What happens to my lost iphone after i get a new replacement and wireless carrier blocks my lost phone? Will the thief be able to use that phone?

I could not be for certain since I'm in the US. Either way, if your iPhone was replaced by insurance, it is no longer yours and nothing to worry about. If it wasn't, you probably won't see it again anyway. The only way to be sure it can't be used is to keep Activation Lock on it.

They may be able to if they try to use a carrier that has not had the IMEI blacklist shared with them. Most of the iPhones stolen in the US are sent to Europe and the US blacklist is not shared there. However, if you did not remove it from your account and Activation Lock is still active, they will be able to wipe it, but they will not be able to activate it. Generally those types of devices are then cannibalized for what useful parts they can get.