Vietnam sold 2.9Mt of cement last month, bringing the total it has sold in the first four months of this year to nearly 9Mt, a Government market managing group has reported.  Despite the huge demand for cement, the selling price of this construction material has remained stable at 725,000-760,000 VND a tonne in the northern market and 840,000-930,000 VND a tonne in the southern market.   Experts predict that the price of cement will not increase in May despite the high prices of constituent materials, including imported clinker, which now stands at around US$28.5 a tonne.