<a href=showthread.php?s=&postid=12757996#post12757996 target=_blank>Originally posted</a> by impur
Not much to screw up - cement, sand, and some rock rubble is all it is.
Actually, there are quite a few things that can be "screwed up".
And since no two cements lots are identical (even from the same plant) chemically speaking, it becomes hard to pinpoint where something went wrong.
An improper cure can produce rock that will leech high pH for a very long time. What is improper? Allowing the rock to dry out before the cure completes is probably the one most people will be guilty of. Rock should be kept moist for at least two weeks, and preferably for a month to ensure that all the cement gets a chance to hydrate. If it doesn't all completely hydrate, then you will have problems as now not only do you have to leech out the Ca(OH)2 (calcium hydroxide) as well as all the unhydrated materials. Simply sealing them in plastic bags or a plastic tote should do the trick, maybe mist them once a week or so to keep the humidity high.
Using too high of a ratio of cement makes a dense cement that is very hard to kure. Water will move through even the densest cement (but not fast set cements, which are considered impermeable by the cement experts), but it might be at a snails pace, only a few millimeters a day, if that. At that rate it will take a very long time indeed to kure out.
Salt itself retards the cure of cement. Cement must cure before it can really start to be kured - otherwise you are simply wasting water and effort. Under normal situations, cement is said to cure in 28 days, but adding sodium chloride has a slowing effect on this process, so it is likely that the actual cure is taking a lot longer than the normal 28 days.
Curing is an ongoing process in the rock - while the cement industry has settled on the "28 day" rule for a testing standard, the chemical reactions going on in the cement can keep going on for decades, and a lot of things can affect this. Humidity and temperature are big one's. For best results, cement should be made in humidity of 50% or higher and in a temp of 65°F or higher. Lower these parameters and you prolong the length of the cure, though going higher than these can have a speeding effect, at least for our purposes.
Larger rocks, esp those with a lot of salt (more than 2 parts of salt), and rock with no aggregates other than salt, can take a coons age to kure as the salt must release before enough water flow can get in to carry the Ca(OH)2 out of the rock. So you can think of it as a two-fold process - one to remove the salt and one to remove the Ca(OH)2, which really, can make it take twice as long to kure.
Making too wet of a cement mix also slows down the kure and actually weakens the cement. Trying to describe the perfect amount of water has stumped us all forever. In the cement industry, they use something called "slump" to determine is there is enough, or too much water in a mix.
http://en.wikipedia.org/wiki/Concrete_Slump_Test
One can sort of adapt this for home use by using either a plastic cup or funnel, filling it up and carefully releasing it. For our purposes, there should be no slump at all.
Packing the rock when you cast it will also make denser rock; when you are casting, try to "drool" the mud into place, in marble sized drops, this will allow natural channels to form in the rock, allowing more water flow and thus allowing it to kure faster; without water flow, little kuring can happen. Using this technique, with suitably "dry" mud will allow water to flow though even non-porous cements like fast set cements or densified cements.
And then there is the question of when to start the kure. Cement is
hydraulic, meaning it will cure underwater. I know from my research that cement cured in water is typically "better" than like cement that cures normally. However I've not been able to find anything that indicates if this will speed or slow the chemical reactions happening in the rock. So I can only speak from observation, mine own and from those of you who post your experiences. What I've gleaned so far says that if you chuck your rock into water too soon, the reaction seems to drastically slow. What is too soon? From what I can tell, anything sooner than two weeks seems to be detrimental to the kure.
When you get to the kure, well, I've had the best luck with adding a air pump and stone as well as power head and heat to the water. I seem to get quicker results when I use all three. Of course this ups the utility bills, but it does seem to help.
Plenty of water volume for the rock is also necessary. Filling up a tote or bucket with rock and topping it off with water just doesn't cut it as you only end up with a few gallons of water. I try to use the same approach we use to calculate how much rock we need in our setups 1lbs to 1gallon and it seems to work well.
And I had such good luck last fall with the daily watering of my rock that I am trying to figure out a "waterfall" idea for kuring (plus then maybe I can justify that water feature I've always wanted, lol). I think that if water is allowed to actually fall through the rock, over and over, without the rock being submerged, the rock will kure faster. I'm planning on trying this on a small scale this summer to see if there is any possibility to it, and if it is, then I might solicit help on devising something that could handle a good sized pile of rock.
I hope this might help - if your having trouble kuring your rock, and you are "guilty" of one or more of these points, then you probably have your answer and can change your approach the next time you cast with hopefully better results...
Thanks, Ocean Size, and Welcome to the Thread! You too, Go200mph :wavehand:
pH was over 10 yesterday.
