Poroplastic properties of calcium-leached cement-based materials

Asymptotic calcium leaching of cement-based materials produces a new material composed of C-S-H with low C/S ratio on the order of 1 and a high porosity generated by the dissolution of Portlandite (CH) crystals, which creates a new pore-size family in the micrometer range. This paper investigates the role of these two phenomena in the multiaxial inelastic and hardening deformation behavior, in compression, of calcium-leached cement pastes and mortars. From triaxial tests and SEM microscopy, it is shown that the low C/S C-S-H matrix is highly plastically deformable, which is consistent with the high degree of polymerization and the effect of the C/S ratio on the intrinsic cohesion of C-S-H. The validity of the effective stress concept is experimentally proven for calcium-leached cement paste and mortar and provides evidence that the low C/S C-S-H solid phase of the cement paste is a pure cohesive incompressible material. In turn, the large pores created by the CH dissolution provides expansion space for the incompressible solid during compressive loading. Once this porosity is filled, the volume deformability is exhausted, and the material dilates to failure. In a similar way, the early tendency of mortars to dilate is found to be a consequence of a competition between plastic material behavior of the matrix (plastic hardening) and porosity-controlled structural deformation (geometrical hardening) triggered by frictional dilation mechanisms in the Interfacial Transition Zone (ITZ).     

A multiscale micromechanics-hydration model for the early-age elastic properties of cement-based materials

The E-modulus of early age cement-based materials, and more importantly, its evolution in time, is one of the most critical material-to-structural design parameters affecting the likelihood of early-age concrete cracking. This paper addresses the problem by means of a multistep micromechanics approach that starts at the nanolevel of the C-S-H matrix, where two types of C-S-H develop in the course of hydration. For the purpose of homogenization, the volume fractions of the different phases are required, which are determined by means of an advanced kinetics model of the four main hydration reactions of ordinary portland cement (OPC). The proposed model predicts with high accuracy the aging elasticity of cement-based materials, with a minimum intrinsic material properties (same for all cement-based materials), and 11 mix-design specific model parameters that can be easily obtained from the cement and concrete suppliers. By way of application, it is shown that the model provides a quantitative means to determine (1) the solid percolation threshold from micromechanics theory, (2) the effect of inclusions on the elastic stiffening curve, and (3) the development of the Poisson's ratio at early ages. The model also suggests the existence of a critical water-to-cement ratio below which the solid phase percolates at the onset of hydration. The development of Poisson's ratio at early ages is found to be characterized by a water-dominated material response as long as the water phase is continuous, and then by a solid-dominated material response beyond the solid percolation threshold. These model-based results are consistent with experimental values for cement paste, mortar, and concrete found in the open literature.     

Experimental study of accelerated leaching on hollow cylinders of mortar

Two mortars, differencing mainly in their initial porosity, were degraded by the use of a chemically accelerated process with ammonium nitrate solution. To specifically study the leached material, the chemical attack was undertaken on thin walled tubes. The leaching effects were evaluated by studying variations in mechanical and hydraulic properties. For both mortars tested, the kinetics of relative loss in strength, in elastic modulus and of increase in permeability were similar. For the same time of degradation, the increase in porosity and the loss in volumetric mass roughly depend on the estimated cement paste proportion of each mortar. The total process of degradation was carried out in three steps: 4, 8 and 16 days. Very sharp variations of all the studied properties were observed until 8 days of leaching followed by a plateau. These two phases are attributed to Portlandite dissolution first then to progressive C-S-H decalcification. At the end of the leaching test, a permeability increase of more than two orders of magnitude and a loss in strength and elastic modulus of more than 85% were observed for both mortars.     

Formulation of a new micromechanic model of three phases for ultrasonic characterization of cement-based materials

The objective of multiphasic models applied to cement-based materials is to estimate the global elastic constants as a function of properties of their constituents. However, these models have limited success for porosity characterization.In this investigation, a new approach that determines the elastic properties of three-phase materials is presented. The proposed model takes into account the microstructural characteristics of the constituent phases, as well as their elastic properties. The micromechanical model of three phases is applied to mortar, considering the material composed by cement paste matrix and two types of inclusions, aggregate and pores. The influence of geometry and elastic properties of both inclusions on the ultrasonic velocity has been evaluated theoretically. The effect of the volume fraction of pores on the ultrasonic velocity is also presented.     

Nanoindentation investigation of creep properties of calcium silicate hydrates

The creep properties of calcium silicate hydrates (C-S-H) are assessed by means of nanoindentation creep experiments on a wide range of substoichiometric cement pastes. We observe that, after a few seconds, the measured creep compliance of C-S-H is very well captured by a logarithmic time function. The rate of the logarithmic creep is found to scale in a unique manner with indentation modulus, indentation hardness, and packing density, independent of processing, mix proportions, indenter geometry and load history. The comparison with macroscopic creep experiments on concrete shows that minutes-long nanoindentations enable a quantitative assessment of the long-term creep properties of cementitious materials, orders of magnitude faster than macroscopic testing. Finally, we show that a strong analogy exists between this logarithmic creep behavior of C-S-H and that of soils, which suggests a granular origin of creep of geomaterials.     

Micromechanical multiscale fracture model for compressive strength of blended cement pastes

The evolution of compressive strength belongs to the most fundamental properties of cement paste. Driven by an increasing demand for clinker substitution, the paper presents a new four-level micromechanical model for the prediction of compressive strength of blended cement pastes. The model assumes that the paste compressive strength is governed by apparent tensile strength of the C-S-H globule. The multiscale model takes into account the volume fractions of relevant chemical phases and encompasses a spatial gradient of C-S-H between individual grains. The presence of capillary pores, the C-S-H spatial gradient, clinker minerals, SCMs, other hydration products, and air further decrease compressive strength. Calibration on 95 experimental compressive strength values shows that the apparent tensile strength of the C-S-H globule yields approx. 320 MPa. Sensitivity analysis reveals that the “C-S-H/space” ratio, followed by entrapped or entrained air and the spatial gradient of C-S-H, have the largest influence on compressive strength.     

Nanogranular packing of C-S-H at substochiometric conditions

Herein, we present a comprehensive nanoindentation investigation of cement pastes prepared at substoichiometric water-to-cement (w/c) mass ratios between 0.15 and 0.4 with and without heat treatment. Based on a statistical indentation technique, we provide strong evidence of the existence of a statistically significant third hydrated mechanical phase in addition to the already known Low-Density (LD) and High-Density (HD) C-S-H phases. The nanomechanical properties of this third phase are found to follow similar packing density scaling relations as LD C-S-H and HD C-S-H, while being significantly greater. This third phase, whose nano-packing density is measured at 0.83 ± 0.01, is therefore termed Ultra-High-Density (UHD) phase. All three phases are present in concrete materials in different volume proportions: LD dominates cement-based materials prepared at high w/c mass ratios; HD and UHD control the microstructure of low w/c ratio materials. In addition, heat treatment favors the formation of HD and UHD. The insight thus gained into the link between composition, processing and microstructure makes it possible to monitor packing density distributions of the hydration products at the nanoscale.     

Effect of hydration temperature on the solubility behavior of Ca-, S-, Al-, and Si-bearing solid phases in Portland cement pastes

The concentrations of Ca, S, Al, Si, Na, and K in the pore solutions of ordinary Portland cement and white Portland cement pastes were measured during the first 28 d of curing at temperatures ranging from 5–50 °C. Saturation indices with respect to solid phases known to form in cement paste were calculated from a thermodynamic analysis of the elemental concentrations. Calculated saturation levels in the two types of paste were similar. The solubility behavior of Portlandite and gypsum at all curing temperatures was in agreement with previously reported behavior near room temperature. Saturation levels of both ettringite and monosulfate decreased with increasing curing temperature. The saturation level of ettringite was greater than that of monosulfate at lower curing temperatures, but at higher temperatures there was effectively no difference. The solubility behavior of C-S-H gel was investigated by applying an appropriate ion activity product (IAP) to the data. The IAP_CSH decreased gradually with hydration time, and at a given hydration time the IAP_CSH was lower at higher curing temperatures.     

Decalcification shrinkage of cement paste

Decalcification of cement paste in concrete is associated with several modes of chemical degradation including leaching, carbonation and sulfate attack. The primary aim of the current study was to investigate the effects of decalcification under saturated conditions on the dimensional stability of cement paste. Thin (0.8 mm) specimens of tricalcium silicate (C₃S) paste, white portland cement (WPC) paste, and WPC paste blended with 30% silica fume (WPC/30% SF) were decalcified by leaching in concentrated solutions of ammonium nitrate, a method that efficiently removes calcium from the solid while largely preserving silicate and other ions. All pastes were found to shrink significantly and irreversibly as a result of decalcification, particularly when the Ca/Si ratio of the C-S-H gel was reduced below ∼ 1.2. Since this composition coincides with the onset of structural changes in C-S-H such as an increase in silicate polymerization and a local densification into sheet-like morphologies, it is proposed that the observed shrinkage, here called decalcification shrinkage, is due initially to these structural changes in C-S-H at Ca/Si ∼ 1.2 and eventually to the decomposition of C-S-H into silica gel. In agreement with this reasoning, the blended cement paste exhibited greater decalcification shrinkage than the pure cement pastes due to its lower initial Ca/Si ratio for C-S-H gel. The similarities in the mechanisms of decalcification shrinkage and carbonation shrinkage are also discussed.     

The nanogranular behavior of C-S-H at elevated temperatures (up to 700 °C)

Cement-based materials are non-combustible, but the complex chemo-physical mechanisms that drive at elevated temperatures the thermal degradation of the mechanical properties (stiffness, strength) are still an enigma that have deceived many decoding attempts. This paper presents, for the first time, results from a new experimental technique that allows one to rationally assess the evolution of the nano-mechanical behavior of cement paste at elevated temperatures. Specifically, the thermal degradation of the two distinct calcium-silicate hydrate (C-S-H) phases, Low Density (LD) C-S-H and High Density (HD) C-S-H, is assessed based on a statistical analysis of massive nanoindentation tests. From a combination of nanoindentation, thermogravimetry and micromechanical modeling, we identify a new mechanism, the thermally induced change of the packing density of the two C-S-H phases, as the dominant mechanism that drives the thermal degradation of cementitious materials. We argue that this loosening of the packing density results from the shrinkage of C-S-H nanoparticles that occurs at high temperatures, most probably due to the loss of chemically bound water.     

A multi-technique investigation of the nanoporosity of cement paste

The nanometer-scale structure of cement paste, which is dominated by the colloidal-scale porosity within the C-S-H gel phase, has a controlling effect on concrete properties but is difficult to study due to its delicate structure and lack of long-range order. Here we present results from three experimental techniques that are particularly suited to analyzing disordered nanoporous materials: small-angle neutron scattering (SANS), weight and length changes during equilibrium drying, and nanoindentation. Particular attention is paid to differences between pastes of different ages and cured at different temperatures. The SANS and equilibrium drying results indicate that hydration of cement paste at 20 °C forms a low-density (LD) C-S-H gel structure with a range of gel pore sizes and a relatively low packing fraction of solid particles. This fine structure may persist indefinitely under saturated conditions. However, if the paste is dried or is cured at elevated temperatures (60 °C or greater) the structure collapses toward a denser (less porous) and more stable configuration with fewer large gel pores, resulting in a greater amount of capillary porosity. Nanoindentation measurements of pastes cured at different temperatures demonstrate in all cases the existence of two C-S-H structures with different characteristic values of the indentation modulus. The average value of the modulus of the LD C-S-H is the same for all pastes tested to date, and a micromechanical analysis indicates that this value corresponds to the denser and more stable configuration of LD C-S-H. The experimental results presented here are interpreted in terms of a previously proposed quantitative “colloid” model of C-S-H gel, resulting in an improved understanding of the microstructural changes associated with drying and heat curing.     

The role of calcium carbonate in cement hydration

Limestone, mainly consisting of calcite, is a permitted additive to Portland cements often up to a 5 wt.% limit. It is shown by experiment and calculation that much, if not all, of this calcite is reactive and affects the distribution of lime, alumina and sulfate and thereby alters the mineralogy of hydrated cement pastes. Calcite affects the mineralogical variant of the AFm phase(s). Calcite additions affect the amount of free calcium hydroxide as well as the balance between AFm and AFt phases, although C-S-H is unaffected in much of the range of compositions. Generic data are shown in graphical form to quantify these mineralogical changes as functions of cement composition and amount of added calcite. Calculations of the specific volume of solids as a function of calcite addition suggest that the space-filling ability of the paste is optimised when the calcite content is adjusted to maximise the AFt content. However, before the calculated data can be used uncritically, certain kinetic constraints on reactivity also need to be assessed. Progress towards the quantification of paste mineralogy suggests that (i) elucidation of the mineralogy of pastes, particularly blended cement pastes, is facilitated by using both theoretical and experimental approaches and (ii) that the ultimate goal, of calculating paste mineralogy from the bulk chemistry, is attainable.     

Microstructure-based micromechanical prediction of elastic properties in hydrating cement paste

Elastic properties of hydrating cement paste can be successfully predicted by combination of the hydration model, percolation theory and micromechanical analysis. Reconstruction of hydrating microstructure is based on the 3D digital NIST model of cement hydration, which is enhanced for the prediction of two C-S-H types. Chemical phases in a percolated microstructure served as an input in a two-level analytical or one-level 3D FEM or FFT elastic homogenization. Special mesh generation for the percolated microstructure is discussed as well as its numerical implementation. Good results from FEM and FFT were found for the size of the representative volume element of 50 × 50 × 50 μm, considering water-to-cement ratio in the range from 0.25 to 0.5. While good predictions in well-hydrated cement pastes were obtained for both analytical and numerical approaches, numerical homogenization was found more accurate and versatile for the whole hydration time.     

Effects of decalcification on the microstructure and surface area of cement and tricalcium silicate pastes

Thin coupons of white portland cement (WPC) and tricalcium silicate paste were decalcified by leaching in concentrated ammonium nitrate solutions, resulting in calcium-to-silicon molar ratios (C/S) ranging from 3.0 (control) down to 0.3. The microstructure and surface area were measured using both small-angle neutron scattering (SANS) and nitrogen gas sorption. The intensity in the SANS data regime corresponding to the volume fractal C-S-H gel phase increased significantly on leaching, and the total surface area per unit specimen volume measured by SANS doubled on leaching from C/S=3.0 to near C/S=1.0. The nitrogen BET surface area of the WPC pastes, expressed in the same units, increased on decalcification as well, although not as sharply. The primary cause of these changes is a transformation of the high-density “inner product” C-S-H gel, which normally has a low specific surface area as measured by SANS and nitrogen gas sorption, into a morphology with a high specific surface area. The volume fractal exponent corresponding to the C-S-H gel phase decreased with decalcification from 2.3 to 2.0, indicating that the equiaxed 5 nm C-S-H globule building blocks that form the volume fractal microstructure of normal, unleached cement paste are transformed by decalcification into sheetlike structures of increasing thickness.     

Analytical Electron Microscopy of Cement Pastes: II, Pastes of Portland Cements and Clinkers

Three hundred sixty-five particles of C-S-H, Ca(OH)₂, AFm phase, and AFt phase from pastes of normally ground portland cements and of finely ground cements and clinkers were analyzed. All the phases, except the Ca(OH)₂, showed significant variation in composition among paste specimens and among particles within each specimen. The C-S-H contains significant amounts of Al, Fe, and S; for that of a normally ground portland cement paste, cured for 28 days, the median Si:AI, Si:Fe, and Si:S ratios were 11, 43, and 15, respectively, whereas the mean Ca:Si ratio for all the particles analyzed was 2.0. The AFm phase in cement pastes is not pure monosulfate but has a mixture of sulfate, hydroxide, and Al- and Si-bearing ions in its interlayer sites; the AFt phase is not pure ettringite but contains Si and its sulfate is probably partly replaced by hydroxide. The Al and Fe contents in the C-S-H and the Si contents in the AFm and AFt phases are greater when finely ground starting materials are used. This fact, together with the marked variation among particles, emphasizes the difficulty of ionic transport in cement pastes.     

Model for the Developing Microstructure in Portland Cement Pastes

A method is proposed for quantitatively predicting the volume of the major phases in hydrated cement pastes as a function of (1) the composition of the cement, (2) the degree of reaction, and (3) the initial water:cement ratio. This procedure is then used to develop a quantitative model for the surface area and volume of porosity that is accessible to nitrogen in C-S-H. Published values for surface areas and volume of pores are compared with the predictions made by the model. An implication of the model is that there are two types of C-S-H, or perhaps regions within the C-S-H: one that nitrogen can penetrate and one that it cannot.     

Identification of viscoelastic C-S-H behavior in mature cement paste by FFT-based homogenization method

A powerful and robust numerical homogenization method based on fast Fourier transform (FFT) is formulated to identify the viscoelastic behavior of calcium silicate hydrates (C-S-H) in hardened cement paste from its heterogeneous composition. The identification is contingent upon the linearity of the creep law. To characterize cement paste microstructure, the model developed by Bentz at the National Institute of Standards and Technology, which has the resolution of 1 μm, is adopted. Model B3 for concrete creep is adapted to characterize the creep of C-S-H in cement paste. It is found that the adaptation requires increasing the exponent of power law asymptote of creep compliance. This modification means that the rate of attenuation of creep with time is lower in C-S-H than in cement paste, and is explained by differences in stress redistribution. In cement paste, the stress is gradually transferred from the creeping C-S-H to the non-creeping components. The viscoelastic properties of C-S-H at the resolution of 1 μm were identified from creep experiments on cement pastes 2 and 30 years old, having the water-cement ratio of 0.5. The irreversible part of C-S-H creep, obtained from these old specimens at almost saturated state, is found to be negligible unless the specimens undergo drying and resaturation prior to the creep test.     

Novel Use of Kaolin Wastes in Blended Cements

This paper describes the influence of different thermally activated clay wastes (ACW) on the hydration phases in cement pastes containing two percentages of addition (10% and 20%). Results show that the main products obtained during hydration of cement pastes containing ACW were portlandite, calcium aluminate hydrates, calcium silicate hydrates, and hydrotalcite-type compounds. Portlandite formation decreases when addition percentage is 20%, contrary to tetracalcium aluminate hydrate, which increases in similar conditions. The ACW that showed the most portlandite consumption was ACW1 (700°C, 2 h) according to thermogravimetric data.     

硫化物对水泥基材料中锌的控制研究

研究了硝酸锌对水泥基材料物理性能的影响以及外加控制剂硫化物对其在水泥基材料中的反应情况和硫化物本身对水泥基材料物理性能的影响。并通过比较硝酸锌和硫化物在2种不同的添加方式下所测得的水泥基材料性能,探讨了硫离子对锌离子的控制作用。研究结果表明,硫化物本身对硬化水泥浆体的宏观性能影响很小,其作为外加控制剂使用能很好地钝化锌离子在硬化水泥浆体中的化学活性。