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.     

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.     

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.     

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.     

Curing temperature and humidity effects on the strength of an aluminous cement

The effects of curing temperature (21 and 60°C), time (1,7,28 days) and relative humidity (25 to 87%) on the strength and phase compositions of specimens prepared from neat Fondu paste with water/aluminous cement ratio of 0.30 were investigated. The room temperature strengths of the materials aged at 60°C, containing C₃AH₆ and AH₃, were higher than pastes cured at 21°C, containing CAH₁₀. Conversion at 60°C of the specimens aged at room temperature resulted in strengths comparable to materials isothermally cured at 60°C. The effects of these curing conditions on the high temperature strength were also investigated.     

Hydration reaction and hardening of calcined clays and related minerals. I. Preliminary investigation on metakaolinite

When mixed with calcium hydroxide and water, metakaolinite obtained by fixed-bed calcination of a commercial kaolinite at 730°C, hydrates and develops, at 28 days, compressive strengths (tests on minicylinders) of about 10–15 MPa. Hydration products are essentially C₂ASH₈ and CSH with some quantities of C₄AH₁₃. The influence of several factors e.g. curing conditions, value of metakaolinite/calcium hydroxide and water/cement ratios, addition of sand to the mix, which can modify the hardening, has been investigated.     

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.     

Compressive strength and hydration of wastepaper sludge ash-ground granulated blastfurnace slag blended pastes

Compressive strength and hydration characteristics of wastepaper sludge ash-ground granulated blastfurnace slag (WSA-GGBS) blended pastes were investigated at a water to binder (w/b) ratio of 0.5. The strength results are compared to those of normal Portland cement (PC) paste and relative strengths are reported. Early relative strengths (1 day) of WSA-GGBS pastes were very low but a marked gain in relative strength occurred between 1 and 7 days and this increased further after 28 and 90 days. For the 50% WSA-50% GGBS blended paste, the strength achieved at 90 days was nearly 50% of that of the PC control paste. Transmission electron microscopy (TEM), X-ray diffraction (XRD) and thermogravimetric (TG) analysis were carried out to identify the mineral components in the WSA and the hydration products of WSA and WSA-GGBS pastes. The principal crystalline components in the WSA are gehlenite, calcium oxide, bredigite and α′-C₂S (stabilised with Al and Mg) together with small amounts of anorthite and calcium carbonate and traces of calcium hydroxide and quartz. The α′-C₂S and bredigite, which phase separate from liquid phase that forms a glass on cooling, are difficult to distinguish by XRD. The hydration products identified in WSA paste are CH, C₄AH₁₃, C₃A.0.5CC?.0.5CH.H_11.5 and C-S-H gel plus possible evidence of small amounts of C₂ASH₈ and C₃A.3CS?.H₃₂. Based upon the findings, a hydration mechanism is presented, and a model is proposed to explain the observed strength development.     

The contribution of class-F fly ash to the strength of cementitious mixtures

The contribution of fly ash to the physical properties of cementitious mixtures has received considerable attention since its inclusion as an essential ingredient of High Performance Concrete (HPC). However, the chemical contribution to the overall structure development has not been fully understood because of the masking of its hydration products by those of cement. In a mixture of class-F fly ash and lime (Ca/Si=2), portlandite diminishes and C₄AH₁₃ forms due to addition of Al to solution. The latter converts to hydrogarnet and C₃ASH₄. CSH is detected at 3 days and continues to increase in intensity. The ²⁹Si magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR) showed that the Al/Si ratio is 0.24 and the average chain length is 10 units. The presence of Al as approximately one-fifth of the Si in a chain length of 10 units suggests that Al tetrahedra may be present in bridging positions.     

Compatibility of hydrogarnet,Ca3Al2(SiO4)x(OH)4(3 − x), with sulfate and carbonate-bearing cement phases: 5–85 °C

The stable existence of hydrogarnet in Portland cement compositions cured at temperatures below 55 °C has long been predicted from application of equilibrium thermodynamics. However hydrogarnet is not often reported in hydrated commercial Portland cements. The substitutions (SO₄–CO₃–OH) in AFm have previously been shown to stabilise AFm to higher temperatures and raise the temperature at which AFm converts to Si-free hydrogarnet, C₃AH₆. But unanswered question remains about the compatibility of AFm and AFm solid solutions with Si-substituted hydrogarnet, Ca₃Al₂(SiO₄)_x(OH)_4(3 − x). Phase relations of C₃AH₆ and Ca₃Al₂(SiO₄)_x(OH)_4(3 − x) at sulfate and carbonate activities conditioned respectively by (gypsum and SO₄-AFt) and (calcite and CO₃-AFt) have been determined experimentally in the range 5–85 °C. The results confirm the instability of Si-free hydrogarnet with carbonate and sulfate-bearing cement phases, but do indicate that a range of silica-substituted hydrogarnet solid solutions are stable under conditions likely to be encountered in blended cement systems.     

CaCO3 addition effect on the hydration and mechanical strength evolution of calcium aluminate cement for endodontic applications

Calcium aluminate cement (CAC) hydrates conversion can be inhibited by adding CaCO₃, leading to C₃A·CaCO₃·11H (3CaO·Al₂O₃·CaCO₃·11H₂O) formation. However, despite its benefits, the stability of this monocarbonate hydrate is not fully understood, especially when the samples are kept in contact with liquid during the curing step. Thus, taking into account the increasing interest in the CAC application as a biomaterial in the endodontic area, this work addresses the evaluation of the mechanical strength and phase transformations of a commercial cement (Secar 71) containing 15 or 20 wt% of CaCO₃. Compressive strength, apparent porosity, dimensional linear changes, X ray diffraction and thermogravimetric tests were carried out to evaluate samples immersed in water and kept at 37 °C between 1 and 30 days of curing. According to the collected results, CAH₁₀ and C₂AH₈ formation were inhibited in CaCO₃ containing compositions and the presence of the C₃A.CaCO₃.11H phase led to a significant cement mechanical strength increase. Nevertheless, the partial decomposition of this monocarbonate hydrate was detected at 37 °C in the range of 1-7 days and the continuous hydration of CA and CA₂ also affected the compressive strength behavior of the evaluated samples.     

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.     

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.     

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.     

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.     

Carbonation of CH and C-S-H in composite cement pastes containing high amounts of BFS

3:1 BFS:OPC, 9:1 BFS:OPC and 9:1 alkali activated BFS:OPC pastes cured at 20 °C and 60 °C for 90 days were submitted to accelerated carbonation under 5% CO₂, 60% relative humidity and 25 ± 5 °C for 21 days. TGA/DTG was used to quantify the amounts of carbonates formed from calcium hydroxide (CH) and calcium silicate hydrate (C-S-H), based on the CH and carbonate contents before and after carbonation. Apparent dry density, apparent porosity and gas permeability were measured before and after accelerated carbonation testing, and the phenolphthalein method used to determine the accelerated carbonation rate. The results showed that samples cured at elevated temperature, i.e. 60 °C, were initially less porous and, therefore, had decreased levels of both total carbonation and C-S-H carbonation. In addition, the carbonation of C-S-H was significantly higher in pastes that contained less CH before carbonation. In the activated 9:1 BFS:OPC, the carbonation of C-S-H was extensive, despite a lower carbonation rate than the analogous non-activated system. In the particular case of activated 9:1 BFS:OPC, a shift in the DTG decarbonation pattern was observed and XRD showed that aragonite was present as one of the calcium carbonate polymorphs.     

Influence of polymer on cement hydration in SBR-modified cement pastes

The influence of styrene-butadiene rubber (SBR) latex on cement hydrates Ca(OH)₂, ettringite, C₄AH₁₃ and C-S-H gel and the degree of cement hydration is studied by means of several measure methods. The results of DSC and XRD show that the Ca(OH)₂ content in wet-cured SBR-modified cement pastes increases with polymer-cement ratio (P/C) and reaches a maximum when P/C is 5%, 10% and 10% for the pastes hydrated for 3 d, 7 d and 28 d, respectively. With wet cure, appropriate addition of SBR promotes the hydration of cement, while the effect of SBR on the content of Ca(OH)₂ and the degree of cement hydration is not remarkable in mixed-cured SBR-modified cement pastes. XRD results illustrate that SBR accelerates the reaction of calcium aluminate with gypsum, and thus enhances the formation and stability of the ettringite and inhibits the formation of C₄AH₁₃. The structure of aluminum-oxide and silicon-oxide polyhedron is characterized by ²⁷Al and ²⁹Si solid state NMR spectrum method, which shows that tetrahedron and octahedron are the main forms of aluminum-oxide polyhedrons in SBR-modified cement pastes. There are only [SiO₄]⁴⁻ tetrahedron monomer and dimer in the modified pastes hydrated for 3 d, but there appears three-tetrahedron polymer in the modified pastes hydrated for 28 d. The effect of low SBR dosage on the structure of aluminum-oxide and silicon-oxide polyhedron is slight. However, the combination of Al³⁺ with [SiO₄]⁴⁻ is restrained when P/C is above 15%, and the structure of Al³⁺ is changed obviously. Meantime, the polymerization of the [SiO₄]⁴⁻ tetrahedron in C-S-H gel is controlled.     

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.     

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.