Bruce wrote:
wrote:
either it sets like rock or it doesnt
On the contrary, in a slightly moist atmosphere, each cement particle
can hydrate (in other words, go off) without joining up to other
particles to form lumps.
********.
I have left bags in 'slightly moist atmospheres' for the odd year, and I
an assure you they set into a lump.
I have been involved in investigating
several projects where such degradation of cement caused major
problems, including complete collapse of a major structure.
I don't think fence posts are 'major structures'.
No one is denying that teh cement aint as good, but your theoretical
attitude is totally at odds with the average DIY persons direct experience.
The idea that cement is perfectly OK to use as long as there are no
lumps in it is an old wives' tale.
No, its a sound piece of experiental knowledge. It is OK to use, and I
have even used it WITH lumps in, breaking them up.
The mortar was not as good, but it was good enough for the purpose at
the time.
I have also eaten food more than one microsecond past its sell by date,
and haven't died in agony either.
If you knew as much about cement as you would have us believe, you would
also know that is degradation begins the moment it leaves the milling
stage and is bagged, and is uniform, and progressive, and the rate
depends on the moisture resistance of the plastic lined sack, and the
amount of moisture in the air, neither of which are guaranteed to be in
any way constants.
The time stamp on the bag is a guide that ensures that even in the worst
of cases the unopened cement should still be to spec at that date.
Given better storeage cement last far longer than that, and anyne who
has actually used it knows that it degrades through clearly identifiable
stages, the first being a slight crust that forms on the outside of the
stuff, where the moisture has penetrated first. Then you get clumping
where there has been generalised partial hydration and there isn't much
in the way of powder left. Nevertheless that lumps if they can be broken
down, still have about 50% ability to rehydrate, and doubling up on the
cement ratio will result in usable mixtures for non demanding applications.
Which 99.99% of D-I-Y mortars are. Certainly adequate for path laying.
fence posts and non critical above ground bricklaying.
If you knew anything about concrete, you would also know that teh
strength is poor in tension, which is why we use rebar, and good in
compression, and is MAINLY down to the actual aggregate in use. The
finction of the cement is not to provide strength as such, but to
prevent the aggregates from shearing past one another.
Most cement failures are due to porisoity, and freszing or ingress of
aor+water to reinfirced structres leading to rusting of te reinfircement.
The point about porosity is the only one you have made that I consider
valid: old cement as I pointed out may not have enough 'good stuff' in
it to totally fill the interstices in the sand, and therefore become
akin to a weaker more porous screed type material. If that is used in
serious structures without compensating in any way there will indeed be
trouble.
However serious structures of that nature are either precast in a
factory, or cast using readymix. I have yet to see a bridge made from
bags of DIY cement on site mixed in a barrow or portable mixer ;-)