Chemical shrinkage is one of the main reasons leading to the early age deformation of concrete. Hence, the prediction of chemical shrinkage is an important issue for evaluating the deformation of concrete and its influence on the durability of concrete structures. There is still room to improve the accuracy for predicting the chemical shrinkage of cement, because some empirical assumptions were involved in traditional methods such as Paulini equation. In this study a discrete algorithm was proposed to deal with the stoichiometry’s conversion of tricalcium aluminate (C3A) and (tetracalcium aluminoferrite) C4AF with ongoing hydration of cement. Based on this discrete algorithm, the volume evolution of phases in cement paste and the chemical shrinkage of cement were calculated. The chemical shrinkage of cement in W/C 0.3 and W/C 0.4 were simulated using this method and compared with the results of experiments and Paulini equation. It was found that the calculated chemical shrinkage of cement in W/C 0.3 and 0.4 are in good agreement with experiments, respectively. In comparison with Paulini equation, the proposed method was more accurate for predicting the chemical shrinkage of Portland cement, because the nonlinear relationship between the chemical shrinkage and degree of hydration of cement can be simulated by dealing with the stoichiometry’s conversion of C3A and C4AF with ongoing hydration.@en

In this study, first of all, the atomistic structure of cement hydration products is estimated via molecular dynamics method and their elastic properties are extracted. Then, cement hydration simulation is done by HYMOSTRUC3D model and the obtained results from both molecular dynamics and HYMOSTRUC3D methods are used for simulation in macro-scales through analytic and lattice methods. Finally, elastic properties of cement paste are estimated with two mentioned methods and compared with each other and also with literature. The study, in fact, aims to investigate an appropriate multi-scale simulation model to examine cement paste elastic properties.

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The HYMOSTRUC3D model has been used successfully to predict hydration and microstructure development of pure Portland cement paste. In recent years, a number of numerical models were proposed for optimizing the use of supplementary cementitious materials. Also HYMOSTRUC3D was extended for simulating the hydration and microstructure development of Portland cement blended with blast furnace slag or/and fly ash (Gao, 2018). This paper summarises the main features of this extended model, called HYMOSTRUC3D-E, and demonstrates the simulation of the hydration process and pore solution chemistry of slag cements.@en

Drying of cement-based overlay systems is a critical issue, because it causes differential shrinkage between the overlay material and the concrete substrate and may induce cracking or debonding of the overlay material. In this paper the mechanisms of moisture transport in hydrating cement-based overlay systems are studied. A model is proposed for simulating the moisture transport. A parameter study has been conducted to quantitatively investigate the influence of the thickness of the overlay material and the curing conditions on hydration of the overlay materials. The evolution of the moisture profile in the overlay system and the development of the degree of hydration (DOH) of the overlay material have been calculated. The change of water content in the overlay material is investigated, in terms of the water absorbed by the substrate, the water consumed by hydration of the overlay material and the water evaporated to the environment. The simulation results show that the water evaporation is a dominant factor that causes water loss of the overlay material, while the water absorption by the substrate plays only a minor role. Moist curing is much more effective than sealed curing (e.g. by using sealing agent) for hydration of the overlay material. The DOH of the overlay material is significantly increased with longer moist curing. Under the same curing condition (e.g. moist curing + drying), thinner overlay materials are more vulnerable to water loss and exhibit a lower DOH. It suggests that for proper hydration of cement-based overlay materials, moist curing is recommended rather than applying a sealing agent.@en

For optimization of the use of Supplementary Cementitious Materials (SCMs), i.e. blast furnace slag (BFS) and fly ash (FA), in cementitious system a numerical model for simulating the hydration and microstructure development of blended cements can be used. Several models have been proposed in recent years to simulate the hydration and microstructure development of blended cements. However, most of these models need further development. For example, the nucleation and growth of calcium hydroxide (CH) particles were often not simulated explicitly in these models.@en

Mercury intrusion porosimetry (MIP) measurements are widely used to determine pore throat size distribution (PSD) curves of porous materials. The pore throat size of porous materials has been used to estimate their compressive strength and air permeability. However, the effect of sample size on the determined PSD curves is often overlooked. In pursuit of a better understanding of the effect of sample size on mercury intrusion into porous materials, a combined experimental and numerical approach was applied. Quartz sand and epoxy resin were mixed to form artificial sandstone. Digital microstructures of the sandstone were obtained by using X-ray computed tomography (CT scan) technique. PSD curves of the artificial sandstone with different sample sizes were determined both by MIP measurement and by simulation of mercury intrusion (i.e., MIP simulation). Percolation analysis was performed on mercury-intruded pores in the digital microstructures. The PSD curves determined both by MIP measurements and by MIP simulations show that there was a significant effect of sample size on mercury intrusion before percolation of mercury-intruded pores. The effect of sample size decreased with the increasing pressure. After the mercury-intruded pores percolated through the samples, the effect of sample size on mercury intrusion became minor. The pore throat size of the artificial sandstone was used to estimate the air permeability using the relation proposed in the literature. The calculated air permeability of the smaller sandstone sample was higher. However, in principle, the air permeability of sandstone samples should be independent of the sample size. Two main conclusions can be drawn: (1) a fixed sample size should be used in MIP measurements or MIP simulation so that the PSD curves of different samples can be properly compared, (2) sample size needs to be considered when the pore throat size determined by MIP measurement is used for estimating air permeability.

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Because the pore plays the primary role in strength development of blended cement paste, the role of filler-hydrates adhesion properties has attracted very little attention. The purpose of this study is to investigate the effect of filler-hydrates adhesion properties on strength development of cement paste. In this study, the development of compressive strength of portland cement paste and cement paste blended with limestone powder and micronized sand was studied experimentally. Parallel with this experimental study, the contact area in these cement pastes was quantified numerically. The relationship between the measured compressive strength and simulated contact area was then analyzed. With this relationship, the effect of filler-hydrates adhesion properties on strength development of cement paste was quantified. The contact area between hydrating cement particle and micronized sand particle had no contribution to the compressive strength. In contrast, the contact area between the hydrating cement particle and limestone particle had a substantial contribution to the compressive strength.

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Graded blended cement made of graded Portland cement (PC), blast furnace slag (BFS) and fly ash (FA) is attractive for cement production. For manufacturing graded blended cement, a suitable mathematical expression should be introduced to describe the particle size distribution (PSD) of its components and control the quality of graded blended cement. This study aims to evaluate Rosin-Rammler-Sperling-Bennet (RRSB) distribution and lognormal distribution for describing the PSD of the components of graded blended cement. RRSB distribution and lognormal distribution are used to fit the PSD of ungraded and graded PC, BFS and FA. It is found that lognormal distribution exhibits smaller fitting errors for describing the PSDs of graded PC, BFS, FA and ungraded FA. What is more, lognormal distribution exhibits good simplicity and popularity. Hence, it is recommended to use lognormal distribution to control the PSD of graded blended cement in manufacturing process.

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Modeling the complex behavior of concrete for a specific mixture is a challenging task, as it requires bridging the cement scale and the concrete scale. We describe a multiscale analysis procedure for the modeling of concrete structures, in which material properties at the macro scale are evaluated based on lower scales. Concrete may be viewed over a range of scale sizes, from the atomic scale (10−10 m), which is characterized by the behavior of crystalline particles of hydrated Portland cement, to the macroscopic scale (10 m). The proposed multiscale framework is based on several models, including chemical analysis at the cement paste scale, a mechanical lattice model at the cement and mortar scales, geometrical aggregate distribution models at the mortar scale, and the Lattice Discrete Particle Model (LDPM) at the concrete scale. The analysis procedure starts from a known chemical and mechanical set of parameters of the cement paste, which are then used to evaluate the mechanical properties of the LDPM concrete parameters for the fracture, shear, and elastic responses of the concrete. Although a macroscopic validation study of this procedure is presented, future research should include a comparison to additional experiments in each scale@en

More and more studies are based on digital microstructures of cement pastes obtained either by numerical modelling or by experiments. A comprehensive understanding of the their pore structures, therefore, becomes significant. In this study, the pore structure of a virtual cement paste (HYMO-1d) generated by cement hydration model HYMOSTRUC 3D is characterized. The pore structure of HYMO-1d is compared to the one of CT-1d that is reconstructed by using X-ray computed tomography technique (CT scan). Both HYMO-1d and CT-1d have the same porosity. Various parameters are taken into account, viz., the specific surface area, the pore size distribution (PSD), the connectivity and the tortuosity of water-filled pores. Regarding the PSD, two concepts (i.e., the “continuous PSD” and the “PSD by MIP simulation”) are adopted. The “continuous PSD” is believed to be a “realistic” PSD; while the “PSD by MIP simulation” is affected by the “throat” and “ink bottle” pores. The results show that HYMO-1d and CT-1d exhibit a similar curve of “continuous PSD”, but distinct curves of “PSD by MIP simulation” and different specific surface areas. A lower complexity of the pore structure of HYMO-1d is indicated by a higher tortuosity of water-filled pores with reference to CT-1d. This study indicates that the comparison of pore structures between the digital microstructures should be based on multiple parameters. It also gives an insight into further studies on digital microstructures, i.e. transport properties of unsaturated materials.

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Autogenous shrinkage is the volume deformation of a closed, isothermal, cementitious material system not subjected to external forces. How to accurately predict the autogenous shrinkage in hardening cement-based materials is an important issue in concrete technology because autogenous shrinkage increases cracking risk and reduces the durability and service life of reinforced concrete structures. Many models simulated the autogenous shrinkage of cement-based materials based on some mechanisms such as capillary pressure and disjoining pressure. These models are normally empirical and cannot reveal the deformation behavior of cement-based materials under the internal load of the driving force of autogenous shrinkage in microscale. As a consequence, the reliability of these models are questionable. This paper proposes a numerical model to simulate the autogenous shrinkage of hardening cement paste. A cement hydration and microstructure model HYMOSTRUC3D is used to simulate the microstructure of cement paste. A pore morphology based method is applied to describe the water and empty capillary pores distribution in the microstructure. Capillary pressure in the microstructure of cement paste is calculated from relative humidity measured by experiment based on Kelvin equation. A discrete algorithm is proposed to divide the hydration time into several static times. At each static time, a lattice finite element fracture method is used to simulate the deformation of simulated microstructure of cement paste under capillary pressure imposing. The autogenous shrinkage of hardening cement paste is equal to the sum of deformation of cement paste at each static time. The autogenous shrinkage of Portland cement paste with water to cement ratio of 0.3 is predicted. The simulation results are in a good agreement with experiments.@en

The heterogeneity of the concrete may be considered on different size scales of observation, ranging from the atomistic scale (10-10m), characterized by the behavior of crystalline particles of hydrated Portland cement, to the macroscopic scale (101 m), where concrete has traditionally been considered homogeneous. The multiscale framework we are proposing in this paper is based on the following models: chemical analyses at the cement paste scale; mechanical lattice model at the cement and mortar scales; geometrical aggregate distribution models at the mortar and concrete scales; and the Lattice Discrete Particle Model (LDPM) at the concrete scale. For that purpose, a set of analysis starting from a known set of parameters of the cement paste. This input is utilized to evaluate the mechanical properties of the mortars (cement and sand), and then these properties are used to evaluate the mechanical properties of the mortar-a4 (mortar-s and aggregate smaller then 4mm). The upscaling in the proposed methodology involved the evaluation of the LDPM concrete parameters based on the mortara4 properties. Here we are suggesting a uni-axial tension "numerical experiments" on the mortar-a4 scale to evaluate the elastic and fracture LDPM mechanical parameters. @en