Growth habit of portlandite crystal in cement paste

The influences of SO₄^2-, SiO₃^2- and supersaturation on the growth habit of portlandite (CH) crystal in coprecipitation systems (CaCl₂+NaOH) were investigated to clarify CH with a hexagonal platelet morphology in cement paste. The results show that the addition of SO₄^2- inhibits the growth of (001) surface and leads to a hexagonal platelet morphology. The addition of SiO₃^2- causes the formation of prismatic morphology. The two-dimensional nucleation results in the formation of hexagonal platelet morphology when the portlandite saturation index is approximately 0.3. When the growth of CH crystal is controlled by the step movement rate, the growth rate of {001} facets is the smallest. In cement paste, the hexagonal platelet morphology of CH is due to the two-dimensional nucleation at a low supersaturation. The presence of SO₄^2- reinforces the platelet morphology.