The effect of dehydroxylation/amorphization degree on pozzolanic activity of kaolinite

The effect of heat treatment parameters on the dehydroxylation/amorphization process of the kaolinite-based materials such as natural and artificial kaolin clays with different amounts of amorphous phase (metakaolin) was investigated. The procedure for quantitative estimation of amorphous phase in the heat-treated kaolinite materials was developed. The process of dehydroxylation/amorphization of kaolinite was characterized by DTA/TGA with mass-spectrometry and X-ray powder diffraction. The influence of the heat treatment temperature and content of the amorphous phase on pozzolanic activity was studied. Finally, the relationships between the chemical activity, activity strength index and the amorphous phase content were found and discussed. The results obtained are important for an optimization of the process of the metakaolin large scale production and it's use as an active pozzolanic admixture.     

新型混凝土掺合料——高活性偏高岭土的研究进展

综述了偏高岭土在混凝土领域国内外的应用现状、研究进展。阐述了具有火山灰特性的偏高岭土在混凝土中的应用机理及其对混凝土微观和宏观性能的影响。     

The pozzolanic activity of a calcined waste FCC catalyst and its effect on the compressive strength of cementitious materials

Equilibrium catalyst (Ecat), one of the spent fluid catalytic cracking (FCC) catalysts from oil companies, shows pozzolanic activity. In this study, the effects on the pozzolanic activity of calcination of Ecat and on the compressive strength of the resulting cementitious materials were examined. The pozzolanic activity of this mineral additive was indicated from DSC measurements. The results show that the pozzolanic activity of Ecat increases with calcined temperature initially, reaches a maximum, and then decreases afterwards. Ecat calcined at about 650 °C becomes the most active. Mortars with 10% calcined catalyst at 3-28 curing days exhibit strength 8-18% greater than that with the untreated. Concrete with a 10% calcined Ecat at 3-28 curing days exhibits strength 7-11% greater than that with the untreated. If the calcined catalyst is further ground, its pozzolanic activity is enhanced, and the compressive strength of the resulting mortars or concrete becomes higher.     

The effect that different pozzolanic activity methods has on the kinetic constants of the pozzolanic reaction in sugar cane straw-clay ash/lime systems: Application of a kinetic-diffusive model

The reaction kinetics of a mixture of sugar cane straw with 20% and 30% of clay burned at 800 and 1000 °C and lime (calcium hydroxide) is studied. A direct method (accelerated chemical method) based on the measurement of the amount of lime reacted as the reaction proceeds is applied. A kinetic-diffusive model published in a previous paper by some authors of this research is used. The fitting of the model by computerized methods allows determining the kinetic coefficients that characterize the process: in particular, the reaction rate constant. The index of pozzolanic activity evaluated according to the obtained values of the reaction rate constant permits to characterize the pozzolanic activity of these materials in a rigorous way. The results are compared with the results obtained applying an indirect method (conductometric method). The kinetic results obtained in the current paper allow affirming that the kinetic-diffuse model used in order to evaluate the pozzolanic reaction is valid, independently of the method used for the evaluation of the pozzolanic activity.     

Geopolymers from Algerian metakaolin. Influence of secondary minerals

The influence of secondary phases (illite, quartz) on the geopolymerization reaction of metakaolin has been investigated by comparing two metakaolins, one prepared from a pure kaolinite and the other from illite- and quartz-containing Algerian kaolin from the Tamazert region, respectively. Geopolymerization was achieved by mixing the metakaolins with an alkaline sodium silicate solution at room temperature and curing at 50 °C. The products were characterized by X-ray diffraction and ²⁹Si and ²⁷Al MAS-NMR. The results show that the secondary phases, at the concentration used in this work, do not prevent the geopolymerization reaction.     

Influence of metakaolin on the properties of mortar and concrete: A review

Supplementary cementing materials (SCM) have become an integral part of high strength and high performance concrete mix design. These may be naturally occurring materials, industrial wastes, or byproducts or the ones requiring less energy to manufacture. Some of the commonly used supplementary cementing materials are fly ash, silica fume (SF), granulated blast furnace slag (GGBS), rice husk ash (RHA) and metakaolin (MK), etc. Metakaolin is obtained by the calcination of kaolinite. It is being used very commonly as pozzolanic material in mortar and concrete, and has exhibited considerable influence in enhancing the mechanical and durability properties of mortar and concrete. This paper presents an overview of the work carried out on the use of MK as partial replacement of cement in mortar and concrete. Properties reported in this paper are the fresh mortar/concrete properties, mechanical and durability properties.     

Study on the pozzolanic properties of rice husk ash by hydrochloric acid pretreatment

The pozzolanic properties of rice husk ash by hydrochloric acid pretreatment are reported in the paper. Three methods have been used to estimate the pozzolanic activity of rice husk ash. The heat evolution and the hydration heat of cement, the Ca(OH)₂ content in the mortar and the pore size distribution of mortar are determined. It is shown that compare with the rice husk ash heated untreated rice husk, the sensitivity of pozzolanic activity of the rice husk ash heated hydrochloric acid pretreatment rice husk to burning conditions is reduced. The pozzolanic activity of rice husk ash by pretreatment is not only stabilized but also enhanced obviously. The kinetics of reaction of rice husk ash with lime is consistent with diffusion control and can be represented by the Jander diffusion equation. A significant increase in the strength of the rice husk ash (pretreated) specimen is observed. The results of heat evolution indicate that the rice husk ash by pretreatment shows the behavior in the increase of hydration of cement. The cement mortar added with the rice husk ash by pretreatment has lower Ca(OH)₂ content after 7 days and the pore size distribution of the mortar with the rice husk ash with pretreatment shows a tendency to shift towards the smaller pore size.     

The influence of structure order on the kinetics of dehydroxylation of kaolinite

The structural order of kaolinite is an important factor that shows a substantial effect on the processes which take place during the thermal treatment of kaolin. The influence of structural order on the dehydroxylation process was investigated by simultaneous thermogravimetry and differential thermal analysis (TG-DTA). The thermal analysis was performed on the samples with gradually decreasing structural order prepared by milling procedure. The apparent activation energy of dehydroxylation process decreases with decreasing structural order according to the exponential function. The extrapolation of experimental data leads to the estimation of apparent activation energy of 76.6 kJ mol^?1 and of frequency factor of 0.12 × 10⁴ s^?1 related to completely disordered form of kaolinite, while the ordered form shows the apparent activation energy of 216.17 kJ mol^?1 and the frequency factor of 9.26 × 10⁴ s^?1. The relationships between features such as the infrared pattern of treated material, the degree of structural order and the apparent activation energy were established.     

The kinetic analysis of the thermal decomposition of kaolinite by DTG technique

The thermal decomposition of kaolinite was studied by differential thermogravimetry (DTG) technique under non-isothermal conditions. Samples of industrially treated (washed) kaolin with high content of the medium ordered kaolinite were calcined using a heating rate from 1 to 40 K min^− 1. The apparent activation energy and frequency factor for the dehydroxylation of kaolinite was evaluated by Kissinger method as 195 ± 2 kJ × mol^− 1 and (8.58 ± 0.33) × 10¹⁴ s^− 1, respectively. Avrami exponent of the process was estimated using Kissinger empirical kinetic models and Carne equation.     

Absorption characteristics of metakaolin concrete

The water absorption (WA) by total immersion and by capillary rise of concrete containing metakaolin (MK) is investigated. Cement was partially replaced with up to 20% MK. The results show that the presence of MK is greatly beneficial in reducing the WA by capillary action. There is a systematic reduction in absorption by capillary action with the increase in MK content in concrete. This reduction is further supported by visual examination of specimens. The absorption by total immersion, however, tends to increase slightly with the increase in MK content. Between 14 and 28 days curing, there is a slight increase in absorption by total immersion and by capillary rise for all MK concretes. Correlation between the absorption characteristics, dynamic modulus of elasticity (E_d), strength and pore size distribution was conducted.     

偏高岭土超细粉对中间包永久衬用高铝浇注料性能的影响

以8～5mm、5～3mm、≤3mm的矾土为骨料,保持颗粒与细粉质量比为65:35不变,研究了用d50=15.91μm,加入量(w)为4%、6%、8%的偏高岭土超细粉,分别取代d50=1.75μm,加入量(w)为1%、2%、3%的Al2O3微粉后,中间包永久衬用高铝质浇注料施工性能、机械强度以及矿物组成的变化。结果认为:用偏高岭土超细粉完全可以取代Al2O3微粉生产莫来石质中间包永久衬用浇注料,添加偏高岭土超细粉的浇注料的流动性、粘聚性、保水性等明显优于加入Al2O3微粉的,且经1400℃煅烧后的机械强度较高;XRD分析表明,以8%偏高岭土超细粉完全取代Al2O3微粉后,试样于1400℃3h烧后的莫来石相含量最高,且其各项性能指标与中间包永久衬要求相一致。     

Effect of dehydroxylation on the specific heat of simple clay mixtures

Specific heats of four clays (standard reference kaolins, commercial kaolin and montmorillonite) before and after dehydroxylation have been measured. The results were compared with handbook data for the thermal chemical properties of solids. Good agreement has been obtained for the reference kaolin before any thermal treatment. Then, following thermal treatments at 500 °C, 600 °C and 700 °C, dehydroxylation leads to a progressive decrease of heat capacity per unit mass. After dehydroxylation, heat capacity values for all the studied materials are rather similar and agree closely with those estimated by the rule of mixtures. Finally, an empirical relation describing the specific heat capacity (C) in J kg⁻¹ K⁻¹ of dehydroxylated kaolin from 40 °C to 1100 °C is proposed: C = 1128 + 0.102T − 36 × 10⁶T⁻² where T is in K.     

Durability of mortars modified with metakaolin

Results of an investigation to determine the effects of metakaolin additions on transport properties of mortars are reported. Comparisons are made to ordinary Portland cement (OPC) to determine the influence of addition and replacement percentage. Cement is replaced on a mass basis of 5-20% for metakaolin. A mixture with natural kaolin is also studied. The transport properties and chemical behaviors are analyzed by means of chloride diffusion tests and sulfate immersion. Observations after more than 100 days are used to prescribe mixtures that reduce the rate of chloride diffusion and sulfate degradation. For metakaolin, the optimum seems to be between 10% and 15% with regard to inhibition effect on chloride diffusion and sulfate attack.     

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.     

Investigation on pozzolanic effect of perlite powder in concrete

The pozzolanic effect of perlite powder (PP) added to concrete can be determined quantitatively with strength indices: specific strength ratio (R), index of specific strength (K), and contribution percentage of pozzolanic effect to strength (P). Besides compressive strength, these indices indicate that perlite powder has a high pozzolanic effect and is an active mineral admixture (MA) for concrete.     

Determination of the pozzolanic activity of fluid catalytic cracking residue. Thermogravimetric analysis studies on FC3R-lime pastes

Spent fluid catalytic cracking catalyst (FC3R) from a petrol refinery played a pozzolanic role in portland cement system as revealed by previous experimental data. In the present study, the pozzolanic activity of FC3R was investigated by means thermogravimetry (TG) of cured lime-FC3R pastes. The influence of pozzolan/lime ratio on the pozzolanic activity was investigated. Due to the chemical composition of FC3R is similar to metakaolin (MK), and knowing that MK has a high pozzolanic activity, the latter was used as a material of comparison in this study. The scope of the study is the determination of the pozzolanic activity of FC3R and the evaluation of amount and nature of pozzolanic products. The products obtained from the reaction between FC3R components (SiO₂/Al₂O₃) and calcium hydroxide (CH) have been characterized, finding that the main pozzolanic reaction product was similar to hydrated gehlenite (calcium aluminosilicate hydrate) CSH and CAH were also formed in the reaction. FC3R showed higher pozzolanic reactivity than metakaolin, for low-lime content pastes and early curing age. Thermogravimetry, Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) became very useful techniques for evaluation of reactivity.     

Cementitious and pozzolanic behavior of electric arc furnace steel slags

The cementitious and pozzolanic behavior of electric arc furnace steel slag, both as received and treated has been studied in detail. The as received slag was completely crystalline and multi-phasic with Fe-substituted monticellite as the predominant phase. Treatment of this slag, remelting and water quenching, results in reduction of Fe-oxide content coupled with an increase in basicity index which makes it more hydraulic compared to the as received slag. The remelted slag has several phases with merwinite as the dominant phase. Thermal analysis of the hydrated slag shows that treating the as received slag increases the water absorption capacity, a property essential for cementitious behavior. Compression strength of the slag blended cements was studied and it was found that substitution of 20% ground granulated blast furnace slag with electric arc furnace steel slag does not decrease the strength beyond 28 days. The control cement has a strength of 58.6 MPa compared to 58 MPa for the cement comprising of 20% untreated slag. The substitution of this untreated slag with treated slag exhibits the highest strength, 61 MPa and a potential for further strength increase after 28 days. In the case of cement mix with no blast furnace slag, substitution of 15% clinker with steel slag does not decrease the strength significantly, 64.4 MPa compared to 66.5 MPa for the control cement. Substituting 30% clinker in the cement mix with electric arc furnace slag however results in significant decrease in strength, 53.4 MPa. The pozzolanic strength of the slag was found to increase significantly due to remelting from 2.0 MPa for the as received slag to 8.0 MPa for the treated slag.     

Assessment of phase formation in lime-based mortars with added metakaolin, Portland cement and sepiolite, for grouting of historic masonry

Lime-based mortars containing pozzolanic additions of metakaolin, sepiolite and white Portland cement are studied in order to determine their performance as historic masonry conservation mortars. Hydration products on metakaolin-lime blended mortars include stable and metastable phases. The presence of such products has been studied by means of DTA and XRD analysis, concluding that the selection between them is mainly related with the water-lime ratio. Sepiolite addition to metakaolin-lime mortars has shown to inhibit C₄AH₁₃ formation. Therefore, the influence of phase distribution on the mechanical resistance is considered. Finally, compounds production on blended lime-white Portland cement was compared to natural hydraulic lime ones, and as a result, no remarkable differences appeared, apart from traces of possible cement Portland addition to the latter, usually not mentioned in the nominal composition supplied by the manufacturers of lime binders.     

The origin of the pozzolanic activity of calcined clay minerals: A comparison between kaolinite, illite and montmorillonite

This paper investigates the decomposition of three clayey structures (kaolinite, illite and montmorillonite) when thermally treated at 600 °C and 800 °C and the effect of this treatment on their pozzolanic activity in cementitious materials. Raw and calcined clay minerals were characterized by the XRF, XRD, ²⁷Al NMR, DTG and BET techniques. Cement pastes and mortars were produced with a 30% substitution by calcined clay minerals. The pozzolanic activity and the degree of hydration of the clinker component were monitored on pastes using DTG and BSE-IA, respectively. Compressive strength and sorptivity properties were assessed on standard mortars. It was shown that kaolinite, due to the amount and location of OH groups in its structure, has a different decomposition process than illite or montmorillonite, which results in an important loss of crystallinity. This explains its enhanced pozzolanic activity compared to other calcined clay–cement blends.