The Effect of an Accelerator on Cement Paste Capillary Pores: NMR Relaxometry Investigations

Abstract

Nuclear Magnetic Resonance (NMR) relaxometry is a valuable tool for investigating cement-based materials. It allows monitoring of pore evolution and water consumption even during the hydration process. The approach relies on the proportionality between the relaxation time and the pore size. Note, however, that this approach inherently assumes that the pores are saturated with water during the hydration process. In the present work, this assumption is eliminated, and the pore evolution is discussed on a more general basis. The new approach is implemented here to extract information on surface evolution of capillary pores in a simple cement paste and a cement paste containing calcium nitrate as accelerator. The experiments revealed an increase of the pore surface even during the dormant stage for both samples with a faster evolution in the presence of the accelerator. Moreover, water consumption arises from the beginning of the hydration process for the sample containing the accelerator while no water is consumed during dormant stage in the case of simple cement paste. It was also observed that the pore volume fractal dimension is higher in the case of cement paste containing the accelerator.

Monitoring the Effect of Calcium Nitrate on the Induction Period of Cement Hydration via Low-Field NMR Relaxometry

“…The formation of these pits does not consume the capillary water as can be observed from the constant capillary peak area ( Figure 2 a). A similar conclusion about the increase was drawn from a fractal analysis of the pore surface based on transverse NMR relaxation measurements [ 24 ]. Figure 4 b shows a faster saturation of the increase in the in the presence of the accelerator (CP + 2% Ca(NO 3 ) 2 , by cement mass), as compared with the simple cement paste (CP).…”

“…Assuming planar pores, this ratio allows finding of another key quantity, the pore size as the distance between the two planes. However, in the case of an evolving porous structure, as is the case of cement-based materials, the change in the surface to volume ratio can be also determined by the change in fractal dimension [ 24 ], and the pore size parameter extracted from the fitting approach must be used here with caution when characterizing the pore size evolution during hydration.…”

“…Some of these techniques require special sample preparation or stopping of the hydration process when utilized. That is why, when studying water evolution and pore structure development inside cement-based materials, it is favorable to use 1 H low-field nuclear magnetic resonance (NMR) relaxometry techniques [ 3 , 22 , 23 , 24 , 25 , 26 ]. The advantage of NMR relaxometry over other techniques is that it allows nonperturbative investigations even during the hydration process without the requirement to stop the hydration process and without any pretreatment of the sample.…”

Triaxial tests on high ductile concrete under different environmental conditions

“…Pichler et al 6 studied the uniaxial compressive strength and hydration degree of concrete samples at different curing temperatures (15-70 C) and used a power law function to characterize the relationship between hydration degree and strength. Xiu et al 7 studied the effects of different curing temperatures (20,35, and 45 C) on the triaxial mechanical properties of cement-based tailing backfilling. The peak deviator stress and cohesion increase as the curing temperature rises, whereas the internal friction angle decreases.…”

“…Low-temperature curing affects the hydration process and does not change the type of hydration products but changes the number of hydration products and promotes the formation of harmful pores. Chen et al 5 studied the triaxial mechanical properties of early-age concrete under different curing conditions (10,20,30, and 40 C) and proposed a curing factor that can reflect the coupling effect of curing temperature and age. The triaxial compressive strength and elastic modulus increase with the rise in the curing factor, and a hydration model is established to predict the triaxial mechanical properties of early-age concrete.…”