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The poor man's sand binder

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vtsteam:
Ironman, I didn't check for how quickly it cured -- waited two days to check the first core. Some of the leftover loose stuff did seem to harden up quickly. But evaporation at the edges of the pile prevented cure there.

I covered the ends of the core boxes with a plate of glass as you do, to prevent evaporation of the ends before cure.

I will check a sample for quick cure next.

The label says it "contains Portland and gypsum cements".

I haven't yet tried the core in a casting.

awemawson:
Rapid setting cement is widely available here in the UK from builders merchants. Frequently used on repairs and alterations to underground salt glazed drains, as use can recommence within an hour so the loo can then be used !

I've used it to repair bitumen lined brick built water tanks when I had my launderettes, as it's impossible to dry the tank wall in a sensible time to make a proper bitumen repair. Also useful for grouting in rag bolts to hold machinery though nowadays epoxy resins are more usually used.

vtsteam:
Well I'm reopening an old topic here, but today I revisited making cores with Portland cement. I actually didn't have a good success rate when I tried this a few years ago. So I went back to baked sand cores using molasses water, etc. The main problem was that the cores weren't strong enough to resist breaking before going into the mold. Some of them were also crumbly.

I'm not sure what was wrong then but today I unmolded a new try at a Portland cement and sand core, and it's perfect, By perfect I mean it looks like Ironman's results!

I think the differences from my first attempts are:

1.) Instead of fine local sand, I used some bagged coarse quartz masonry sand (actually labeled as "medium")

2.) I properly used 87% sand to 9% cement to 4% water and mixed very thoroughly.
(I'm not sure if in the past I might have tried the simple 100 to 9 to 4 mix, which would have reduced the cement and water proportions.)

I think the real difference is the sand. If you think about it, a fine sand has much more surface area than coarse sand. Since you are trying to coat every grain of sand with cement when molding an aggregate like this, there may have been too little cement to properly coat the fine sand I used in my earlier tries.

Coarse sand is also more porous. I can easily blow lengthwise through this finished core, so I don't think it needs any additional venting. And coarse angular sand is just generally stronger when packed.

Anyway here's the core I just made. It will be used in casting an iron engine cylinder, when casting weather eventually gets here. I just packed a second cement/sand core today, and we'll check it as well after 3 more days of curing.

Very happy with this result!

ironman:
Another satisfied customer, when I first tried this method I had the same problems.

vtsteam:
It's great Ironman. I owe you a lot for the successes I'm getting, as do others.  :beer:

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