Cement is one of the most versatile and widely used materials in the construction industry and plays a key role in any infrastructure development [Bye, 1999]. Due to the increasing demand, the cement manufacturing industry has been growing rapidly. However, the production of cement involves high energy consumption and CO2 emission which has resulted in major sustainability and environmental issues [Devi et al., 2017]. As a result of its large-scale production, the cement industry contributes to about 5% of total greenhouse gases in the atmosphere. Furthermore, due to the increased demand for raw materials, the prices of cement have been increasing over the years. Supplementary materials like fly ash are promising low-cost alternatives that can act as partial cement replacement and help to enhance the properties of concrete [ASTM C618-19, Prabhakar et al., 2005, Cheriyan et al., 2014]. Fly Ash is an industrial by product produced during the combustion of coal, which when collected and utilized properly can be resourceful in reducing the use of cement, thereby minimizing energy consumption and resulting in significant saving of natural resources and economy.
The cement clinker when mixed with water in the desired ratio undergoes a series of complex chemical reactions known as cement hydration to produce a final hardened material. These reactions may take place separately or simultaneously to produce a collection of hydration products [Mehta et al., 2006, Bullard et al., 2011]. The study of cement hydration with supplementary cementitious materials enables development of concrete with improved workability, durability and strength. However, the experimental techniques employed to understand the hydration mechanism offer significant challenges. In the past decades, advances in computer based numerical models have allowed users to create virtual cement compounds and investigate the hydration kinetics. Furthermore, the global pandemic, which resulted in lockdowns and disrupted laboratory studies, necessitated the adoption of an integrated approach to precisely model the experimental mechanism for obtaining hydration properties of cement compounds using simulation tools. With continuous development and improvement in computational techniques, the complex hydration phenomenon of cement mortars and concrete can be studied through 3-D microstructure development. The Virtual Cement and Concrete Testing Laboratory (VCCTL) is a computer model developed by the National Institute of Standards and Technology (NIST) that can be used for the hydration and microstructure development of existing and new cement-based compounds [Bullard et al., 2009]. VCCTL is a web-based virtual laboratory that allows users to create virtual materials and control the simulation environment to hydrate, and evaluate a cement based microstructure. The accuracy of the model depends on the accuracy of the input data added about cement powder and properties of binder material used. It is necessary to register proper physical and chemical compositions of materials in the VCCTL software to achieve reliable results. The software takes as input of following experimental data: cement phase composition; particle size distribution, material phase volume and surface area fraction, aggregate grading, cement particle dispersion in microstructure, detailed study of which is required to achieve reliable results.
The hydration model of VCCTL, CEMHYD3D, created by NIST in 1997 uses digital lattice based methods to create a 3D digital microstructure where each 3D cubical element, called voxel of the lattice consists of physical and chemical characteristics of the materials being modeled [Bentz et al., 1997, Hiller and Lipson 2009]. The chemical reactions during the process of mixing, hydrating, and hardening can be simulated through computer modeling. Each voxel undergoes an independent reaction based on its composition, which then nucleates with other voxels to form the hydrated compound. After the compound has been hydrated, the hydration kinetics can be studied and parameters like the heat of hydration, degree of hydration, porosity, chemical shrinkage can be predicted. The microstructure present in the form of voxel allows the model to be integrated with finite element methods for the calculation of mechanical properties like modulus of elasticity. The strength and durability of cement compounds can further be calculated using empirical relations.
Many studies have been conducted to examine the cement hydration model and properties of hydrated cement obtained using VCCTL software. Bullard et al. demonstrated the software interface and performed two studies examining the software’s application for solving practical problems. The first study included the replacement of coarse clinkers with limestone and testing the effects on elastic moduli and compressive strength. The second study estimated the 28-day compressive strength for different clinkers [Bullard et al., 2009]. Watts et al. (2013a) evaluated the effectiveness of VCCTL to predict various properties of Portland cement concrete with water reducing admixtures. The study emphasized the importance of proper characterization of the materials which are essential to run a simulation. The study conducted a sensitivity analysis to determine and enhance the accuracy of the results. Valentini et al., 2014 studied the hydration kinetics and elastic moduli results obtained using VCCTL software and compared the numerical results with the experimental observations. The software proved to adequately predict the result with observed differences of less than 10% variation Watts et al. (2013b) compared the compressive strength and heat of hydration for 20 different cements with the experimental data obtained through the proficiency sample of the Cement and Concrete Reference Laboratory. The software proved to be practical and reliable with a mean absolute difference of 5% and 6.5% respectively [Valentini et al., 2014]. Abzaev et al. (2019) explored the curing mechanism and strength properties of CEM I 42.5B for different water cement ratio. The study also conducted experimental analysis to investigate the correlation between strength properties at various w/c ratio. The long-term asymptotic value of the sample compressive strength under saturated and adiabatic hydration may also be ascertained via virtual experiments [Bejar et al., 2018]. Models like VCCTL attempt to simulate the experimental tests virtually and it needs the particle size distribution, shape distribution, and elastic moduli to create virtual material properties and make accurate calculations of the composite [Erdogan et al., 2017].
One of the popular methods to calculate compressive strength is the Power’s gel-space ratio [Taylor, 1997].The relationship between the microstructure and compressive strength is as follows:
$${\sigma }= {{\sigma }}_{{o}} {{X}}^{{A}} \left(1\right)$$
Where, σ = compressive strength; σo = experimentally measured compressive strength at 3d; X = gel-space ratio and A = constant
The method is limited since it requires data of experimentally measured compressive strength to predict strength at later stages. The elastic properties can be used to measure the strength and determine the durability of cement mortars. VCCTL performs finite element calculation on each voxel of the simulated cement mortar to calculate elastic properties based on material input data and hydration parameters [Abzaey et al., 2019]. In the VCCTL user guide, Bullard et al., 2014 suggests following the empirical equation described by Neville (1995) to calculate the compressive strength. It is a relation between the elastic modulus and the compressive strength and is as followed:
$${{E}}_{{c}}=4.73 \sqrt{{{f}}_{{c}}}$$
2
The relation between strength and modulus can be rearranged to:
$${{{f}}_{{c}}=0.0447\left({{E}}_{{c}}\right)}^{2}$$
3
Where, fc = compressive strength & Ec = modulus of elasticity
Kaur (2016) conducted an experimental study to analyze the effects on Indian cement mortar due to partial replacement of OPC with fly ash and it examinates experimental techniques to investigate the hydration mechanism including the characterization of cement and fly ash using X-ray diffraction (XRD) and X-ray fluorescence (XRF) analysis. The hydration kinetics along with the strength development of cement mortar was investigated in the study. The data required as input parameters (phase composition, phase fraction of clinker, particle size distribution of cement) for creating virtual cement and fly ash compounds in VCCTL software are obtained from existing literature (Kaur, 2016). It is already established using proof of concept that computational modeling can emulate the performance of physical testing laboratories for heat of hydration and compressive strength testing of type I, II and III portland cements [Watts et al., 2017]
In this paper, the researcher wishes to explore the employability of the Virtual Cement and Concrete Testing Laboratory (VCCTL) to predict the hydration properties of Ordinary Portland and Pozzolanic cement and validated the same for adopted literature (Kaur, 2016). Again, this study further extends to obtain the cement hydration phenomena for variation supplementary replacement with altered temperature. The use of fly ash as supplementary materials is very common, but the complete understanding of hydration kinetics of cementitious material is still very few. Hence, this study includes the successive partial replacement of fly ash to general OPC cement to understand the hydration parameters. In addition to that, effect of temperature over the hydration kinetics for cementitious materials also simulated through VCCTL for better understand and also, microstructures are developed to support the experimental verification of hydration phenomena.