环境扫描电镜用于硅酸盐水泥早期水化的研究

INVESTIGATION ON THE EARLY STAGE HYDRATION OF PORTLAND CEMENT USING ENVIRONMENTAL SCANNING ELECTRON MICROSCOPY

采用环境扫描电镜对硅酸盐水泥的早期水化过程进行了连续观察。将硅酸盐水泥早期水化过程分为预诱导期、诱导期、加速期、减速期和稳定期个阶段加以描述。预诱导期阶段水泥开始水解,释放出离子,硅酸三钙(C3S)颗粒表面形成低n(Ca)/n(Si)层,第一批水化产物产生,水泥颗粒表面生成一层水化物的保护膜,使水化反应速度降低。诱导期阶段保护膜逐渐推进直至覆盖整个颗粒表面,膜内外产生渗透压力差。当渗透压力大到足以使薄膜在薄弱处破裂,缺钙的硅酸盐离子就被挤入液相,并和钙离子结合,生成各种不定形的CSH。加速期阶段钙离子和硅酸盐离子浓度相对于CSH来说达到过饱和,CSH高速生长,在颗粒表面附近形成类似于网状形貌的产物(高密度CSH),而在颗粒间的原充水空间里形成近球状形貌的产物(低密度CSH)。减速期阶段水化产物继续生长,由不定形富水的凝胶状转变为不定形的颗粒状,显微结构继续发展。稳定期阶段水化产物颗粒个数几乎保持不变,但单个颗粒均逐渐生长变大,显微结构逐渐致密化。保护膜的形成和破裂分别可以解释诱导期的产生和结束。CSH的生长速度是加速期水化反应速度控制的主要因素。

Continuous observations on the early stage of Portland cement (PC) hydration were made with the environmental scanning electron microscopy (ESEM). The early stage hydration of PC is divided into five periods, the pre-induction, induction, acceleration, deceleration and stabilization periods. In the pre-induction period, the hydrolysis of cement takes place and the ions are released into the solution. A layer with low n(Ca)/n(Si) is formed on the surface of tricalcium aluminate (C3S), and the first hydration products appear, then, a protective membrane is extending gradually until it covers the whole grains, resulting in generating the difference of osmotic pressure outside and inside the membrane. When the osmotic pressure increases large enough to burst the membrane in the weak point, the silicate ions lack of calcium ions are squashed into the solution and combined with calcium ions, and polymorphic C-S-H starts to precipitate. In the acceleration period, the concentrations of calcium and silicate ions reach supersaturation compared to that of C-S-H in the solution, which results in the high-speed growth of C-S-H. Reticulation-shaped C-S-H (high-density C-S-H) forms near the grain surface while spherulitic-shaped C-S-H (low-density C-S-H) forms in the interspaces of grains formerly filled by solution. In the deceleration period, the hydration products change from unshaped water-abundant gels to unshaped particles and the microstructure continues to evolve. In the stabilization period, the amounts of crystals output under 3 d of hydration remain almost unchanged compared with that under 1 d of hydration. However, as the hydration progresses, single crystals grow up progressively almost in every direction, and the microstructure gradually becomes compact. The formation and the rupture of the protective membrane probably respectively account for the beginning and ending of induction period. The growth rate of C-S-H is a main factor that controls the hydration rate of the acceleration period.