Re: Four Rivers Revisited

Jonathan Clarke (
Fri, 30 Apr 1999 07:19:26 +1000

Greetings Allen

Apologies for the slow response, I have been away and have had little chance to
follow the discussion until now.

Allen Roy wrote in part:

> If the rocks were limestones or sandstones cemented by calcium carbonates,
> then there is not a real problem. Concrete, using Portland Cement which
> made from lime, reaches 98% maximum strength in 28 days. The proposed
> impact storm is thought to have lasted for some 150 days. So at least some
> of the limestones or sandstones could have reached pretty near maximum
> strength during the impact storm.

Portland cement is composed of aluminosilicates formed from a mixture of clay
iron oxides, bauxite, and limestone calcined at about 1500 degrees C. The
result is a mixture of mainly dicalcium and tricalcium silicates with minor
amounts of tricalcium aluminate and tetracalcium aluminoferrite. Gypsum
(hydrated calcium sulphate) is then added.

When portland cement is mixed with water and aggregate the slurry sets, forming
concrete. This occurs when the calcium silicates hydrate, forming interlocking
crystals of calcium silicate hydrate and calcium hydroxide. The main phase is
the calcium silicate hydrate, which bind the aggregate together to form
concrete. The gypsum acts as a curing (setting) retardant.

Primary cements in carbonate sediments include high and low magnesium calcite
and aragonite. These typically are stablised as calcite and are sometimes
partly to wholly replaced by dolomite or quartz. Sandstones may be cemented by
carbonates, quartz, feldspars, clays or micas. All cements are precipitated
from saturated solutions that percolate through the sediment.

Portland cement has absolutely no counterpart in the non-human world. Its
composition and the processes by which it is manufactured and forms concrete
bears no relationship to sedimentary cements and cementation whatsoever.

Hope this clarifies things.

God Bless