Categories: Applied Chemistry , News |  Comments
Just when we all thought the tube couldn’t get any worse, frustrated commuters in London last week were treated to the news that, due to an engineering mishap, a signal control room for the Victoria line had become flooded with fast-setting concrete, forcing the line to temporarily halt.
Then things got even weirder… the news reported that when the sludgy mess was discovered, staff had rushed to nearby shops to buy bags of sugar to throw on it. This, they said, ‘stops the concrete from setting so quickly’ so it could be cleaned up before it damaged equipment. This intrigued us in CW office – why sugar? It seems bizarre that something so simple and readily available could have this effect.
It turns out this trick is well known in the construction industry, and builders often use small amounts of sugar or sugary liquid as an instant retardant for concrete on hot days, to stop it setting too quickly and cracking.
Sugar disrupts the setting process by preventing the hydration reaction between water and cement – a key ingredient of the concrete mix containing calcium, silicon and aluminium oxides. Dry concrete mix contains cement together with a coarse aggregate – usually sand or crushed gravel. When water is added it reacts with the cement’s components to make a thick paste which hardens to bind the aggregate together.
Throwing sugar into the mix interferes with the hydration process, although the exact mechanism is still a bit of a mystery. One theory is that the sugar molecules coat cement particles and prevent them clumping together to form a smooth paste. Another suggests that the sugar reacts with aluminium and calcium in the cement to make insoluble complexes. These interfere with the hardening process and leave less Al and Ca available to react with the water. Some sugars work better than others (white refined sugar works well, while the milk sugar lactose only has a moderate retarding effect), and salt acts in a similar way. The cement hydration process itself is not fully understood, and there are likely to be several interactions at play.
The more sugar you mix in, the longer the concrete takes to set, and if the sugar concentration exceeds 1% of the cement mixture it will refuse to harden altogether. While this effect can be useful, it does have its downsides. Because sugar is not usually considered hazardous, dry concrete mix can easily become contaminated while it is being transported. The Australian company CSR, which used to produce both sugar and building materials, once had to recall a whole shipment of cement after it used one of its bulk sugar boats to transport aggregate.
As for the signal control room at Victoria, it seemed the sugar did its job long enough for the concrete to be scooped out – the trains were back to normal the following day.