Many of the fire trucks seen at the site hours after the blasts use a kind of firefighting foam.
Authorities have not said what caused the explosions, saying only that they originated at the warehouse owned by Ruihai International Logistics. Its website says the company is authorized to handle chemicals ranging from flammable gases and liquids, including compressed natural gas and ethyl acetate, to chemicals that explode on contact with water, including sodium cyanide and calcium carbide.
However, the warehouse was a transit point and authorities have no clear picture of what was there at the time of the blasts. Records at the site were destroyed in the disaster, meaning that authorities must rely on customs documents, said Gao Huaiyou, deputy director of occupational safety in Tianjin.
Fire officials so far have confirmed only that calcium carbide, potassium nitrate and sodium nitrate were at the warehouse.
Zhou said further firefighting efforts must go slowly because of the potentially complex mix of chemicals at the site.
"Many types of different materials with different characteristics are mixed together and could at any time result in a chemical reaction or explosion," Zhou said.
At one point early Friday evening, a small explosion could be seen inside the cordoned-off zone, in what may have been a controlled blast by firefighters.
In an interview with the Shanghai-based publication The Paper, a fire official at the Ministry of Public Safety, Lei Jinde, was quoted as confirming that the first wave of firefighters used water to cool down areas on fire. When asked whether they knew there was calcium carbide at the warehouse, he said yes and that firefighters would not have sprayed water on the substance.
When asked whether the initial firefighter response could have sparked the blasts, Lei said "No. The response procedure must have been scientific."
On the day after the blasts, Tianjin environmental protection chief Wen Wurui told a news conference that there had been no apparent contamination of the air, based on samples taken at five air monitoring stations throughout the city. He also said all waterways leading from the disaster site to the sea had been sealed to contain any potential water leaks.
Tianjin, with a population of about 15 million, is being promoted by the Chinese government as a center for finance and high-tech industry. The Tianjin Economic Development Area has attracted foreign investors including Motorola, Toyota, Samsung and Novozymes.
Toyota spokesman Itsuki Kurosu said Friday that more than 50 of its employees in Tianjin were among the injured, though the automaker was still confirming the extent of the injuries.
Many of the hundreds of people hurt in the explosions were cut by shattered glass.
Associated Press writers Ian Mader, Didi Tang and news assistant Yu Bing in Beijing and writer Erika Kinetz in Shanghai contributed to this report.