Industrial conglomerate Siam Cement PCL said Wednesday its board has approved a plan to invest THB3.2 billion in a new integrated cement plant in Cambodia.  Siam Cement will hold a 90 per cent stake in the venture, while Khaou Chuly Group, a Cambodian investor, will hold the remainder, Siam Cement said in a statement to the Stock Exchange of Thailand.  The cement plant is expected to commence operations by 2008 with an annual production capacity of 850,000t the statement said.