Development of Rheo-IR: Combination of a Strain-Controlled Rheometer with an IR Spectrometer for in-situ Mechanical and Chemical Analysis of Cement Paste

Fourier-transform infrared (FTIR) spectroscopy is a common tool for determining the chemical composition of a material in the solid, liquid, or gas phases, both qualitatively and quantitatively. It is also employed as a technique for monitoring the rate of chemical changes as a function of time, concentration, temperature etc. These chemical changes can have rheological implications, such as polymerization kinetics, rubber crosslinking, or epoxy curing, to name a few. In this thesis, a unique set-up that is able to simultaneously measure rheology and IR spectra with an improved sensitivity, to correlate mechanical behavior with microstructural changes, was developed. The stepwise development of chemically and mechanically sensitive Rheo-IR set-up is presented. This includes the design of an IR transparent upper-plate rheological geometry used as an attenuated total reflectance (ATR) sampling tool and a description of the technical and methodological adaption of the ATR crystal into an ARES G2 rheometer. In this new set-up, a strain-controlled rheometer is combined in a novel configuration with an ATR crystal and the IR beam is guided through two off-axis parabolic mirrors to the quasi-static upper plate of the rheometer to gain maximum IR sensitivity. ... mehrThereby online and directly correlated real-time FTIR spectra can be acquired whilst simultaneously conducting rheological measurements. This allows for 'in-situ' correlation of macroscopic rheological properties with microscopic, molecular chemical changes. These experiments are conducted for a material under controlled conditions having exactly the same sample time evolution for the simultaneous measurement. In addition, this set-up allows to study the effect of shear under steady state and oscillatory shear conditions, both in the linear (SAOS) and nonlinear regime (steady shear and LAOS).

As a proof of concept and to demonstrate its potential, this newly developed method was applied to correlate the polymer network formation for a free radical co-polymerization of acrylic acid and methylenebis(acrylamide) as a crosslinking agent via IR spectroscopy and the respective mechanical time evolution, in a dilute water-based solution. In addition, the newly developed Rheo-IR technique was applied to cement paste hydration and structural build-up. The IR results were compared to those from offline FTIR. Along with the Rheo-IR and FTIR measurements other rheological techniques, including FT-Rheology, Rheo-NMR and Rheo-Dielectrics were applied to study early cement paste hydration and structural build-up. The strain deformation and flow of fresh cement paste on the basis of microscale processes during early hydration was of interest. Applying FT-rheology, the intensity at mechanical higher harmonics are quantified and normalized to the fundamental intensity. A model with a quadratic scaling in the strain amplitude is used to predict two critical strains, that can be associated with the solid ($\gamma_{cs} \sim 0.1\% $) and mobile ($\gamma_{cm} \sim 0.01\% $) parts of cement paste. Interestingly, the effect of hydration time has minimal relevance on these critical strains. The influence of the microstructure seen through the mechanical evolution is attributed to the formation of hydrates, as determined spectroscopically through FTIR. Although, the hydration products (i.e. ettringite) do form within the first hour of hydration, the relative amount seems not to be the main contributing factor to structural build up. The structural build-up as a function of applied strain as investigated by Rheo-NMR, shows that although a relatively large strain for cement ($\gamma_{c} = 0.3\%$) is applied, the molecular mobility is the same as that of a lower strain ($\gamma_{c} = 0.01\%$), within the first hour of hydration. Through Rheo-Dielectrics, a similar trend is observed for the dc-conductivity. However, after 2 to 5 hours a 100 to 1000 times decrease in conductivity is seen, due to setting. The sample exposed to a higher strain, sets faster. Based on the observations from the Rheo-combined methods as a well as FT-Rheology the assumption is that physical interactions such as colloidal interactions could be more dominant than chemical bond formation for structural build-up in the first few minutes to the first hour of cement paste hydration.