The hydration of Portland cement,C3S and C2S in the presence of a calcium complexing admixture (EDTA)

The effect of EDTA, a calcium chelating agent, on the early hydration of Portland cement, C₃Sand β-C₂S has been studied by solution analysis and electron microscopy. EDTA is a retarded of cement hydration. Under normal conditions of hydration, the silica levels in solution are very low (<0.05 M) but in the presence of EDTA an initial flush of silica appears in the bulk aqueous phase. On continued hydration, following the saturation of EDTA with calcium, the appearance of ‘free’ calcium causes precipitation of C-S-H gel from the bulk solution and changes in microstructure of the colloidal gel around clinker particles in C₃S and β-C₂S pastes are observed. The action of EDTA as a retarding admixture is explained in terms of the membrane model of cement hydration.     

Hydration kinetics and microstructure development of normal and NaAlO2-activated Al-doped β-C2S pastes

Sodium aluminate (NaAlO₂) can be used to accelerate hydration of Portland cements. Here, we compare the use of NaAlO₂ solutions with deionized water on the hydration of Al-doped β-C₂S. The microstructure development of hydration product C-S-H obtained from hydrated Al-doped β-C₂S was investigated. NaAlO₂ significantly improved the hydration kinetics of Al-doped β-C₂S due to the enhancement in the precipitation of calcium hydroxide and stimulation of C-(A)-S-H nucleation and increased the early-age mechanical strength of Al-doped β-C₂S pastes. NaAlO₂ promoted the incorporation of Al in the C-(A)-S-H structure and accelerated the formation of C-(A)-S-H phases containing tetra-, penta- and hexa-coordinated Al in its composition. The alkali cations modified the electrostatic equilibrium of the C-(A)-S-H, thus promoting the development of the nano-mechanical properties.     

Hydration of B2O3-stabilized α′-and β-modifications of dicalcium silicate

Paste hydration of B₂O₃-stabilized α′- and β-C₂S was studied at reaction times between 5 hours and 28 days. Data on the degree of hydration determined by X-ray diffraction at different ages were used to evaluate hydration rate constants. The higher rate of α′-C₂S hydration resulted in the higher compressive strengths of the α′-C₂S-alite paste as compared with the β-C₂S-alite paste. Differences in crystal imperfections of the two C₂S-polymorphs were determined using electron microscopy and electron diffraction.     

C4A3Š hydration in different alkaline media

This study explored the behaviour of laboratory-synthesised calcium sulphoaluminate (C₄A₃Š) in alkaline media. C₄A₃Š was hydrated in three liquid media: water, 8-M NaOH and 4 (wt.%) Na₂CO₃ added to the C₄A₃Š + water mix. Hydration kinetics were studied via isothermal conduction calorimetry and 2- and 28-day mechanical strength values were found. The reaction products were characterised with XRD and FTIR. The findings showed that whilst C₄A₃Š hydration kinetics were accelerated in the presence of alkalis, the resulting pastes had lower mechanical strength than the pastes hydrated with water and exhibited severe decay in some cases. An analysis of the hydration products revealed the presence of ettringite in the water-hydrated C₄A₃Š pastes, whereas under alkaline conditions the main calcium sulphoaluminate hydrate detected was U phase.     

Magnesium sulfate attack on hardened blended cement pastes under different circumstances

This paper describes the sulfate resistance of some hardened blended Portland cement pastes. The blending materials used were silica fume (SF), slag, and calcium carbonate (CaCO₃, CC?). The blended cement pastes were prepared by using W/S ratio of 0.3. The effects of immersion in 10% MgSO₄ solution under different conditions (room temperature, 60 °C, and drying-immersion cycles at 60 °C) on the compressive strength of the various hardened blended cement pastes were studied. Slag and CC? improve the sulfate resistance of ordinary Portland cement (OPC) paste. Mass change of the different mixes immersed in sulfate solution at 60 °C with drying-immersion cycles was determined. The drying-immersion cyclic process at 60 °C accelerates sulfate attacks. This process can be considered an accelerated method to evaluate sulfate resistance of hardened cement pastes, mortars, and concretes.     

Scanning electron micrographic studies of ettringite formation

The morphology of ettringite, formed by various reactions (C₃A + gypsum, C₃A + anhydrite, CA + magnesium sulfate, in the presence and absence of calcium hydroxide) was studied by scanning electron microscopy. Methods included immersion of solid samples into saturated solutions of the other reactant, and paste hydration. It is concluded that slender needles and spherulites are formed only if sufficient space is available; alternately (e.g. in pastes of lower water-to-solid ratios) ettringite occurs as short, prismatic crystals. Ettringite is formed in C₃A-gypsum pastes near the surface of C₃A grains by a through-solution mechanism; a topochemical mechanism can be definitely excluded. Results are significant for a better understanding of mechanism of expansion and set retardation by gypsum.     

The hydration characteristics of MSWI fly ash slag present in C3S

This paper reports on an investigation of the hydration characteristics of C₃S and the mixing of C₃S with municipal solid waste incinerator (MSWI) fly ash slag. The results can be summarized as follows: TGA observations show lower amounts of CSH and Ca(OH)₂ in samples that incorporated MSWI slag into C₃S, possibly due to the partial replacement of the C₃S by slag with less activity. In general, the incorporation of slag into C₃S decreases the initial hydration reactions, but it increases the pozzolanic reactions at a later stage by consuming Ca(OH)₂. As the hydration precedes, the C₃S peaks decrease, up to 90 days, due to the activation of the slag by liberated Ca(OH)₂. As well, the amount of hydration products (Ca(OH)₂) from the pure C₃S pastes are more than for the C₃S-slag pastes. Moreover, the degree of hydration of the C₃S pastes increases with the curing time, the C₃S-slag paste also shows that lower hydration degree values at all ages.     

Hydration of C3S in presence of CA: Mineral-pore solution interaction

C₃S and CA are the main phases of OPC and Fe-rich CAC, respectively. The objective of this research was to investigate the influence of CA on C₃S hydration, representing an under sulfated OPC-rich binder, and to shed light on the underlying hydration mechanisms. To this end, C₃S was blended with 1-30 wt-% CA and the pastes (w/c 0.5) were investigated by heat flow calorimetry, in situ X-ray diffraction and analysis of the pore solution chemistry. CA additions ≥5 wt-% reveal a separation into three distinct heat flow maxima, whereas additions ≤3 wt-% just retard the start of the main reaction. The silicate reaction (dissolution of C₃S and precipitation of C–S–H with or without CH) can be retarded for 4 to ≥22 hours in comparison to pure C₃S depending on the admixed CA content. The start of the silicate reaction seems to be related to a decrease in Al- and increase in Ca-concentration in the pore solution. However, it can be shown in this study that C₃S is able to dissolve even at high Al concentrations in the pore solution.     

The role of alumina on performance of alkali-activated slag paste exposed to 50 °C

The strength and microstructural evolution of two alkali-activated slags, with distinct alumina content, exposed to 50 °C have been investigated. These two slags are ground-granulated blast furnace slag (containing 13% (wt.) alumina) and phosphorous slag (containing 3% (wt.) alumina). They were hydrated in the presence of a combination of sodium hydroxide and sodium silicate solution at different ratios. The microstructure of the resultant slag pastes was assessed by X-ray diffraction, differential thermogravimetric analysis, and scanning electron microscopy. The results obtained from these techniques reveal the presence of hexagonal hydrates: CAH₁₀ and C₄AH₁₃ in all alkali-activated ground-granulated blast-furnace slag pastes (AAGBS). These hydrates are not observed in pastes formed by alkali-activated ground phosphorous slag (AAGPS). Upon exposure to 50 °C, the aforementioned hydration products of AAGBS pastes convert to C₃AH₆, leading to a rapid deterioration in the strength of the paste. In contrast, no strength loss was detected in AAGPS pastes following exposure to 50 °C.     

Effect of Temperature on C3S and C3S + Nanosilica Hydration and C–S–H Structure

This study aimed to monitor the effect of temperature and the addition of nanosilica on the nanostructure of the C–S–H gel forming during tricalcium silicate (C₃S) hydration. Two types of paste were prepared from a synthesized T₁ C₃S. The first consisted of a blend of deionized water and C₃S at a water/solid ratio of 0.425. In the second, a 90 wt% C₃S + 10 wt% of nanosilica blend was mixed with water at a water/solid ratio of 0.7. The pastes were stored in closed containers at 100% RH and 25°C, 40°C, or 65°C. The hydration reaction was detained after 1, 14, 28, or 62 d with acetone, and then pastes were studied by ²⁹Si magic angle spinning nuclear magnetic resonance (²⁹Si MAS NMR).The main conclusion was that adding nSA expedites C₃S hydration at any age or temperature and modifies the structure of the C–S–H gel formed, two types of C–S–H gel appear. At 25°C and 40°C, more orderly, longer chain gels are initially (1 d) obtained as a result of the pozzolanic reaction between nSA and portlandite (CH) (C–S–H_II gel formation). Subsequently, ongoing C₃S hydration and the concomitant flow of dimers shorten the mean chain length in the gel.     

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.     

Novel high-resistance clinkers with 1.10 < CaO/SiO2 < 1.25: production route and preliminary hydration characterization

New amorphous calcium silicate binders, hydraulically active, were produced by a process consisting in fully melting and rapid cooling of a mixture of typical raw materials (limestone, sand, fly-ash and electric furnace slag) with overall CaO/SiO₂ molar ratios (C/S) comprised between 1.1 and 1.25. Pastes were produced from these materials by mixing them with water in a water/binder ratio of 0.375. Compressive strength was determined at the ages of 7, 28 and 90 days and the hydration of these pastes was followed during this period by XRD, FTIR and ²⁹Si MAS-NMR. Tobermorite-like structures with low C/S and semi-crystalline character were observed to develop upon hydration of these new amorphous calcium silicate hydraulic binders. Moreover, no Portlandite was formed during hydration of these materials. The maximum compressive strength after 90 days is above 40 MPa. TGA was performed in order to determine the amount of structural water present in the pastes and their content related to the amount of hydrated products obtained. The relation between compressive strength and the amount of hydration products was investigated and some considerations about the mechanical properties of the hydration products and paste microstructure were inferred.     

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.     

Properties and hydration of blended cements with steelmaking slag

The present research study investigates the properties and hydration of blended cements with steelmaking slag, a by-product of the conversion process of iron to steel. For this purpose, a reference sample and three cements containing up to 45% w/w steel slag were tested. The steel slag fraction used was the “0-5 mm”, due to its high content in calcium silicate phases. Initial and final setting time, standard consistency, flow of normal mortar, autoclave expansion and compressive strength at 2, 7, 28 and 90 days were measured. The hydrated products were identified by X-ray diffraction while the non-evaporable water was determined by TGA. The microstructure of the hardened cement pastes and their morphological characteristics were examined by scanning electron microscopy. It is concluded that slag can be used in the production of composite cements of the strength classes 42.5 and 32.5 of EN 197-1. In addition, the slag cements present satisfactory physical properties. The steel slag slows down the hydration of the blended cements, due to the morphology of contained C₂S and its low content in calcium silicates.     

The effects of Cr2O3, Cu(OH)2, ZnO and PbO on the compressive strength and the hydrates of the hardened C3A paste

The purpose of this paper is to investigate the effects of Cr₂O₃, Cu(OH)₂, ZnO or PbO on the hydration of C₃A and characterization of its hydrates. C₃A pastes adding the above compound were examined on the basis of the hydration products and their structure, compressive strength and rate of early hydration.     

Effect of mechanical activation on the hydraulic properties of stainless steel slags

This work aims to assess the possibility of using ladle metallurgy and argon oxygen decarburization stainless steel slag as a hydraulic binder after mechanical activation. Prolonged milling in ethanol suspension resulted in 10-fold increase of the surface area and increase of the amorphous phase. Calorimetric analysis of slags mixed with water indicated the occurrence of exothermic reactions. XRD results revealed that periclase, merwinite, γ-C₂S and bredigite, decreased with hydration time. Thermogravimetric analyses indicated that the main hydration products are most probably C–S–H, CH and MH. The hydrated products in both slags were similar to C–S–H gel. WDS analysis demonstrated Ca and Si to be widespread in the structure. Formation of M–S–H gel or incorporation of Mg in the C–S–H gel remains uncertain. The 90 days compressive strength of mortars prepared from slags reached approximately 20% for LM and 10% for AOD of the compressive strength of mortars prepared from OPC.     

Structure and mechanical properties of aluminosilicate geopolymer composites with Portland cement and its constituent minerals

The compressive strengths and structures of composites of aluminosilicate geopolymer with the synthetic cement minerals C₃S, β-C₂S, C₃A and commercial OPC were investigated. All the composites showed lower strengths than the geopolymer and OPC paste alone. X-ray diffraction, ²⁹Si and ²⁷Al MAS NMR and SEM/EDS observations indicate that hydration of the cement minerals and OPC is hindered in the presence of geopolymer, even though sufficient water was present in the mix for hydration to occur. In the absence of SEM evidence for the formation of an impervious layer around the cement mineral grains, the poor strength development is suggested to be due to the retarded development of C-S-H because of the preferential removal from the system of available Si because geopolymer formation is more rapid than the hydration of the cement minerals. This possibility is supported by experiments in which the rate of geopolymer formation is retarded by the substitution of potassium for sodium, by the reduction of the alkali content of the geopolymer paste or by the addition of borate. In all these cases the strength of the OPC-geopolymer composite was increased, particularly by the combination of the borate additive with the potassium geopolymer, producing an OPC-geopolymer composite stronger than hydrated OPC paste alone.     

Characteristics of pastes from a Portland cement containing different amounts of natural pozzolan

This work is concerned with assessing the influence of natural pozzolan on the physical, mechanical and durability properties of blended Portland cement pastes. The results indicate that final setting times of natural pozzolan blended Portland cement pastes range from 4 to about 5 h. Naphthalene-type superplasticizer tends to retard the hydration process of plain and natural pozzolan blended Portland cement pastes. These blends show slightly higher setting times than those without superplasticizer. The use of superplasticizer is found to have a significant influence on the workability. At a lower level of Portland cement replacement by natural pozzolan, the addition of 1% superplasticizer by weight of blended Portland cement leads to a significant decrease in the water to Portland cement plus natural pozzolan ratio for a given workability. However, for the blended Portland cement with a high proportion of natural pozzolan, the increase in water content causes the porosity to increase with an accompanying decrease in compressive strengths. The variations in composition and cure time are found to provide significant changes in compressive strength. Depending on these parameters, the variation in compressive strength can be estimated by using the equation, σ=σ₀/[1+exp(a+bp+cp²)]ⁿ, where σ is the compressive strength of natural pozzolan blended Portland cement paste at a given cure time and natural pozzolan replacement level (MPa); σ₀ is the compressive strength of plain Portland cement pastes with or without superplasticizer at a given cure time (MPa); p is the natural pozzolan replacement level (%); a, b, c, n are the empirical constants to be determined. The blend with a composition of 80% Portland cement and 20% natural pozzolan and 1% superplasticizer provides superior strength and durability characteristics in comparison to the counterparts without superplasticizer and to the blends with a high proportion of natural pozzolan. At high contents of natural pozzolan, the resistance to freezing and thawing is found to be impaired. Moreover, these blended cements do not provide high durability performance against sulfate attack.     

Influence of amorphous silica on the hydration of tricalcium aluminate

Amorphous silica influences tricalcium aluminate (C₃A) hydration both in pastes and in suspensions. Two heat peaks are found by isothermal calorimetry during the paste hydration of C₃A. The addition of amorphous silica causes the second heat peak to shift towards shorter reaction times and become more pronounced. In suspensions, the change in ion concentration in the water phase is not influenced by the presence of amorphous silica except that the change in concentration occurs more quickly. Quantitative X-ray analysis shows that more 3CaO.Al₂O₃.6H₂O is present in suspensions containing amorphous silica than in silica-free suspensions at equal hydration times.     

Hydration of calcium aluminates at 60°C – Development paths of C2AHx in dependence on the content of free water

The influence of water loss during the hydration of calcium aluminates on the phase development is investigated at 60°C. This is relevant for applications in which calcium aluminate cement (CAC) based formulations are exposed to quick drying during hydration. The presented results provide new insights into the well-known conversion processes occurring in CAC pastes. Using in situ XRD two different routes of the development of initially formed C₂AH₈ are determined: (a) transformation to C₃AH₆ + AH₃ in the presence of sufficient free water and (b) dehydration to C₂AH₅ at a lack of free water. Moreover, the influence of precuring of the pastes at 23°C before heating to 60°C is investigated. The increasing loss of free water with increasing precuring time resulting from both, precipitation of hydrate phases and evaporation, causes incomplete hydration of CA or CA₂ as well as dehydration of C₂AH₈ instead of conversion into C₃AH₆. Comparative investigations of sealed samples always revealed complete hydration of CA and CA₂ as well as complete conversion of C₂AH₈.