Lafarge Corp., a Reston-based cement and construction materials company, agreed yesterday to purchase $135 million worth of cement plants and transportation terminals from a Swiss company.

Ownership of Lafarge and the Swiss company, Cementia Holding AG, was already intertwined. Last year, Lafarge's French majority owner, Lafarge Coppee, purchased a 60 percent stake of Cementia. The properties Lafarge is buying are the bulk of Cementia's U.S. holdings.

The acquisition appears to be a move by the parent company to consolidate its U.S. operations, said David L. Beeghly, an analyst who follows Lafarge for the regional brokerage firm Ferris, Baker Watts Inc. in Baltimore. "It would just make sense for Lafarge to oversee them in the United States," he said.

But Katrina Luba Farrell, a Lafarge spokeswoman, said that is not the case. The deal was negotiated directly by Lafarge and Cementia.

The deal, which is scheduled to close in January, will increase Lafarge's annual cement production capacity by 20 percent to more than 15 million tons. The company had sales of $1.5 billion last year and currently has 8,500 employees, only 70 of whom work in the Washington area.

Among the assets Lafarge will acquire are two Mississippi River Valley cement producers and 16 transportation terminals along the river.

Since cement production relies on the health of the construction industry, it is cyclical. Access to the transportation network along the Mississippi will allow the company to shift distribution between markets without closing plants, Farrell said.

To finance the purchase, Lafarge will issue 4 million shares of common stock and assume up to $95 million of Cementia's debt. That move concerned Beeghly, who said the company has already over-leveraged itself with acquisitions over the past five years. "My worry is that the share price is below book value. ... You're diluting shareholders' stock a lot more than you should." In the past five or six years, Lafarge has increased the amount of stock it has outstanding by 50 percent, Beeghly said.

Lafarge's earnings have been hurt by the slowdown in the construction industry. The company's profit in the first nine months of this year was $43.3 million, down 45 percent from $79.6 million during the same period of 1989. Its stock price, off 34 percent this year, jumped 50 cents yesterday to close at $11.87 1/2.