State-owned savings bank Caixa Geral de Depositos bought 20 million shares of cement company Cimpor SA more than tripling its stake to 4.6 per cent.   In a regulatory filing Cimpor said CGD bought 20 million shares in an over-the-counter transaction, bringing its total holding to 30.8 million shares. That represents 4.59 per cent of Cimpor, the statement said.  Cimpor shares finished down 0.7 per cent Friday, at Euro 5.66.