As a rule, I don’t get worked up over oil refineries. But the one gradually taking form on 2,500 hectares of swampland outside Lagos, Nigeria’s Mad Max commercial capital, is so big, so audacious and so potentially transformative that it is like Africa’s Moon landing and its Panama Canal — a Pyramids of Giza for the industrial age.

If Aliko Dangote, the billionaire businessman behind what even he calls his “crazy” $12bn project, can pull it off, he will go down as the continent’s John D Rockefeller, Andrew Carnegie and Andrew Mellon combined. And once he’s built it, he intends to treat himself to a small indulgence: he’ll buy Arsenal, his favourite football club.

“When we finish this project, for the first time in history Nigeria will be the largest exporter of petroleum products in Africa,” he tells me, summoning the drabbest of platitudes for a project of pharaonic ambition. I am sitting with Africa’s richest man discussing his life of superlatives over Thai food on his 108-foot yacht, moored in Lagos Lagoon. Yet the image he projects is more like a modestly successful encyclopedia salesman. When I arrive at the dock, Dangote, a Muslim, is praying in his quarters. He soon comes out to greet me and turns out to be the most solicitous of hosts. “Feel at home,” he says. “We can hang that for you,” he adds, when I place my crumpled jacket on the yacht’s white leather couch. “Can we offer you something to drink?” Dangote goes through the options arranged before us: “There’s vegetable spring rolls, chicken wings in a barbecue sauce, green Thai curry, some kind of seafood salad, noodles,” he says. “You are my guest, so what do you want? You want rice? Plain jasmine or with egg?” I plump for the jasmine.

“You don’t eat egg?” he asks. You asked me to pick one, I protest. “That looks like satay,” I say, pointing to one of the plates. “It is satay, actually.” He spoons me out several of the dishes — it’s always fun to be served by a billionaire — and we dig in. It’s tasty, and there’s a plate of green chilli sauce to liven up proceedings. Dangote crunches into a spring roll and ignores the gently buzzing phone on the table. A few numbers on the refinery will help illuminate the scale of his “craziness”. When it is up and running — if it gets up and running — it will process 650,000 barrels of oil a day, a third of every drop Nigeria produces and approaching 1 per cent of planetary production. That will make it the biggest oil refinery of its type in the world. As a sort of side concern, it will pump out all the plastic Nigeria’s 190m people need (or imagine they need), plus 3m tonnes of fertiliser a year, more than all its farmers currently sprinkle on their fields. To make things more interesting, Dangote is building the whole thing on a swamp. (It’s a tax-friendly swamp, at least.) That requires sinking 120,000 piles, on average 25 metres in length. No port in Nigeria is big enough to take delivery of the massive equipment, which includes a distillation tower the height of a 30-storey building, and no road is strong enough to bear its weight. Dangote has had to build both, including a jetty for which he has dredged the seabed for 65m cubic metres of sand.

There is not enough industrial gas in the whole country to weld everything together, so Dangote will build his own industrial gas plant. There aren’t enough trucks, so he’s producing those in a joint venture with a Chinese company. The plant will need 480 megawatts of power, about one-tenth of the total that electricity-starved Nigeria can muster. You guessed it. Dangote is building his own power plant too. For years — and absurdly — Nigeria has exported all its oil as crude and then reimported refined petroleum, such as petrol and benzene. That has been a lucrative racket for the middlemen who scheme over import contracts and who concoct ways to scam a system distorted by subsidies. “I’m sure you know about this game,” Dangote says. Because of its reputation for skulduggery, he says, he has shunned the oil trade. “It is very simple to destroy a name,” he adds, referring to a family business that stretches back to his great-grandfather on his mother’s side, Alhassan Dantata, a prodigiously wealthy merchant who imported kola nuts from Ghana and exported groundnuts from Nigeria. “But it’s very difficult to build it.”

He tries to fast at least once a week, he says, looking guiltily at our feast. “It helps to clean your system. More peanut sauce?” Many of today’s billionaires spin their fortunes from intangibles: the internet, the media, banking or hedge funds. Dangote has made his money from more prosaic things: salt, sugar, flour and, above all, cement. An awful lot of cement. He was born in Kano, an ancient trading town in northern Nigeria, where he was brought up by his grandparents after his father died when Dangote was eight. After studying business at Al-Azhar University in Cairo, he moved to Lagos to strike out on his own. He too became a trader, but unlike the other businessmen whose fortunes were built on import licences available to the friends of politicians, Dangote had a hankering to make things.

Now 61, building his refinery is the culmination of that ambition. It will produce every litre of refined petroleum Nigeria needs, which could end the import business at a stroke, saving the country billions of dollars in foreign exchange. Won’t he make enemies of those he is depriving of easy money? “You can’t just come and remove food from their table and think they’re just going to watch you doing it,” he says. “They will try all sorts of tricks. This is a very, very tough society. Only the toughest of the tough survive here.”

Most Nigerians assume that Dangote is tougher than the next guy. While to many he is a hero who builds factories, employs thousands and reinvests his money at home, to others he is a villain: a ruthless monopolist who squeezes favours from the government of the day and crushes competition like limestone in a cement mixer. Some accuse him of avoiding taxes by invoking an investment incentive known as “pioneer status”.

Others say he is more of a rentier than an entrepreneur, gouging the country with high prices and raking in ludicrous profits. “People throw a lot of mud at you and you have to see how you can clean it up,” he says of his detractors. In person, he is charm itself, a soft-spoken man with a pleasantly round face, close-cropped hair and a greying moustache so delicately trimmed that it is almost not there. He projects integrity and humility, even piety. I’ve met mere millionaires with more swagger than him. Yet Dangote is a billionaire 14 times over and the 100th richest person in the world, according to Forbes. He is a networker extraordinaire. To watch him work a room is to witness a kind of genius. He irradiates a Dickensian bonhomie as he glides from table to table, picking up goodwill — and intelligence — with each pressing of the flesh. If there are competing obligations — the wedding reception of the daughter of a Big Man, a dinner for the vice-president, a foreign investors’ post-conference gala — he manages to be at all three events at once, an apparition moving unhurriedly through the room as though he has all the time in the world. Like Bill Clinton, he remembers your name; like Al Capone, he’s got your number. Even Dangote’s yacht — named Mariya, after his mother — manages to be understated, if such a thing is possible in a 108-foot vessel with a price tag, according to Lagos’s gossipy tabloids, of $43m.

It was styled after a boat owned by fellow Nigerian billionaire, Femi Otedola, though intriguingly Dangote had his built a few feet shorter. He makes no secret of how he got his big break, one that transformed him from a wealthy man — and by all accounts a bit of a dilettante — into a business colossus whose interests straddle the continent. It happened one day not long after the election in 1999 of Olusegun Obasanjo, the former military leader who had embraced the country’s lurch to democracy by running for the presidency. Dangote contributed both to that campaign and to his subsequent re-election in 2003. “Obasanjo called me very early in the morning and said, ‘Can we meet today?’”Š” says Dangote, recalling the presidential summons.

He wanted to know why Nigeria couldn’t produce cement, instead importing it by the boatload. Dangote told him it was more profitable to trade than to produce. Only if imports were restricted would it be worthwhile. Obasanjo agreed. Dangote has never looked back. Now Africa’s undisputed King of Cement, he produces in 14 countries. I hear that the business makes 60 per cent margins, I say. He waves the number away. “We have a margin of 47 per cent,” he says, as if that were a mere bagatelle. No one else can compete on efficiency, he says. Critics say Nigeria pays more for cement than it ought to, slowing investment in construction and housing.

When I put that to him, he immediately reaches for his phone, checking out today’s prices in Ghana, Benin and Ivory Coast. His own price is competitive, he says, adding that people often forget the high transport costs of importation. Muhammadu Buhari, the current president, despairs of a manufacturing base that has shrivelled as a consequence of oil addiction, bemoaning that Nigeria even imports toothpicks. “What Nigeria needs is to produce locally what we can produce locally,” Dangote says, nibbling at a skewered satay, and defending the thinking that has made him rich. “Nigeria still imports vegetable oil, which makes no sense. Nigeria still imports 4.9m tonnes of wheat, which does not make sense.

Nigeria still imports 97 or 98 per cent of the milk that we consume.” Of the latter (astonishing, considering the country’s roughly 20m cows), he says, “The government needs to bring out a draconian policy to stop people importing milk, just like they did with cement.” His phone is still vibrating. This time he takes it. He’s flying to India the following morning on his private jet and is making final arrangements. While he’s talking, I take a second helping of the seafood salad, a ceviche-like dish of calamari and succulent prawns marinated in a wincing sauce. “It has been very, very, very hectic,” he says of his recent schedule. Only that morning, his doctor warned him to slow down and get more sleep. He reckons he rarely gets five hours a night. “The heart, it keeps pushing and pushing and pushing, but there must be a limit.”

Often he’s firefighting. Problems erupt in one country or another and he is constantly criss-crossing the continent by jet. In Tanzania, where he’s built a $650m cement plant, he’s battled with the president over a threat to seize assets. Not long after I met Dangote, his country manager in Ethiopia was murdered. When he’s not dealing with crises, he’s fending off friends and relatives, who are often seeking help of a pecuniary nature. “People call me in the middle of the night to tell me about their problems,” he smiles wryly. Tony Blair, the former British prime minister and a friend of Dangote’s, told him he needed to screen his calls. “Tony said he only makes three phone calls a day,” Dangote says incredulously, helping himself to noodles. Each day scores of emails come rat-tat-tatting in. “You try to be polite and reply but they come back to you with a longer email, not minding that here is a very, very busy person,” he says mournfully. He reckons that he takes more than 100 calls a day. “”Š‘Look Aliko’,” he says Blair told him, “”Š‘the world is not going to fall apart if you don’t answer your phone.’”Š”

Dangote’s schedule is also inhibiting romance. Twice divorced and with three grown-up daughters, he’s on the lookout for a new bride. “I’m not getting younger. Sixty years is no joke,” he says, “but it doesn’t make sense to go out and get somebody if you don’t have the time. Right now, things are really, really very busy, because we have the refinery, we have the petrochemicals, we have the fertiliser, we have the gas pipeline.”

With sweet talk like that, I think to myself, it can’t be long before he wins some lucky woman’s heart. “I need to calm down a bit.” His ambitions are changing. He is talking about pulling back from the business, concentrating on strategy and letting others run things day-to-day. “I’m trying to step back from some of the boards.” He will float the cement business in London, perhaps by the end of this year, and has already appointed independent directors — including Blair’s wife Cherie — to help satisfy London’s pesky governance requirements. He remains Nigeria’s strongest advocate, though he consistently denies political ambition. If he ran for president, you wouldn’t bet against him. “Nigeria has always had a lack of visionary leadership,” is the closest he’ll come to declaring political intent. “There’s no country in Africa that has the energy of here. Nowhere, I’m telling you.”

He is less coy about another ambition: his designs on Arsenal, a Premier League football team he has long supported. “I love Arsenal and I will definitely go for it,” he says matter-of-factly, as though discussing the latest model of iPhone. He reckons it’s worth about $2bn. Long frustrated with the club’s decline under Arsène Wenger, the recently replaced manager, he says that as owner, he would involve himself in rebuilding the team — “chipping in my own advice”, as he puts it. “When I buy it, I have to bring it up to the expectations of our supporters.” But first he has a refinery to build. “When you visit, you’ll see what a headache I am talking about,” he says of a project into which he has sunk more than $6bn of his own money. “Once I have finished with that headache, I will take on football.”