What is wrong with this battery
The calcium battery has a higher open circuit voltage than a regular lead-acid battery. Thus it also requires a higher voltage to charge it. The fact that the voltage reading was 12 vdc confused the poster, who was used to regular lead-acid batteries. The calcium battery was NOT bad - it was depleted of all charge. The reason that the battery was believed to be dead was that it had 0 current output with 12 vdc reading and attempts to charge it had failed.
A good lead-acid reads 12.6 vdc open circuit. When depleted of charge with all good cells the same battery reads 11 vdc or more, but less than 12 vdc. If the battery reads 10.5 vdc, the chances are that one of its cells is bad.
The calcium battery on the other hand reads 13.5 vdc open circuit. When depleted of charge with all good cells the same battery reads 12 vdc or more, but less than 13 vdc. If the battery reads 11.5 vdc, the chances are it has one dead cell.
The net is that the calcium battery as seen by the op was completely discharged. When the smart charger was put on it, it charged for a significant amount of time at very high amperage of 20 amps so that the smart charger decided something was wrong with the battery. The op thought so too. But after reading more about calcium batteries, it was discovered that a higher charge voltage was needed.
A different old standard charger was employed. This charger began charging at almost 15 vdc. After an hour the battery was being charged at the max that the charger puts out and the current was down to 2 amps. This continued for some time, but the current eventually was down to nearly 0 amps and the voltage remained at the maximum of about 15.6 vdc.
In all probability, the charging circuit of the vintage John Deere lawn tractor it was on does not put out a high enough voltage to charge the calcium battery (if it was working at all). The owner of the lawn tractor gave it away so it was removed from the op's reach, though it appeared to be operating with the different battery installed (the battery installed was not new, but it might be capable of starting the tractor many times without being charged).
The calcium battery after being charged with a higher voltage, will be placed into service on a newer lawn tractor and the voltage checked to see to it that the battery is charging properly. The battery now tests good at 350 cca, and has 13.5 vdc open circuit reading.