> Jonathan wrote:
> > Portland cement is composed of aluminosilicates formed from a mixture of
> > iron oxides, bauxite, and limestone calcined at about 1500 degrees C.
> > result is a mixture of mainly dicalcium and tricalcium silicates with
> > amounts of tricalcium aluminate and tetracalcium aluminoferrite. Gypsum
> > (hydrated calcium sulphate) is then added.
> I worked for several years as a Civil Engineering Tech inspecting the
> construction of Interstate roadways and concrete bridges. I learned a
> little something about cement and concrete. :)
In this case you should not be confusing concrete with rock cements.
> > Primary cements in carbonate sediments include high and low magnesium
> > and aragonite. These typically are stablised as calcite and are
> > partly to wholly replaced by dolomite or quartz. Sandstones may be
> cemented by
> > carbonates, quartz, feldspars, clays or micas. All cements are
> > from saturated solutions that percolate through the sediment.
> In a catastrophic setting, the sandstones would be deposited from carbonate
> or silicate rich waters. The deposit would become cemented by precipitated
> minerals from the water solution which saturates the sands. One need not
> wait for reletively slow percolation to do the job.
Fast or slow, the cement must be deposited from fluids percolating through the
sediment (unless you believe it materialises from the 4th dimension).
> > Portland cement has absolutely no counterpart in the non-human world.
> > composition and the processes by which it is manufactured and forms
> > bears no relationship to sedimentary cements and cementation whatsoever.
> The ancient Romans made concrete, while not as good as our Portland cement,
> from simpler recepies consisting primarily of lime. Deposits of lime mud
> would not take too long to harden.
You did not mention Roman cement in the first instance, but specifically said
Portland cement. Roman cement was indeed made from lime. On addition of
water lime forms calcium hydroxide, or slaked lime. Like the hydrated
calcium-aluminosilicates of Portland cement this has no counterpart in the
geological world (at least that I am aware of).
I have no problem with rapid cementation by carbonates. Cementation rates are
very variable, some deep sea oozes are uncemented even at great depth, other
sediments cement instantaneously. There are ways of determining which is
relevant to a particular example. The types of sediments and theenvironments in
which you get rapid cementation are well known, as are those unlikely to have
it. If we are to discuss cementation, let us use realistic examples. Please do
not bring in red herrings about concrete or roman cement, they are not