20% of fine limestone and also to PC up to 7 days. Strength results were explained by the phase assem-blage using gel space ratio. Metakaolin reaction slows down in LC3 cement paste at late ages. The results indicated that the lack of water-filled capillaries is the most likely mechanism slowing down metakaolin reaction. Internal relative humidity measurements showed that at 28 days pore radius above 13 nm are not saturated with water. Based on Kelvin-Laplace equation, the growth of AFm and C-A-S-H phases were suggested to be limited below a range of pore radius around 3 to 8 nm. The strength and phase assemblage of LC3 with clinker content from 50% to 30% were studied. The substitution of 70% of clinker by 30% calcined clay and 40% limestone showed strength de-velopment passing the criteria for 32.5 N class, using a CEM I 42.5 R for the cement part. In the blend with 30% of clinker, metakaolin reaction did not slow down despite the depletion of port-landite at 7 days. Calcium was most likely consumed from C-A-S-H phases. Strength results of LC3 blends were explained by the combined water fraction approach since gel space ratio requires the quantification of the C-A-S-H composition, which is very challenging for low-clinker systems. The slow kinetics of slag reaction induced low strength development at early age. The dissolution of slag in NaOH solution with water solid ratio of 100 showed that the addition of calcium signifi-cantly slows down the rate of dissolution. In blended cements, slag dissolution might be delayed at early age due to the high concentration of calcium in the pore solution. At late ages, slag reaction in cement paste seems to be limited in capillaries above pore radius of 6 nm due to the lack of wa-ter. The main findings of this research should allow to increase the replacement level of clinker in blended cements.