Hydration reactions in pastes C3S+C3A+CaSO4.2aq+water at 25°C. II

Variation of C₃A/C₃S ratio in pastes if C₃S+C₃A+CaSO₄.2aq+water influences the hydration reactions in a way compatible with retardation of C₃A hydration by amorphous Al (OH)₃, but not compatible with retardation by dissolved ions or by a “C₄AH₁₃” retarding layer.     

Fundamental mechanisms for polycarboxylate intercalation into C3A hydrate phases and the role of sulfate present in cement

The fundamental reactions leading to the intercalation of polycarboxylate (PC) superplasticizers into calcium aluminum hydrates were studied by hydration of pure C₃A in the presence of PC at 75 °C. It was found that the amount of dissolved sulfate present in cement pore solution determines whether organo-mineral phases are formed or not. In the absence of sulfate, PCs easily intercalate during C₃A hydration in alkaline solution. Under these conditions, only excessive steric size of the PC will prevent intercalation. At low sulfate concentrations (SO₄²⁻/C₃A molar ratios of 0.1-0.35), PC intercalates with intersalated alkali sulfate, are formed. At high sulfate concentrations (SO₄²⁻/C₃A molar ratios of 0.7-2), PC can no longer intercalate. Instead, sulfate, because of its higher negative charge density, fills the interlayer space and monosulfoaluminates with different water contents are formed.Anion exchange experiments confirm that from the initially formed C₄AH₁₃, PC will exchange the interlayer OH⁻ anion whereas with monosulfoaluminates, no replacement of sulfate by PC was found. Consequently, in alkaline solution, PC intercalates will not exchange their PC against OH⁻ anions whereas sulfate will gradually replace the PC.Generally, intercalation of PC is an unwanted process because it consumes superplasticizer which is effective only when it adsorbs onto the cationic surfaces of AF_m and AF_t phases. Our experiments demonstrate that intercalation can be avoided by using PCs with long side chains or highly sulfated cements (SO₄²⁻/C₃A molar ratio ≥ 0.75) containing alkali or calcium sulfates which dissolve fast. In undersulfated cements, however, PC intercalates can be formed, either directly during the stacking process of the [Ca₂Al(OH)₆]⁺ main layer, with PC acting as the template which determines the interlayer distance, or by anion exchange between initially formed aluminate hydrates (e.g. C₄AH₁₃ or C₂AH₈) and the PC anion.     

C3A hydration

C₃A is hydrated with time and temperature as variable parameters. The solid hydration products were observed using the scanning electron microscope and determined by XRD. The heat development was followed by means of isothermal microcalorimetry.The first hydration product is a gel-like material with no detectable XRD lines. The hexagonal phases which follow have a better crystallinity when formed below than above room temperature. In the latter case distinct XRD lines are only obtained after some time. C₃AH₆ as single crystals or aggregates develops earlier at high than at low temperatures. The morphology of C₃AH₆ varies with hydration time and temperature.This sequence of reactions occurs slowly in suspensions if a small amount of C₃A is used. In pastes, and in suspensions if a larger amount of C₃A is used, C₃AH₆ is formed very quickly and no hexagonal hydrates were detected by XRD.     

Microstructural and hydration resistance study of CaO with powder surface modification by Al coupling agents: Alkoxy type and phosphate type

The surface of Ca(OH)₂ powders were pre-treated with alkoxy type aluminium coupling agent (ALC) and phosphate type aluminium phosphorus coupling agent (ALPC), respectively in this work. The shaped specimens were calcinated at 1600 °C for 3 h and then the phase compositions and microstructures of CaO specimens were investigated. The results revealed that both ALC and ALPC could conspicuously enhance the hydration resistance of CaO specimens by modifying surface microstructures in different ways. The boundaries of CaO grains in the specimens were covered with C₃A glass phase after introducing of ALC, which was replaced by calcium phosphate when the ALC was replaced by ALPC. The hexagonal barrier layer, which was the hydration product of C₃A, played a protective role in CaO grains. The obtained results in our work indicated that ALC was more effective in improving hydration resistance of CaO materials.     

Effect of organic compounds on the interconversions of calcium aluminate hydrates. Hydration of monocalcium aluminate

Organic compounds can sorb into the structure of C₂AH₈, as was found previously for C₄AH₁₃, and thereby restrict its conversion to C₃AH₆. The presence of interlayer aluminate ions inhibits complex formation at room temperature, but at 75°C aluminum hydroxide is expelled from the structure as interlayer complexes form. The conversion of CAH₁₀ to C₃AH₆ can also be restricted by organic molecules although complex formation does not appear to take place under the conditions studied. Those molecules which effectively stabilize CAH₁₀ so strongly retard the hydration of CA that rapid strength development is no longer possible.     

Tricalcium aluminate hydration in the presence of lime,gypsum or sodium sulfate

The influence of Ca(OH)₂, CaSO₄·2H₂O and Na₂SO₄ on the C₃A hydration was examined in order to study the retardation mechanism of C₃A hydration caused by lime and/or gypsum additions. When C₃A hydrates in the presence of gypsum, the results do not confirm the retardation mechanism based on sulfate ions adsorption or C₄AH_x impervious coating. They substantially confirm the mechanism based on ettringite crystals coating C₃A grains. In the absence of gypsum C₃A hydration is retarded by C₄AH_x formation coating C₃A grains.     

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.     

Bond strength between C3A paste and iron,copper or zinc wire and microstructure of interface

The object of this report is to grasp the fundamental data with interaction between cement and metal. Bond strength between C₃A paste and iron, copper or zinc wire and microstructure of the interfacial region were examined by pull-out test and scanning electron microscope respectively.The bond strength between C₃A paste and zinc wire in 28-day of curing is larger than that between C₃A paste and iron or copper wire and hexagonal hydration products and C₃AH₆ are formed in paste at interface. On the other hand, C₃AH₆ is formed in that between C₃A paste and iron or copper wire.     

The influence of Na2O on the hydration of C3A. I. Paste hydration

The influence of Na₂O on the hydration of C₃A was studied both by following the hydration of xNa₂O. (3?x) CaO.Al₂O₃ (0<x<0.25) in water, and of C₃A in solutions of NaOH. Low NaOH concentrations prevent a very early appearance of the second heat evolution peak, indicating a more controlled formation of C₃AH₆ nuclie. Higher NaOH concentrations advance the second peak; this is ascribed to a decreased stability of the hexagonal hydrates with increasing NaOH concentrations.     

Study of the strength developed by stable carbonated phases in high alumina cement

The direct hydration of high alumina cement to the cubic phase, C₃AH₆, at temperatures above 30°C is proposed to avoid the conversion from CAH₁₀ to C₃AH₆. The carbonation of C₃AH₆ with CO₂ between 20°C and 90°C causes an increase in strength which is higher at higher temperatures. It is caused by the presence of thermodynamically stable carbonated phases. The optimum hydration times are the minimum necessary to get C₃AH₆: 3 hours at 80°C and 6 hours at 60°C. Subsequent carbonation up to a total time of 24 hours after the end of mixing provides 80% of the strength obtained after 28 days of hydration at the same temperature, with the same compounds being formed. If CaCO₃ is added to the cement before mixing, the strengths are even higher than for specimens without it. The effect of a super-water-reducing admixture on strength has also been studied.     

The impact of Al2O3 amount on the synthesis of CASH samples and their influence on the early stage hydration of calcium aluminate cement

In this work the impact of Al₂O₃ amount on the synthesis (200?°C; 4–8?h) of calcium aluminium silicate hydrates (CSAH) samples and their influence on the early stage hydration of calcium aluminate cement (CAC) was examined. It was found that the amount of Al₂O₃ plays an important role in the formation of calcium aluminate hydrates (CAH) because in the mixtures with 2.7% Al₂O₃ only calcium silicate hydrates (CSH) intercalated with Al³⁺ ions were formed. While in the mixtures with a higher amount of Al₂O₃ (5.3–15.4%), calcium aluminate hydrate – C₃AH₆, is formed under all experimental conditions. It is worth noting that the largest quantity of mentioned compound was obtained after 4?h of hydrothermal treatment, in the mixtures with 15.4% of Al₂O₃. It was proved that synthesized C₃AH₆ remain stable up to 300?°C and at higher temperature (945?°C) recrystallized to mayenite (Ca₁₂Al₁₄O₃₃), which reacted with the rest part of CaO and amorphous structure compound, resulting in the formation of gehlenite (Ca₂Al₂SiO₇). Moreover, the synthesized C₃AH₆ addition induced the early stage of CAC hydration. Besides, in the samples with an addition, the induction period was effectively shortened: in a case of pure CAC (G70) paste, hydration takes about 6–6.5?h, while with addition – only 2–2.5?h. The synthesized and calcinated compounds was characterized by using XRD and STA analysis.     

Hydration characteristics of tricalcium aluminate phase in mixes containing β-hemihydate and phosphogypsum

The tricalcium aluminte phase was prepared from pure chemicals on a laboratory scale. Five mixes were formulated from the prepared C₃A phase, β-hemihydate, phosphogypsum, calcium hydroxide and quartz. Different mixes were hydrated at various time intervals, namely, 6, 24, 72 and 168 h. The kinetics of hydration was measured from chemically combined water and combined lime contents. The phase compositions and microstructures of the hydrated products were studied by X-ray diffraction (XRD), differential thermal analysis (DTA)/TG, scanning electron microscopy (SEM) techniques and FT-IR spectroscopy. This work aimed to study the effect of partial to full substitution of phosphogypsum by β-hemihydate on the hydration characteristics and microstructures of tricalcium aluminte phase. The results showed that the combined lime slightly increases with the increase of amounts of phosphogypsum. The XRD patterns showed the increase in the intensities of monosulphate and different forms of calcium aluminate (C₄AH₁₃ and C₄AH₁₉) with phosphogypsum content. Ettringite is less stable than monosulphoaluminate, so it transformed into monosulpho-aluminate after 24 h, which persisted up to 168 h. The mechanism of the hydration process of C₃A phase in the presence of phosphogypsum proceeds in a similar path as with β-hemihydate. Phosphogypsum reacts with C₃A in the presence of Ca(OH)₂ forming sulphoaluminate hydrates, which are responsible for setting regulation in cementitious system.     

Gas-phase and liquid-phase hydration of C3A

In cement manufacturing an important problems is the tendency of cement to combine with water vapour during the grinding, transport and storage. Prehydrated cement may result in retardation of the strength development of the concrete.As it is mainly the clinker mineral C₃A which reacts with acqueous vapour, some experiments concerning hydration of C₃A in the gas phase and liquid phase have been carried out. Variable parameters were temperature, relative humidity and hydration time. The morphology and composition of the hydration products were characterized by using scanning electron microscopy, XRD and thermal analysis.During gas-phase hydration gel, hexagonal and cubic phases were formed. The liquid hydration products were shown to be identical whether the C₃A was almost pure or contaminated with minor components such as C₁₂A₇, free lime or chemically bound water formed during water-vapour hydration. However, if the amount of chemically bound water exceeds 3% the hydration products were anomalous showing rounded, irregular C₃AH₆ aggregates regardless of hydration conditions.The properties of the water-vapour hydrated C₃A might be connected with the retardation of strength when using prehydrated cement, but no possible mechanism has been found.     

Stability in the system CaO–Al2O3–H2O

The solubility of AH₃, CAH₁₀, C₂AH_7.5, and C₃AH₆ was determined experimentally at 7 to 40 °C and up to 570 days. During the reaction of CA, at 20 °C and above initially C₂AH_7.5 formed which was unstable in the long-term. The solubility products calculated indicate that the solubilities of CAH₁₀, C₂AH_7.5 and C₄AH₁₉ increase with temperature while the solubility of C₃AH₆ decreases. Thus at temperatures above 20 °C, C₃AH₆ is stable, while at lower temperature also CAH₁₀ and C₄AH₁₉ are stable, depending on the C/A ratio.At early hydration times, CAH₁₀ can be stable initially at 30 °C and above, as the formation of amorphous AH₃ stabilises CAH₁₀ with respect to C₃AH₆ + 2AH₃. With time, as the solubility AH₃ decreases due to the formation of microcrystalline AH₃, CAH₁₀ becomes unstable at 20 °C and above.     

Influence of metastable hydrated phases on the pore size distribution and degree of hydration of MK-blended cements cured at 60 °C

When MK reacts with calcium hydroxide, cementitious products are formed. It has been reported that CSH, C₂ASH₈ and C₄AH₁₃ are the most important hydrated phases formed. These phases are stable at 20 °C. However, some of them (C₂ASH₈ and C₄AH₁₃) are metastable phases, converting to hydrogarnet (C₃ASH₆) for long curing times at elevated temperatures. The partial or total conversion reaction could produce a negative effect on the performance and durability of blended cements, due to a volume decrease associated with the process of transformation.Due to the influence that this conversion could have on the microstructure and durability of a cement paste containing MK, the current paper presents the results of a research programme carried out on blended cements containing 10%, 20% and 25% of MK, cured at 60 °C up to 124 days of hydration.The total, partial porosity and average pore diameter evolution vs. time is determined using mercury intrusion porosimetry (MIP). An estimated degree of hydration of MK-blended cements cured at 60 °C is proposed.The results show that there is no increase in porosity values and average pore diameters with time. Therefore, the hydrated phases produced in MK-blended cements under the test conditions used do not have a negative effect on the microporosity. A suitable correlation between porosity and degree of hydration has been found.     

Hydrates of calcium ferrites and calcium aluminoferrites

The series of substances C_xF, where x = 1, 2, 3, 4 and C₂A_yF_1?y, where $\text{y}\text{=}\text{1}\text{3}\text{,}\text{1}\text{2}\text{,}\text{2}\text{3}$, have been studied by X-ray diffraction, scanning electron microscopy and Mossbauer spectroscopy, after hydration at temperatures from 4° to 180°C. A new phase, probably C₄FH₁₉, was formed at 4°C. C₃FH₆ forms in the CaOFe₂O₃H₂O system, but it is metastable and decomposes to form calcium hydroxide and hydrous ferric oxide or, at higher temperatures, hematite. The hydration of C₂A_yF_1?y at 4°C produces hexagonal tetracalcium aluminoferrite hydrates and hydrous ferric oxide, which convert at 21°C and above to the cubic C₃(A,F)H₆. The substitution of iron for aluminium in the lattice destabilizes this compound, which decomposes to form calcium hydroxide, hematite and C₃AH₆.     

Elastic properties of high alumina cement castables from room temperature to 1600°C

High-alumina refractory castables with compositions in the systems CaO–Al₂O₃ and CaO–Al₂O₃–SiO₂ were studied using an ultrasonic technique. The technique allows in-situ, non-destructive measurement of Young's modulus from room temperature to 1600°C. Elastic and dilatometric properties were investigated in relation to phase changes (followed by XRD) and sintering phenomena. The conversion of CAH₁₀, the hydration of still-anhydrous cement phases, and the dehydration of C₃AH₆ and AH₃ are related with events in Young's modulus evolution. Addition of 1 wt% of silica fume strongly decreases the high-temperature mechanical properties.     

Effect of micro-sized alumina powder on the hydration products of calcium aluminate cement at 40 °C

In this work, the effect of different micro-sized alumina powders on the hydration products of calcium aluminate cement (CAC) during hydration at 40 °C is studied. The cement hydration at the designated times is terminated by the freeze-vacuum method. The phase development and microstructure evolution during the cement hydration are investigated by XRD and DSC, and SEM, respectively. It is found that 3CaO·Al₂O₃·6H₂O (C₃AH₆) is the dominant product of the pure CAC after hydration at 40 °C for 3.5 h. But 2CaO·Al₂O₃·8H₂O (C₂AH₈) is the dominant hydrate and C₃AH₆ is not found in the mixtures of CAC and micro-sized alumina powder under the same condition. The results indicate that the addition of alumina powders promotes the formation of C₂AH₈ and retards the conversion of C₂AH₈ to the C₃AH₆ phase. Moreover, such phase development with alumina addition is discussed.     

Effect of porosity on carbonation and hydration resistance of CaO materials

CaO pellets with different porosity were carbonated at 700 °C in CO₂ atmosphere. The carbonation rate was controlled by the diffusion of CO₂, regardless of the difference in porosities. For the low-porosity pellet, carbonation reaction only occurred on the surface, with a dense CaCO₃ film thus formed, which combined well with the substrate material; while for the pellet of high-porosity, the carbonation reaction occurred simultaneously both on surface and inside pores, and each CaO grain was surrounded by CaCO₃ film that contained microfissures. Hydration test results showed that carbonation treatment could effectively improve the hydration resistance of CaO materials regardless of porosity, but the carbonated high-porosity pellet was prone to breakage due to poor combination between the carbonated CaO grains. Therefore, for the purpose to improve the hydration resistance by carbonation treatment, it is recommended that the CaO materials should be either with less appreciable apparent porosity or with a limited carbonation ratio for the high-porosity CaO material.     

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.