Early strengthening effect of synthetic ettringite and C-S-H composited crystal seeds in cement
Objective Utilizing crystal seeds is one of the most effective means to accelerate the hydration of cementitious materials,which can subsequently shorten the setting time of a certain cement or upgrade the mechanical strength of cement-based composites at an early age.With regard to Portland cement,calcium silicate hydrate(C-S-H)at the nanoscale is widely employed as a seed-ing crystal,and some encouraging results have been obtained.Although hydrated calcium sulphoaluminate,i.e.,ettringite(Al2O3-Fe2O3-tri,AFt),is also a major early-stage product of Portland cement,fewer studies have explored the seeding effect of superfine ettringite in the hydration of Portland cement,especially with the incorporation of nano C-S-H seeds.In this study,superfine ettringite and nanoscale C-S-H,prepared by the through-solution method,were mixed into the paste of ordinary Port-land cement to investigate the synergistic effect of these two kinds of crystal seeds on the cement hydration.Methods Superfine ettringite and nanoscale C-S-H,synthesized by a similar through-solution method but with different starting reactants,were characterized microstructurally using Zeta-potential analysis,X-ray diffraction(XRD),scanning electron microscopy(SEM),transmission electron microscopy(TEM),etc.Thereafter,superfine ettringite and nanoscale C-S-H in an aqueous system were employed as combined seeds in the preparation of paste blocks using ordinary Portland cement(Shanshui P.O 42.5)as the cementitious material.The ettringite content varied within the range of 0～0.8％relative to the weight of P.O 42.5.The content of nanoscale C-S-H increased incrementally from 0 to 5％.Cubic blocks in the size of 20 mm×20 mm×20 mm were cured under standard conditions(20±2)℃ in temperature and at least 90％in relative humidity).The compressive strength of the cement paste at 1 and 3 d were investigated and discussed in details to reveal the synergistic effect between these two kinds of crystal seeds and their influence on the mechanical properties of cement paste.Results and Discussion Both superfine ettringite and nanoscale C-S-H showed excellent dispersibility and long-term suspen-sion stability in water.Zeta-potential analysis showed that the mean diameters of ettringite and C-S-H in the aqueous system were 1 423 nm and 117 nm.SEM revealed that ettringite particles,typically separated from the suspension,showed a short rod-shaped crystalline morphology(Fig 3).The length of these crystals was in the range of 200～600 nm and 50～80 nm in diameter,resulting in an aspect ratio(length-to-diameter ratio)of around 5～12.TEM revealed the microscopic morphologies of nanoscale C-S-H with a diameter of about 270 nm,which was made up of flake-like nanostructures but with poor crystallinity.Superfine ettringite and nanoscale C-S-H in aqueous suspension were employed as the combined seeds for the hydration of ordinary Port-land cement,Grade P.O 42.5,which was prepared with a water/cement weight ratio of 0.32,as shown in Fig.4.The C-S-H suspension maintained a consistent content of 5％relative to the weight of P.O 42.5.The compressive strength of the cement paste at early ages increased and then decreased apparently as the superfine ettringite content rose incrementally from 0 to 0.8％.The maximum strength of the cement paste at 1 and 3 d was obtained with ettringite contents of 0.6％and 0.4％,respec-tively,which were 125％and 57％higher than those of the reference specimens.In addition,with the ettringite content fixed at 0.4％in weight,the 1 and 3 d compressive strength of the cement paste increased evidently with the increase in C-S-H content.However,further increases in C-S-H content beyond 5％resulted in poor workability,i.e.,serious bleeding in the cement paste prepared with a water/cement ratio of 0.32,thus discouraging the use of higher C-S-H content in the combined seeds for P.O 42.5.Conclusion The combined use of superfine ettringite and nanoscale C-S-H evidently improves the mechanical strength of P.O 42.5 cement at early ages,which is apparently higher than using either superfine ettringite or nanoscaled C-S-H as the crystal seed alone.The experimental results suggest a strong synergistic effect between superfine ettringite and nanoscale C-S-H,which improves the compressive strength of ordinary Portland cement at early ages.
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