Trimethylsilylation as a tool for the study of cement pastes (2) quantitative analysis of the silicate fraction of Portland cement pastes

The trimethylsilyl derivatives of the silicate fractions of three fully hydrated Portland cement pastes have been studied by gas-liquid and gel-permeation chromatography. The pastes had 9–11% of the silica as monomer, 22–30% as dimer and 44–57% as polysilicate. Gel-permeation chromatograms of the polysilicate derivatives all had maxima corresponding to a degree of polymerisation (dp) of about 4 and shoulders at higher values; the weight average dp's were 14–17 and although the maximum dp's were high, only about 2% of the silica had dp's greater than 50. The mean connectivity of the polysilicates was 2.3.     

The composition of the C-S-H phases in portland cement pastes

The composition of the calcium silicate hydrate (C-S-H) present in Portland cement pastes from eight days to five years old, and in concrete samples ten years old, has been determined by electron probe micro-analysis. The dependence of C-S-H composition on water/cement ratios has also been studied. When the cement is only partially hydrated inner and outer hydrate layers are seen around alite grains. These hydrates have different chemical compositions. With progressive hydration it becomes impossible to distinguish the inner from outer hydrates without a reference alite grain. At this stage two distinct C-S-H compositions can still be found by microanalysis. After further hydration a single CSH composition predominates. The calcium content of C-S-H varies significantly with water/cement ratio and there is new evidence to suggest that in the C-S-H structure Mg replaces Ca, and that Al, S and Fe replace Si.     

Roughness of fracture surfaces and compressive strength of hydrated cement pastes

A new type of roughness number Rn_o is formulated as a possible analytical tool for surface studies using confocal microscopy. The formulation accommodates fractal dimension and both of the boundary length scales limiting the fractal region of the fracture surface under investigation. Besides the number Rn_o other roughness characteristics are discussed and their effectiveness in the field of cementitious materials is tested. 3D-surface profile SP and surface roughness SR parameters designed for surface 3D-analysis were calculated for fracture surfaces of hydrated Portland cement pastes with different values of water-to-cement ratio. The surface profile SP parameters monotonically increased with water-to-cement ratio and monotonically decreased with compressive strength. A short discussion of possible reasons for such a behavior is presented.     

On the hydration of MgO in cement pastes hydrated up to eight years

The present work extends the knowledge on the behaviour of high magnesia cements, pure or with pozzolanic additives cured for 8 years in potable water at 18+2°C. For this purpose, clinker containing 10% MgO was prepared in an appropriate furnace. The raw materials used were all of industrial origin. Four cements were prepared by cogrinding clinker with gypsum and the following pozzolanic materials: Santorin Earth (S.E.), Fly Ash I (F.A.I.), Fly Ash II (F.A.II) and Fly Ash III (F.A.III), in a proportion of 30%.The Mg(OH)₂ produced during the hydration of the above cements was determined quantitatively by thermogravimetric method, while the hydration phenomena were studied by X.R.D. It is concluded that when clinker containing ca. 10% MgO is coground with a good pozzolan, the hydration of MgO reaches a limited value at the age of ca. 6 years. The percentage of hydrated MgO permits the safe use of these cements, as for the long term unsoundness due to MgO hydration.     

Studies of the trimethylsilyl derivatives of unhydrated cement and hydrated cement pastes by fourier transform infrared spectroscopy

Trimethylsilyl derivatives (T-derivatives) of unhydrated cement and hydrated cement pastes were studied by Fourier transform infrared spectroscopy. The infrared spectra of all these derivatives are very similar. However, when the spectrum of the T-derivatives of the unhydrated cement is subtracted from those of the hydrated pastes, a difference in the characteristics of the siloxane bands is observed in the subtracted spectra. The implications of these observed changes to cement hydration process is discussed.     

Degree of depolarisation,an indicator for water bonding in hydrated cement pastes

Permittivity (dielectric constant), weight loss and shrinkage of hydrated cement pastes was measured after firing the samples up to 1000°C. The degree of depolarisation (DD) can be calculated from measured data. DD can be used as an indicator of water bonding in the cement paste.     

The effects of fly ash composition on the chemistry of pore solution in hydrated cement pastes

This paper reports the findings of an investigation to determine the influence of fly ash composition on the evolution of the pore solution chemistry in Portland cement/fly ash systems. Twelve fly ashes, selected to represent the wide range of composition of North American ashes, were used in the study. In addition to pore solution expression and analysis, inner hydration products were analyzed using energy-dispersive X-ray analysis. The study shows that the alkalinity of pore solution increases as the calcium and alkali content of the fly ash increase, and decreases as the silica content of the ash increases. However, there is no consistent trend between the composition of the inner calcium-silicate hydrate and fly ash composition.     

On the determination of rat constants for hydration processes in cement pastes

A procedure has been developed to evaluate the actual rate constants for the hydration of individual phases in polysize cementitious systems. The evaluation is based on the comparison between observed and calculated degrees of hydration, the latter being obtained by taking into account the particle size distribution of the original sample and the effects of possible overlapping of different rate-determining processes. The determination of rate constants for C₃S-hydration is described in order to illustrate the proposed procedure.     

Characterization by solid-state NMR and selective dissolution techniques of anhydrous and hydrated CEM V cement pastes

The long term behaviour of cement based materials is strongly dependent on the paste microstructure and also on the internal chemistry. A CEM V blended cement containing pulverised fly ash (PFA) and blastfurnace slag (BFS) has been studied in order to understand hydration processes which influence the paste microstructure. Solid-state NMR spectroscopy with complementary X-ray diffraction analysis and selective dissolution techniques have been used for the characterization of the various phases (C₃S, C₂S, C₃A and C₄AF) of the clinker and additives and then for estimation of the degree of hydration of these same phases. Their quantification after simulation of experimental ²⁹Si and ²⁷Al MAS NMR spectra has allowed us to follow the hydration of recent (28 days) and old (10 years) samples that constitutes a basis of experimental data for the prediction of hydration model.     

DTA-TG study on the Ca(OH)2 - pozzolan reaction in cement pastes hydrated up to three years

In order to study the way in which Santorin earth (pozzolan) acts during its hydration with portland cements and specifically, the rates of its action and its optimum content, the amount of Ca(OH)₂ derived during the hydration has been quantitatively determined, by means of thermogravimetry. Thus, cement pastes have been prepared with mixtures of portland cement containing proportions of Santorin earth up to 40% of various finenesses. These pastes were cured in water up to three years.     

The analysis of ettringite in hardened cement paste

The solubilities of pure hydrates and the mixture of hydrates existing in hardened cement paste to the ethylene glycol-methanol solution, the effects of co-existing Ca(OH)₂ upon the solubility and rate of dissolution of each hydrate, suitable concentration of ethylene glycol, and the limit of the amount of ettringite capable of being analysed in paste were investigated. From the results obtained, the reliable analytical method for the determination of ettringite in hardened cement paste was established.     

Methods for determination of degree of reaction of slag in blended cement pastes

To measure the degree of reaction of slag in blended pastes, five methods were studied. Selective dissolution and differential scanning calorimetry are shown to be unreliable, SEM-BSE-IA-mapping is time consuming, but does provide good results with a reasonable degree of precision. The difference in cumulative calorimetry and chemical shrinkage curves of slag blends in comparison to blends with inert filler shows potential to isolate the reaction of the slag. These methods have the advantage of being continuous, techniques with good precision, but the absolute heat of hydration, or contribution to chemical shrinkage of any particular slag is not known. Calibration of the calorimetry technique with SEM-BSE-IA-mapping seems to be a promising method to understand and quantify the degree of reaction of slag.     

Analysis of triethanolamine in cement pastes

Triethanolamine (TEA) in hardened cement paste has been analyzed by infrared spectrophotometry. The technique is sensitive for the qualitative detection of small amounts of TEA. However, the percent recovery is very much concentration dependent and, until the reproducibility of the method has been established, reliable quantitative data from this technique cannot be expected.     

Optimization of strength in cement pastes

It has been demonstrated that porosity is by far the dominant controlling factor limiting strength of hydrated cement paste. Mechanical means have been employed in the present study to minimize this porosity, “hot pressing” under rather modest temperatures and pressures, producing materials having very low porosity and unusually high strength. A new relationship to describe the interrelation of strength and porosity is given, and the effect of maturity of specimens, composition and microstructure are illustrated. Though theoretical density has not yet been achieved, the cement pastes have compressive strengths (as well as tensile and shear strengths) an order of magnitude higher than in normally hydrated cements, are stable and durable, and have dense interpenetrating microstructures.     

Investigations on the relationship between porosity,structure and strength of hydrated portland cement pastes I. Effect of porosity

In a series of cement paste specimens made with different water-cement ratios and hydrated for different times the relationship between porosity and strength was determined. For a range of porosities between 5 and 28 per cent this relationship can be best expressed in the form of a linear plot. At equal porosities strengths of specimens obtained by pressing lie distinctly below those obtained by casting.     

Composition, morphology and nanostructure of C-S-H in white Portland cement pastes hydrated at 55 °C

The C-S-H present in water- and alkali-activated hardened pastes of white Portland cement hydrated at 55 °C has been characterized. The mean length of the aluminosilicate anions in the C-S-H was similar in both systems and increased with age. Inner product C-S-H generally had a fine scale, homogeneous morphology. Outer product C-S-H was generally fibrillar with water, and foil- or lath-like with alkali. There were some regions of C-S-H with coarse morphology. It was not possible to determine the chemical composition of C-S-H using the SEM; TEM-EDX was necessary. The C-S-H formed in the alkali-activated paste had a lower mean Ca/(Al + Si) ratio than that formed with water, which was offset by a larger quantity of calcium hydroxide. The potassium in the KOH-activated paste was present either within the C-S-H structure charge balancing the substitution of Al³⁺ for Si⁴⁺, or adsorbed on the C-S-H charge balancing sulfate ions.     

Investigation on the structure of fully hydrated portland cement and tricalcium silicate pastes. III. Specific surface area and permeability

On a series of d-dried OPC pastes hydrated at 20 and 90 °C and Ca₃SiO₅ pastes hydrated at 20 °C the specific surface area was measured using four different methods i.e. BET_{N 2} and those calculated from adsorption isotherms and BET_{H 2 0} mercury porosimetry and water permeability data. Values found using different methods were not identical. In addition to that, the specific surface area of the same pastes in non-dried state was estimated from their permeability to water. A drying and resaturation of the pastes resulted in a decrease of the specific surface area indicating a coarsening of the pore structure in the course of drying.     

On the viscometric examination of cement pastes

The use of flow curves and hysteresis loops in studying the rheology of cement pastes is examined critically. The shape of flow curves obtained when cement pastes are sheared in a coaxial cylinders viscometer has been studied. Hysteresis loops of three main types were found, depending on the length of time taken to complete a shear cycle. At short cycle times the downcurve is at lower stress levels than the upcurve (structural breakdown), while at long cycle times they are reversed (structural build up). At intermediate cycle times the lines cross, and double or treble loops may be obtained. The variation of loop shape with cycle time is different for different cements.     

Formation of magnesium silicate hydrate (M-S-H) cement pastes using sodium hexametaphosphate

Magnesium silicate hydrate (M-S-H) gel is formed by the reaction of brucite with amorphous silica during sulphate attack in concrete and M-S-H is therefore regarded as having limited cementing properties. The aim of this work was to form M-S-H pastes, characterise the hydration reactions and assess the resulting properties. It is shown that M-S-H pastes can be prepared by reacting magnesium oxide (MgO) and silica fume (SF) at low water to solid ratio using sodium hexametaphosphate (NaHMP) as a dispersant. Characterisation of the hydration reactions by x-ray diffraction and thermogravimetric analysis shows that brucite and M-S-H gel are formed and that for samples containing 60 wt.% SF and 40 wt.% MgO all of the brucites react with SF to form M-S-H gel. These M-S-H cement pastes were found to have compressive strengths in excess of 70 MPa.