Bad news for the burned clay I fear......
Reported from another paper:
"the heat treatment up to 500 °C of sepiolite implies the loss of the four water molecules coordinated to magnesium two of them being lost at around 300 °C and the second two at 500 °C. The loss of water produces structural modifications in sepiolite, causing the channels to collapse to a folded structure, the microporosity now being inaccessible to nitrogen and carbon dioxide molecules. The loss of microporosity is responsible for the initial decrease observed in the volume of micropores of the pellet with increasing heat treatment"
Ok carbon dioxide molecules are 232 pm (picometers) and N2 are 300 pm. NH4+ are 175 pm they could enter in the microporosity ? I Do not know.
Mogens, keep control on Nh3, Nh4+ NO2- and NO3-
Maybe even if the NH4+ ions cannot enter microporosity they can be attracted on the surface of the sepiolite granules and the system should work also with burned clay.
Moreover
"On the other hand, the experimental results show that the pellets resulting from the heat treatment at 500 °C are consistent when kept under water: this is not the case for pellets dried at 110 °C or heat treated at 300 °C."