Abstract:This study evaluates two calcium-silicate-hydrate (C-S-H) nanoseeds: tobermorite (TOB) and foshagite (FOS), for accelerating hydration and strength gain in cement systems. TOB and FOS seeding at 1.5%wt generated 7 and 3 times more heat of hydration than the control at hour four. In addition, 1.5%wt TOB and 1%wt FOS seeding increased 1- and 3-day compressive strength (fc) by 40 and 30% and flexural strength (ff) by 20 and 23%. Twenty-eight-day fc and ff increased by up 30 and 17% with 1.5%wt TOB. The contribution of C-S-H seeds to the total global warming potential of seeded mortars was 9–15% and down to 2–5% using recycled steam. After the improvements in 28-day fc were factored in, the carbon intensity index of seeded mortars was lower than the control by up to 20%. Based on these initial results, the studied hydrothermally synthesized C-S-H seeds appear sensible from the strength development and environmental stances.