Tetracalcium Aluminate Research Articles

Improving the strength of metakaolin-lime based binder

Pozzolanic reaction of low-calcium metakaolin (MK) with calcium hydroxide (CH) at ambient temperature in the presence of water forms a series of hydrated phases such as tetracalcium aluminate hydrate (C4AH13), calcium silicate hydrate (C-S-H) and calcium aluminum silicate hydrate (stratlingite - C2ASH8). Stratlingite is the main crystalline phase and is responsible for the strength of the binder. Tetracalcium aluminate hydrate (C4AH13) is carbonated upon contact with air or converted to hemicarboaluminate and/or monocarboaluminate if the system contains excess carbonate phases (calcite or calcareous aggregates). However, the calcium aluminate hydrates formed after the reaction of MK with lime lose their strength over time due to their instability. Especially in a high humidity environment where there is no carbonation, the presence of both stratlingite and (CH) in the binder creates weak phases called katoite (Ca3Al2(SiO4)(OH)8) and can reduce the mechanical strength and durability. On the other hand, it has been determined that using chemical activators to increase the pozzolanic reactivity is the most feasible method, although it increases the cost of the material. In this study, it is aimed to eliminate the phases that cause katoite formation and improve the performance of the binder by using alkaline hydroxide solution in a mixture of metakaolin and slaked lime, similar to the pore solution caused by hydrated cement. For this aim, the effect of sodium carbonate (N-Na2CO3), quicklime (C-CaO) and calcite (CC-CaCO3) on the (CH-Ca(OH)2) activated metakaolin system is investigated through the experimental campaign. The preparation of the mixtures is done by the novel one-part mixing method.

Effect of reverse-osmosis brine and sodium aluminate on the hydration properties and strength of alkali-activated slag cement

The effects of sodium aluminate (NA) and reverse-osmosis brine (RW) on alkali-activated slag cement (AASC) using sodium hydroxide and sodium silicate as activators were investigated. NA was mixed at 0%, 2.5%, and 5.0% by binder weight, and the mix waters tested were tap water (TW) and RW. In the experiment NA showed the effect of improving the compressive strength, with the highest strength value obtained at 2.5%. However, the strength improvement effect of NA was more clearly demonstrated in RW samples than in TW samples. Changes in the C4AH13 (Tetracalcium aluminate hydrate; Ca4Al2(OH)14.6 H2O), C2ASH8 (stratlingite; Ca2Al2SiO7.8 H2O), and C3AH6 (katoite; Ca3Al2(OH)12) phases were observed according to the increase in NA through hydration reaction analysis. Friedel’s salt (Ca4Al2O6Cl2.10 H2O) was additionally observed in the RW sample. In the TW samples, more of the C3AH6 phase was formed than in the RW samples, which affected the strength reduction. In the RW samples, increases in the chloride ions and calcite phase in the pore solution due to carbonation of Friedel’s salt were confirmed. This phenomenon of the RW samples became more pronounced as the NA concentration increased. The technique of utilizing RW in AASC showed the possibility of being used as a novel stabilization/solidification method for reverse osmosis brine.

Impact of the grinding process on the granulometric properties of triclinic alite

Four major clinker minerals, alite, belite, tricalcium aluminate and tetracalcium aluminate ferrite are crucial for cement properties. This work deals with alite as the most important clinker minerals. Alite is affected by the milling process technology more than other clinker minerals. It contains micro-cracks, which are formed during fast cooling, and is therefore less resistant to grinding than other clinker minerals. The paper deals with the influence of length and the technology of the milling process in 3 types of laboratory mills on the particle size and distribution, agglomeration and the change of the grain shape of the synthetically prepared triclinic alite.

Carbonation-Induced Mineralogical Changes in Coal Mining Waste Blended Cement Pastes and Their Influence on Mechanical and Microporosity Properties

The worldwide pursuit of new eco-efficient pozzolans is ongoing. Kaolinite-based waste is an eco-friendly source of recycled metakaolinite, a highly pozzolanic product. In this study, a blended cement paste containing 20% activated coal waste (ACW) was exposed to a 100% CO2 atmosphere at 65% RH for 7 days. The variations in its phase composition and strength were studied and compared to an OPC control. Both pastes were cured for 28 days prior to the carbonation test. Reaction kinetics were assessed using XRD, SEM/EDX, TG/DTG, FT-IR, Micro-Raman spectroscopy, pore solution pH and the cumulative carbonated fraction. The blended cement carbonated 68% faster than the control. While portlandite carbonation was the main reaction in both cements, decalcification was also observed (more intensely in the 20% ACW paste) in other hydraulic calcium phases (C-S-H gel, monocarboaluminate (C4AcH12), ettringite and tetracalcium aluminate (C4AH13). The end product of this reaction was calcium carbonate, mainly in the form of calcite, although traces of aragonite and amorphous carbonate were also detected. Compressive strength values rose with accelerated carbonation time and pore size reduction in both cement pastes.

Influence of ZnO on the activation of kaolinite-based coal waste: Pozzolanic activity and mineralogy in the pozzolan/lime system

Abstract One inconvenience presented by the thermal activation of kaolinite-based wastes is their low content of metakaolinite, a highly pozzolanic product listed in current standards for the manufacture of commercial cements. The addition of a chemical activator during the thermal activation process is a priority line of research to increase the reactivity of the recycled metakaolinite. In this paper, an additional chemical activator, ZnO, is studied and its effect on both pozzolanic properties and the evolution of mineralogical phases in the thermal activation of coal waste with a reaction time of up to 90 days in the pozzolan/lime system. To do so, activation temperatures of between 550 °C/650 °C were selected and additions of chemical activator (ZnO) in percentages of between 0.0% and 3.0% by weight of coal waste, because it is an activator with a positive effect on a 100% natural kaolinite. The results showed that the incorporation of ZnO inhibited the reactivity of the recycled metakaolinite and in consequence, the capacity of the metakaolinite to react with the surrounding lime; even more so when the content of added chemical activator was raised, albeit with some exceptions, in the samples activated at 550 °C and 650 °C with 0.5% of chemical activator. In none of the cases under analysis was the chemical activator able to improve the properties of the metakaolinite in comparison with the properties of the reference sample activated only with temperature. The hydrated phases that appeared in the pozzolanic reaction were tetracalcium aluminate hydrate, stratlingite, monosulfoaluminate hydrate and LDH (phyllosilicate/carbonate).

From coal-mining waste to construction material: a study of its mineral phases

The study from the recycling of coal mining waste from NW of Spain as pozzolans materials in cement industry is the scope of this present study. The thermal activation of coal-mining wastes, the formation of their hydrated phases and their evolution are analyzed, over pozzolanic reactions at 1, 7 and 28 days following activation. The coal-mining waste showed good pozzolanic activity, following thermal activation at 600 and 900 °C for 2 h. Total destruction of kaolinite is produced after treatment at 600 °C/2 h retention in the oven. This temperature and time are best suited for activation of the best environmental way. The compounds arising during the pozzolanic reaction in coal-mining waste activated at 600° and 900° in a Ca(OH)2 system were C–S–H gels, stratlingite (C2ASH8), tetracalcium aluminate hydrate (C4AH13), layered double hydroxides and monosulfoaluminate. When the temperature is 600 °C appeared layered double hydroxides via the metastable phase is favored during the first 7 days of reaction, while stratlingite is present as the major crystalline phase at 28 days into the pozzolanic reaction.

Fly ash/paper sludge as constituents of cements: hydration phases

Ternary cements prepared with equal amounts of thermally activated paper sludge and fly ash in proportions of 35% and 50%, in substitution of ordinary Portland cement, are mixed to study their phase stability. The underlying intention is to increase the proportional substitution of waste products in these types of cement, thereby consigning less waste to landfill sites. The hydration phases of the cement specimens were analysed using thermogravimetric/derivative thermogravimetric (TG/DTG) analysis, X-ray diffraction, scanning electron microscopy/energy-dispersive X-ray (SEM/EDX) spectroscopy. The phases identified in the cement blends include Portlandite, monocarboaluminate compound C4AC̅H12, and layered double hydroxides–type compounds (LDH), as well as ettringite and calcium silicate hydrate gels. Increased formation of both tetracalcium aluminate carbonate 12 hydrate (C4ACH12) and LDH-type compounds and reductions in ettringite and portlandite were observed when the proportional substitution of cement by waste products rose from 35% to 50%. A characteristic compound of pozzolanic reactions, bicalcium silicate (C2ASH8), was not observed, suggesting that it is inhibited in these types of substitutions.

Rice Husk as a Substitute Fuel in Cement Kiln Plant

A simple mathematical model of heat balance was applied to a cement kiln plant with a precalciner to estimate the effect of using rice husk as a substitute fuel for natural gas on specific heat consumption. Effects of the husk ash on the characteristics of the raw mix and clinker of ordinary Portland cement were also evaluated. Referring to Egyptian kiln plants data, it was found that the weight of rice husk required to supply fuel heat in the precalciner represented about 11-13% of the raw mix weight and about 90% of the total fuel weight (natural gas + husk). Specific heat consumption increased by about 3.7%, and the amount of combustion flue gases increased by about 20% when natural gas was completely substituted by rice husk. The use of rice husk as a substitute fuel in a cement kiln plant was accompanied by a marked decrease of lime saturation factor of the raw mix, a drastic decrease of tricalcium silicate and an increase of dicalcium silicate in the clinker. This data can provide the basis for the formation of a new type of cement such as high belite cement. The raw mix design was adjusted using pyrite as a correcting factor to keep the characteristics of the raw mix and the clinker similar to the factory data. KeywordsAlternative Fuel; Rice Husk; Natural Gas; Raw Mix Design; Clinker Mineral Composition; Raw Mix Parameters; Specific Heat Consumption Nomenclature ms1 = mass of the inlet raw mix to the preheater (kg) hs1 = specific enthalpy of the inlet raw mix to the preheater (kJ/kg) mf = mass of the fuel (kg) hf = low calorific value of the fuel (kJ/kg) hu = specific enthalpy of the fuel (kJ/kg) mA3 = mass of tertiary air from the cooler to the calciner (kg) hA3 = specific enthalpy of tertiary air from the cooler to the calciner (kJ/kg) mA5 = mass of secondary air from the precooling zone in the kiln (kg) hA5 = specific enthalpy of secondary air from the precooling zone (kJ/kg) mA6 = mass of secondary air from the cooler to the kiln (kg) hA6 = specific enthalpy of secondary air from the cooler to the kiln (kJ/kg) mG1 = mass of outlet flue gas from the preheater (kg) hG1 = specific enthalpy of outlet flue gas from the preheater (kJ/kg) ms6 = mass of clinker leaving the firing zone to the cooler (kg) hs6 = specific enthalpy of clinker leaving the firing zone (kJ/kg)  HR.c, HR.K = heat of reactions in the calciner and the kiln, respectively (kJ) Qw.p.c, Qw.k, Qw.c. = wall heat losses from the preheatercalciner, kiln, and cooler, respectively (kJ) Vaf = theoretical amount of fuel combustion air (Nm /kg fuel) Vgf = theoretical amount of fuel combustion gases (Nm /kg fuel)  = excess air factor for fuel combustion MCO2 = mass of evolved CO2 from calcination Cpg, Cpa = specific heat of flue gases and air, respectively (kJ/Nm 3 C) Tg, Ta = temperature of flue gas and combustion air, respectively ( C) h.f.o. = heavy fuel oil cli. = clinker LSF = lime saturation factor SIM = silica modulus AM = alumina modulus C3S = tricalcium silicate C2S = dicalcium silicate International Journal of Energy Engineering Apr. 2015, Vol. 5 Iss. 2, PP. 16-27 17 DOI: 10.5963/IJEE0502001 C3A = tricalcium aluminate C4AF = tetracalcium aluminate ferrite Borders of the heat balance zones, as shown in Fig. 2: [1] [2] = preheating zone [2] [3] = precalciner zone [3] [4] = bottom stage cyclone [4] [5] = heating zone of the rotary kiln [5] [6] = precooling zone [6] [7] = cooler

Mineralogical study of calcined coal waste in a pozzolan/Ca(OH)2 system

Activated carbon mining waste influences the formation of hydrated phases, their saturation indexes and mineral stability fields during pozzolanic reactions. The behavior of these reactions is predicted in this study by examining the influence of carbon waste at 600°C over 2h, by means of a thermodynamic model running on a computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations (PHREEQC program: pH-REdox-Equilibrium in Program C). Experimental analysis of the solid phases shows early formation of monosulfoaluminate hydrate, C–S–H phases and subsequent precipitation of laminar minerals, tetracalcium aluminate hydrate, layered double hydroxide compounds (phyllosilicate/carbonate) (LDH) and stratlingite. Monosulfoaluminate hydrate was formed on day one of the reaction for samples with activated coal mining waste. The thermodynamic calculations confirmed the experimental observations.

Chemical and mineral transformations that occur in mine waste and washery rejects during pre-utilization calcination

The mineralogical content of coal mine waste consists primarily of inorganic compounds which can be converted into a metakaolin-based product under controlled activation conditions that is also a highly pozzolanic material. Activation temperatures ranging from 500 to 900°C over 2h retention time affect the mineralogy of coal mine waste, as well as the formation and evolution of the hydrated phases that form during the pozzolanic reaction. The hydrated phases formed during the pozzolanic reaction in the activated coal mine waste (ACMW)/Ca(OH)2 system were C–S–H gels, stratlingite, tetracalcium aluminate hydrate, LDH compounds (phyllosilicate/carbonate) and monosulfoaluminate. Low temperatures (600°C) favored the formation of LDH compounds and stratlingite; whereas monosulfoaluminate formed during the hydrated phase at higher temperatures (900°C) during the first day of the pozzolanic reaction, and tetracalcium aluminate hydrate appeared as the dominant crystalline phase at 7 and 28days.

Palygorskite as an admixture to air lime–metakaolin mortars for restoration purposes

As mortars for rehabilitation purposes should have a similar composition to the historical materials, this work focussed on the preparation of mortars with binder:aggregate 1:3 volumetric ratio (vr) traditionally used in old buildings in central parts of Portugal. Mortars based on air lime binder with additions of 5mass% of palygorskite (Pal) and 20mass% of metakaolin (MKaol) were prepared to provide 1:3 vr, in which lime binder or sand was substituted.Needle-like Pal particles are functional as reinforcing fibres both in lime–Pal mortars and in air lime–MKaol–Pal systems. Chemical and mechanical properties of mortars were analysed at 28, 90 and 180days of curing. Pozzolanic activity of MKaol was revealed by the presence of tetracalcium aluminate hydrate C4AH13 and monocarboaluminate C4A C¯ H11 at early ages of curing. Top value of flexural strength at 180days shows lime–Pal mortar, in which sand was substituted by Pal, while the highest value of compressive strength was reached by lime–MKaol–Pal mortar, in which MKaol:lime mass ratio was 1:4.75. It was confirmed that differences in MKaol:lime mass ratios, even very small, cause considerable variances in the obtained mechanical properties.

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 asX‐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)2systems. 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.

The effect of elevated temperature on bond performance of alkali-activated GGBFS paste

The main reaction products were investigated by analysis of microstructure of alkali-activated ground granulated blast furnace slag (GGBFS) paste. An experimental research was performed on bond performance of alkali-activated GGBFS paste as a construction adhesive after exposure to 20–500 °C. Through XRD analysis, a few calcium silicate hydrate, hydrotalcite and tetracalcium aluminate hydrate were determined as end products, and they were filled and packed each other at room temperature. In addition, akermanite dramatically increased at 800 °C and above. The two key parameters, the ultimate load Pu.T and effective bond length Le, were determined using test data of carbon fiber-reinforced polymer (CFRP)-to-concrete bonded joints at elevated temperature. The experimental results indicate that the ultimate load Pu.T remains relatively stable initially and then decreases with increasing temperature. The effective bond length Le increases with increasing temperature except at 300 °C. The proposed temperature-dependent effective bond length formula is shown to closely represent the test data.

Evolution of Mineralogical Phases by27Aland29SiNMRinMK‐Ca(OH)2System Cured at 60°C

The evolution of the metastable phases in metakaolin/Ca(OH)2systems 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 theMK/Ca(OH)2system over 123 d at 60°C using nuclear magnetic resonance spectroscopy (27Aland29SiNMR). The results obtained by27Aland29SiNMRshow that during the first 30 h, the metastable phasesC2ASH8andC4AH13, 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.

Mechanism analysis and effect of styrene–acrylate copolymer powder on cement hydrates

In this paper a styrene–acrylate copolymer (SAE) powder was used to modify Portland cement paste. The concentration of the products of hydration was investigated using X-ray diffraction (XRD). Also the physical and chemical interactions occurring in the modification process were studied using Fourier-transform infrared spectrometer (FTIR). The results show that the addition of SAE powder facilitates the formation of ettringite (AFt). The hydration product tetracalcium aluminate hydrate (C4AH13) begins to appear in the cement paste after 7days, its formation slightly decreases with an increase in the SAE powder–cement ratio (mp/mc). Single-sulfur hydrated calcium sulphoaluminate (AFm) is also found in the cement paste beginning after 7days; however its content is very small. Also as the mp/mc is increased the amount of calcium hydroxide (Ca(OH)2) in the hydrating cement paste decreases, this effect is more pronounced at earlier ages. FTIR analysis indicates that Ca2+–carboxyl complexes form during the hydration of cement paste containing SAE powder.

Hydration products of lime–metakaolin pastes at ambient temperature with ageing

Mortars constituted of lime mixtures with pozzolanic additions have been extensively used in the past for the construction of historic and traditional buildings.This paper presents the results of blended pastes of lime and metakaolin (MK), namely compounds formed and their stability over time. This research is part of an extensive study aiming at the formulation of lime based mortars for restoration purposes.It has been shown for several years that MK has been applied in inorganic binders due to its capacity to react vigorously with calcium hydroxide (CH). In the presence of water originating a series of major hydrated phases, namely tetra calcium aluminate hydrate (C4AH13), calcium silicates hydrates (CSH) and calcium aluminium silicate hydrates (stratlingite – C2ASH8).Several blended pastes of lime and MK, with different substitution rates of lime by MK (wt%) were prepared and cured at a temperature of 20°C and relative humidity RH>95%.The phase composition of the formed hydrated phases was determined by X-ray diffraction (XRD) and simultaneous thermal analysis (TG–DTA).The obtained results showed that lime/MK pastes compositions displayed different reaction kinetics during curing time, being the pozzolanic products content directly proportional to the substitution rate of lime by MK. Also, a relationship between the increase stratlingite content and the MK substitution rate of lime by MK was found.

Influence of some admixtures on the formation of primary and secondary ettringite

Different types of admixtures (2% each of superplasticiser, barium nitrate and stearic acid) were added to examine their effect on the hydration reactions of pure tricalcium aluminate (C3A) in presence of gypsum (G). The effect of 5% magnesium sulfate solution on the formation of secondary ettringite was examined for different time intervals, 3, 7, 28, 60 and 90 days. The hydration products were investigated using X-ray diffraction, Fourier transform infrared and scanning electron microscopy. The time taken for ettringite formation was determined without admixture and in the presence of admixtures. The results showed that superplasticiser, barium nitrate and stearic acid retard ettringite formation whereas they accelerate the formation of calcium mono-sulfoaluminate hydrate phase and tetracalcium aluminate.

Blended cements containing high volume of natural zeolites: Properties, hydration and paste microstructure

In this study, properties and hydration characteristics as well as paste microstructure of blended cements containing 55% by weight zeolitic tuff composed mainly of clinoptilolite mineral were investigated. Free Ca(OH)2 content, crystalline hydration products and decomposition of zeolite crystal structure, pore size distribution and microstructural architecture of hydrated cement pastes were examined. Superplasticizer requirement and compressive strength development of blended cement mortars were also determined. The blended cements containing high volume of natural zeolites were characterized with the following properties; (i) no free Ca(OH)2 in hardened pastes at the end of 28 days of hydration, (ii) less proportion of the pores larger than 50 nm when compared to portland cement paste, (iii) complete decomposition of crystal structure of zeolite at the end of 28 days of hydration, (iv) presence of tetra calcium aluminate hydrate as a crystalline product of pozzolanic reaction, (v) more compatibility with the melamine-based superplasticizer when compared to the naphthalene based product, and (vi) similar 28 days compressive strength of mortars to that of reference portland cement.

Mineral phases in an activated kaolinitic waste blended cement system

Addition of pozzolans to blended cements is well known to have a positive effect due to the pozzolanic reaction. For some pozzolanic materials such as clays, the activation temperature plays an important role in the mineralogy and in the pozzolanic reaction. This paper reports the influence of the activation conditions (700 to 800 °C for 2 or 5 h in a furnace) of a clay waste on the mineralogy of a 20% blended cement matrix at 28 days of normalised curing. The mineralogical composition changes were analysed by X-ray diffraction (XRD) and scanning electron microscopy (SEM) — energy dispersive X-ray spectroscopy (EDX). The presence of an unstable LDH-like compounds (without magnesium) and the presence of tetracalcium aluminate hydrate (C 4AH 13) and calcium hydroxide, and the persistence of alite (Portland cement compound), was identified.

Novel Use of Kaolin Wastes in Blended Cements

This paper describes the influence of different thermally activated clay wastes (ACW) on the hydration phases in cement pastes containing two percentages of addition (10% and 20%). Results show that the main products obtained during hydration of cement pastes containing ACW were portlandite, calcium aluminate hydrates, calcium silicate hydrates, and hydrotalcite‐type compounds. Portlandite formation decreases when addition percentage is 20%, contrary to tetracalcium aluminate hydrate, which increases in similar conditions. The ACW that showed the most portlandite consumption was ACW1 (700°C, 2 h) according to thermogravimetric data.