Shashank Bishnoi

Numerical Simulation of Porosity in Cements

The pores in cementitious materials, their sizes and connectivity have an important influence on the durability of concrete. Several microstructural models have been developed to simulate the three-dimensional pore network in cement pastes. In this article, microstructures with the

Numerical benchmark campaign of COST Action TU1404 – microstructural modelling

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Micro-Chemo-Mechanical Characterization of a Limestone-Calcinated-Clay Cement Paste by Statistical Nanoindentation and Quantitative SEM-EDS

μic model are compared with experimental results. It is seen that despite having a resolution for the capillary pores very close to reality, the experimentally observed breakthrough diameter from mercury intrusion is much lower than the values obtained by applying an algorithm of mercury intrusion to the simulated microstructure. The effect of some of the most important input parameters on the pore sizes in the simulated microstructure explored. The phenomenon which seems best able to explain this discrepancy is that C–S–H is not in fact a phase with a smooth surface as represented in microstructural models, but a phase which grows as needles into the pore space, leading to very small water-filled capillary pores from quite young ages. The results demonstrate it will be extremely challenging to represent the porosity of real microstructures in microstructural models on the scale of hundreds of microns necessary to study macroscopic transport.

Sulphate Optimization of Binders with Calcined Clay Using Isothermal Calorimetry

AbstractThis paper presents a new approach to model the creep behavior of cement paste at early ages. The creep behavior is simulated by applying a time-varying generalized Maxwell model on the individual elements of a finite-element mesh of a simulated three-dimensional microstructure and compared with results in the literature. All mechanical properties of the constituent phases are taken from literature and Maxwell chain parameters are obtained by fitting the intrinsic creep of calcium silicate hydrate (C-S-H). A reasonable agreement between the simulations and the experimental results are obtained by assuming a constant C-S-H density of 2.0  g/cm3. It was found that better agreements could be obtained at low degree of hydrations, by assuming a loosely packed C-S-H growing in the microstructure. It was also found that the short-term creep characteristics of C-S-H from nanoindentation can be used to reproduce macroscopic creep at least over a few days. The results show how numerical models can be used t...

Hydration and Mechanical Properties of Limestone Calcined Clay Cement Produced with Marble Dust

This paper presents the results of the numerical benchmark campaign on modelling of hydration and microstructure development of cementitious materials. This numerical benchmark was performed in the scope of COST Action TU1404 “Towards the next generation of standards for service life of cement-based materials and structures”. Seven modelling groups took part in the campaign applying different models for prediction of mechanical properties (elastic moduli or compressive strength) in cement pastes and mortars. The simulations were based on published experimental data. The experimental data (both input and results used for validation) were open to the participants. The purpose of the benchmark campaign was to identify the needs of different models in terms of input experimental data, verify predictive potential of the models and finally to provide reference cases for new models in the future. The results of the benchmark show that a relatively high scatter in the predictions can arise between different models, in particular at early ages (e.g. elastic Young’s modulus predicted at 1 d in the range 6-20 GPa), while it reduces at later age, providing relatively good agreement with experimental data. Even though the input data was based on a single experimental dataset, the large differences between the results of the different models were found to be caused by distinct assumed properties for the individual phases at the microstructural level, mainly because of the scatter in the nanoindentation-derived properties of the C-S-H phase.

Limestone Calcined Clay Cement: The Experience in India This Far

As the market for cement has slowly moved from Portland cements towards pozzolanic cements and continues to move towards ternary cements, the tests for cements and the components blended in them remain largely unchanged. While it is recognised that certain mineral additives, e.g. limestone, do not meet the traditional definition of pozzolanic materials, the amount of processing and testing conditions play an important role in the measured reactivity of other additives. In addition, the current testing methods are unable to take the interaction of different components and their effects on the overall properties of cements into account. This paper reviews the methods currently available for the characterisation of materials and their suitability for application to ternary cements. Possible modifications to these tests, in order to take the cements of future into account are suggested. The inclusion of test methods to ascertain the suitability of the materials mixed in the cements is also suggested.

Effective Clinker Replacement Using SCM in Low Clinker Cements

To address the sustainability concerns associated with Portland cement clinker production, the ternary blend of limestone, calcined clay and cement (which was named LC3) has recently been demonstrated to be an efficient solution. This work aims to contribute to the development of this promising material by applying latest techniques to characterize the chemo-mechanical properties of the complex heterogeneous microstructure of an LC3 paste by combining statistical nanoindentation and quantitative SEM-EDS techniques (SNI-QEDS). The results showed that the mechanical properties of LC3 come from a complex microstructure assemblage of C-A-S-H, Al-rich hydrates and anhydrous grains. Thus, the LC3 microstructure is composed by large anhydrous grains (clinker and calcined clay) embedded in a cementitious paste made of hydrates incorporating finely graded grains of anhydrous calcined clay, limestone and quartz. Moreover, the partial reaction of the calcined clay, limestone and Portlandite formed C-A-S-H and other Al-rich hydrates (likely including carboaluminates). Notably, the latter exhibited higher mechanical properties than those of C-A-S-H. Finally, the present work provides new knowledge for better understanding the complex LC3 microstructure towards advanced modelling and mix design optimization.

High Level Clinker Replacement in Ternary Limestone-Calcined Clay-Clinker Cement

The concept of using isothermal calorimetry for sulphate optimization of Portland cement was developed by Lerch more than 70 years ago. In this paper, we demonstrate a new calorimetry based approach for modern low clinker blended cements grounded in Lerch’s concept and that can be used for both periodic sulphate optimization and continuous process control to ensure optimum performance during daily cement production. More frequent sulphate optimization, coupled with daily process control, is especially important for binders that rely on optimum aluminate hydration, such as the novel LC3 cement that utilizes aluminate phases from both clinker aluminates and calcined clay. In the LC3 cement, the sulphate component is the major phase controlling the early hydration, strength development and admixture compatibility of the alumina bearing phases. Moreover, the calorimetry based approach described herein can be substantially automated and does not require a traditional laboratory setting, nor air conditioning, since the calorimetry itself provides a laboratory environment in its temperature control chamber. The only manual labour required for this concept is the weighing of binder, water, calcium sulphate and optional admixtures. The mixing, data collection, calibration, evaluation of the sulphate optimum and subsequent process control can all be automated, such as in Calmetrix Inc’s software suite for cement research and development.