The addition of 1–4% s/s (dry solids by mass of C3S) of afwillite (C3S2H3) seeds to C3S pastes made with two different commercial polyacrylate-based superplasticizers (SP) allows the pastes to be cast at low water/C3S mass ratios (w/c) and overcomes the hydration retardation produced by the SPs. SP-free C3S pastes seeded with afwillite at an initial w/c of 0.50 gave about 30% lower 28-day compressive strengths than the unseeded controls, due to higher porosities. However, at w/c = 0.35, with the addition of 0.4% s/s SP, the afwillite-seeded pastes gave similar or higher strengths than the unseeded controls at all ages tested. Hydration rate data obtained by chemical shrinkage measurements suggest that this is because the degrees of hydration of the C3S in the low w/c afwillite-seeded pastes made with added SP reach higher values than in the unseeded controls, compensating for the difference in density of the hydrates.