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.     

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.     

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.     

Influence of calcium additions on the compressive strength and microstructure of alkali‐activated ceramic sanitary‐ware

The ceramic sanitary‐ware market generates large amounts of waste, both during the production process and due to construction and demolition practices. In this paper, the effect of different amounts and calcium sources (calcium hydroxide Ca(OH)₂, calcium aluminate cement CAC, Portland cement PC) on the alkaline activation of ceramic sanitary‐ware waste (CSW) was assessed. Blended samples were activated with NaOH and sodium silicate solutions and cured for 3 and 7 days at 65°C. The maximum amount of calcium source‐type added to the system varied according to its influence on the compactability of the mortars.CSW was physico‐chemically characterized and the compressive strength development of activated samples was assessed on the mortars. The nature of the reaction products was analyzed in pastes, by X‐ray diffraction, thermogravimetric analysis, infrared spectroscopy and microscopic studies. The results show a great positive influence with the addition of moderate amounts of Ca(OH)₂, PC and CAC on the mechanical properties. Among the typical hydrates usually observed in plain water‐hydrated PC or CAC, only AH₃ and a small amount of C₃AH₆ were identified in the alkali‐activated CSW/CAC blended pastes, which indicates that Al and Ca from PC, CAC and Ca(OH)₂ are taken up in the newly formed (N,C)‐A‐S‐H or C‐A‐S‐H gels.     

Hydration of Ca7ZrAl6O18 phase

The hydraulic properties of the Ca₇ZrAl₆O₁₈ (C₇A₃Z) phase as well as the hydration products and thermal decomposition mechanism of this hydrated phase were studied. Microcalorimetric analysis has shown that the C₇A₃Z phase reacts with water very quickly, especially in the first 2 h after the start of the experiment. Hydration of calcium zirconium aluminate proceeds with the formation of high refractory calcium zirconate (with melting point 2345 °C), apart from the hydrated, nearly amorphous material. According to the DTA–TG–EGA, FT-IR and SEM/EDS examinations it has been found that not only the hydrates CAH₁₀, C₂AH₈ and C₄AH₁₉ are present, but also C₃AH₆ (C = CaO, A = Al₂O₃, H = H₂O), the only hydrated calcium aluminate which is a thermodynamically stable phase above 40 °C. Unhydrated Ca₇ZrAl₆O₁₈ and CaZrO₃ phases have been found by XRD, but crystalline hydrates have not been detected.     

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.     

Study of stabilized phases in high alumina cement mortars part II effect of CaCO3 added to high alumina cement mortar subjected to elevated temperature curing and carbonation

The effects of adding ground limestone to high alumina cement mortar were studied. The hydration was carried out at 60°C to form the cubic hydrate C₃AH₆ directly, hence avoiding the conversion of CAH₁₀ to C₃AH₆, and at 20°C. Other experiments involved subsequent carbonation with CO₂ and thermal treatment. The formation of stable carbonated phases was evident even at very early ages.     

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.     

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 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.     

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.     

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.     

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.     

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.     

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.     

Stabilite thermodynamique des phases dans le systeme CaOAl2O3CO2H2O

The paper presents a theoretical investigation on the nature of thermodynamical stable precipitating phases in the CaOAl₂O₃CO₂H₂O system when monocalcium aluminate (CA) with or without an excess of lime or alumina is mixed with water in equilibrium with gaseous carbon dioxide. Basing on the nature of ions in solution (A10₂⁻, Ca⁺⁺, CO_{3_}⁻⁻, OH⁻ and H⁺). and supposing that only $\text{C}\text{C}$, AH₃, CH, CAH₁₀, C₂AH₈ and C₄ACH₁₁ are the possible stable phases, the system was mathematically described taking into account the values of solubility products, the equilibrium between atmospheric carbon dioxide and the solution, ionic dissociation constant of water, matter balance of calcium and aluminate ions and condition of the solution electroneutrality. For a given system of initial parameters, computer calculation has allowed to determine the only possible solid-liquid system among the 22 theoretically possible cases (zero, one or two solids in equilibrium with the solution). So, varying the initial parameters has led to draw isobaric and isosteric phase diagramms, what shows that, in the conditions of the study, CAH₁₀ is the only stable phase at 21°C. The result suggests that the precipitation of C₂AH₈ and $\text{C}{}_4\text{A}\text{C}\text{H}{}_{11}$ can only be a consequence of saturations of the solution with respect to CAH₁₀.     

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.     

Hydration reactions in suspensions of C3A + C3S + CaSO4.2aq. in water

In suspensions of C₃A in water, C₃AH₆ formation is counteracted by the addition of CaSO₄.2aq.or of large additions of C₃S but promoted by small additions of C₃S. The composition of the hexagonal calcium aluminate hydrates precipitating is influenced by the dissolution rate of the C₃A.     

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.