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Cement-Based Nano-Hybrids

A part of my doctoral project is focused on the development of new cement-based materials. The objective is to develop novel materials that can increase the service-life of concrete infrastructures and contribute to the resolution of sustainable development issues.

The research plan involves preparation and characterization of the polymer-modified C-S-H, which has been produced by the intercalation of various types of polymers into the layers of C-S-H. My work will utilize the application of techniques such as Differential Scanning Calorimetry, X-ray Diffraction, Scanning Electron Microscopy, Dynamic Mechanical Analysis and Nuclear Magnetic Resonance in order to investigate the performance and durability of these novel cementitous materials.

C-S-H polymer nanocomposite
Interaction of polymer molecules with the nanostructure of C-S-H
Triangles: Silicate tetrahedra, P: Polymer molecule grafted at defect locations

Based on the results, a new set of polymer-modified C-S-H materials would be produced and characterized. Finally, the knowledge of the characteristics of the modified C-S-H will be applied to Portland cement paste samples. These materials will be fabricated and experiments performed to assess their viability for use in the construction industry.

Calcium silicate hydrate
  There are various indications that confirm the layered nature of C-S-H. Using helium as a nanoprobe in a pycnometer the inflow of this gas is measured after incremental removal of water from cement and C-S-H (I). First conducted by R. F. Feldman, the inflow increases as water is removed until a point at which it decreases. This point is and indication of a possible collapse in the nano-structure of hydration products. The results from helium inflow test shows that there is an analogy between C-S-H (I) and the calcium silicate hydrate that is produced in the hydration of Portland cement.

C-S-H model
Feldman-Sereda model for the microstructure of C-S-H
Black lines: C-S-H sheet, Circles: Adsorbed water, Crosses: Interlayer water

Various techniques such as dynamic mechanical analysis and high resolution nuclear magnetic resonance spectroscopy are employed to evaluate the nanostructural changes during the dehydration of calcium silicate hydrate systems. This would provide some evidences of the nature and role of interlayer water in C-S-H (a feature that is not considered in the colloid model.

Hedvall Effect
  The increased reactivity of materials in their crystalline transition state is called Hedvall effect. When mixed together, two materials in the solid state have no reaction in most cases. But, when the mixture is heated up to the crystalline or melting temperature of one of them, a chemical reaction might occur.

Hedvall effect in cement chemistry

Silver nitrate has been used in this regard that easily reacts with calcium based inorganic materials. The calibration of the endothermal peak of silver nitrate on cooling provides a method to estimate the unreacted silver nitrate after the Hedvall effect test and calculate the material reactivity. Due to the large difference in the atomic number of the atoms in the reactions (Ag, Ca and Si), the backscattered electron images were proven very useful in identifying various species such as silver silicate in the reaction products.

C-S-H seeding

The addition of the C-S-H seeds accelerates the hydration of Portland cement manifested by the shift in the heat of hydration peaks to earlier times as well as an increase in the amount of total heat released. The hydration of Portland cement containing synthetic calcium silicate seeds (C-S-H) can be monitored in a conduction calorimeter.

C-S-H seeding
SEM image of the C-S-H seeding at 80K magnification
C-S-H crystals from the hydration of C3S formed on the surface of C-S-H seeds

The extent of the acceleration depends on the amount and composition of the C-S-H seeds. The analysis of hydration products at various times using XRD, SEM and DSC techniques reveals more details about the hydration kinetics and the formation of various products. It is suggested that the synthetic C-S-H seeds promote the nucleation and the crystallization of the C-S-H that forms during the hydration of Portland cement.

Mechanical Properties of C-S-H
  The relation between the chemistry and mechanical performance of the cement-based materials has not been fully examined. The variety of species in hydrated Portland cement is one of the main issues in this regard. Working with phase pure C-S-H of variable stoichiometry may help us obtain more details about the influence of chemistry of cementitious systems on their engineering characteristics. In order to achieve this goal it is necessary to use powder compaction technique to make solid bodies of synthetic C-S-H. Various mechanical measurements such as microhardness, modulus elasticity, dynamic mechanical response, creep and stress relaxation can then be made on the compacted samples.

Dynamic Mechanical Properties of C-S-H
Various stages in the DMA response of C-S-H systems
upon the removal of adsorbed and interlayer water.

Results from ongoing research studies have revealed interesting features from the mechanical behaviour of C-S-H in relation to its C/S ratio. Similar features have been identified between synthetic C-S-H and that produced in the hydration of Portland cement which supports the validity of using a layered model for C-S-H with the assignment of structural roles to the interlayer water.


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