###### Investigating the nanocrystalline structure and properties of C-(N)-A-S-H gels:

The dire need for sustainability and environmental protection, as well as the recent environmental degradation around the planet call for increasing efforts towards alternative binder systems in concrete in order to address one of the biggest sources of CO2 emissions worldwide. The nanocrystalline structure of calcium (-sodium) aluminosilicate hydrate (C-(N)-A-S-H) gels and their occurrence in various alkali-activated cements has been contested in past decades. Various claims have been made about the similarities between calcium silicate hydrate in ordinary Portland cement (OPC) systems and such gels, but the differences in the nano-scale structures as well as their chemical properties cannot be overlooked. In this investigation, a combination of X-ray diffraction (XRD), pair distribution function analysis (PDF), attenuated total reflectance (ATR) - Fourier transform infrared spectrometry (FTIR) and thermogravimetric analysis (TGA) was deployed to offer a more holistic understanding of the structural properties of gels. It is shown that C-(N)- A-S-H follows well-ordered patterns in short-ranges, and sodium (in the form of an alkaline solution) is catalytic in the formation and growth of the nanocrystalline structure over time. The elucidation of such similarities and differences can influence our understanding of the macroscale material properties of the gels, such as durability and weather resistance of alkali-activated cements. The research was conducted at the Adlinger Center for Energy & the Environment as well as the Argonne National Laboratory.

###### Background theory:

X-Ray Diffraction:
X-ray diffraction is an ideal analytical technique to characterize crystalline, fine-grained materials. The individual cement phases yielded from the analysis offer insight in the crystalline structural properties of the material under investigation. The crystalline structure diffracts X-rays and produces an XRD pattern of peaks and valleys at specific diffraction angles. The diffraction angle is determined via symmetry and sizing of the unit cell using Bragg’s Law,
$$n\lambda = 2dsin\theta$$
where n is an integer and λ is the wavelength of the incident wave, θ is the scattering angle and d is the interplanar distance between Specifically with regards to cements, whether anhydrous or hydrated are present in the material30. Therefore, this technique determines the various peaks that appear in the data are indicative of various phases present in the mix30. However, this alone is extremely insightful in understanding the exact nanocrystalline structure of the materials and potentially improve upon it.

PDF analysis:

The pair distribution function (PDF) G(r) is obtained via a sine Fourier transform of the total scattering function S(Q),
$$G(r)=2/\pi \int_{Q_{min}}^{Q_{max}} Q[S(Q)-1]sin(Qr)\,dQ$$
where Q is the momentum transfer. PDF data is typically obtained using synchrotron radiation due to the need for a high $$Q_{max}$$.

Thermogravimetric analysis:

Thermogravimetric analysis is used to identify the amount of bound water and portlandite in the mix and thus follow the reaction of Portland cement or to benchmark the reactivity of supplementary cementitious materials (SCM’s)30. Its ability to detect X-ray amorphous hydrates (which would not show up in a typical X-ray diffraction measurement due to their lack of a long-range structure) such as C-S-H or $$AH_{3}$$ makes it a suitable complementary method to other measurements such as X-ray diffraction above. The TGA measurement is strongly dependent on the exact conditions present at the time of the experiment, such as the pan used, the heating vessel, the heat rate, the gas environment etc. This makes it difficult to replicate TGA experiments from one lab in another, but with enough accuracy the results can be replicated within the same lab for multiple samples (as is the case in this investigation).

ATR-FTIR Spectroscopy:

FTIR Spectroscopy is an extremely useful technique in identifying the kinds of molecules and atomic arrangements present in the sample and their exact amounts. In the same way that TGA uses heat and XRD X-rays, FTIR uses light. Light or electromagnetic radiation is composed of the electric and magnetic vectors (electric and magnetic waves propagating at 90◦ from each other. The beam hits an interferometer which splits it into two paths of 4cm and 10cm, and then recombines into a single outgoing beam that is interpreted by the light receiver. More specifically, with regards to attenuated total reflectance (used in this investigation), the instrument is based on internal reflectance.

The critical angle $$θ_{c}$$ mentioned above can be calculated as follows,
$$\theta_{c}=sin^{-1}\frac{n_{s}}{n_{c}}$$
The critical angle for diamond is n=2.42 and the material angle will be influencing exactly how much light will penetrate the sample and return back to the receiver.

The depth of penetration contains the final wavenumber W, which is what shows up on a typical ATR-FTIR graph and what constitutes the most important piece of quantitative information in the result. The depth of penetration is,
$$\frac{1}{2\pi W_{n}(sin^{2}\theta-n^{2}s_{c})}$$
Where, DP = Depth of Penetration, W = Wavenumber, $$n_{c}$$ = Refractive index of ATR crystal, ”$$\theta$$ = angle of incidence and $$n_{sc}$$ = $$\frac{n_{sample}}{n_{crystal}}$$.

Summary of findings:

The primary objective of this investigation was to synthesize C-(N)-A-S-H gels of various alkalinities and explore their nanocrystalline structure and properties. The changes in alkalinity introduced differences in the composition and behavior of the constituents in the samples.

In summary:

• C-S-H and portlandite phases increase with higher alkalinities;
• the atom-atom distances in the various formed phases decreases with alkalinity, leading to a denser morphology;
• Silica interconnectivity increases within the nanocrystalline structure;
• Highly alkaline samples contain less H2O as shown in the TGA experiments;
• There is a strong presence of unreacted silica fume in the matrix at Q4;
• The methods outlined in chapter 2 did not fully allow for complete carbonation prevention;
• More rigorous methods of transfer and handling should be established to eliminate C-O bands in ATR-FTIR and other measurements;
• From the XRD measurements we conclude that atom-atom relation $$\geq 8$$ are nonexistent due to the amorphous nature of the gel at longer-range structures.

Future work:

The realm of C-(N)-A-S-H gel research is open for more additions and further expansion. This investigation followed a holistic approach and tackled a wide range of questions surrounding the nanocrystalline structure of the gel. More advanced methods of preventing carbonations of the samples can be incorporated in future iterations of the work. More advanced X-ray diffraction setups can also be incorporated to yield higher resolution XRD graphs (such as the ones obtained at Argonne National Laboratory but for the entire range of samples measured) and thus a more complete understanding of the individual phases present in the structure. Finally, additional techniques such as 29-NMR spectroscopy in addition to the aforementioned techniques can offer more insight in the exact Si sites and their behavior over time.

Please refer to the full manuscript for a detailed discussion and illustration of the results.

###### Team:

Principal Investigator: Claire E. White
Research Team: Nishant Garg, Theo Dimitrasopoulos

###### Tools & methods:

Bruker D8 DISCOVER diffractometer, PerkinElmer Pyris 1 Thermogravimetric analyzer, PerkinElmer Frontier FTIR Spectrometer with a Universal ATR Sampling Accessory, PDF Analysis using data we collected from the Advanced Photon Source at the Argonne National Laboratory.