The largest U.S.-owned cement company filed a federal lawsuit accusing a Dallas suburb of unconstitutional discrimination for favoring the purchase of cement made in a more environmentally friendly process.

Plano is the latest of several local governments, including Dallas and Fort Worth, to be sued by Overland Park, Kan.-based Ash Grove Cement Co. over "green cement" laws. Ash Grove claims purchasing polices that give preference to cement from "dry kilns" that typically pollute less than older "wet kilns" violate state competitive bidding laws and don't clean up the air.

"With little to no consideration of state and federal public contracting rules, Plano adopted a cement purchasing resolution that is not only unconstitutional, but also highly prejudicial to Ash Grove," the company said in a statement.

Ash Grove has only wet kilns at its operation just south of Dallas that is part of the largest single concentration of cement plants in the nation.

Plano's city attorney did not return a message seeking comment Wednesday, but environmentalists said the suit and others like it are simply ploys designed to stop other cities from taking action.

"Ash Grove has no hope of winning this lawsuit," said Jim Schermbeck, a member of the environmental group Downwinders at Risk that closely monitors the cement plants. "What they do have hope of is putting this in the freezer so long that nobody passes any more green cement rules. It's hard to convince a city council or any governmental body to run headlong into a federal lawsuit."

Schermbeck said Ash Grove filed its first suit in November out of desperation, only after local governments repeatedly and overwhelmingly adapted the green laws.

Ash Grove said in its suit that Plano is acting illegally by giving preferential treatment based on something outside the competitive bidding process. The lawsuit said "only the competence of the bidder and quality of the price" can be considered. The company seeks an injunction stopping Plano from enforcing its resolution, which it says passed after just 10 minutes of discussion in April 2008.

Ash Grove said in the lawsuit that Plano's objective to clean up the notoriously dirty air in the Dallas-Fort Worth area was "laudable," but that the resolution "has done little, if anything to actually improve the air quality. Instead, what the resolution has accomplished is to unfairly stifle competition in the cement industry and jeopardize jobs and economic growth."

Ash Grove said in its suit that it has reduced smog-causing nitrogen oxide emissions drastically since 1996.