One of my friends had this question in a BARC-2014 interview. The interviewer asked him “What if I threw my tea on the concrete.” See, that was an indirect form of what you asked. It slows the setting of the concrete.
And, yes he got it right.
In simple terms it is based on a glucose catalytic principle. According to its formula, it belongs to polyhydroxy group and its surface has a strong polarity that results in solid liquid adsorption, this molecular hydroxy group slows the hydration process by hindering the crystal contact and delays the tricalcium silicate hydrate reaction.
Approximately
.1 % sugar slows the rxn by ~ 5 hours.
.2 % sugar slows the rxn by 9-11 hours.
For more info you can check in Neville, M.S. Sherry or journals.

Technically speaking,

Sugars have been categorized as ‘cement destroyers’ and when sugar is added to Portland cement paste at the onset of mixing, hardening may be delayed indefinitely.

I don’t want to get into chemical reactions and stuff..

Concrete hardens due to formation of calcium silicate hydrate.

The retarding action of sugar is probably by the prevention of the formation of calcium silicate hydrate.

Sugar exceeding 0.2% by weight of concrete will slow down the reaction drastically.