Answer
0.7–2.0 L per 100 kg of cementitious material for normal concrete; 1.5–3.5 L per 100 kg for high-performance low-w/c mixes — confirm with trial batches, since w/c ratio, cement content, C3A and alkali content and temperature all change the retardation.
Detailed question
What is the dose of a retarding admixture?
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