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 Deck Post Anchor Mix | Concrete Alternative and Replacement for Deck Post, Fence Post, Mailbox, & Playground Equipment Installs. 2.5lb Bag of Fast-Setting, Expanding Composite Footing Compound

Product ID: 339257619

Details

Brand
H.B. Fuller
Colour
Yellow
Item dimensions L x W x H
38 x 216 x 191
Material
Adhesive
Shape
Rectangular

⚡Sets in 15 minutes

💧Hydrophobic, waterproof protection

🪶2.5lb lightweight bag replaces 100lbs concrete

€ 80.14

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Sold byAmazon CyprusDelivered byDesertcartCustomer service byDesertcartReturns14 days · 30 with PRO

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Description

⚡ Build faster, lighter, and stronger—ditch concrete for good!

PRO GRADE STRENGTH - Uses the same expanding composite tech trusted by utility pole pros for rock-solid stability.
LIGHTNING FAST SETUP - Mixes in seconds and ready to build in just 15 minutes—no downtime, no mess.
COMPACT ECO FRIENDLY - No heavy lifting, no harmful chemicals, and easy storage—perfect for modern, sustainable projects.
WATERPROOF ROT PROOF - Hydrophobic formula seals out moisture to prevent rust, rot, and deterioration—posts last longer.
ULTRA LIGHTWEIGHT POWER - A single 2.5lb bag replaces up to 100lbs of concrete—save your back and your energy.

Fast 2K Deck Post Anchor Mix is a revolutionary 2.5lb expanding composite footing compound that replaces traditional concrete for setting deck, fence, mailbox, and playground posts. It mixes quickly in seconds, sets in 15 minutes, and offers superior waterproof, hydrophobic protection to prevent rot and rust. Lightweight and easy to handle, it delivers professional-grade strength with less mess, no heavy lifting, and eco-friendly benefits.

Specifications

ASIN	B08LMS1VNK
Best Sellers Rank	#138,913 in Tools & Home Improvement ( See Top 100 in Tools & Home Improvement ) #169 in Decking & Fencing Supplies
Brand Name	H.B. Fuller
Color	Yellow
Customer Reviews	4.3 4.3 out of 5 stars (2,057)
Grade Rating	Commercial/Industrial
Item Diameter	4 Inches
Item Dimensions	38 x 216 x 191 millimeters
Item Form	Foam
Item Shape	Rectangular Prism
Item Weight	2.5 Pounds
Manufacturer	H.B. Fuller
Material	Adhesive, Sealant
Model Number	10016571
Number of Items	1
Part Number	732470-200541
Specification Met	Partially assembled
UPC	644216539390
Unit Count	1 Count

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€ 80.14

Duties & taxes included

In stock

Sold byAmazon CyprusDelivered 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 Cyprus.

Secure transaction

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