From the Portland
cement association, portland cement is a "hydraulic cement (cement that not only hardens by reacting with water
but also forms a water-resistant product) produced by pulverizing clinkers consisting essentially of hydraulic calcium silicates, usually containing one or more of the forms of calcium sulfate as an inter ground addition." Got that.......
In other words, Portland cement is the 'glue' that holds concrete together. There are a variety of ways that portlland cement can be formulated and when used for ferrocement it is mixed as very thick formulation, high strength mixture. Ferrocement behaves just the same as any other form of reinforced concrete with the steel taking the tension and the portland cement taking the compression
What makes ferrocement unusual relative to conventional reinforced concrete is the very high proportion of steel relative to cement which makes it stonger per square inch than normal concrete and weaker per square inch than steel.
The good news is that ferrocement is substanially lighter than steel and also, when properly applied, the portland cement creates a water proof membrane over the steel. There in lies the problem, as cement ages it gets continuously harder and stronger in compression
but more brittle. Over time small hairline cracks from in the thin shell protecting the steel and those hairline cracks allow moisture and air to reach the steel reinforcing. Once that happens the rust spreads down the length of the steel and deteriorates the bond between the steel and the concrete and that pretty much ends the useful reliable lifespan of a ferro
Of course the length of time required for that deterioration varies with the quality of the construction. On big boats (over 50 feet or so) the shell can be thick enough that it takes a very long time for moisture to reach the steel. But on small boats, in an effort to keep weight down, the thickness of the shell is often reduced to the point that the lifespan is quite short, OR the shell is made thicker to provide a longer lifespan at the price of ending up with an extremely heavy boat.
Which gets us back to the strength of Ferrocement relative to its weight. Ferrocement is one of the densest boat building materials that there is (just over twice the density of fiberglass) while offering roughly 5%-15% greater strength by cross sectional area. The numbers are easy to run, pound for pound, at least in the usual steel to cement ratios employed on smaller boats..... pound for pound ferro
cement is the weakest of the boat building materials currently used in boat construction.
While no material is perfect, modern composites, when properly engineered, offer the most strength (including impact resistance) per pound of any of the other choices and when coupled with vinylester resin, the longest potential for a low maintenance