Abstract: The invention provides for metering the pumping rate of BFS/mud mixtures pumped to the cementing head on a rig for sealing off the formation from the casing. Since BFS/mud mixtures require an activation agent, preferably sodium hydroxide is pumped into the flowline to the cementing head. Feedback from the BFS/mud slurry flowrate is fed to the activator injector pump drive system to regulate the addition rate of the activating agent in response to varying conditions, such as mud flowrate. The injection rate target can also be adjusted to compensate for any changes in downhole temperature. The risk of fouling surface equipment and the casing internal diameter is minimized because the activation agent is added very close in time to when the BFS/mud mixture enters the cementing head.