Could this be used to fill a plaster of Paris mold?

Not that I'm aware of, it doesn't really say on the package but it does sit up very fast and is very durable. Sorry but that's the best I can answer your question.

Could this be used to hold an 8x8 supporting a substantial weight?

I had to replace a 4x4 post for my mailbox and used this 2K Post Back Fill. I already had a deep hole in the cement, so this stuff fills up the gaps beautifully all around and keep the post solidly planted. I'm not sure about 8x8, weight is not the issue here but you would need a "solid" hole like in my case to hold to post, then this foam would then fill up any gaps. Good luck.

Can this be used to form the deck list foundation where the 8x8 post then sits on top held by a bracket anchored in the foundation?

No, Fast 2K can only be used for installing posts that are sunk into the ground. It cannot be used as a foundation material nor can any brackets be anchored into it.

Would this be good to use for posts for a horse fence strong and sturdy?

Yes, also sets up quick and should be good for a horse fence.

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Fast 2K Fence Post Mix and Concrete Alternative and Replacement for Fence Post and Mail Post Installs. 32.8 oz Bag of Expanding Composite Footing Compound

Fast 2K Fence Post Mix and Concrete Alternative and Replacement for Fence Post and Mail Post Installs. 32.8 oz Bag of Expanding Composite Footing Compound

Product ID: 334097086

Details

Brand
H.B. Fuller
Colour
No Color
Item form
Foam
Material
Adhesive
Special features
Waterproof

⚡Ready to build in 15 minutes

💧Waterproof & hydrophobic to prevent rot

🪶Lightweight 32.8 oz bag replaces 80-100 lbs concrete

13219 kr

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Sold byAmazon IcelandDelivered byDesertcartCustomer service byDesertcartReturns14 days · 30 with PRO

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Description

🚀 Build Faster, Stronger & Smarter with Fast 2K!

PRO GRADE STRENGTH - Uses the same expanding composite tech trusted by utility pros for rock-solid, long-lasting post stability.
HYDROPHOBIC DEFENSE - Waterproof formula protects posts from rot, rust, and deterioration—extend your fence’s lifespan effortlessly.
SPEEDY SETUP ZERO WAIT - Mixes in seconds and sets in 15 minutes so your project moves at your pace, not the other way around.
UNIVERSAL COMPATIBILITY - Bonds seamlessly with wood, vinyl, and metal posts—one solution for all your fencing needs.
BACK SAVING LIGHTWEIGHT POWER - Ditch the heavy concrete—just 32.8 oz of patent-pending mix replaces up to 100 lbs of mess and strain.

H.B. Fuller’s Fast 2K Fence Post Mix is a revolutionary 32.8 oz expanding composite footing compound that replaces traditional concrete for fence and mail post installations. Lightweight and mess-free, it sets in just 15 minutes, offers superior strength with professional-grade technology, and features a waterproof, hydrophobic formula that protects posts from water damage and decay. Compatible with wood, vinyl, and metal, it’s the smart, efficient choice for modern DIYers and pros alike.

Specifications

ASIN	B00LAFUC34
Brand Name	H.B. Fuller
Color	No Color
Compatible Material	Metal, Vinyl, Wood
Container Type	Bag
Customer Reviews	4.3 4.3 out of 5 stars (2,081)
Global Trade Identification Number	00669602000322
Included Components	Multitool Accessory
Item Form	Foam
Item Package Quantity	1
Item Type Name	Fast 2K 26OZ Back Fill
Item Weight	2.2 Pounds
Manufacturer	H.B. Fuller
Material Type	Adhesive, Sealant
Model	250
Number of Pieces	1
Other Special Features of the Product	Waterproof
Part Number	254-20-S
UPC	713976908484 669602000322
Unit Count	1 Count
Volume	32.8 Fluid Ounces
Water Resistance Level	Waterproof

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13219 kr

Duties & taxes included

In stock

Sold byAmazon IcelandDelivered byDesertcartCustomer service byDesertcartReturns14 days · 30 with PRO

Buyer Protection · Full refund if your order doesn't arrive as described.

Desertcart purchases this item on your behalf and handles shipping, customs, and support to Iceland.

Secure transaction

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TrustScore 4.5 | 7,300+ reviews

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