Travelling to Saskatoon (Canada) from Colombo (Sri Lanka)

If Beijing is your only stop in mainland China, you are definitely eligible for 24 hour visa free transit and you don't need any kind of visa. By the way, you should be more precise when presenting your itinerary because both of us know that there is no direct flight from Beijing to Saskatoon. In other words, you will definitely have additional landing(s) somewhere else. OK, it is important that Beijing remains your only stop since your layover is 18 hours long.

Assuming that you will take UL868 from Colombo to Beijing, you will probably travel on two separate tickets. If this is a case, then you will apply for a temporary entry permit upon your landing in Beijing at 24 hour visa free transit counter to be allowed to exit the airport and to collect your luggage. Also, you are advised to book your accommodation for an overnight in Beijing and to have a confirmation with you.