In order to prevent the excessive gassing that can happen with normal cells which can be topped up, maintenance free batteries have calcium in the plates. This helps prevent the gassing and helps the hydrogen given off to recombine to water inside the cells. The calcium also reduces the self-discharge rate of the battery.
It seems however there is an issue that can arise termed 'Antimony free effect' or 'passivation' which can affect batteries with calcium instead of antimony in the plates such as mine. Fulle details can be read in the comments of this page:
But in short, a thin layer of lead oxide can form on the plates in the battery. Battery desulphators are not actually removing sulphation, but removing oxidation instead. So I'm hoping that the battery of mine with the desulphator connected might improve.
Doing some more reading, this page describes it in much more detail:
We believe it to be highly significant that pulsing began gaining popularity in the early 1990s, only after all the major battery manufacturers had introduced low-maintenance and maintenance-free automobile batteries. These batteries have lead-calcium alloy grids. Lead-calcium batteries overwhelmingly fail due to something known in the trade as "passivation" or "open circuit". It has been described by battery technologists as the "antimony-free effect".
Lead-calcium alloy develops an ultra-thin, very poorly conducting tetragonal lead oxide, (alpha-PbO), layer on the surfaces of the positive plate grids over time, leaving the positive active material in the positive plates isolated from the supporting positive grid structure. Battery manufacturers use tin to control this oxide layer. This is far from straightforward. Addition of more than 1.5% tin to the alloy reduces the passivation effect - critically, at 0.6%, tin actually provides a worsening of the effect. Tin is expensive, so as little as possible is used, with unpredictable results. There are more than 90 patents describing tin, as well as other metals, alloyed with, or plated onto positive grids - suggesting this too, is a highly problematic technology. Silver is even more beneficial but is excruciatingly expensive. Before lead-calcium, lead-antimony was the preferred alloy. Lead-antimony has always been absolutely 100% trouble-free in this regard.
There is also a long article on different charging regimes for sealed lead acid batteries:
They suggest that the typical approach of holding a calcium/lead battery on float might cause excessive corrosion of the plates and cause premature failure ie. 3-4 year life rather than 20 year design life. They suggest that the battery voltage is allowed to fall and then is periodically given a small current charge at regular intervals to bring the voltage up.
There is also a report done by the US Navy on Calcium batteries used in submarines, which is also an interesting read:
They manufactured some identical cells half with lead-antimony and half with lead-calcium and put them through some charge cycles to see what happened. The lead-calcium ones ended up with very soft paste on the plates that very easily smeared off.
So it does seem that the addition of calcium to batteries versus antimony really is for convenience, but at a trade off of battery life.