a continuing series for the gravitationally impaired
Life for a rocket engine nozzle throat is pretty rough. The heat is intense, the pressures are high, and the gas velocities are at, well, Mach 1. With the hot combustion chamber gas trying to squeeze through the throat there isn't much of a calm cool boundary layer of gas to protect the throat wall. The sonic shock wave that forms in the throat moves forward and back depending on the local gas conditions literally scrubbing the throat walls. Yep it's a rough life.
Regenerativly cooled chambers are designed to concentrate their cooling ability at the throat. Most simple uncooled chambers resort to graphite the withstand the abuse. The use of cheap common cement as a nozzle material would result in throat erosion, lowering the combustion chamber pressure and reducing thrust. However, in our pursuit of a Minimum Cost Design rocket engine we have discovered that a simple vapor pressure system with decaying pressure levels is the cheap way to go. Plugging the decaying combustion chamber pressure into the nozzle equations indicates that you need, you guessed it, a throat that gets larger as the burn progresses to keep the mass flow through the throat and the thrust levels up. Soooo, throat erosion is a good thing, and cheap concrete is the way to go. The only high tech part of the design is picking the right cheap concrete to get the throat to erode at the proper rate.