Purely for educational purposes.

I'm not a concrete expert, but a tonne of concrete is less than half a cubic metre.
A concrete truck carries 10 cubic yards, or nearly 18 metric tons of concrete.
If this "educational fact" is true, then that amount of sugar might cause an issue with a piece of sidewalk, but it's unlikely to get noticed on anything being built with concrete, unless you bring a metric shit ton of sugar to the party.
As it happens, sugar appears to be added to concrete on purpose, specifically to increase the working time at the potential cost of weakening the structure, but research into that is ongoing.
Source: https://concretecaptain.com/what-does-sugar-do-to-concrete-mix/
In other words, this post is bollocks.
Edit: After it was pointed out to me by @Lupus@feddit.org that my link was slop, which I agree after reading more than the first two paragraphs, I went looking for better information and found this actual research:
https://www.sciencedirect.com/science/article/pii/S221450952030036X
Interestingly during my search for information in relation to sugar added to concrete, my results appeared overwhelmingly generated by LLM, like the top link I found initially.
Also, adding sugar appears to increase the compressive strength and that might be more significant than the increased work time.
- Can any concrete experts suggest another additive for us to avoid?
  
  Not a concrete expert, but dead bodies. They decompose, can create pockets of pressurised gas, and leave a hollow cavity of no structural strength.
  Source: Me. I made it up, but seems plausible.
  
  There's two broad categories of ways that concrete can get messed up: things that apparently ruin a batch on the spot, and things that compromise the resulting structural integrity in ways that might not be readily apparent.
  The biggest things I can think of for the first category are water and time. More water means more "workability", so it flows better, but it means the overall strength goes down. You basically want to use a minimal amount of water you can get away with. More water also means more time to set, so concrete workers will add water to the mix in the truck to slow it down if they need to. You can always add more water, but you can never take water out (or add more cement/aggregate on site). Concrete trucks have a water tank on board to add water, and also clean off equipment after pours because job sites often don't have water. If enough water is added to the concrete in the truck, the batch is ruined. I'm sure there are compounds that could ruin a batch, but the water is right there.
  Time also hurts because there is a ticking clock for getting the mix out of the truck once it has been made, particularly if the batch plant is far from the work site, and weather is hot. If the site is not completely prepped and ready to go when the truck gets there, enough delays will force the whole batch to be scapped before it ruins the truck. A lot of work goes into getting forms set properly and squared, and the underlying gravel compacted, so that is all delicate before the truck gets there.
  The second category, things that compromise the integrity over time has a lot of potential, but there is no guarantee that strength will be compromised enough to cause failure, and there's a lot of potential for collateral damage if it's in a bridge, building, or something people could be on/in. The first things that come to mind here are stuff that will decay over time resulting in voids, like woodchips. I know there's been research into woodchips as an intentional additive, but I'm sure there's more science that goes into that than just tossing it it. Something that causes oxide jacking would also really increase failure rates. This is when a material like steel rusts and expands, splitting concrete apart. This happens often if rebar is not fully encased in concrete. This does happen eventually to any reinforced concrete, which is why modern concrete structures have design lifetimes unlike Roman concrete which lasts indefinitely, but has to be much more massive. Adding a bunch of nails or something like that to concrete would probably speed up the process, but enough nails/woodchips to make a difference would most likely be noticed by the people doing the pour (which could actually be a benefit).
  If you wanted it to be unnoticeable, you would probably want to get a roll of rebar tying wire, and figure out a way to get it placed in the prepped site in such a way that it would be exposed to the elements after the final pour without being easily noticeable by the people doing the pour. Bonus points if the effort is focused on areas of stress concentration like corners or joints in the concrete.
  
  Well what you could do is put calcium chloride into the concrete and that'll fuck up the chemical composition of Portlandcement and most steel reinforcements found in concrete
- My dad told me stories of him chucking a bag of sugar into mixers when pouring slabs if the first truck was taking too long and the next truck got going too early. So a fully loaded mixer is going to need more than one bag to fuck up the whole batch. Fortunately there’s a nice cone at the top which makes it easier to chuck stuff into the mixer, I’m sure you and maybe 5 of your close friends could take one or two bags each.
- In other words, this post is bollocks.
  I don't know, after reading through that AI slob of an article it says a good amount to add while still retaining sufficient strength after curing is between 0,1 to 0,5% sugar.
  So let's assume that more than 1% gets you into trouble, that's still a lot, but sticking to your 18 tons concrete truck example - 180kg of sugar will ruin a whole truckload of concrete.
  I think I could smuggle 180kg of sugar into a concrete truck without anyone noticing until it's too late.
  I'd say with enough people and dedication the story in the post could be true, not super likely but also not impossible.
  
  2 pounds of sugar is not the same as 10 kg of sugar.

2 lbs of sugar would fit in two Venti size drink cups.
1 ton of concrete wouldn't fill up a VW beetle.
So if you want to sabotage even a small office building, you're gonna need a fuckload of sugar.
- How am I expected to be able to afford a fuckload of sugar in this economy?
  
  The secret ingredient... is crime
- That depends where the concrete was going to be used. On the first floor of a multi floor building? Will probably cause some structural issues after it's nearly build or fully built.
  
  So if you want to sabotage a building, the idea would be to get a job where you're assigned to haul the wheelbarrow full of concrete to the back of the jobsite or drag the end of the chute to where it's going to dump out, so you know exactly where that ton of concrete is going. Then you surreptitiously pour out a couple pounds of sugar from your orange vest or fake beer gut and stir it around with a trowel without arousing suspicion. Zoidberg takes notes: "surrep...titiously..."
- A concrete truck can hold up to about 50000lbs or 25 tons so you only need 50lbs of sugar to ruin a truck load. That sounds like a pretty easy amount to throw into a concrete truck
  
  It might not sound like much, but the French people who committed the prison concrete sabotage weren't ordinary people who showed up with sugar in their pockets, they were a well organized group that had also broken into offices to steal construction plans. One of their documents describes sabotaging the concrete as "a simple matter" of adding sugar, but that's all - it doesn't say whether they added it to a running truck (which would be quite a trick because everybody on a truck crew knows who's in the crew) or mixed it dry with sacks of concrete in a storeroom, or what. It also doesn't mention observing any actual results. People's vision of how this would work is way too simple - "Just toss a 50-lb bag of something into a truck!"

Pouring a good amount of sugar into an ICE vehicle's gas tank will also ... uh... give that car a ... tummy ache.
EDIT: Water is more effective, and sand even more so.
- Adding sugar doesn't do anything beyond possibly blocking up the fuel filter. It won't dissolve in gasoline, so it's no more effective than sand. Water in quantity (1L or more) works great because it won't dissolve in gasoline and is denser, so it'll sink to the bottom of the tank and get sucked up by the fuel pump.
  https://auto.howstuffworks.com/fuel-efficiency/fuel-consumption/sugar-in-gas-tank.htm
  
  AFAIK, the whole point of adding sugar and sand to a gas tank is that they clog up the filters, and that can result in the in the engine stalling... after an indeterminate amount of time.
  The idea being if you can add enough, stealthily, now the car fails without an immediately apparent time-proximal cause.
  Do that to an entire motor pool, or a good chunk of it, and you can functionally force a decision between a significant amount of logistics and repair costs, and fleet downtime... or, deal with a random number of vehicles failing randomly.
  Sand is... probably better at clogging up fuel pump filters faster than sugar, and if any actually gets into the actual engine, can cause more damage there.
  Water, on the other hand, is more likely to rapidly cause an ICE vehicle to either sputter or stall... but you have to use a good bit more water than sand or sugar.
  Anyway, back to my jam.
  
  if you can get into fuel tank you can do so many more interesting things. chlorinated solvents will burn giving hydrogen chloride, and this will corrode everything downstream, pretty fast at that because it's hot
  also if you can fuck with fuel tank, you might be able to do the same with engine oil. fine alumina or silica added to it will cause quick weardown of moving parts that at the same time looks natural
  not sure what kind of (permanent) damage water will do to engine, maybe it'll look like it's flooded if enough gets in
- PSA: never mix bleach with gasoline
  
  what chemical reaction is that?

Two liter bottle of soda directly into the mixer while the driver is agitating the load.
Just don't stop the barrel from spinning you don't want to get the teamsters in trouble.
Once the load leaves the shute the laborers can deal with it not setting up.
At least that's how it works in Minecraft.
- So one thing Ive seen when they go to pour concrete they always fill up like a coffee can of what they pour as a sample. That sample sets quicker than the rest and they send that to a shop that beats the hell out of those coffee can shaped logs of concrete. If it doesn't stand up to those tests they alert the construction and the concrete is removed and redone. At least that's my understanding.
  So maybe straight in the mixer isn't the best. It would show up in the tests and they'd be onto it quickly.
  But maybe that's just how my minecraft modpack works
  
  Are they only coffee can sized now? My buddy did a stint as a grunt with an engineering firm where all he did was load those things into a crusher and document how they broke, and the logs were more like human torso sized.

I get sticking it to the man, and down with fascism, but I'm not seeing any major buildings going up we need to sabotage? Our issue is more with current gov being complete fuckheads. No concrete associated with that.
- They are planning to construct more deportation camps I would say that's a good thing to stop.
  
  In Guantanamo Bay for now.

My hands are kinda shaky, like, legit. I keep dropping my phone lol. Who's to say I wouldn't accidentally drop 2 pounds of sugar? 😉

That reminds me of when they accidentally poured concrete into the server room that controlled, I think, the Victoria line. When they realised their mistake, workers bought all the sugar around the site to pour it in before the concrete would set to make the cleanup easier.
- How did they mix it in, though?
  
  Either it’s bullshit, or the amount of concrete in question was less than an inch thick.
  Though sugar spread in front of the concrete would prevent it from adhering to surfaces, so maybe that was the goal. The wood forms they use to hold concrete in place are coated with a sugar and oil mixture so they come off easy. So that would make sense.
  
  Right? That seems to be the biggest missing piece in this 'advice' to sabotage things. You can't just dump the sugar on top after they've already poured it, and when is a concrete truck just left at the construction site unattended?
  
  Just looked it up again... Pictures suggest that it wasn't that much volume:
  https://www.independent.co.uk/news/uk/home-news/victoria-line-cement-flooding-fixed-workers-used-sugar-to-stop-spilled-concrete-from-setting-9082206.html

Just chuck tomatoes in for effect, it's funnier.

This sounds like a great way to get innocent people killed

okay so adding sugar leads to cracks in the construction I guess? this might be dangerous for carrying tensile-stressed elements like bridges or arcs, but a cracked sidewalk or flooring is not that bad.