Effect of the variations of clinker composition on the poroelastic properties of hardened class G cement paste

The effect of the variations of clinker composition on the poroelastic properties of class G oil-well cement pastes is studied using a multiscale homogenization model. The model has been calibrated in a previous work based on the results of a laboratory study. Various compositions of class G cements from literature are used in a hydration model to evaluate the volume fractions of the microstructure constituents of hardened cement paste. The poroelastic parameters such as drained bulk modulus, Biot coefficient, and Skempton coefficient are evaluated using the homogenization model. The results show that the variations in chemical composition of class G cements have no important effect on the variations of the poroelastic properties.     

Melt differentiation and crystallization of clinker minerals in a CaO-SiO2-Al2O3-Fe2O3 pseudoquaternary system

In the CaO-SiO₂-Al₂O₃-Fe₂O₃ pseudoquaternary system, the solid solutions of Ca₃SiO₅ [C₃S(ss)], Ca₂SiO₄ [C₂S(ss)], Ca₂(Al_xFe_1 − x)₂O₅ with 0.40 ≤ x ≤ 0.57 (ferrite) and Ca₃Al₂O₆ [C₃A(ss)] were crystallized out of a complete melt with 52.9 mass% CaO and Al₂O₃/Fe₂O₃ = 0.70. When the melt was cooled from 1673 K at 80 K/h, the crystals of ferrite with x = 0.40, C₃S(ss) and C₂S(ss) would start to nucleate from the melt at 1630 K. During further cooling, the x value of the precipitating ferrite would progressively increase and eventually approach 0.57 at 1613 K. The resulting ferrite crystals showed a zonal structure, the x value of which successively increased from the cores toward the rims. Actually, the x values of 0.43 and 0.52 were confirmed for, respectively, the cores and rims by EPMA. As the simultaneous crystallization of zoned ferrite, C₃S(ss) and C₂S(ss) proceeded, the coexisting melt would become progressively enriched in the Al₂O₃ component. After the termination of the ferrite crystallization, the C₃A(ss), C₃S(ss) and C₂S(ss) crystallized out of the differentiated melt. The end result was the four phase mixture of ferrite, C₃A(ss), C₃S(ss) and C₂S(ss), being free from the nucleation of Ca₁₂Al₁₄O₃₃ solid solution.     

Effect of the clinker composition on the threshold limits for Cu,Sn or Zn

The threshold limit corresponds to the maximum amount of trace element that could be incorporated into clinker whilst reaching the limit of solid solution of its four major phases (C₃S, C₂S, C₃A and C₄AF). For Cu, Zn or Sn, these threshold limits in a standard clinker (65% C₃S, 18% C₂S, 8% C₃A and 8% C₄AF) were equal to 0.35, 0.7 and 1 wt.% respectively (Gineys et al., in press). This paper presents the effect of the clinker composition on these threshold limits. Laboratory made clinkers having different mineralogical compositions was characterised by XRD and SEM. Results showed that the threshold limits for Cu, Zn or Sn were consistent. The threshold limit for Sn was affected by the Bogue content in interstitial phases. On the other hand, the threshold limit for Zn was affected by the Bogue content in C₃S of clinker while that of Cu was unaffected by any modifications of clinker composition.     

High intakes of Cr, Ni, and Zn in clinker: Part I. Influence on burning process and formation of phases

The influence of high intakes of Cr, Ni, and Zn on the burning process and the formation of clinker phases is described. Three different raw meals were used and were burned with the metal oxides in four concentrations from 200 to 25,000 ppm. The resulting clinker was analysed for the content of free lime and X-ray diffraction analysis was done. Some of the clinker material was crushed and polished, and quantitative point counting was performed to provide the content of the different clinker phases. These samples were also analysed by scanning electron microscope connected with an energy-dispersive X-ray-spectrometer to detect the composition of the clinker phase. The results show that only very high intakes of heavy metals have measurable effects on the formation and composition of the clinker.     

Modeling the degradation of Portland cement pastes by biogenic organic acids

Reactive transport models can be used to assess the long-term performance of cement-based materials subjected to biodegradation. A bioleaching test (with Aspergillus niger fungi) applied to ordinary Portland cement pastes during 15 months is modeled with HYTEC. Modeling indicates that the biogenic organic acids (acetic, butyric, lactic and oxalic) strongly accelerate hydrate dissolution by acidic hydrolysis whilst their complexation of aluminum has an effect on the secondary gel stability only. The deepest degradation front corresponds to portlandite dissolution and decalcification of calcium silicate hydrates. A complex pattern of sulfate phases dissolution and precipitation takes place in an intermediate zone. The outermost degraded zone consists of alumina and silica gels. The modeling accurateness of calcium leaching, pH evolution and degradation thickness is consistently enhanced whilst considering increase of diffusivity in the degraded zones. Precipitation of calcium oxalate is predicted by modeling but was hindered in the bioleaching reactor.     

Early hydration and setting of Portland cement monitored by IR, SEM and Vicat techniques

Diffuse Reflectance Infrared DR-FTIR spectroscopy is employed to monitor chemical transformations in pastes of Portland limestone cement. To obtain a sufficient time resolution a freeze-dry procedure is used to instantaneously ceasing the hydration process. Rapid re-crystallization of sulphates is observed during the first 15 s, and appears to be complete after ~ 30 min. After ~ 60 min, spectroscopic signatures of polymerizing silica start to emerge. A hump at 970-1100 cm^− 1 in conjunction with increasing intensity in the water bending mode region at 1500-1700 cm^− 1 is indicative of the formation of Calcium Silicate Hydrate, C-S-H. Simultaneously with the development of the C-S-H signatures, a dip feature develops at 800-970 cm^− 1, reflecting the dissolution of Alite, C₃S. Setting times, 180 (initial) and 240 (final) minutes, are determined by the Vicat technique. Combining DR-FTIR, SEM and Vicat measurements it is concluded that the setting is caused by inter-particle coalescence of C-S-H.     

High intakes of Cr, Ni, and Zn in clinker: Part II. Influence on the hydration properties

This work presents the results of the hydration of cements with high intakes of Cr, Ni, and Zn. The cements were produced from clinkers that were doped with 200 to 25,000 ppm of heavy metal. Investigations on the clinkers were presented in Part I. In this paper the rate of heat generation of the cements in the first 2 days was analysed by differential scanning calorimetry. The hydration products were investigated by scanning electron microscopy combined with energy-dispersive X-ray spectrometer and also by X-ray powder diffraction. The initial setting of some samples was tested, as well as the strength. The results show that heavy metals only have an influence on the hydration properties of the cements if the dosage is much higher than in ordinary Portland cement.     

Assessment of phase formation in lime-based mortars with added metakaolin, Portland cement and sepiolite, for grouting of historic masonry

Lime-based mortars containing pozzolanic additions of metakaolin, sepiolite and white Portland cement are studied in order to determine their performance as historic masonry conservation mortars. Hydration products on metakaolin-lime blended mortars include stable and metastable phases. The presence of such products has been studied by means of DTA and XRD analysis, concluding that the selection between them is mainly related with the water-lime ratio. Sepiolite addition to metakaolin-lime mortars has shown to inhibit C₄AH₁₃ formation. Therefore, the influence of phase distribution on the mechanical resistance is considered. Finally, compounds production on blended lime-white Portland cement was compared to natural hydraulic lime ones, and as a result, no remarkable differences appeared, apart from traces of possible cement Portland addition to the latter, usually not mentioned in the nominal composition supplied by the manufacturers of lime binders.     

Tobermorite/jennite- and tobermorite/calcium hydroxide-based models for the structure of C-S-H: applicability to hardened pastes of tricalcium silicate, β-dicalcium silicate, Portland cement, and blends of Portland cement with blast-furnace slag, metakaolin, or silica fume

The purpose of this article is to discuss the applicability of the tobermorite-jennite (T/J) and tobermorite-‘solid-solution’ calcium hydroxide (T/CH) viewpoints for the nanostructure of C-S-H present in real cement pastes. The discussion is facilitated by a consideration of the author's 1992 model, which includes formulations for both structural viewpoints; its relationship to other recent models is outlined. The structural details of the model are clearly illustrated with a number of schematic diagrams. Experimental observations on the nature of C-S-H present in a diverse range of cementitious systems are considered. In some systems, the data can only be accounted for on the T/CH structural viewpoint, whilst in others, both the T/CH and T/J viewpoints could apply. New data from transmission electron microscopy (TEM) are presented. The ‘inner product’ (Ip) C-S-H in relatively large grains of C₃S or alite appears to consist of small globular particles, which are ≈4-8 nm in size in pastes hydrated at 20 °C but smaller at elevated temperatures, ≈3-4 nm. Fibrils of ‘outer product’ (Op) C-S-H in C₃S or β-C₂S pastes appear to consist of aggregations of long thin particles that are about 3 nm in their smallest dimension and of variable length, ranging from a few nanometers to many tens of nanometers. The small size of these particles of C-S-H is likely to result in significant edge effects, which would seem to offer a reasonable explanation for the persistence of Q⁰(H) species. This would also explain why there is more Q⁰(H) at elevated temperatures, where the particles seem to be smaller, and apparently less in KOH-activated pastes, where the C-S-H has foil-like morphology. In blended cements, a reduction in the mean Ca/Si ratio of the C-S-H results in a change from fibrillar to a crumpled-foil morphology, which suggests strongly that as the Ca/Si ratio is reduced, a transition occurs from essentially one-dimensional growth of the C-S-H particles to two-dimensional; i.e., long thin particles to foils. Foil-like morphology is associated with T-based structure. The C-S-H present in small fully hydrated alite grains, which has high Ca/Si ratio, contains a less dense product with substantial porosity; its morphology is quite similar to the fine foil-like Op C-S-H that forms in water-activated neat slag pastes, which has a low Ca/Si ratio. It is thus plausible that the C-S-H in small alite grains is essentially T-based (and largely dimeric). Since entirely T-based C-S-H is likely to have different properties to C-S-H consisting largely of J-based structure, it is possible that the C-S-H in small fully reacted grains will have different properties to the C-S-H formed elsewhere in a paste; this could have important implications.     

Polymerised high internal phase emulsion cement hybrids: Macroporous polymer scaffolds for setting cements

We polymerised the continuous styrene/divinylbenzene monomer phase of high internal phase emulsions (HIPEs) containing 70 vol.% cement slurry as internal phase to synthesise polymer cement hybrid materials. These novel cement containing poly(merised)HIPEs have an interconnected bi-phasic structure consisting of an interpenetrating network of set cement and polymer. Incorporating 14 wt.% of polymer into the cement resulted in an increased compressive strain to failure as compared to pure set cement but both elastic modulus and crush strength decreased. These novel polymer cement hybrid materials have a better chemical resistance against acetic acid then pure cement and showed also no shrinkage when exposed to xylene and dodecane.     

Ultrasound monitoring of the influence of different accelerating admixtures and cement types for shotcrete on setting and hardening behaviour

The possible use of ultrasound measurements for monitoring setting and hardening of mortar containing different accelerating admixtures for shotcrete was investigated. The sensitivity to accelerator type (alkaline aluminate or alkali-free) and dosage, and accelerator-cement compatibility were evaluated. Furthermore, a new automatic onset picking algorithm for ultrasound signals was tested. A stepwise increase of the accelerator dosage resulted in increasing values for the ultrasound pulse velocity at early ages. In the accelerated mortar no dormant period could be noticed before the pulse velocity started to increase sharply, indicating a quick change in solid phase connectivity. The alkaline accelerator had a larger effect than the alkali-free accelerator, especially at ages below 90 min. The effect of the alkali-free accelerator was at very early age more pronounced on mortar containing CEM I in comparison with CEM II, while the alkaline accelerator had a larger influence on mortar containing CEM II. The increase of ultrasound energy could be related to the setting phenomenon and the maximum energy was reached when the end of workability was approached. Only the alkaline accelerator caused a significant reduction in compressive strength and this for all the dosages tested.     

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.     

An ESEM investigation of latex film formation in cement pore solution

Environmental scanning electron microscopy (ESEM) and complementary methods were employed to study the time dependent film formation of a latex dispersion in water and cement pore solution. First, a model carboxylated styrene/n-butyl acrylate latex dispersion possessing a minimum film forming temperature (MFFT) of 18 °C was synthesized in aqueous media via emulsion polymerization. Its film forming property was at a temperature of 40 °C, studied under an ESEM. The analysis revealed that upon removal of water, film formation occurs as a result of particle packing, particle deformation and finally particle coalescence. Film formation is significantly retarded when the latex dispersion is present in cement pore solution. This effect can be ascribed to adsorption of Ca²⁺ ions onto the surface of the anionic latex particles and to interfacial secondary phases. This layer of adsorbed Ca²⁺ ions hinders interdiffusion of the macromolecules and subsequent film formation of the latex polymer.     

Temperature dependence, 0 to 40 °C, of the mineralogy of Portland cement paste in the presence of calcium carbonate

Thermodynamic calculations disclose that significant changes of the AFm and AFt phases and amount of Ca(OH)₂ occur between 0 and 40 °C; the changes are affected by added calcite. Hydrogarnet, C₃AH₆, is destabilised at low carbonate contents and/or low temperatures < 8 °C and is unlikely to form in calcite-saturated Portland cement compositions cured at < 40 °C. The AFm phase actually consists of several structurally-related compositions which form incomplete solid solutions. The AFt phase is close to its ideal stoichiometry at 25 °C but at low temperatures, < 20 °C, extensive solid solutions occur with CO₃-ettringite. A nomenclature scheme is proposed and AFm-AFt phase relations are presented in isothermal sections at 5, 25 and 40 °C. The AFt and AFm phase relations are depicted in terms of competition between OH, CO₃ and SO₄ for anion sites. Diagrams are presented showing how changing temperatures affect the volume of the solid phases with implications for space filling by the paste. Specimen calculations are related to regimes likely to occur in commercial cements and suggestions are made for testing thermal impacts on cement properties by defining four regimes. It is concluded that calculation provides a rapid and effective tool for exploring the response of cement systems to changing composition and temperature and to optimise cement performance.     

Re-use of drinking water treatment plant (DWTP) sludge: Characterization and technological behaviour of cement mortars with atomized sludge additions

This paper aims to characterize spray-dried DWTP sludge and evaluate its possible use as an addition for the cement industry. It describes the physical, chemical and micro-structural characterization of the sludge as well as the effect of its addition to Portland cements on the hydration, water demand, setting and mechanical strength of standardized mortars.Spray drying DWTP sludge generates a readily handled powdery material whose particle size is similar to those of Portland cement. The atomized sludge contains 12-14% organic matter (mainly fatty acids), while its main mineral constituents are muscovite, quartz, calcite, dolomite and seraphinite (or clinoclor). Its amorphous material content is 35%.The mortars were made with type CEM I Portland cement mixed with 10 to 30% atomized sludge exhibited lower mechanical strength than the control cement and a decline in slump. Setting was also altered in the blended cements with respect to the control.     

Three-dimensional visualization of pore structure in hardened cement paste by the gallium intrusion technique

It is widely known that the pore structure of concrete strongly influences its physical properties. Therefore, we developed a technique for the visualization of the pore structure because of clearing it correctly. However, this visualization is limited to two-dimensional imaging for sections of the specimen. As a result, in this study, we developed a technique for reconstructing the acquired 2D images of the pore structure into 3D form by stacking them. By using this image, the relationship between water permeability and pore connectivity was clarified, and it was shown clearly that the pore connectivity strongly affects the water permeability.     

New cements for the 21st century: The pursuit of an alternative to Portland cement

Preparation of this article entailed authors analyzing the contents of quite a number of papers, although the main objective was never to review the state of the art of new cements. Rather authors intend to discuss why they believe alkaline activated cement can be positioned at the epicentre of a new and necessary transition from today's Portland cement to the new cements of the future. A brief history of alkaline cements serves as an introduction to the technology itself. The interest roused around calcium sulfoaluminate-based cements is also reviewed, albeit summarily. The greater part of the article focuses, however, on alkaline cements which are classified into five categories. The fundamental chemical and structural characteristics of aluminosilicate-based alkaline cements are also described, and the key advances made in the understanding of synthetic gels are discussed. The paper ultimately finds hybrid cements to be technologically viable materials for contemporary construction.     

Study of mineralogy and leaching behavior of stabilized/solidified sludge using differential acid neutralization analysis: Part II: Use of numerical simulation as an aid tool for cementitious hydrates identification

In this work, we propose a methodology coupling differential acid neutralization analysis, chemical analysis of selected leachates and numerical simulation to identify the minerals controlling the leaching behavior of stabilized hydroxide sludge. This second part deals with the use of numerical simulation as an aid tool for the identification of the minerals. The framework for minerals identification is based on the study of minerals stability in function of the geochemical context using numerical simulation. A mineral assemblage permitting the simulation of a pH dependence leaching test (acid neutralization and release of elements) has been identified for the four studied cement pastes. Therefore, the proposed methodology is a pertinent tool for the modeling of the leaching behavior of inorganic wastes.     

Early hydration of clinker–slag–metakaolin combination in steam curing conditions, relation with mechanical properties

High strength can be obtained at early ages for precast concrete elements by the use of CEMI 52.5R cement (OPC) and thermal treatment (steam curing). To compensate for the announced withdrawal of CEM I cements because of high CO₂ emissions during their production and the ecotax that this will imply, one attractive alternative is the use of composed cements resulting from the combination of clinker with mineral admixtures. In steam curing conditions, previous studies have shown an increase in the compressive strength at one day of age for mortars incorporating an OPC/blast furnace slag (GGBS)/metakaolin (MK) combination, in comparison with mortars incorporating OPC only. The present study investigates the connection between the compressive strength, at one day of age, of steam cured mortars made with various binders and the hydration of these binders. The progress of the hydration was characterised by means of XRD, thermal and microprobe analyses. The results indicate that the increase in compressive strength when MK is incorporated (OPC/MK or OPC/MK/GGBS) can be explained by an increase in the amount of C-S-H, C-A-H, C-A-S-H phases, a decrease in the amount of CH and a change in the chemical nature of the matrix (decrease in C/S ratio). The decrease in compressive strength of OPC/slag-based material can be explained by a reduction in the amount of hydrated phases (particularly C-S-H) and compactness.These are promising results for precast concrete manufacturers who are concerned about preserving the environment.