The city government's move caught the construction sector off-guard and cast a shadow over the prestigious multibillion-dollar Ping An project, with many analysts now wondering if it will be completed on time.

There were 92 concrete-mixing operations in the city, the Shenzhen Housing and Construction Bureau said, including the 15 tainted companies.

Last year, the bureau revoked the licences of another 16 concrete companies for failing to meet the required standards.

One Shenzhen-based property agent said: "I think it's going to be the milk powder scandal of our industry. If the Ping An project is found to use even one particle of substandard sea sand, market confidence will collapse."

About 200 buyers of flats in Jiayu Garden in the city's Longgang district took to the streets on Sunday calling for a refund after CCTV named the complex as an example of housing projects using untreated sea sand.

Some owners of flats at the Spring Land development in Baoan district said sea sand was also used in their buildings.

"We'll petition the city government along with other affected property owners," one flat owner said.

In December, a TV station in Foshan reported that sea sand was in concrete that was used in more than 80 per cent of the city's residential and commercial blocks. More than 1,000 property owners of four estates in the city launched protests in December but the municipal government dismissed the complaints, saying there was not enough evidence.

In June, reported a triad trial gang had been illegally digging and trading sea sand in Shanwei.