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 Post Anchor Mix | Expanding Composite Footing Compound (Fence, 1)

Fast 2K Post Anchor Mix | Expanding Composite Footing Compound (Fence, 1)

Product ID: 17019209

Details

Brand
H.B. Fuller
Color
Yellow
Item Form
Foam
Material
Adhesive
Special Feature
Lightweight

⚡2.5lb replaces 80lbs concrete

⏱️Sets in 5 minutes

🌡️-5°F to 105°F use range

R$628

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Sold byAmazon BrazilDelivered byDesertcartCustomer service byDesertcartReturns14 days · 30 with PRO

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Description

🚀 Fast, strong, and weatherproof—build smarter, not harder!

RAPID SET GO - Mix in 30 seconds, set in 5 minutes, and build on your post in just 30 minutes—perfect for busy pros.
ROT RESISTANT BOND - Hydrophobic formula adheres tightly to prevent rot and corrosion, extending your post’s lifespan.
ALL WEATHER WARRIOR - Reliable performance from freezing -5°F to scorching 105°F, so your project never waits.
LIGHTWEIGHT POWERHOUSE - Ditch the heavy lifting—2.5lbs of Fast 2K replaces 80lbs of concrete for effortless installation.
UNIVERSAL COMPATIBILITY - Works seamlessly with wood, metal, and vinyl posts—deck, fence, mailbox, or pole barn, nailed it.

Fast 2K Post Anchor Mix is a lightweight, expanding composite footing compound designed to replace traditional concrete for setting posts. It mixes quickly without water, sets in 5 minutes, and works across a wide temperature range (-5°F to 105°F). Compatible with wood, metal, and vinyl, it prevents rot and meets building code requirements, making it the go-to solution for fast, durable post installations.

Specifications

ASIN	B00LAFUC34
Best Sellers Rank	#39,947 in Tools & Home Improvement ( See Top 100 in Tools & Home Improvement ) #76 in Decking & Fencing Materials
Brand Name	H.B. Fuller
Color	Yellow
Compatible Material	Metal, Vinyl, Wood
Container Type	Bag
Customer Reviews	4.3 4.3 out of 5 stars (1,769)
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	Lightweight
Part Number	254-20-S
UPC	713976908484 669602000322
Unit Count	33.8 Ounce
Volume	32.8 fluid ounces
Warranty Description	Warranty

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TrustScore 4.5 | 7,300+ reviews

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Excellent communication throughout the order process. Product is perfect.

2 weeks ago

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The MOLLE sheath is of exceptional quality. Very happy with my purchase.

2 weeks ago

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R$628

Duties & taxes included

In stock

Sold byAmazon BrazilDelivered 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 Brazil.

Secure transaction

Trustpilot

TrustScore 4.5 | 7,300+ reviews

Imran F.

Very reliable shop with genuine products. Will definitely buy again!

2 weeks ago

Vikram D.

The MOLLE sheath is of exceptional quality. Very happy with my purchase.

2 weeks ago

View all Trustpilot reviews