A day before, crews forced a slow torrent of heavy mud down the broken wellhead from ships a mile above to push the crude back to its underground source. The cement was the next step in this so-called "static kill" and is intended to keep the oil from finding its way back out.
"This is not the end, but it will virtually assure us that there will be no chance of oil leaking into the environment," retired Adm. Thad Allen, who oversees the spill response for the government, said in Washington.
The progress was another bright spot as the tide appeared to be turning in the months-long battle to contain the oil, with a federal report this week indicating that only about a quarter of the spilled crude remains in the Gulf and is degrading quickly.
Even so, Joey Yerkes, a 43-year-old fisherman in Destin, Fla., said he and other boaters, swimmers and scuba divers continue to find oil and tar balls in areas that have been declared clear.
"The end to the leak is good news, but the damage has been done," Yerkes said.
If the mud plug in the blown-out well is successfully augmented with the cement, the final step involves an 18,000-foot relief well that intersects with the old well just above the vast undersea reservoir that had been losing oil freely since the Deepwater Horizon rig exploded off Louisiana on April 20, killing 11 workers.
The hope has been to pump mud and possibly cement down the relief well after its completion later this month, supplementing the work in this week's static kill and stopping up the blown-out well from the bottom.
It could take at least a day for the cement pumped into the blown well to dry, and another five to seven days for crews to finish drilling the final 100 feet of the relief well. Then the pumping process in the relief well could last days or even weeks, depending on whether engineers find any oil leaks, Allen said.
Despite the progress on the static kill, BP executives and federal officials won't declare the threat dashed until they use the relief well - though lately they haven't been able to publicly agree on its role.
Federal officials including Allen have insisted that crews will shove mud and cement through the 18,000-foot relief well, which should be completed within weeks. Crews can't be sure the area between the inner piping and outer casing has been plugged until the relief well is complete, he said.
But for reasons unclear, BP officials have in recent days refused to commit to pumping cement down the relief well, saying only that it will be used in some fashion. BP officials have not elaborated on other options, but those could include using the well simply to test whether the reservoir is plugged.
"We have always said that we will move forward with the relief well. That will be the ultimate solution," BP Senior Vice President Kent Wells said Wednesday afternoon. "We need to take each step at a time. Clearly we need to pump cement. If we do it from the top, we might alter what we do with the relief well, but the relief well is still a part of the solution. The ultimate objective is getting this well permanently sealed."
The game of semantics has gone back and forth this week, with neither yielding.
Allen clearly said again Thursday that to be safe, the gusher will have to be plugged up from two directions, with the relief well being used for the so-called "bottom kill."
"The well will not be killed until we do the bottom kill and do whatever needs to be done," he said, adding: "I am the national incident commander and I issue the orders. This will not be done until we do the bottom kill."
Whether the well is considered sealed yet or not, there's still oil in the Gulf or on its shores - nearly 53 million gallons of it, according to the report released Wednesday by the Interior Department and the National Oceanic and Atmospheric Administration. That's still nearly five times the size of the Exxon Valdez spill, which wreaked environmental havoc in Alaska in 1989.
But almost three-quarters of the nearly 207 million gallons of oil that leaked overall has been collected at the well by a temporary containment cap, been cleaned up or chemically dispersed, or naturally deteriorated, evaporated or dissolved, the report said.
The remaining oil, much of it below the surface, remains a threat to sea life and Gulf Coast marshes, NOAA Administrator Jane Lubchenco said. But the spill no longer threatens the Florida Keys or the East Coast, the report said.
Some outside experts have questioned the veracity of the report, with at least one top federal scientist warning that harmful effects could continue for years even with oil at the microscopic level.
But Allen said the estimates are based on the best data scientists had available and that they could be "refined" as more research is completed.
"Models are an approximation of reality and are therefore never perfect," he said.
An experimental cap has stopped the oil from flowing for the past three weeks, but it was not a permanent solution.
The static kill - also known as bullheading - probably would not have worked without that cap in place. It involves slowly pumping the mud and now the cement from a ship down lines running to the top of the ruptured well a mile below. A similar effort failed in May when the mud couldn't overcome the flow of oil.
Contributing to this report were Associated Press writers Jennifer Kay in Pensacola Beach, Fla., Mary Foster in Grand Isle, Tamara Lush in Tampa, Fla., Annie Greenberg in Miami, and Jay Reeves in Birmingham, Ala.