Siam Cement, one of Thailand's premier blue chip stocks, said it will press on with new petrochemical projects despite a bleak market outlook stemming

from lower-priced foreign competition and local surpluses.

The Thai construction materials giant has invested more than $300 million in wholly owned or joint venture petrochemical projects since 1987, and will spend at least another $134 million in the next two years.Siam Cement's consolidated and unconsolidated third-quarter results are expected later this week and are anticipated to be sharply lower than last year's.

Siam Cement executives said the company will stick to its investment

plans even though most downstream products from its new projects face competition from foreign producers.

A large chunk of Siam Cement's expansion budget in the next few years will go to its petrochemical projects, including a $60 million polystyrene plant, owned equally by it and Dow Chemical Co.

Siam Cement is also spending $55 million on expanding a plant of its Thai Polyethylene Co. unit to add a plastic resin, a low-density polyethylene, to its product line.

Siam Cement is building a $20 million midanine plant through a joint venture with Sri Thai Superware PLC, Nippon Carbide Industries Co. and Nomura Trading.

"Prices of most petrochemical products we're involved in are at their rock bottoms . . . Cutthroat rivals in this sector are crying for blood," said Apiporn Pasawat, Thai Polyethylene's managing director.

Foreign rivals have a lower cost edge over Siam Cement, because of their economies of scale. This means foreign products are price competitive here, despite Thailand's 40 percent import tariff.

Mr. Apiporn said polyol, another plastic material produced by a separate Siam Cement-Dow Chemical venture, faces a saturated market in Thailand, where two suppliers are producing 40,000 metric tons a year against a domestic demand of 10,000 tons.

''We, as a Thai company, are investing in these plants to meet the country's future needs even though they may not make much commercial sense now," a Thai Polyethylene executive said.

The surpluses have prompted Dow Chemical to pull back from an earlier plan to hold a 49 percent stake in Siam Styrene Monomer Co., another joint venture with Siam Cement, in a $230 million styrene plant tentatively due to open in 1998.

Dow Chemical has told Siam Cement it will reduce its equity share in the project to 25 percent, partly because of a similar but bigger facility planned by Royal Dutch Shell Group in Singapore.

"We're concerned about the styrene monomer project," said Dow Chemical executive Noel Williams. "We'd normally not need another plant . . . but we're committed to taking at least a 25 percent interest in it."

"Prices of petrochemical products are competitively low," the executive said. "The issue is Thai import tariffs. Although our plants here need to be competitive, an industry in its infancy should have some tax protection" against imports.