The force of the explosion was so great that it blew Mr Reynolds out through the side of the building onto the road 10 metres below. An ambulance crew attended but he was pronounced dead at the scene.
Leamington Spa Crown Court heard the married man, whose daughter Kayleigh was 18 months old at the time of his death, was treating waste cement dust in the bypass dust plant at the company's Rugby Cement Works in Lawford Road.
While he was clearing a blockage in the lower mixer, there was a violent explosion of steam and dust from inside the machine.
HSE's investigation into the incident found that Cemex had recognised the potential for blockages to cause explosions as steam pressure built up within the mixer, but took no action to prevent them.
The court also heard the company had failed to review its risk assessment following a previous incident in May 2006, when another man was injured using the same machine. This explosion bent a metal-cladded external wall, pushing it out by 50cm.
Speaking after the hearing, HSE principal inspector Neil Craig said: "This was an entirely avoidable tragedy, which has left a young family without a husband and a father. If CEMEX had investigated the previous incident properly, Mr Reynolds would still be alive today.
"Cemex's protection against the build up of pressure was for the plant to be continuously vented when processing waste cement dust, but it frequently blocked. These blockages then caused steam to build up to a high pressure.
"The company could have made a number of changes to the mixer to reduce the flow of dust and improve the venting and cooling systems, or devised a new system of work. However, no action was taken and employees were expected to operate this dangerous piece of machinery.
"Employers must make sure that they take proper precautions to protect their workers, especially when there is the potential for explosion."