The warehouse, designed to house dangerous and toxic chemicals, was storing mainly ammonium nitrate, potassium nitrate and calcium carbide at the time of the blasts, according to police.
Chemical safety experts said calcium carbide reacts with water to create acetylene, a highly explosive gas. An explosion could be caused if fire fighters sprayed the calcium carbide with water, they said.
The official Xinhua news agency has said several containers in the warehouse had caught fire before the blasts.
Lei Jinde, the deputy propaganda department head of China's fire department, a part of the Ministry of Public Security, told state-backed news website ThePaper.cn that the first group of fire fighters on the scene had used water.
"We knew there was calcium carbide inside but we didn't know whether it had already exploded," he said. "At that point no one knew, it wasn't that the fire fighters were stupid," Lei said, adding that it was a large warehouse and they didn't know the exact location of the calcium carbide.
Xinhua has reported 18 firefighters remain missing, with 66 among the hundreds of people hospitalized.
David Leggett, a chemical safety expert based in California, told Reuters the acetylene explosion could have detonated the ammonium nitrate. The two blasts were about 30 seconds apart, the second much larger than the first.
"In my mind, the presence of ammonium nitrate makes it easier to explain the level of devastation," he said.
Read MoreScenes from the devastating Tianjin explosion
The explosions at the port, the world's 10th largest, were so big they were seen by satellites in space and registered on earthquake sensors.
A team from the International Atomic Energy Agency's Beijing environmental emergency response center, as well as 214 Chinese military nuclear and biochemical materials specialists, had gone to Tianjin, Xinhua said.
Several thousand residents were moved to 10 nearby schools after apartment buildings and homes were damaged, mainly by shockwaves from the explosions, it said.
Foreign companies from across the globe were trying to determine the damage to their facilities in and around the port, a gateway to northeast China.
Toyota said windows were broken at its passenger vehicle assembly plant but no one was hurt. The Nikkei newspaper said numerous other Japanese companies sustained minor damage to their operations.
French carmaker Renault said more than 1,000 of its cars were damaged but no staff were hurt. Around 4,000 cars belonging to Hyundai were at the site but damage had not yet been assessed, a spokesman said.
Mining giant BHP Billiton said its port operations and shipments were disrupted but its iron ore discharging berth had not been damaged.
Oil tanker arrivals and departures were also disrupted.
John Deere & Co, the U.S. farm and construction equipment maker, said several workers who were at home at the time were injured, some critically.
Xinhua identified the owner of the warehouse as Tianjin Dongjiang Port Ruihai International Logistics. The state-backed China Daily newspaper said its manager had been detained.
The Tianjin Maritime Safety Administration said the company violated packaging standards during a safety inspection two years ago.
Death toll in China's Tianjin explosion rises to 50: Reports
Of 4,325 containers that were checked, five failed the inspection because packaging was sub-standard, according to a report posted on the administration's website in January 2014.
The company's website said it was a government-approved firm specializing in handling "dangerous goods". Phone numbers listed on its website were disconnected and an email to the company went unanswered.
President Xi Jinping said those responsible should be "severely handled".
Tianjin city officials had met recently with companies to discuss tightening safety standards on the handling of dangerous chemicals, authorities said.
Industrial accidents are not uncommon in China following three decades of breakneck economic growth. A blast at an auto parts factory in eastern China killed 75 people a year ago when a room filled with metal dust exploded.
Wednesday's blasts sent shockwaves through apartment blocks kilometres away in the port city of 15 million people. Internet videos showed fireballs shooting into the sky.
Anxious residents rushed to hospitals to seek news about injured loved ones. Dozens of police guarded the entrance of the TEDA hospital, a Reuters witness said.
The blasts shattered windows in buildings and cars and knocked down walls in a 2-km radius around the site.