A sand scandal is brewing in China, with concerns that low-quality concrete has been used in the construction of many of the country's largest buildings – putting them at risk of collapse.

The recipe to make concrete is pretty simple – cement, aggregate and water – but the strength of the final batch can vary wildly depending on the kinds of aggregate and cement used and the proportions they're mixed in. Commonly the aggregate used in many modern building projects consists of crushed gravel or other rock, including sand, and that's the cause of so much distress in the Chinese construction industry at the moment. Inspections by state officials have found raw, unprocessed sea sand in at least 15 buildings under construction in Shenzhen, including a building which, when finished, was set to become China's tallest.

[partner id="wireduk"]The Ping'an Finance Center is planned to top out at 660m, making it not only China's tallest building but the second-tallest building in the world after the Burj Dubai. 80m has been built so far, but construction has been halted in the wake of the revelation from Shenzhen's Housing and Construction Bureau that substandard sea sand concrete had been used in its construction. According to a notice on the Bureau's website posted on March 16, 31 companies had had their licenses to work revoked for at least six months.

Many of the companies named in the notice took heavy hits to their share prices on the Hong Kong stock exchange. Bloomberg reports that the financial incentive to use illegal sea sand, which is far cheaper than legal river sand, is the likely reason for the problem. Untreated sea sand is unsuitable for construction because it still contains chlorine and salt, which corrodes steel – river sand from freshwater channels doesn't have that problem.

It can take only a few decades for a building to become dangerously unsafe if untreated sea sand is used in its concrete – including the possibility of collapse. While this scandal has been confined only to Shenzhen thus far, the possibility of it spreading to other Chinese cities is cause for concern. The country currently has nine of the 20 tallest buildings in the world under construction, while there were reportedly so many skyscrapers under construction in 2011 that it worked out as a new one being topped out every five days right through into 2014.

The Ping'An Finance Center is one of several very tall skyscrapers under construction in China, but it might not have the title of the country's tallest for too long even if construction resumes. The highly publicized plan by Broad Sustainable Building to build the world's tallest building, Sky City One, in only 90 days in Changsha, attracted skeptical responses from engineers who felt it was completely unrealistic – the head of the company behind construction of London's Shard even joked that he'd retire if Broad pulled off the stunt. However, he can rest easy, as the skyscraper has yet to break ground, as it continues to be delayed.