DONALDSONVILLE — High above the expanding CF Industries fertilizer complex in Ascension Parish, two new stacks rise 395 feet, well above the sprawling facility’s other shimmering towers and twisting pipes of steel.

Encased in a supporting metal framework, the dual stacks serve as one of many exhaust pipes for the plant and are one of most visible indicators of the $2.1 billion expansion that company officials say is rounding into completion after more than three years of construction.

Government officials in Ascension have been bracing for the day when CF Industries and a multi-billion dollar run of other expansions wind down and, with them, end a major haul in sales tax collections. But the expected slowdown in tax revenue hasn’t happened just yet.

Ascension Parish school officials said this week they expect the 2015-16 fiscal year, which ended Thursday, to bring in about $72.5 million in sales tax revenue.

Though they had planned on $3 million decline in collections from the school system’s 2-cent sales tax, school officials now say they will end up $1.3 million above last fiscal year’s all-time record.

Diane Allison, the school system’s director of business services, said the higher-than-expected collections will contribute to a $4 million surplus. Based on board policy, at least a portion of that money could wind up earmarked for school construction in the fast-growing district of more than 22,000 students.

Even with the $120 million bond issue that parish voters approved in April for new schools, Chad Lynch, school system director of planning and construction, said the system still has a long list of other needs, including roofing, parking lot paving and smaller school expansions.

“I got miscellaneous onesies and twosies out there for days,” Lynch said.

Mark West, administrator of Ascension Parish’s sales tax collection agency, said he had cautioned local governments last year to budget conservatively as major projects wind down.

But he said the strong collections seen by the School Board still largely reflect these big projects as plants, though finished with construction, may be ordering equipment, processing chemicals and other items related to the new facilities. The equipment may not show up until near the end of construction or even afterward — when the sales tax is collected — as plants are being started up.

“Just to use as an analogy: You buy a brand new house, and the next thing you know you’re buying new furniture and you’re buying window treatments and all this other stuff, which costs you money,” West said. “So even though you put your money into buying the house, that doesn’t mean you’re not seeing additional expenses. Same thing with the plants.”

West noted that collections from industrial suppliers are down this year while collections from the plants themselves remain strong. That’s a sign, he said, that construction-related activity is ebbing and plants are making those equipment and chemical purchases.

Separate from the plant expansions, though, sales tax collections from retail and automotive sales continue to see steady increases, West said, an indication other parts of Ascension’s economy continue to grow.

Despite the upbeat returns through the first half of the calendar year and a $717 million expansion of Shell’s Geismar complex starting this year, school officials expect a slowdown for the 2016-17 fiscal year.

Diane Allison, the school system’s director of business services, said she is projecting a $9.5 million drop in sales tax revenue and state funding through the Minimum Foundation Program, Louisiana’s formula to pay for public schools.

Chris Close, CF Industries spokesman, said the company’s expansion in Donaldsonville is expected to end sometime in 2016, though the company has not set a more precise date.

CF Industries has two of the three units built in the expansion already in production. The third, an ammonia unit, is “mechanically complete” and going through startup procedures.

Though a few large cranes remain at the site, Close said those are being used to move equipment around and set up and take down scaffolding for painting.

“We’re nearing the finish line there,” Close said.

Follow David J. Mitchell on Twitter, @NewsieDave.