Comparative assessment of shrinkage and hydration behaviour in traditional cement and low carbon cement by using NMR relaxometry

This study presents a comparative assessment of the hydration behaviour and pore structure evolution in OPC and the low carbon alternative, LC3, using low field 1HNMR relaxometry. The main objective was to non destructively characterize the microstructural development and water dynamics in these binders over 28 days curing period under drying conditions, to infer their potential for shrinkage. Samples with a water to cement ratio of 0.5 were prepared and exposed to open air condition. NMR transverse relaxation time T2 decay curves were acquired at regular intervals (1, 2, 7, 14, 21, and 28 days) using a CPMG pulse sequence on a 0.2 Tesla NMR relaxometry. The raw data were processed using the UpenWin software to invert the measured NMR signal into distributions of T2 relaxation times, on which populations of water confined in gel, interhydrate or capillary pores can be observed. The findings show that the hydration mechanisms are different. OPC exhibited rapid microstructural densification, with the gel pore water fraction of the T2 distribution dominating at day 1 and increasing over by day 28, indicating complete consumption of capillary water and high potential for autogenous shrinkage. On the contrary LC3 showed a more porous structure at day 1, with significant capillary pore water fraction. It underwent a rapid pozzolanic reaction, leading to a swift increase in gel pore formation by day 2. LC3 maintained a small but persistent fraction of capillary water up to 28 days, indicating a more gradual, continuous hydration process that may mitigate self drying stresses. Both systems retain water in gel pores under intense drying condition, but LC3’s capacity to retain a more distributed pore network highlights its different shrinkage behaviour. The findings underscore LC3’s viability as a sustainable binder, capable of developing a refined pore structure over time while potentially offering dimensional stability compared to OPC due to its pore evolution characteristics.