Pozzolanic properties of a residual FCC catalyst during the early stages of cement hydration

The catalyst used in fluidized catalytic cracking (FCC) units of refineries after several recovery cycles in regeneration units, reduces its activity and it is partially substituted by new catalyst in the process. As it has a high silicon and aluminum oxides content, the pozzolanic properties of a Brazilian FCC spent residual catalyst, used in different substitution degrees to cement, were evaluated by three thermal analysis techniques during the early stages of hydration of a type II Portland cement. NCDTA curves show in real time that the residual catalyst, accelerates the stages of cement hydration. TG and DSC curves of respective pastes after 24 h of hydration evidence the pozzolanic activity of the waste, respectively, by the lower water mass loss during the dehydroxylation of the residual calcium hydroxide and by the lower dehydroxylation endothermal effect. Within the analyzed period, the higher is the cement substitution degree, the higher is the pozzolanic activity of the residual catalyst.     

Swelling effects in cross-linked polymers by thermogravimetry

A Brazilian coal power plant generates a waste composed by the fly and bottom ashes produced from coal combustion and by a spent sulfated lime generated after SO₂ capture from combustion gases. This work presents a study of the early stages of the hydration of composites formed by this waste and a type II Portland cement, which will be used for CO₂ capture. The cement substitution degrees in the evaluated composites were 10, 20, 30 and 40%, and the effect of the coal power unit waste on the hydration reaction was analyzed on real time by NCDTA, during the first 40 h of hydration. The results show that the higher is the substitution degree, the higher is the retarding effect on the cement hydration process. Actually, by respective thermogravimetric (TG) and derivative thermogravimetric (DTG) analysis on initial cement mass basis, this effect is caused by double exchange reactions among Ca and Mg components of the waste, during the first 4 h of hydration, which promote a much higher exothermic effect in the NCDTA curve, simultaneously to respective induction periods. The pozzolanic reactions, due to the presence of the waste silica and alumina containing amorphous phases, consume part of the original Ca(OH)₂ content existent in the waste in the case of 30 and 40% substituted pastes, and also from part of the Ca(OH)₂ produced in cement hydration reactions, in the case of the 10 and 20% substituted pastes.     

Importance of quantitative thermogravimetry on initial cement mass basis to evaluate the hydration of cement pastes and mortars

Thermogravimetric analysis (TG) curves of cement pastes and mortars are obtained by default on their respective initial sample mass basis. This fact does not allow a direct comparison of TG data of percentual mass losses due to the dehydration of a same hydrated component of differently aged pastes or mortars of same cement because the initial masses of the differently aged sample usually have different initial compositions. To solve this problem, one can transform the original thermal analysis curves from the initial sample mass basis to the initial cement mass basis, to have the same composition basis for any hydration time. This paper presents in detail how this can be done graphically and analytically and applies the method to study the evolution of cement hydration during the first 28 days of pastes and mortars prepared from the same type II cement. It also shows how to compare quantitatively the main cement hydrated phases formed during solidification and setting processes of pastes and mortars with different initial compositions as a function of hydration time.     

Simultaneous effect of silica fume,metakaolin and ground granulated blast-furnace slag on the hydration of multicomponent cementitious binders

Portland cement was partially replaced by metakaolin (MK), silica fume (SF) and ground granulated blast-furnace slag (BFS). Globally, two amounts of SF (5 and 10 mass%) and total substitution level of 35 mass% were used to prepare blended samples. Their early and 28 days hydration was studied by means of isothermal calorimetry and thermal analysis. Developed phase composition was assessed using compressive strength measurements. Acceleration of cement hydration in early times was proved and reflected higher amounts of finer additives. Despite dilution effect, the presence of more reactive SF and MK resulted in pozzolanic reactions manifesting already before 2 days of curing and contributing to the formation of strength possessing phases. The influence of BFS addition showed later and thanks to the synergic effect of all the used additives; it was possible to increase its content up to 25 mass% by keeping the compressive strength values near that of referential one.

     

A study of CO<Subscript>2</Subscript> capture by high initial strength Portland cement pastes at early curing stages by new non-conventional thermogravimetry and non-conventional differential thermal analysis

Although the literature presents intensive studies based on monitoring cement hydration in adiabatic and semi-adiabatic environments, such as non-conventional differential thermal analysis (NCDTA) systems, studies of cement hydration in controlled climatic chambers are very rare. Using three W/C ratios (0.5, 0.6 and 0.7) and three relative humidity conditions (60, 80 and 100%) at 25 °C, the authors analyzed in real time the evolution of cement hydration reaction during the first 24 h in an environmental-controlled chamber. The main objective of this paper is to present two new developed systems of NCDTA (NNCDTA) and non-conventional TG and to show, using high-strength sulfate-resistant Portland cement pastes in a controlled chamber as application examples, how the developed systems measure on real time the thermal effects and the mass changes that occur during hydration and carbonation reactions. The captured CO₂ mass can be quantified as it occurs by carbonation curves. The results are in agreement with the mechanical and structural properties of the used pastes and with their TG/DTG data, after being processed by different operational conditions. Carbonation for 24 h alters significantly the cement hydrated paste composition, resulting in final poor mechanical resistance properties. However, carbonation for 1 h, in specific conditions, enhances them when compared to a non-carbonated reference paste, due to a final higher content of silica and alumina hydrated phases and to a lower mass ratio between that of their combined water and their total mass as well.     

Pozzolanic reaction of a spent fluid catalytic cracking catalyst in FCC-cement mortars

This work presents the relation between the pozzolanic activity, the hydration heat and the compressive strength developed by blended mortars containing 10 and 35% of a spent fluid catalytic cracking catalyst (FCC). The results show that, in comparison with 100% Portland cement mortar, a mortar with 10% FCC increases the hydration heat all over the period of testing. This hydration heat increasing is due to the pozzolanic effect, therefore the resulting compressive strength is higher than the reference mortar. Whereas, in a mortar with 35% of FCC, the hydration heat is higher than 100% PC mortar, until 10 h of testing. After this age, the substitution degree predominates over the pozzolanic activity, showing in this case, lower hydration heat and developing lower compressive strength than 100% PC mortar.     

Study of a Brazilian spent catalyst as cement aggregate by thermal and mechanical analysis

Fluidized catalytic cracking units of refineries normally use zeolite catalysts to treat heavy oil fractions. This catalyst is regenerated continuously, but due to the reduction of its activity during the process, it is partially substituted by a new catalyst make-up. The spent residue has a high content of silicon and aluminum oxides and usually presents pozzolanic properties. This paper presents the study of a Brazilian spent catalyst, which is being tested as a pozzolanic aggregate in partial substitution to cement. Pastes were prepared with 15, 20 and 25% in substitution to cement mass and analyzed after 28 days of hydration. Hydrated paste samples were analyzed by simultaneous thermogravimetry and differential thermal analysis, to quantify the calcium hydroxide consumption, as well as the content of other main hydrated cement phases. Compressive strength analysis was also performed after 28 days of hydration. Although, as spent catalyst content is increased, the pozzolanic activity is confirmed by the increase of calcium hydroxide consumption on cement mass basis, unlikely to other studied spent FCC catalysts, tested for the same purpose, the compressive strength of respective paste specimens decreases, due to the increase of other hydrated phases formation.     

Artificial pozzolanic cement pastes containing burnt clay with and without silica fume

Pozzolanic cement blends were prepared by the partial substitution of ordinary Portland cement (OPC) with different percentages of burnt clay (BC), Libyan clay fired at 700 °C, of 10, 20, and 30%. The pastes were made using an initial water/solid ratio of 0.30 by mass of each cement blend and hydrated for 1, 3, 7, 28, and 90 days. The pozzolanic OPC–BC blend containing 30% BC was also admixed with 2.5 and 5% silica fume (SF) to improve the physicomechanical characteristics. The hardened pozzolanic cement pastes were subjected to compressive strength and hydration kinetics tests. The results of compressive strength indicated slightly higher values for the paste made of OPC–BC blend containing 10% BC The results of DSC and XRD studies indicated the formation and later the stabilization of calcium silicates hydrates (CSH) and calcium aluminosilicate hydrates (C₃ASH₄ and C₂ASH₈) as the main hydration products in addition to free calcium hydroxide (CH). Scanning electron microscopic (SEM) examination revealed that the pozzolanic cement pastes made of OPC–BC mixes possesses a denser structure than that of the neat OPC paste. Furthermore, the addition of SF resulted in a further densification of the microstructure of the hardened OPC–BC–SF pastes; this was reflected on the observed improvement in the compressive strength values at all ages of hydration.     

Microstructure of Portland cement mortar amended by burnt kaolin sand

Two types of raw materials, original kaolin sand OKS I and OKS II were used for experiment. They were transformed (1 h at 650 °C with 10 °C/min temperature increase) to burnt kaolin sand (BKS I and BKS II) with pozzolanic properties. Contents of decisive mineral—metakaolinite—in BKSs are as follows: BKS I (fraction below 0.06 mm) 20%; BKS II (fraction below 0.06 mm) 36% and BKS II (fraction below 0.1 mm) 31% by mass. Mortars with blends of Portland cement (PC) and BKS were prepared announced as: MK I (0.06) with 5 and 10% cement substitution by metakaolinite; MK II (0.06) with 5 and 10% cement substitution by metakaolinite and MK II (0.1) with 5, 10, 15 and 20% cement substitution by metakaolinite. The reference mortar with 100% of PC was made for comparison. All mortars were adjusted on the constant workability 180 ± 5 mm flow. Besides significant increase in compressive strengths—the refinement of pore structure in mortars with BKS connected with decreases in permeability and Ca(OH)₂ content were revealed. The above facts confirm pozzolanic reaction of BKS in contact with hydrated PC and indicate perceptiveness of BKS for the use in cement-based systems as a pozzolanic addition.     

Simultaneous solidification of two catalyst wastes and their effect on the early stages of cement hydration

Two catalyst wastes (RNi and RAI) from polyol production were considered as hazardous, due to their respective high concentration of nickel and aluminum contents. This article presents the study, done to avoid environmental impacts, of the simultaneous solidification/stabilization of both catalyst wastes with type II Portland cement (CP) by non-conventional differential thermal analysis (NCDTA). This technique allows one to monitor the initial stages of cement hydration to evaluate the accelerating and/or retarding effects on the process due to the presence of the wastes and to identify the steps where the changes occur. Pastes with water/cement ratio equal to 0.5 were prepared, into which different amounts of each waste were added. NCDTA has the same basic principle of Differential Thermal Analysis (DTA), but differs in the fact that there is no external heating or cooling system as in the case of DTA. The thermal effects of the cement paste hydration with and without waste presence were evaluated from the energy released during the process in real time by acquiring the temperature data of the sample and reference using thermistors with 0.03 °C resolution, coupled to an analog–digital interface. In the early stages of cement hydration retarding and accelerating effects occur, respectively due to RNi and RAl presence, with significant thermal effects. During the simultaneous use of the two waste catalysts for their stabilization process by solidification in cement, there is a synergic resulting effect, which allows better hydration operating conditions than when each waste is solidified separately. Thermogravimetric (TG) and derivative thermogravimetric analysis (DTG) of 4 and 24 h pastes allow a quantitative information about the main cement hydrated phases and confirm the same accelerating or retarding effects due to the presence of wastes indicated from respective NCDTA curves.     

Middle stage of Portland cement hydration influenced by different portions of silica fume,metakaolin and ground granulated blast-furnace slag

Present study deals with the influence of metakaolin (MK), silica fume (SF) and ground granulated blast-furnace slag (BFS) on middle hydration of ordinary Portland cement replaced by 45 mass% of particular supplementary cementitious materials (SCMs). Acceleration of cement hydration by SF and MK was proved up to the first 12 h by isothermal calorimetry as well as by thermogravimetric analyses. From the beginning of deceleratory period, when SCMs stopped to act as accelerators, more evident influence of the dilution effect was observed. Nevertheless, the presence of pozzolanic reactions was demonstrated already after 15 h of curing and even when SF and MK were used in the amount equal to 5 mass%. Synergic effect of the used SCMs allowed to increase the quantity of BFS up to 35 mass% without significant changes in their positive action.

     

Early stages hydration of high initial strength Portland cement

This work complements a quantitative thermogravimetric study of the first 24 h of hydration of a high initial strength and sulphate resistant Portland cement (HS SR PC) using non-conventional differential thermal analysis (NCDTA) and Vicat needle method. Different water/cement (W/C) ratios from 0.35 to 0.85 were used to evaluate the most indicated operating conditions to maximize calcium hydroxide production for further use in CO₂ capture. Thermogravimetric analysis data performed at 4 and 24 h of hydration were also compared to the NCDTA and Vicat data for each kind of paste, to analyze the influence of the W/C ratio on the simultaneous hydration and setting process. The increase of the W/C ratio increases the induction time retards the solidification and setting processes but increases the hydration degree as the W/C ratio is increased from 0.45. At 24 h, products prepared with 0.35 W/C ratio present a little higher hydration degree than those prepared with W/C = 0.45, because of the highest level of temperature in the reacting mixture in the former case, during the first 8 h. There is a practical limit of W/C = 0.66 to prepare the pastes, due to a limit of the miscibility between HS SR PC and water, above which, the excess of water forms a separated phase that does not interfere in the hydration process.     

A Study of the Hydration of Lime-Pozzolan Binders

DTA/TG and TG/DTG thermal studies and XRD investigations were carried out on pastes of lime-pozzolan binders to examine the relative hydration process. The binders were prepared with two natural pozzolans and hydrated lime, mixed together in different proportions. The main hydrated phases formed in the pastes are calcium silicate hydrate (CSH) and mono-carboaluminate. The growth of CSH was greater for the paste of the pozzolan richer in reactive constituents. For pastes with a higher lime/pozzolan ratio, a slowing of the formation of CSH was observed, while the formation of monocarboaluminate was more intense. The calcium hydroxide contents of the pastes obtained by thermogravimetric analysis made it possible to determine the pozzolanic activities of the two pozzolans under examination. The mortars for the various lime-pozzolan binders displayed characteristic hardening. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermogravimetric investigation on the chloride binding behaviour of MK-lime paste

The hydration products of 2.5, 5 and 10% Cl⁻ containing metakaolin (MK)-lime pastes are compared with the same obtained from MK-lime paste to understand the chloride binding behaviour of MK during the hydration of cement. Results indicate that 2.5% Cl⁻ addition into the MK-lime paste initially enhances the formation of Friedel's salt (Ca₂Al(OH)₆Cl·2H₂O), but Friedel's salt decomposes at later stages due to the formation of stratlingite (C₂ASH₈). In 5 and 10% chloride containing pastes, Friedel's salt is observed throughout the reaction periods along with the high amount of CSH. Small amount of stratlingite is also formed on or after 60 day hydration of 5% Cl⁻ containing MK-lime pastes. On the other hand, MK-lime-10% Cl⁻ containing pastes show the complete absence of stratlingite and C₄AH₁₃ through out the hydration period, which are the major hydration products of MK-lime paste. Mesuarements of pH of the simulated pore fliuds help to understand the decomposition behaviour of Friedel's salt. From the experimental results, chloride binding mechanism of MK-lime paste is also discussed.     

Effect of nano-metakaolin addition on the hydration characteristics of fly ash blended cement mortar

The effect of nano-metakaolin (NMK) addition on hydration characteristics of fly ash (FA) blended cement mortar was experimentally investigated. The amorphous or glassy silica, which is the major component of a pozzolan, reacts with the calcium hydroxide liberated during calcium silicate hydration. It is believable to add FA and NMK particles in order to make high performance concrete. The physico-mechanical properties of FA blended cement mortars made with different percentages of NMK were investigated. The experimental results showed that the compressive and flexural strengths of mortars containing NMK are higher than those of FA blended cement mortar at 60 days of hydration age. It is demonstrated that the nanoparticles enhances strength than FA. In addition, the hydration process was monitored using scanning electron microscopy and thermal gravimetric analysis (TG). The results of these examinations indicate that NMK behaves not only as a filler to improve microstructure, but also as an activator to promote the pozzolanic reaction.     

Influence of the calcined paper sludge on the development of hydration heat in blended cement mortars

The present study is based on the influence of the addition of a pozzolanic material as a result of the activation of an industrial waste coming from the Spanish paper industry on the heating as well as hydration heat of the cement mortars made with 10 or 20% of active addition. Once the sludge has been calcined at different temperatures (700–800°C) and stays in furnace (2 and 5 h), the calcined products showed high pozzolanic activity. The maximum activity corresponded to the paper sludge calcined at 700°C for 2 h (S1). Besides, it can be proved that there was an increase both of the heating and also of the hydration heat in the first 23–25 h for both additions (10 and 20% of S1) regarding the reference cement mortar. This behaviour would be related to the influence of different effects: filler and pozzolanic during the first hours of reaction, and by the dilution effect for longer hydration times, mainly when 20% of S1 was added.     

Physical and chemical studies on cement containing sugarcane molasses

Molasses is generally used as a grinding aid in cement and as a water reducer and retarder in concrete. In China, the output primarily consists of sugarcane molasses. In this paper, the effects of sugarcane molasses on the physical performance and hydration chemistry of conventional Portland cement were investigated. The setting times, the normal consistency of cement pastes, the compressive strengths and fluidities of the mortars were respectively determined according to Chinese Standard GB/T 1346, GB/T17671 and GB/T 2419. The effect of molasses on the hydration kinetics of cement was investigated using a calorimeter. The hydration products and pore size distribution of the cement pastes were analysed by X-ray powder diffraction, differential scanning calorimetry and a mercury injection apparatus. The results show that a small amount of sugarcane molasses retards the setting and hardening of cement paste and increases the fluidity of cement mortar, while excess molasses accelerates the setting and hardening. Molasses improves significantly the compressive strength at 3d due to the decrease of porosity. The addition of 1.0 % molasses accelerates the formation of ettringite, prevents the second hydration of aluminate phase and delays the hydration of C₃S.     

Evaluation of pozzolanic activity of natural and artificial pozzolans by thermal analysis

In this research, the pozzolanic activity of natural and artificial pozzolan used for preparation of restoration mortars was evaluated. For this purpose, several pastes were prepared, by mixing two artificial pozzolans and a natural one with commercial hydrated lime, in different ratios. The pastes were cured in standard conditions (RH = 98%, T = 25 °C). The pozzolanic activity was evaluated by using simultaneous differential thermal and thermogravimetric analysis (DTA/TG) after curing for 3, 7, 14, 28 days. The obtained results revealed that the various lime/pozzolan pastes displayed different reaction kinetics and therefore the various pozzolans present different reactivity, in proportion to its mineralogical, physical and chemical characteristics.     

Heat of hydration of high reactive pozzolans in blended cements: Isothermal conduction calorimetry

A study was carried out comparing silica fume (SF) and dealuminated kaolin (DK) as pozzolanic materials in blended cements. Ten, 20 or 30 wt% of SF or DK were substituted for Portland cement. The kinetics of hydration up to 45 h were studied using isothermal conduction calorimetry. Blends containing pozzolanic materials usually have decreased heats of hydration compared to pure cement during the period of C₃S hydration, i.e. during the main hydration peak. Depending on the chemical composition and the activity of the pozzolan, the reaction taking place with the lime typically contributes to the heat output after the main hydration peak.The pozzolanic activity of DK is the principal factor and heat evolution increases with respect to pure PC mortar, during the first 15 h. The presence of hydrated silica (silanol groups) in DK increases the pozzolanic activity especially before and during induction period. The acidic silanol sites are capable of a fast acid-base reaction with the alkalis and with any Ca(OH)₂ present in cement during the induction period.     

Evaluation of pozzolanic activity and physico-mechanical characteristics in ceramic powder-lime pastes

Ceramic powder has been used as an artificial pozzolanic addition, in preparing pozzolanic mortars for the historic/traditional structures’ construction. In order to evaluate the pozzolanic activity of ceramic powder, several pastes were prepared, by mixing it with hydrated lime, in different ratios. The pastes were stored in standard conditions (RH=99±1%, T=25±1°C) and evaluated using thermal analysis (DTA/TG), X-ray diffraction (XRD), compressive strength tests and mercury intrusion porosimetry (MIP), in time. The obtained results revealed that the compounds formed were CSH and C₄ACH₁₁ (monocarboaluminate) after 270 days of curing. The calcium hydroxide consumption increases as the initial amount of the ceramic powder in the paste augments. The maximum strength development is obtained for ceramic powder/hydrated lime ratio 3:1.