The effect of high curing temperature on the reaction kinetics in MK/lime and MK-blended cement matrices at 60 °C

It is well known that the pozzolanic reaction between metakaolin (MK) and calcium hydroxide produces CSH, C₂ASH₈ (stratlingite), C₄AH₁₃ and C₃ASH₆ (hydrogarnet). However, the presence or absence of these hydrated phases depends on different parameters, such as curing temperature, matrix used, etc. This paper shows the results of a study in order to know the effect of high curing temperature (60 °C) on the kinetics of the pozzolanic reaction in different matrices. MK/lime (calcium hydroxide) and MK-blended cement matrices were studied in samples stored and cured at 60 °C and up to 123 days of hydration. The nature, sequence and crystallinity of the hydrated phases were analysed using differential thermal analysis (DTA) and X-ray diffraction (XRD) techniques.Results showed that the sequence and formation of the hydrated phases was different in both matrices cured at 60 °C. In an MK/lime matrix, C₂ASH₈, C₄AH₁₃ and C₃ASH₆ were the main hydrated phases; while in an MK-blended cement, stratlingite was the sole hydrated phase issued from pozzolanic reaction. The DTA and XRD data also reveal an important fact: there is no evidence of the presence of hydrogarnet in blended cements.     

The effect of temperature on the hydration rate and stability of the hydration phases of metakaolin-lime-water systems

The paper presents the results of a study carried out to determine the effect of curing temperature on the kinetics of reaction of a metakaolin (MK)/lime mixture. MK and analytical grade Ca(OH)₂ were mixed in a ratio of 1:1 by weight and with a water/binder ratio of 2.37. Specimens were cured at 20 and 60 °C. In the first case, the curing time varied from 2 h up to 180 days and, in the second case, from 2 h up to 123 days. A mathematical model was applied to calculate the rate constant for the hydration reaction. The identity and the amount of the phases present were determined from thermal analysis (TG and DTA) data. The results showed that the rate constant for the samples cured at 60 °C was 68 times greater than the rate constant at 20 °C for the same curing period (up to 9 days). At 20 °C, the sequence of appearance of the hydrated phases was C-S-H, C₂ASH₈ and C₄AH₁₃; while at 60 °C, the sequence was C-S-H, C₂ASH₈, C₄AH₁₃ and hydrogarnet (C₃ASH₆). There is no evidence of further C₂ASH₈ and C₄AH₁₃ transformation into hydrogarnet in the mixture studied for 123 days at 60 °C.     

Study of hydrated phases present in a MK-lime system cured at 60 °C and 60 months of reaction

This research presents the experimental results of a study carried out to determine the effect of curing temperature on the reaction kinetics in a metakaolin/lime mixture cured at 60 °C and after 60 months of hydration. The stabilities of hydrated phases formed during the pozzolanic reaction in these working conditions were evaluated. The results obtained in current paper showed that metastable hexagonal phases (C₂ASH₈ and probably C₄AH₁₃) coexist with stable cubic phase (hydrogarnet) in the absence of lime. Also, there is evidence of the possible presence of a calcium aluminum silicate hydroxide hydrate (vertumnite).     

Evolution of Mineralogical Phases by 27Al and 29Si NMR in MK‐Ca(OH)2 System Cured at 60°C

The evolution of the metastable phases in metakaolin/Ca(OH)₂ systems cured at high temperatures, remains mostly unknown, newer techniques may now help to establish both the kinetic mechanism of the pozzolanic reaction and the thermodynamic stability of the main hydrated hexagonal phases: Stratlingite (C₂ASH₈) and tetra calcium aluminate hydrate (C₄AH₁₃). For this reason this work examines the kinetics of the pozzolanic reaction in the MK/Ca(OH)₂ system over 123 d at 60°C using nuclear magnetic resonance spectroscopy (²⁷Al and ²⁹Si NMR). The results obtained by ²⁷Al and ²⁹Si NMR show that during the first 30 h, the metastable phases C₂ASH₈ and C₄AH₁₃, coexist with the cubic phase (C₃ASH₆) obtained directly from the pozzolanic reaction. The gel C–S–H is clearly identified after 21 h of reaction, whereas at shorter times the C–S–H bands overlap those with the unreacted metakaolin ones. After 123 d of pozzolanic reaction, the first signs of the cubic phase are detected, a consequence of the conversion reaction of the metastable phases, and a phenomenon not previously identified.     

Conversion of calcium aluminate cement hydrates at 60°C with and without water

There have been different hypotheses about the transformation mechanisms of calcium aluminate cement hydrates and this work aims to clarify the long‐running debate about the conversion approaches. In this work, CAH₁₀ and C₂AH₈ were produced from the pastes of calcium aluminate cement (CAC) cured for 24 hours at 10 and 20°C separately. And the cured pastes were continually cured at 60°C for 3 days with water and without water, respectively. The hydration of the pastes was halted by freeze‐drying, and the phases and microstructure of hydrates were investigated by XRD and SEM, respectively. The results indicate that CAH₁₀ and C₂AH₈ converted into C₃AH₆ and AH₃ in water presence at 60°C, but did not transform into C₃AH₆ and AH₃ without water. It is confirmed that the conversion of CAH₁₀ and C₂AH₈ to C₃AH₆ and AH₃ happens through preceding solution of CAH₁₀ and C₂AH₈ and subsequent precipitation of C₃AH₆ and AH₃.     

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.     

The Influence of Activated Coal Mining Wastes on the Mineralogy of Blended Cement Pastes

In recent years, kaolinite‐based wastes are focusing the attention of researchers to obtain recycled metakaolinite, with consequent environmental and socioeconomic benefits. One of these lines of research is based on coal mining waste, which once activated thermally, it becomes a highly pozzolanic product (ACM). This study reports the influence of activated carbon mining waste on the formation and evolution of the mineralogical phases in the ACM/cement system as well as their influence on the microstructure up to 90 d of reaction. Mineralogical analyses clearly show that the addition of ACM modified mineralogical compounds of blended cements. The C₄AH₁₃ and C₄AH₁₂ were the predominant phases in this type of cements; while in the ordinary portland cement cements, portlandite, ettringite, and carboaluminate were main hydrated phases. Two differential zones in the pore size distribution of the C–S–H gels at 12 and 4.5 nm were observed, predominating the formation of C–S–H gels at 12 nm when 20% of ACM was added to the cement.     

Etude au microscope electronique de pates de ciment alumineux hydratees en C2AH8 et en CAH10

Fractures of monocalcium aluminate and high alumina cement pastes, hydrated at 30°C (C₂AH₈) or 12°C (CAH₁₀) are examined by electron microscopy. Water-cement ratio determines the specimen porosity. Hydrates are well crystallized near the pores. The formation of CAH₁₀ results from the reaction between solution and anhydrous material, while C₂AH₈ is able to precipitate from the bulk solution.     

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.     

Mineralogical Evolution of Kaolin‐Based Drinking Water Treatment Waste for Use as Pozzolanic Material. The Effect of Activation Temperature

This study reports on exhaustive scientific research into the influence of the activation temperature of inert waste from drinking water treatment plants for use as supplementary cementing material in cements. The effect of activation temperature on the mineralogy of the reactive products resulting from pozzolanic activity and on the evolution of the hydrated phases formed during the pozzolanic reaction at 28 d of curing was analyzed with the assistance of different instrumental techniques such as X‐ray fluorescence, X‐ray diffraction, thermogravimetric analysis, infrared spectroscopy, and scanning electron microscopy. The results show that all the activated products (based on metakaolinite) presented high pozzolanic activity at all ages of the reaction (up to 90 d), although 600°C at 2 h are the recommended ideal activation conditions from an energetic and economic viewpoint. The activation temperature (600°C–900°C for 2 h of retention) plays an important role in the reaction kinetics in activated drinking water waste/Ca(OH)₂ systems. The hydrated phases identified under these activation conditions were very similar, but with important differences in the crystalline aluminates phases content. Thus, the formation of stratlingite (C₂ASH₈) is favored at low temperatures (<800°C); whereas at higher temperatures (at 900°C), tetra calcium aluminate hydrate (C₄AH₁₃) appears as the only crystalline phase. Finally, this type of treatment of drinking water waste (based on kaolinite) is ideal to obtain future pozzolans based on recycled metakaoline, a product that is currently listed in international standards for the manufacture of commercial cements.     

Mechanical properties and micro-structures of calcium aluminate based ultra high strength cement

This paper describes the mechanical properties and microstructure of calcium aluminate based ultra high strength cement at early age. By using silica fume, polycarboxylate based superplasticizers and a hybrid defoaming mixer, which is anon-contact mixer, cement paste with water to powder ratio of 0.1 can be cast in a mold. When the water to powder ratio is 0.1, the bending strength of hardened samples can be obtained over 30 MPa. Samples were cured at 40 or 60 °C for 7 days. At 60 °C, C₃AH₆ is mainly produced, whereas C₃AH₆ and C₂AH₈ are produced at 40 °C. The mechanical properties of hardened samples with low water to powder ratio are related to the pore volume and pore size distribution.     

Influence of metakaolin on the conversion and compressive strength of quaternary phase paste

Application of calcium aluminate cement in construction faces the challenges of high manufacturing cost and volumetric instability associated with hydrates conversion. This work introduces a newly developed high-performance Ca₂₀Al₂₆Mg₃Si₃O₆₈ (Q phase)-metakaolin (MK) composite binder. The influence of MK on the conversion and strength development of Q phase paste cured at 40°C was investigated. The mechanism of MK on the stability of synthetic hydrate was studied by solution chemistry, XRD, and NMR. The pure Q phase paste experiences a significant strength reduction due to hydrates conversion, whereas the Q phase paste containing 15% MK exhibits a continuous increase in strength. MK promotes the formation of CAH₁₀, contributing to the refinement of pore structure and enhanced mechanical property. The Al^V and Al^IV dissolved from MK increase the concentration in the pore solution, and then the solubility of CAH₁₀ decreases due to the common-ion effect, thus inhibiting the subsequent precipitation of C₃AH₆. In addition, the release of dissolved alumina from MK considerably impedes silica dissolution, and consequently, the formation of C₂ASH₈ is hindered at a higher content of MK.     

Significance of low water—solid ratio and temperature on the physico-mechanical characteristics of hydrates of tricalcium aluminate

Hydration characteristics are reported of tricalcium aluminate studied at 20 and 80 °C using water-solid ratios of 0.2 and 1.0. Hydration products were subjected to differential thermal analysis, thermogravimetric analysis, scanning electron microscopy, length change, porosity and pore size distribution, and micro-hardness measurements. Hydration proceeds faster at 80 °C than at 20 °C and the cubic C₃AH₆ phase is detected in the first few seconds. At 20 °C hydration occurs with the initial formation of the hexagonal phases and expansion is continuous. At higher temperatures and a water-solid ratio of 0.2, after an initial small expansion, the dimensional change is low. The product at 80 °C develops more than fourfold the hardness value developed at 20 °C. After 2 days of hydration, porosity, per cent C₃AH₆ and microhardness are 15.2%, 83% and 38.9 kg/mm²; corresponding values for the product obtained at 20 °C are 21.5%, 75% and 9.1 kg/mm². Microstructural examination of the material formed at 80 °C indicates a closely welded and continuous network of the cubic phase. These results reveal that at a low water-solid ratio and higher temperatures the formation of the cubic phase from C₃A results in an enhancement of strength.     

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.     

Phase evolution and hydraulic properties of low cement castables matrixes

Abstract

To identify the hydration products of aggregate-free, low cement castables (LCC), cement matrixes were examined. Two series of cementing batches based on 33·3 and 50·0 wt-% high alumina cement (HAC) were processed by adding ultra-fine calcined alumina/fumed silica mixtures (FA/FS) with weight ratios of 1·0, 1·5, 2·3, 4·0, and 9·0 to each series. The hydrated batches were investigated for their hydraulic properties. Batches showing the highest cold crushing strength with minimum water of consistency (WOC) and reasonable setting time (ST) were selected and characterised with respect to phase composition, microstructure, and microchemistry, before and after firing up to 1400°C, by XRD, DTA, and SEM techniques. Cementing batches containing ≈33 wt-% HAC, and ≈67 wt-% FA/FS mixtures with ratios of 4·0–9·0 show optimum particle packing and hydration conditions with the least amount of WOC. This results in increases in cold crushing strength (CCS) of cementing batches up to 58 MPa after hydration for 3 d. The hydrated batches are composed mainly of unreacted α-Al₂O₃ particles bonded by CAH₁₀, AH₃, and C₂ASH₈ phases. On firing up to 1400°C, the hydrated phases are transformed into anhydrous CA₂ and CA₆ patches enclosing limited amounts of CAS₂ and/or C₂AS phases. Such batches are suitable for application as cementing matrixes for high alumina low cement castables. The low HAC content with high FA/FS ratio in the presence of more fine alumina in the matrixes of such castables leads to significant improvement in their hot mechanical properties.     

Hydration behavior of spinel containing high alumina cement from high titania blast furnace slag

Hydration behavior of the spinel containing high alumina cement prepared from high titania blast furnace slag via smelting reduction method is studied. Cooling condition has considerable effect on the phase compositions and hydration behavior of the prepared cements. Hydraulic CA, CA₂, inert spinel and gehlenite are the main mineral phases of the naturally cooling cement. Glassy phase, CA and some spinel are the main phases of the splat cooling cement. Both of the prepared cements have controllable setting time, water requirements. Strength of splat cooling cement develops slowly than naturally cooling cement. The naturally cooling cement has satisfactory compressive strength, which is higher than splat cooling cement, but lower than commercial CA80 and Secar71. XRD and SEM observation confirms that CAH₁₀ is the main hydrate of splat cooling cement. Metastable CAH₁₀, C₂AH₈, are the main hydrates of naturally cooling cement, which will convert to stable C₃AH₆ with continuing hydration.     

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.     

The impact of mechanical grinding on calcium aluminate cement hydration at 30 °C

Calcium aluminate cement (CAC) was ground for 1 and 2 h to investigate the impact of mechanical grinding on CAC hydration at 30 °C and CAC-bonded castable strength. Phase composition and microstructure of unground and ground cements after hydration for predetermined times and terminated by the freeze-vacuum drying were compared. The results indicate that the particle size and particle size distribution of CAC were reduced and narrowed, respectively by grinding, thereby favoring the hydration rate and the conversation of C₂AH₈ to C₃AH₆. Then enhanced cement hydration also increases the strengths of castables bonded with milled CAC after drying and firing.     

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.     

Thermodynamic modelling of the effect of temperature on the hydration and porosity of Portland cement

The composition of the phase assemblage and the pore solution of Portland cements hydrated between 0 and 60 °C were modelled as a function of time and temperature. The results of thermodynamic modelling showed a good agreement with the experimental data gained at 5, 20, and 50 °C. At 5 and at 20 °C, a similar phase assemblage was calculated to be present, while at approximately 50 °C, thermodynamic calculations predicted the conversion of ettringite and monocarbonate to monosulphate.Modelling showed that in Portland cements which have an Al₂O₃/SO₃ ratio of > 1.3 (bulk weight), above 50 °C monosulphate and monocarbonate are present. In Portland cements which contain less Al (Al₂O₃/SO₃ < 1.3), above 50 °C monosulphate and small amounts of ettringite are expected to persist. A good correlation between calculated porosity and measured compressive strength was observed.