Protection For Exterior Marble
I read in another Q&A that you don't recommend sealing marble that will be used in wet/exterior conditions. Can you give me more details on the reasons for that? Will standing rain water yellow the white marble (honed I believe) or stain it in some other way?
First,sealing is only necessary when an absorbent stone will be consistently exposed to staining foods, drinks, products.
You will read/hear that "all stones should be sealed always" ... usually from a sealer company.
It's not necessarily bad advice, it's just not entirely true and some stones just can't be sealed.
For wet areas, showers, baths, patios sealing is not typically recommended because it's possible that the sealer may not allow the marble to breath well enough and water will get trapped in the stone causing it to break down and fall apart over time.
Sealer technology is getting better about letting the marble breath, but considering the risk of staining is so small in a shower or bath, why bother or risk trapping water.
Outside, even if you risk some stains from parties, bbq's, etc. weather (rain and sun) effectively wash out and bleach out stains pretty quick and relative to an indoor kitchen the exposure/stain risk is still much smaller.
So, again why bother or risk interfere with the stone's natural ability to handle the elements.
That's the thinking. It isn't necessarily a hard and fast rule. It's more a matter of assessing risk vs. reward considering the time and cost to apply.
Now, white marble can yellow when constantly exposed to water.
The reason is that white marble often has iron deposits within it. The iron gets exposed to water and rusts. The rusty water leaches through the marble to the surface causing it to yellow.
Sealing marble may help prevent this, but outdoors water can come up from under the stone as well, so you'd want to seal all sides of the tile.
Or better, go with a travertine or granite.
I'd say the bigger question and problem is that you have standing rain water. Really, you need to solve this grading/draining problem before doing anything.
Hope this sheds more light on the question.