German insurer Allianz AG has decreased its stake in building materials group HeidelbergCement AG stepwise to 4.6 per cent in a move to optimise its portfolio, Allianz said on December 23, 2004.   The sale could result in a possible book profit for the insurer, an Allianz spokesperson said without elaborating.   Based on its market price on December 22, 2004, the stake’s value has been estimated at almost Euro130m ($173.6m).  In February 2004, the insurer held a 7.5 per cent stake in HeidelbergCement.