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cement concrete res.: Crystallization of c-s-h and c-a-s-h in artificial seawater at room temperature
Topic
crystallization of c-s-h and c-a-s-h in artificial seawater at ambient temperature
: Crystallization of c-s-h and c-a-s-h in artificial seawater at room temperature
Keywords:
c-s-h；c-a-s-h；Tobemollite;Crystallization;Sea water;ph
Year of publication: 2023.
*：cement and concrete research
Research Group: Warda Ashraf Group, University of Texas, USA.
Background:
Hydrated calcium silicate (C-S-H) is one of the main hydration products in ordinary Portland cement (OPC). If a high-alumina-silicon auxiliary cementitious material (SCMS) is incorporated into the OPC or an alkali-activated slag system is used, the main hydration product is C-A-S-H. The c-s-h or c-a-s-h phase is the main cement hydration product, which largely determines the strength and long-term durability of cement
C-S-H exhibits a mixed structure of amorphous and crystalline phases。Specifically, the amorphous c-s-h formed in the early stage of cement hydration can be used as a nucleation site for the crystalline phase c-s-hc-s-h can evolve from amorphous to a crystalline phase with a tobemolite structure。Tobemollite has a layered structure similar to the ideal layered structure known for c-s-h. Topbemullite has orthorhombic subunit symmetry, and its layered structure consists of a CaO2 layer and a 7-coordinate Ca2+, in which oxygen coexists with silicon in the silicon-oxygen tetrahedral chain on either side of the CaO2 layer. Water molecules and external Ca2+ are present in the CaO2 layer. If aluminum is mixed into c-s-h, aluminum-containing tobermolite can be obtained。Among them, aluminum tends to replace silicon at the silicon-oxygen tetrahedral bridging position and promotes the merging of c-a-s-H dimers, which ultimately results in longer chain lengths of aluminium-bearing tobemullite. During the ancient Roman period, the Romans discovered 11 tobemulite in the hydration products of ancient concrete, and the large amount of tobemulite and its self-healing ability gave ancient Roman concrete excellent durability. On the other hand, aluminum-containing tobemollite fills the concrete pores and enhances the bonding of pumice stone chips in Roman concrete. In addition, aluminum-containing tobemollite is also found in nuclear power plant concrete and autoclaved concrete that has been used for a long time. In short,Learn what it takes for c-s-h or c-a-s-h to be converted into tobemollite or aluminum-containing tobemollite to produce durable and sustainable cementitious composites
The results showed that aluminum ions, alkali metal ions and sulfate ions promoted the crystallization of c-s-h c-a-s-h and the formation of tobemollite. Among them, aluminum ions and alkali metal ions are the most important factors affecting the formation of tobemullite and its final structure. Seawater is rich in alkali metal ions and sulfate ions, and the use of aluminosilicate cementitious materials can also ensure the aluminum content. Therefore,Seawater is expected to accelerate the conversion of c-s-h and c-a-s-h to tobemollite
The starting point of the study
To date,There are few studies on the effect of seawater on the crystallization of C-S-H or C-A-S-H。In addition, most studies on the formation of tobemollite have been conducted at high temperatures and pressuresThere is no literature on the crystallization of tobemullite at room temperature and pressure
What to study
In this paper, the effects of different pH conditions and aluminum content in artificial seawater on the crystallinity of C-S-H and C-A-S-H were studied。Specifically, transmission electron microscopy (TEM) and selective electron diffraction (SAED) are usedScanning electron microscopy (SEM) energy spectroscopy (EDS), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) were used to characterize the morphology of C-S-H and C-A-S-H synthesized by different pH and different Al (Al+Si).C-S-H and C-A-S-H were observed to form tobemollite in artificial seawater at ambient temperatures.
Key conclusions:
In this paper, the crystallization of C-S-H and C-A-S-H with different aluminum contents in artificial seawater of different pH was evaluated。The main conclusions are as follows:
1) At ambient temperatures, tobemollite can be formed in artificial seawater, but this depends on the optimal combination of aluminum content and pH. Among them, the relatively low pH (82-10) is conducive to the formation of tobemollite;While the aluminium content contributes to the crystallization of tobemollite, excess aluminum forms other aluminium-bearing phases than tobemollite.
2) Although the role of aluminum in accelerating the crystallization of tobemollite has been well established, this paper also confirms the significant effect of pH on tobemollite crystallization. In the future, more in-depth studies (e.g., molecular dynamics studies) will help to fully understand the crystallization mechanism of c-s-h and c-a-s-h and the specific effects of the above conditions on the crystallization of c-s-h and c-a-s-h. At the same time, understanding the formation pathways of tobemullite at room temperature will help optimize the use of low-carbon resources and develop sustainable and durable cementitious materials.
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Shen Yinong, Lecturer, Ningbo University.
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Minjie Xu, graduate student, Jilin Jianzhu University.