A Chinese multi-millionaire who built himself an Egyptian pyramid and a replica of Versailles vows to construct the world's tallest building in just six months -- despite authorities preventing work amid safety concerns.

Zhang Yue is worth an estimated 1.1 billion yuan ($180 million)and has grandiose aspirations, the biggest of them to build an 838 metre tall tower he calls "Sky City" by the year's end.

It is designed to be 10 metres higher than the current title-holder, the Burj Khalifa in Dubai -- which took five years to construct.

But he admits the project has run into fierce opposition. "There are not many people who support us," Zhang told AFP. "There are too many bad people."

Zhang, 53, made a fortune selling air-conditioners and was the first Chinese entrepreneur to own a private helicopter, but has sought to reinvent himself as a green crusader.

He retains a down-to-earth manner, eating in a staff canteen and spitting casually into a tissue as he talks, but sees himself as a visionary hoping to reshape China's cities.

The decades-long movement of hundreds of millions of people from China's countryside to its towns and conurbations is the largest migration in human history, and both a cause and effect of its economic boom, but he sees it as a road to environmental disaster.

"We have to quickly move out of this mistaken kind of urbanisation," he said, describing Sky City, with energy-saving materials and reduced use of land, as "one such way out".

Zhang's company Broad Sustainable Building has already built a 30 storey hotel in 15 days in the central Chinese city of Changsha.

A time-lapse video of the construction has been viewed more than five million times on Youtube and shows the concrete and metal sections being slotted into place and bolted together, akin to a gigantic Lego set.

"Our aim is not making money," he said, lounging in bare feet on the hotel's top floor, as thick grey smog -- a common sight in Chinese cities -- blurred his view of surrounding fields.

"Once you have environmental consciousness, money loses meaning."

A short man who appears to have difficulty staying still, Zhang sipped from a giant cylinder of tea as a chauffeur drove him past the 130-foot high pyramid he built on his corporate campus.

Opposite it stands a replica of France's Palace of Versailles, designed by his wife, which Zhang plans to turn into an "environmental philosophy academy", although for now it hosts a display of North Korean paintings.

Zhang -- who has renounced his helicopter citing concerns about climate change -- is "not far off being an environmental activist", said Rupert Hoogewerf, compiler of the Hurun Report, an annual Chinese rich list.

China's wealthiest -- many of whom have been millionaires for more than a decade -- are attempting to influence social and other issues, he said.

"These are people who have the sense they have everything financially and materially they could ever want and are now looking beyond that, to legacy and extended status."

"I could make an even taller building"

Close to Zhang's office, workers in a cavernous hangar welded together the pre-fabricated building sections, and he insisted there would be "no problems" using the method to build Sky City.

"We will be finished by December," he said. "I could make an even taller building."

Construction was formally launched last year, but rapidly suspended and state-run media reported authorities in Changsha had ordered a halt as it lacked proper permits.

Independent engineering experts say the Sky City concept faces a host of problems, from lift design and fireproofing to the physical compression caused by the monumental weight of the completed building.

An audience "laughed" when Zhang's plans were first presented at a meeting of the US-based Council on Tall Buildings and Urban Habitat (CTBUH), said David Scott, a structural director at engineering firm Laing O'Rourke.

But now "people understand it's a much more serious offer", he said. "I wouldn't say it's not feasible... Properly thought through, it will work."

China's earthquake-proofing standards for skyscrapers are amongst the world's strictest, he added, and construction may have been delayed by a mandatory expert review of the project.

CTBUH director Antony Wood said: "I'm still sceptical, but it's with a massive amount of respect for what (Broad Group) has done so far. I'm not inclined to write it off."

In the hotel, Zhang -- who studied art and first worked as an interior decorator -- brandished one of his firm's egg-shaped smartphones, which can gauge levels of tiny air pollutants known as PM2.5.

It was an attempt to demonstrate his building's immaculate air quality -- although the demonstration was rendered more difficult by the cigarette he had just smoked.

Pollution is a hot-button issue in China, but Zhang still feels victimised and misunderstood.

"In this society, if you try to do something good, no one will believe you," he said. "Society lacks basic trust, and sees everything good as bad."