Pore size seen through water
Water molecules in a small pore are always close to the wall, and the wall makes the NMR signal fade quickly. In a large pore most of the water is far from the wall, and the signal lasts longer. A porous sample filled with water therefore shows a range of decay times, which reflect its range of pore sizes.
Where it is used
- Cement, concrete and plaster, including their hydration
- Rocks and rock cores
- Wood, paper and cellulose materials
- Catalyst supports, ceramics and powders
- Soils and other natural porous media
Typical tasks are comparing pore structures between samples, following how the structure changes during hardening or ageing, and studying how fluids sit in a rock.
From a decay to a distribution
The sample is saturated with water and the decay of the signal is recorded with a train of echoes. An inverse Laplace transform turns the decay into a distribution of relaxation times, where each peak is a group of pores of similar size and its height shows how much water sits in it.
Pore size and surface binding together
A distribution of T2 times alone cannot always tell water in a small pore from water bound to a surface, because both relax quickly. Measuring T1 and T2 together and inverting in two dimensions gives a map that pulls them apart. Along the diagonal, where T1 equals T2, the position shows how small or large the pores are. Above the diagonal, the distance shows how strongly the water is held at the surface.
The same approach separates water from oil in a rock or a catalyst, and maps can also combine diffusion with relaxation.
What it adds
- Non-destructive and quick, so many samples can be compared
- Water is the probe, so no mercury, high pressure or special gases are needed
- The measurement works on wet samples as they are
- The same sample can be measured again as it changes
Points that matter in practice
- To turn a relaxation time into a pore size, the instrument has to be calibrated against a reference technique such as gas adsorption or mercury intrusion
- Paramagnetic minerals, for example iron compounds in rocks, shorten the relaxation and can distort the result
- Pores that are well connected exchange water, which can merge peaks
- The inversion is a mathematically delicate step. It uses smoothing, so the distributions are smooth by construction and the exact width of a peak should not be over-interpreted
- Good signal quality matters. Two-dimensional maps need longer measurements and software that includes the inversion, which should be checked for your instrument
- The sample must be fully saturated and kept at a defined temperature
From your question to a routine method
Describe your materials and your question. MSY Consulting runs feasibility measurements, helps you plan the calibration against your reference technique, and supports you with the analysis until the method is part of your routine.
Let's talk about your application
Send an email describing your instrument and your application, and you will get a reply.
contact@msyconsulting.euMSY CONSULTING SAS · Wissembourg, France · LinkedIn