李佳全¹，佟 钰¹，冷雨虹¹，王 刚²，赵明宇¹，房延凤¹

(1. 沈阳建筑大学 材料科学与工程学院，辽宁 沈阳，110168；2. 中建八局第二建设有限公司，山东 济南，250022)

引用格式：

DOI：10.13732/j.issn.1008-5548.2024.04.006

收稿日期：2024-04-15，修回日期：2024-05-23，上线日期：2024-06-26。

基金项目：国家自然科学基金项目，编号：52378252；辽宁省教育厅高等学校基本科研项目，编号：JYTZD2023165。

第一作者简介：李佳全（1997—），男（壮族），硕士生，研究方向为高性能水泥混凝土材料。E-mail：932370142@qq. com。

通信作者简介：房延凤（1988—），女，副教授，博士，硕士生导师，辽宁省百千万人才万人层次，主要从事工业废渣综合利用和特种水泥方面的研究。E-mail： fangyf@sjzu. edu. cn。

摘要：【目的】 解析钙矾石-水化硅酸钙晶种的协同效应，改善普通硅酸盐水泥的早期力学强度。【方法】 以溶液法合成的超细钙矾石和纳米水化硅酸钙为复合晶种，在微观结构表征基础上，探究钙矾石和水化硅酸钙的掺量以及水灰比对水泥净浆试块 1、3 d抗压强度的影响与作用机制。【结果】 溶液法合成的超细钙矾石和纳米水化硅酸钙的纯度高，结晶完整，在水中可长时间保持稳定悬浮状态，适合作为复合晶种使用。当纳米水化硅酸钙质量分数固定为5%情况下，随着超细钙矾石掺量的增大，水泥净浆试块的早期抗压强度先升高后降低，在钙矾石掺量（质量分数，下同）为0. 6%时，试块的1 d抗压强度提高125%；当钙矾石掺量为0. 4%时，水泥净浆试块的3 d抗压强度提高57%。在超细钙矾石掺量一定情况下，水泥净浆试块早期强度随水化硅酸钙掺量的提高而明显增长，但掺量高于5%会导致泌水现象的发生。【结论】 超细钙矾石与纳米水化硅酸钙作为复合晶种具有明显的协同效应，可有效提高普通硅酸盐水泥的早期力学强度，早强效果明显优于单独使用的钙矾石或水化硅酸钙晶种。

Abstract

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 seeding 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 Portland 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）o C 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 suspension 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 about270nm， 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 Portland 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%， respectively， 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.

Keywords：ettringite； calcium silicate hydrate （C-S-H）； crystal seeds； portland cement； early strength

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