Sodium gluconate has been shown to compensate the impact of alkalis on the long term strength development of cement pastes. This work studies the impact of sodium gluconate combined with Na2SO4 on the microstructural development of white cement-slag systems compared to the addition of only Na2SO4 to better understand its effect on strength development. At long ages the addition of sodium gluconate in systems with Na2SO4 increases the strength compared to the systems with only Na2SO4. Differences in strength cannot be explained by changes in the degree of hydration of cement, degree of reaction of slag, phase assemblage or C-S-H morphology and chemical composition, since these parameters are not affected by sodium gluconate. H-1 NMR shows that sodium gluconate increases the water amount in C-S-H and image analysis confirms the formation of a less dense C-S-H, which can better fill the space. Consequently, the porosity decreases and strength development increases.