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.     

Reactive pozzolana from Indian clays—their use in cement mortars

Studies were undertaken to produce reactive pozzolana i.e. metakaolin from four kaolinitic clays collected from different sources in India. The metakaolin produced from these clays at 700-800 °C show lime reactivity in between 10.5 to 11.5 N/mm² which is equivalent to commercially available calcined clay Metacem-85. The microstructure of the metakaolin has been reported. The effect of addition of metakaolin up to 25% in the Portland cement mortars was investigated. An increase in compressive strength and decrease of porosity and pore diameter of cement mortars containing metakaolin (10%) was noted over the cement mortars without metakaolin. The hydration of metakaolin blended cement mortars was investigated by differential thermal analysis (DTA) and scanning electron microscopy (SEM). The major hydraulic products like C-S-H and C₄AH₁₃ have been identified. Durability of the cement mortars with and without metakaolin was examined in different sulphate solutions. Data show better strength achievement in cement mortars containing 10% MK than the OPC mortars alone.     

Pore solution in alkali-activated slag cement pastes. Relation to the composition and structure of calcium silicate hydrate

In this work, the relationship between the composition of pore solution in alkali-activated slag cement (AAS) pastes activated with different alkaline activator, and the composition and structure of the main reaction products, has been studied. Pore solution was extracted from hardened AAS pastes. The analysis of the liquids was performed through different techniques: Na, Mg and Al by atomic absorption (AA), Ca ions by ionic chromatography (IC) and Si by colorimetry; pH was also determined. The solid phases were analysed by XRD, FTIR, solid-state ²⁹Si and ²⁷Al NMR and BSE/EDX.The most significant changes in the ionic composition of the pore solution of the AAS pastes activated with waterglass take place between 3 and 24 h of reaction. These changes are due to the decrease of the Na content and mainly to the Si content. Results of ²⁹Si MAS NMR and FTIR confirm that the activation process takes place with more intensity after 3 h (although at this age, Q² units already exist). The pore solution of the AAS pastes activated with NaOH shows a different evolution to this of pastes activated with waterglass. The decrease of Na and Si contents progresses with time.The nature of the alkaline activator influences the structure and composition of the calcium silicate hydrate formed as a consequence of the alkaline activation of the slag. The characteristic of calcium silicate hydrate in AAS pastes activated with waterglass is characterised by a low structural order with a low Ca/Si ratio. Besides, in this paste, Q³ units are detected. The calcium silicate hydrate formed in the pastes activated with NaOH has a higher structural order (higher crystallinity) and contains more Al in its structure and a higher Ca/Si ratio than those obtained with waterglass.     

Initial hydration of tricalcium silicate as studied by secondary neutrals mass spectrometry: II. Results and discussion

The hydration of tricalcium silicate (C₃S) was studied by secondary neutrals mass spectrometry (SNMS), a method that enables determination of the Ca/Si ratio of the formed calcium silicate hydrate (C-S-H) phase with an extremely low information depth. It was found that the magnitude of this parameter within the hydrate layer formed at the surface of the nonhydrated C₃S is not constant and increases with increasing distance from the liquid-solid interface. It was also found that, at a constant distance from the surface, the Ca/Si ratio declines with hydration time. The kinetics of the hydration process is characterized by a very fast initial reaction, followed by a dormant period and a subsequent period of renewed hydration. The rate of hydration becomes distinctly accelerated by elevated temperature and retarded by the presence of sucrose, while NaCl affects the initial hydration kinetics only to a small degree.     

Calcium silicate structure and carbonation shrinkage of a tobermorite-based material

Carbonated autoclaved aerated concretes (AACs) show no shrinkage at a degree of carbonation approximately less than 20%. The ²⁹Si MAS NMR spectrum showed that at a degree of carbonation less than 25%, the typical double-chain silicate anion structure of tobermorite-11Å was well maintained and interlayer Ca ions were exchanged with protons. This corresponded to the absence of carbonation shrinkage at a degree of carbonation less than 20%. When the degree of carbonation increased from 25% to 50% up to 60%, the double-chain silicate anion structure of tobermorite-11Å was decomposed and Ca ions in the Ca-O layers were dissolved, showing a possible mechanism of carbonation shrinkage.     

Examining the relationship between the microstructure of calcium silicate hydrate and drying shrinkage of cement pastes

Cement paste undergoes a volumetric contraction called drying shrinkage when placed in a low relative humidity (RH) environment. Only a portion of this shrinkage is reversible upon rewetting. In order to understand better the mechanisms responsible for the irreversible portion of drying shrinkage, a quantitative comparison was made between shrinkage values and microstructural properties of cement pastes. Drying shrinkage, surface area and pore volume were manipulated using curing temperature and chemical admixtures. It was observed that total and irreversible drying shrinkage increase with surface area and pore volume as measured by nitrogen (1-40 nm pore radius range), when degree of hydration and water-to-cement ratio (w/c) are held constant (0.55 and 0.45, respectively).     

The calcium silicate hydrates

This article is concerned with the calcium silicate hydrates, including crystalline minerals and the extremely variable and poorly ordered phase (C-S-H) that is the main binding phase in most concrete. Up-to-date composition and crystal-structure information is tabulated for the most important crystalline calcium (alumino) silicate hydrates and related phases. A number of models for the nanostructure of C-S-H are summarized and compared and it is shown that there is much more of a consensus than might seem apparent at first sight. The value of the recently solved structures of 1.4 nm tobermorite and jennite, together with those of jaffeite and metajennite, for visualizing the nanostructural elements present in the models is demonstrated. The importance of Hal Taylor's contribution to the solution of the structure of jennite is highlighted. The applicability of Richardson and Groves' model is demonstrated using experimental composition-structure observations on the nature of C-S-H in a Portland cement-fly ash blend.     

X-ray photoelectron spectroscopic investigation of nanocrystalline calcium silicate hydrates synthesised by reactive milling

X-ray photoelectron spectroscopy (XPS) has been used to analyse a series of mechanochemically synthesised, nanocrystalline calcium silicate hydrates (C-S-H). The samples, with Ca/Si ratios of 0.2 to 1.5, showed structural features of C-S-H(I). XPS analysis revealed changes in the extent of silicate polymerisation. Si 2p, Ca 2p and O 1s spectra showed that, unlike for the crystalline calcium silicate hydrate phases studied previously, there was no evidence of silicate sheets (Q³) at low Ca/Si ratios. Si 2p and O 1s spectra indicated silicate depolymerisation, expressed by decreasing silicate chain length, with increasing C/S. In all spectra, peak narrowing was observed with increasing Ca/Si, indicating increased structural ordering. The rapid changes of the slope of FWHM of Si 2p, Δ_Ca-Si and Δ_NBO-BO as function of C/S ratio indicated a possible miscibility gap in the C-S-H-solid solution series between C/S 5/6 and 1. The modified Auger parameter (α′) of nanocrystalline C-S-H decreased with increasing silicate polymerisation, a trend already observed studying crystalline C-S-H. Absolute values of α′ were shifted about − 0.7 eV with respect to crystalline phases of equal C/S ratio, due to reduced crystallinity.     

A thermodynamic model of dissolution and precipitation of calcium silicate hydrates

A thermodynamic incongruent dissolution/precipitation model of calcium silicate hydrate (C-S-H) is proposed, assuming a binary nonideal solid solution of Ca(OH)₂ and SiO₂. Using this model, both dissolution and precipitation of the C-S-H phase, with a continuous change in the Ca / Si ratio of the solid phase, can be predicted. The notable features of the model are its good continuity and simplicity so that calculation can be easily compiled in a calculation code. A series of experiments were carried out. C-S-H precipitates were prepared using two techniques: precipitation by contacting Ca(OH)₂ solution with C-S-H gel and hydrolysis in a mixture of Ca and Si solutions. The equilibria in these experiments were predicted well by the proposed model. A calculation using the model also predicted well the dissolution of ordinary Portland cement hydrate with water exchange.     

Chemical alteration of calcium silicate hydrate (C-S-H) in sodium chloride solution

The effect of sodium chloride on the chemical alteration of calcium silicate hydrate (C-S-H) was measured and discussed. The release of calcium from C-S-H was increased as the concentration of sodium chloride in the solution increased. It was observed that sodium sorbed onto the C-S-H phases and some sodium replaced calcium in C-S-H so that the release of calcium was enhanced. An integrated modelling approach employing an ion-exchange model and an incongruent dissolution model of C-S-H is developed. It reasonably and accurately predicted the release of calcium from C-S-H in sodium chloride solution by considering cation exchange and the effect of the ionic strength on the solubility of C-S-H.     

The structure and stoichiometry of C-S-H

This review relates to the models describing the structural evolution of calcium silicate hydrate (C-S-H) at the crystal-chemical level as a function of composition in terms of calcium to silicon ratio. The different models are compared and discussed in the light of recent spectroscopic and microscopic data. Taking into account the structure and the morphological properties of C-S-H, a surface reaction thermodynamic model has been proposed and discussed to predict and correlate the chemical and structural evolution of C-S-H with solution chemistry.     

Incorporation of zinc into calcium silicate hydrates, Part I: formation of C-S-H(I) with C/S=2/3 and its isochemical counterpart gyrolite

We have investigated the incorporation of zinc into both nanocrystalline and crystalline calcium silicate hydrates with starting C/S ratios of 2/3 (0.66). Zinc was added replacing calcium in the starting mixtures [Zn/(Zn+Ca)=0-1/4; 0-10 wt.% Zn], and the resultant phases were characterised using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), differential thermal analysis-thermogravimetry (DTA-TG) and environmental scanning electron microscopy (ESEM).In both groups of samples, increasing zinc content led to gradual structural changes, until eventually a second phase was formed. Zinc was incorporated to similar limits in both sets of samples. The thermal stability of the structures increased to a certain zinc content, beyond which there was structural destabilisation. Zinc incorporation is possible up to ∼6 wt.%. Our observations strongly indicate similar zinc incorporation mechanisms in both sample series, namely incorporation of zinc into the interlayer of C-S-H(I) and the X-sheet of gyrolite for nanocrystalline and crystalline samples, respectively.     

Autogenous shrinkage in high-performance cement paste: An evaluation of basic mechanisms

In this paper, various mechanisms suggested to cause autogenous shrinkage are presented. The mechanisms are evaluated from the point of view of their soundness and applicability to quantitative modeling of autogenous shrinkage. The capillary tension approach is advantageous, because it has a sound mechanical and thermodynamical basis. Furthermore, this mechanism is easily applicable in a numerical model when dealing with a continuously changing microstructure. In order to test the numerical model, autogenous deformation and internal relative humidity (RH) of a Portland cement paste were measured during the first week of hardening. The isothermal heat evolution was also recorded to monitor the progress of hydration and the elastic modulus in compression was measured. RH change, degree of hydration and elastic modulus were used as input data for the calculation of autogenous deformation based on the capillary tension approach. Because a part of the RH drop in the cement paste is due to dissolved salts in the pore solution, a method is suggested to separate this effect from self-desiccation and to calculate the actual stress in the pore fluid associated with menisci formation.     

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.     

Influence of bleed water reabsorption on cement paste autogenous deformation

The effects of bleed water reabsorption and subsequent early age expansion on observed autogenous deformation are investigated in this research. Bleeding was induced by varying superplasticizer and shrinkage-reducing admixture dosages and by increasing the water-to-cement ratio. This research revealed that significant early age expansion occurs with increasing chemical admixture dosages and higher water-to-cement ratios, as expected, due to increasing bleeding of those samples. When samples were rotated, negligible early age expansion was observed. Thus, bleed water reabsorption is shown to be the primary mechanism causing initial expansion in sealed autogenous deformation samples. Thermal dilation and ettringite growth appear to have a minimal influence on the observed expansion. Rotating the samples during setting eliminates the potential for bleed water reabsorption and is recommended for all autogenous deformation testing.     

Alteration of fractured cementitious materials

The alteration of cement materials in a fractured repository was investigated by experimental and modelling techniques to predict the long-term evolution of a cementitious repository for the safety assessment of radioactive waste disposal. A flow-through experiment with an artificially fractured cement column sample was carried out, and the evolution of a chemical composition in discharged water and the distribution of mineral components in a solid matrix, which was dominated by the dissolution of portlandite and calcium-silicate hydrate (C-S-H), were observed. A coupling transport and chemical equilibrium calculation code, which includes a thermodynamic incongruent dissolution model of C-S-H, was developed to predict the alteration of the fractured cement materials. The advection transport of a component in a solution within a crack and the diffusion of a component in a solid matrix were modelled in the calculations. With the proposed model, the possible alteration of cement materials along a crack was described.     

Accelerated carbonation of Friedel's salt in calcium aluminate cement paste

The stability of Friedel's salt with respect to carbonation has been studied in calcium aluminate cement (CAC) pastes containing NaCl (3% of Cl⁻ by weight of cement). Carbonation was carried out on a powdered sample in flowing 5% CO₂ gas at 65% relative humidity to accelerate the process. At an intermediate carbonation step, a part of the sample was washed and dried up to 10 cycles to simulate a dynamic leaching attack. The two processes were followed by means of X-ray diffraction (XRD), pH and Cl⁻ analyses in the simulated pore solution.     

In situ time-resolved X-ray diffraction of tobermorite formation in autoclaved aerated concrete: Influence of silica source reactivity and Al addition

The hydrothermal formation of tobermorite during the processing of autoclaved aerated concrete was investigated by in situ X-ray diffraction (XRD) analysis. High-energy X-rays from a synchrotron radiation source in combination with a newly developed autoclave cell and a photon-counting pixel array detector were used.To investigate the effects of the silica source, reactive quartz from chert and less-reactive quartz from quartz sand were used as starting materials. The effect of Al addition on tobermorite formation was also studied. In all cases, C-S-H, hydroxylellestadite and katoite were clearly observed as intermediates.Acceleration of tobermorite formation by Al addition was clearly observed. However, Al addition did not affect the dissolution rate of quartz. Two pathways, via C-S-H and katoite, were also observed in the Al-containing system. These results suggest that the structure of initially formed C-S-H is important for the subsequent tobermorite formation reactions.     

Changes in the size of pores during shrinkage (or expansion) of cement paste and concrete

The often-argued and seldom-resolved issue of relating the bulk shrinkage of concrete to changes in the size of pores is analyzed using a simple model. It is shown that the pores in a composite containing both shrinking and nonshrinking solid phases can themselves either shrink or expand when the matrix shrinks, depending on the amount of restraint in the system. The analysis also applies to expansive deformations, which occur with some types of chemical attack of cement paste. A mathematical relationship between the degree of restraint in a composite and the relationship between bulk volume changes and pore volume changes is given. These observations provide guidelines for interpreting the meaning of gaps that often form between aggregate and paste in concrete.