5 ms·

My understanding is that desert sand gets blown around by the wind, which (heh) sands it smooth. Smooth sand doesn't grip the concrete (I think), so it's not am

by eszed 3y ago

My understanding is that desert sand gets blown around by the wind, which (heh) sands it smooth. Smooth sand doesn't grip the concrete (I think), so it's not amenable for building. When I first read that my question was whether a different concrete formula or amalgam could work with smooth(er) sand, or if that's a hard limit that cannot be overcome. Does anyone know?

engineer_22 3y ago
Concrete strength is a function of several factors Strength of Cement - portland cement is typically used. Fire limestone to get lime, add fly ash and other additives for increased strength. Ratio of Water to Cement is important to get a complete reaction, but dont add too much water. Aggregate - angular aggregates provide the cement something to stick to and distribute the stresses evenly. You want a well graded aggregate, including sand particles up to larger chunks of rock. In my part of the world the aggregate is typically crushed limestone. Add clean sand for different mixes. There are other chemical additives that will increase strength, workability, and the speed of cure. Since reaction of water to cement is exothermic - on the really high tech monolithic pours they're using technology to carry away the waste heat. You want the whole mass to be at the same temperature to reduce the internal stresses. There are actually spherical glass orbs you can add to your concrete if you want it light-weight. Floating concrete is a thing. This comes at the expense of strength.
- lostlogin 3y ago
  Adding to this, when making concrete, ‘sharp sand’ is preferable to ‘building sand’ as it’s chunkier. Building sand is more commonly used for mortar and gives a better finish. However this is a distant memory of what a brick layer was instructed me on when I was getting supplies for him a long time ago. The below link seems to agree. https://buildingadvice.co.uk/sharp-sand/ https://buildingadvice.co.uk/sharp-sand/
gtvwill 3y ago
Yo it's cement + aggregate makes concrete. As in concrete is the sum of two or more products. Concrete doesn't grip anything. Cement binds and aggregate shape doesn't count for bugger all. You use tie in rods of rebar to bind one slab to another, not the aggregate because the aggregate does sh*t all for structural strength. It's more for the compressive strength. Tbh you can make a form or concrete out of perlite and cement. It's light af, great for ship hulls when reinforced with steel. Still concrete,just doesn't need any sand.
- eszed 3y ago
  Thanks. Yeah, I've poured a few slabs and built a few walls, but construction engineering is waaay out of my normal competencies, so I appreciate the vocabulary corrections. You say "aggregate shape doesn't count for bugger all". So why does river / crushed sand work, and desert sand doesn't? It sounds more like there are different aggregate mixes / types of aggregate material that work for particular applications (hadn't thought that through, but it makes perfect sense), and that "rough" sand is better for the most-common concrete use-cases. Do you know of specific types of concrete where "smooth" sand would be preferred?
  - gtvwill 3y ago
    Someone covers it further down. There's a whole bunch more to the strength of concrete than the aggregate. Man last major concreting I did smooth sand would have been awesome. We were already trying to use the finest grade sand we could find. It was on mad max 4. Making concrete not look like concrete (think underwater caverns, cliffs, waterfalls). So smooth is awesome, makes it easy to texture to look like different types of weathered rock. All the structural strength was from steel rebar with chook mesh wired to it. The crete and scratch coats were about 100mm thick at the most.
- margalabargala 3y ago
  > Cement binds and aggregate shape doesn't count for bugger all. This isn't true at all. Using your example of rebar, this is why any piece of rebar has lugs down it; to bond better with the cement. If you use a smooth cylinder of steel, it won't bind to the cement as well and you wind up with a weaker finished product. The ribs of the rebar increase the surface area and make the rebar-cement bond stronger. Same thing with desert sand. If you have rounded sand grains, you've got reduced surface area per unit volume, and thus reduce the strength of the bond between the aggregate and the cement. That weaker bond becomes a fracture point.
PaulHoule 3y ago
https://en.wikipedia.org/wiki/Geopolymer_cement https://en.wikipedia.org/wiki/Geopolymer_cement is the ultimate example of a technology suppressed by regulatory capture.
- gottorf 3y ago
  What are the regulatory burdens around this? Is it simply that building codes only recognize Portland cement, and anything else is treated like you're trying to build with mud?
  - PaulHoule 3y ago
    Exactly.
- bboygravity 3y ago
  From your link: Generally, one of the main problems with geopolymer binder is its poor workability: Alkali-activated fly ash has a much greater plastic viscosity than ordinary Portland cement[31] and is prone to fast setting. In a matter of minutes, it can produce “highly viscous, unmanageable concrete mixtures”.[32] These problems were faced with Portland cement as well, leading to the development of mix designs and admixtures that increase workability; to a limited extent, those techniques can be applied to geopolymer binder.
  - namibj 3y ago
    And in Roman times, concrete was laid in lifts like how foundation soil is handled today. Yeah, it's more effort than just pouring a thick soup into a vibrating mould, but e.g. precast slabs could handle a non-pourable paste. The side effect of rapid hardening would also be rather easy to handle and even allows shorter mould cycle times (and thus afford more expensive moulds). Another example would be railroad ties that could compression-mould. Those are produced in the millions per year.

by eszed 3y ago

engineer_22 3y ago

Concrete strength is a function of several factors Strength of Cement - portland cement is typically used. Fire limestone to get lime, add fly ash and other additives for increased strength. Ratio of Water to Cement is important to get a complete reaction, but dont add too much water. Aggregate - angular aggregates provide the cement something to stick to and distribute the stresses evenly. You want a well graded aggregate, including sand particles up to larger chunks of rock. In my part of the world the aggregate is typically crushed limestone. Add clean sand for different mixes. There are other chemical additives that will increase strength, workability, and the speed of cure. Since reaction of water to cement is exothermic - on the really high tech monolithic pours they're using technology to carry away the waste heat. You want the whole mass to be at the same temperature to reduce the internal stresses. There are actually spherical glass orbs you can add to your concrete if you want it light-weight. Floating concrete is a thing. This comes at the expense of strength.

lostlogin 3y ago
Adding to this, when making concrete, ‘sharp sand’ is preferable to ‘building sand’ as it’s chunkier. Building sand is more commonly used for mortar and gives a better finish. However this is a distant memory of what a brick layer was instructed me on when I was getting supplies for him a long time ago. The below link seems to agree. https://buildingadvice.co.uk/sharp-sand/ https://buildingadvice.co.uk/sharp-sand/

gtvwill 3y ago

Yo it's cement + aggregate makes concrete. As in concrete is the sum of two or more products. Concrete doesn't grip anything. Cement binds and aggregate shape doesn't count for bugger all. You use tie in rods of rebar to bind one slab to another, not the aggregate because the aggregate does sh*t all for structural strength. It's more for the compressive strength. Tbh you can make a form or concrete out of perlite and cement. It's light af, great for ship hulls when reinforced with steel. Still concrete,just doesn't need any sand.

eszed 3y ago
Thanks. Yeah, I've poured a few slabs and built a few walls, but construction engineering is waaay out of my normal competencies, so I appreciate the vocabulary corrections. You say "aggregate shape doesn't count for bugger all". So why does river / crushed sand work, and desert sand doesn't? It sounds more like there are different aggregate mixes / types of aggregate material that work for particular applications (hadn't thought that through, but it makes perfect sense), and that "rough" sand is better for the most-common concrete use-cases. Do you know of specific types of concrete where "smooth" sand would be preferred?
- gtvwill 3y ago
  Someone covers it further down. There's a whole bunch more to the strength of concrete than the aggregate. Man last major concreting I did smooth sand would have been awesome. We were already trying to use the finest grade sand we could find. It was on mad max 4. Making concrete not look like concrete (think underwater caverns, cliffs, waterfalls). So smooth is awesome, makes it easy to texture to look like different types of weathered rock. All the structural strength was from steel rebar with chook mesh wired to it. The crete and scratch coats were about 100mm thick at the most.
margalabargala 3y ago
> Cement binds and aggregate shape doesn't count for bugger all. This isn't true at all. Using your example of rebar, this is why any piece of rebar has lugs down it; to bond better with the cement. If you use a smooth cylinder of steel, it won't bind to the cement as well and you wind up with a weaker finished product. The ribs of the rebar increase the surface area and make the rebar-cement bond stronger. Same thing with desert sand. If you have rounded sand grains, you've got reduced surface area per unit volume, and thus reduce the strength of the bond between the aggregate and the cement. That weaker bond becomes a fracture point.

PaulHoule 3y ago

https://en.wikipedia.org/wiki/Geopolymer_cement https://en.wikipedia.org/wiki/Geopolymer_cement is the ultimate example of a technology suppressed by regulatory capture.

gottorf 3y ago
What are the regulatory burdens around this? Is it simply that building codes only recognize Portland cement, and anything else is treated like you're trying to build with mud?
- PaulHoule 3y ago
  Exactly.
bboygravity 3y ago
From your link: Generally, one of the main problems with geopolymer binder is its poor workability: Alkali-activated fly ash has a much greater plastic viscosity than ordinary Portland cement[31] and is prone to fast setting. In a matter of minutes, it can produce “highly viscous, unmanageable concrete mixtures”.[32] These problems were faced with Portland cement as well, leading to the development of mix designs and admixtures that increase workability; to a limited extent, those techniques can be applied to geopolymer binder.
- namibj 3y ago
  And in Roman times, concrete was laid in lifts like how foundation soil is handled today. Yeah, it's more effort than just pouring a thick soup into a vibrating mould, but e.g. precast slabs could handle a non-pourable paste. The side effect of rapid hardening would also be rather easy to handle and even allows shorter mould cycle times (and thus afford more expensive moulds). Another example would be railroad ties that could compression-mould. Those are produced in the millions per year.