Cemento adicionado con un residuo del proceso de craqueo catalítico ( FCC ) : hidratación y microestructura Blended cement containing fluid catalytic cracking catalyst residue ( FCC ) : hydration and paste microstructure

The effect of the incorporation of a petrochemical industry waste, named spent fluid catalytic cracking (FCC) in the hydration process and microstructure of cement paste, was studied. Portland cement (OPC) pastes, with and without addition of meta-kaolin (MK) and silica fume (SF), were used as reference materials. The characterization of the type of hydration products was performed by using X-ray diffraction (XRD), thermo-gravimetric analysis (TG), scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR). This study is complemented with the evaluation of compressive strength of blended cement mortars up to 90 days of curing. The main products of hydration in cement pastes, added with FCC, were calcium silicate hydrate (CSH); calcium aluminate hydrates (CAH) and hydrated calcium aluminosilicates (CASH). The addition of 10% of FCC reports lime consumption from 61% to 360 days of curing; this value is higher than the ones reported by the additions of MK and HS in the same percentage of addition. This indicates a higher reactivity of the FCC, which is manifested even at early ages. It should be noted that, considering the dilution factor, the addition of 10% FCC as cement replacement contributes to the strength of the blended material in the order up to an additional 30%. This is indicative of the potential use of this waste as supplementary material in cementitious mixtures.


Introduction
In the construction sector worldwide, Portland cement is the main cementitious material.However, its production requires the use of high temperatures, high energetic consumption, as well as the employment of fossil fuels and high consumption of natural resources, which affect environmental sustainability.Therefore, such process is considered as highly pollutant.This situation has led the scientific community and producer companies to consider the use of other less-pollutant energetic sources and, to incorporate blended material as a partial replacement of cement.In that sense, different kinds of blended material have been analyzed, considering that their addition to cement represents an economic, ecologic and technical solution (Carrasco et al., 2005).
Silvia Izquierdo, Jherson Diaz, Ruby Mejía, Janneth Torres Such blended material may come from different sources, either natural (pozzolans, limestone), natural and activated by temperature (clay, meta-kaolin) or from industrial byproducts (fly ashes, high furnace slag or rice husk) (Kumar and Monteiro, 2006).It is important to highlight that some of such products are considered as industrial waste materials.Their use as blended products has some environmental and economical benefits, since waste material is not dumped into a garbage deposit, thus increasing their aggregate value.Generally, blended products may contribute by modifying green mortars and concrete behavior, as well as their performance in the long term (ACI 201).Some alternative materials work as microfillers by increasing cement paste density and by delivering a dense and homogeneous microstructure in the paste-aggregate transition zone.They contribute to the permeability reduction, to increase mechanical properties and, in general, they facilitate the production of blended mortars and concretes of proper mechanical and durability properties.In some countries, specifications and standards have incorporated blended products by admitting them as part of the so-called blended cements.
Over the past few years, the use of a fluid catalytic cracking material has been reported (FCC, English initials).This residual material comes from petrochemical industry, where the virgin catalyst is used to refine crude for gasoline and other fuels production.The worldwide production is estimated in 400,000 tons/year approximately (Zornosa et al., 2008), which is mainly used by some refineries as inert element for roads paving and filling, as well as refill or aggregate for cement blocks.In Colombia the generation of FCC catalyst residue is estimated in 15,000 tons per year.In spite of being in contact with products of the petrochemical industry and, although some heavy metals have been found in its structure, this material is not environmentally dangerous; therefore, it is classified as an inert solid residue (Torres et al., 2012).
The FCC is a semi-crystal material formed by particles of spherical or spheroid shape, of highly porous nature, thus providing an increased specific surface which is quite typical of high reactivity (Chen et al., 2004;Paya et al., 2009;Tseng et al., 2005).The chemical composition of this spent catalyst is quite similar, no matter what its origin is.It is mainly formed by Al 2 O 3 within a range of 37 -46% and, SiO 2 within a range of 47 -58%.It has an average particle size ranging from 60-70 μm (Chen et al., 2004;Tseng et al., 2005;Torres et al., 2009;Paya et al., 2003;Paya et al., 2003a;Pinto et al., 2007).
Different studies suggest that the optimum FCC percentage, as cement blended material, is between 10 and 15%, values that increase cement compressive strength from 10% to 26%, respectively (Chen et al., 2004;Paya et al., 2009;Tseng et al., 2005).It is important to indicate that such increases may be observed at early ages, due to the effects of blended material on the acceleration reactions during the hardening process.
Although it is demonstrated that this is a highly reactive element, it is possible to increase its reactivity by means of thermal or mechanical activation processes (Paya et al., 2000).Furthermore, some researchers have demonstrated that blended mortars and cements with 20% FCC have a proper behavior, when facing chlorides and sulfates attacks (Pacewska et al., 2000;Zornosa, 2007).
The purpose of the present paper is to analyze the microstructural development and the hydration process of blended cement pastes at 10% and 20% FCC, as replacement for cement in the same proportions; consequently two widely studied pozzolans have been employed as reference materials: one of aluminum nature (meta-kaolin, MK) and the other o siliceous nature (silica fume, SF) containing 10% of blended material.

Materials and experimental procedure 2.1 Materials
For the execution of this study, a fluid catalytic cracking residue (FCC) was employed, which was supplied by a Colombian petro-chemical industry.Its physical and chemical characteristics are shown by Table 1. Figure 1 shows the pictures from scanning electron microscopy (SEM) for the FCC.It is demonstrated that particles have a spherical or spheroid shape, with a high porosity, which provides a high specific surface reflected by a high reactivity.

Blended pastes preparation and employed techniques
In order to monitor the hydration process, Ordinary Portland Cement pastes (OPC) were elaborated with and without blended materials.Pastes were prepared with a water/ binder ratio (w/B) of 0.35% and the replacement of cement by FCC, in the proportion of 10% and 20%.Besides non-blended Portland cement (OPC), cements were used with addition of meta-kaolin (MK) and silica fume (SF) in a proportion of 10%, as reference materials.The samples were cured at room temperature in saturated water with Ca (OH) 2 during different periods comprising 7, 28, 90, 180 and 360 days.When curing stage was completed, the samples were grinded and "frozen" by using acetone and ethanol, so as to avoid the progress of reactions.Finally the materials were characterized by their mineral and micro-structural composition by using the thermogravimetric analysis (TG), x-ray diffraction (DRX), nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM) analyses.Furthermore, the mechanical compressive strength for FCC blended cements was also assessed.

Discussion and results analysis 3.1 Study by means of Thermo-gravimetric analysis -Portlandite consumption
The thermo-gravimetric analysis (TG/DTG) was carried out by a TA Instrument device SDT Q600, at heating speed of 10°C/min, under a nitrogen atmosphere with a flow of 100 ml/min by employing an alumina crucible.Figure 2 shows the DTG curves of reference paste (100% OPC) and pastes blended with FCC, MK, SF at different curing ages, 7, 28, 90, 180 and 360 days.
The peak number 1 on Figure 2, located at a temperature between 115-120°C, corresponds to the dehydration of calcium silicate hydrate (CSH) and, it is overlapped with peak number 2 corresponding to ettringite, which is located at a temperature ranging from 100-180°C.Peak 3 in the range of 180-240°C is attributable to the dehydration process of calcium aluminate hydrates (CAH) and; hydrated calcium alumino-silicates (CASH).Finally peak number 4, around 400°C, corresponds to the de-hydroxylation of calcium hydroxide (CH), and a minor peak in the area at 700°C corresponds to the decomposition of blended carbonate for the test sample.It can be observed that as long as material hydration progresses some changes take place on the DTG graph.Such changes are provoked by the increase of hydration products.As from seven ( 7) curing days, it is observed that peak 3 is higher for FCC and MK blended pastes, due to the high alumina content (Table 1) and the progress of pozzolanic reactions.This behavior matches with the one reported by other researches (Pacewska et al., 1998;Pacewska et al., 2000;Borrachero et al., 2002;Dweck et al., 2008).DTG graphs allow us to conclude that hydration products obtained from FCC addition are quite similar to the ones reported by the reaction between meta-kaolin, silica fume and cement.
By analyzing the peak around 400°C, corresponding to the dehydration of calcium hydroxide (CH), it is observed that blended samples contain less amount of calcium hydroxide in comparison to non-blended samples, throughout all curing ages, therefore, proving the pozzolanic nature of FCC, MK and SF.In general, 10% FCC blended cement shows a lower amount of CH, in comparison to the ones containing MK and SF at the same percentage, thus showing a higher reactivity by FCC.
The progress of dehydration of blended cement is associated to the pozzolanic activity of blended material, which can be confirmed by the calcium hydroxide activity consumption assessed from the weight loss calculation between 400 and 500°C on Figure 2. Affixed lime percentage reported by FCC, MK and SF, due to pozzolanic reaction, was calculated by means of the Equation 1 (Tseng et al., 2005;Paya et al., 2003;Zhang et al., 2012).
Where (CH) 0 is the initial amount of Ca(OH) 2 contained by the paste OPC/FCC and; (CH) p is the amount of Ca(OH) 2 in the same paste, at a given age.
The results obtained by Equation 1 are presented by Table 2, which shows that 10% FCC blended cement at has the greatest reactivity, throughout different curing ages under study.Consequently, at 7, 28, 90, 180 and 360 curing days, consumptions of 38.84%, 44.07%, 46.46%, 57.71% and 61.44%, are reported respectively.Generally, such values are higher than the ones reported by MK and SF blended pastes.It is important to mention that the 20% FCC addition also reports higher lime consumption in comparison to other blended pastes, excepting for 90 and 180 days of curing, where the consumption was slightly lower.This behavior is in line with the one reported by other researchers (Torres et al., 2009;Pacewska et al., 1998;Pacewska et al., 2002).The values here presented explain some unusual results reported by former studies regarding curing ages up to 28 days (Trochez et al., 2010).Above results are in line with the ones obtained from mechanical compressive strength (see Figure 3), highlighting the performance of 10% FCC paste.

NMR -NMR 29 Si analysis on hydrated pastes
The 29 Si Nuclear magnetic resonance technique (NMR) is used to identify the level of silicate polymerization on the hydrated paste.The obtained spectrum signals depend on the length and angle of the Si-O-Si link, as well as on the adjacent atoms.Initially, the cement anhydrous correspond to Q0 (-66 -74 ppm), where silicon atom does not share oxygen with adjacent atoms.As long as hydration progresses, polymerization takes place delivering dimers (Q1, -75 -82 ppm) or chains (Q2, -85 a -95 ppm), making room for CHS.The replacement of Si in the tetrahedron by aluminum atoms (Qn (xAl)) displaces the spectrum signal more positive values.Figure 4 the NMR 29 Si spectrums on the reference paste and 10% FCC, MK and SF blended pastes at ages of 1, 7 and 90 curing days.All spectrums contain non-reactive anhydrous cement components at close resonances ranging from -71.3 ppm and -73.3 ppm attributable to belite (C3) and alite (C2), respectively.The spectrums also contain wide resonances, still unresolved, with peaks pertaining to Q n tetrahedrons of calcium silicate hydrate (CSH) (Kjorth et al., 1988;Brough et al., 1994;Cong et al., 1998).Resistencia a la compresión, MPa

days 60 days 90 days
This phase resonances are in the SiO 4 tetrahedrons groups located at the end of chains (Q1, a -78ppm), at the center chains (Q 2 , a -85 ppm) and, located at bridging position (Q 2p , a -82 Finally is a peak at ppm, pertaining to the resonance of a SiO 4 tetrahedron and AlO 4 tetrahedron the closest neighbor (Q ( 1Al)) (Richardson et al., Andersen et al., 2003).

days (Ref-Cem) c). 90 days (Ref-Cem) d). 1 day (FCC-Cem) e). days (FCC-Cem) f). 90 days (Ref-Cem) g). 1 day (MK-Cem) h). days (MK-Cem) i). 90 days (MK-Cem) j). 1 day (SF-Cem) k). days (SF-Cem) l
). 90 days.C2: bicalcium silicate (SC 2 ), C3: tricalcium silicate (SC 3 ) By raising curing ages, a decrease of peaks C2 and C3 is observed, followed by an increased amount of SiO 4 Q 1 , Q 2 (1Al), Q 2 and Q 2p , which correspond to the CSH phase, thus confirming the progress of hydration process.Particularly, the highest reaction level is observed on FCC and MK blended pastes at 90 curing days.It is worth mentioning that in such a case, Q (1Al) units are highly increased.Such behavior is derived from the high percentage of Al 2 O 3 available in the chemical composition of such blended pastes, in comparison to pattern pastes and SF pastes.By increasing the level of Si by Al in bridging positions, in CSH chain, may lead to the acceleration of Belite hydration speed (Andersen et al., 2004).

NMR -NMR 27 Al analysis on hydrated pastes
NMR 27 Al studies on hydrated cement pastes allow us to monitor calcium aluminate hydration phases, by means of the observation of combined tetrahedral Al samples, Al (IV); and octahedral, Al (VI) (Andersen et al., 2003;Faucon et al., 1998;Faucon et al., 1998a).Figure 5 presents NMR 27 Al spectrums of blended pastes at 1, 7 and 90 curing days, where a signal of about 13 ppm is available for all the spectrums corresponding to Al(VI) of ettringite (Brunet et al., 2010).The presence of such phase is proven by the results obtained from DRX analysis.On the other hand, a resonance close to 74 ppm, which is attributed to the Al(IV) added to CSH chains, shall be considered.The replacements of Si by Al in the chain tetrahedrons are supported by the observation of Q 2 (1Al) on NMR 29 Si.This behavior is prominent at 90 curing days, as much for reference pastes as for different blended pastes.Some researches state that the Al(IV) resonance of tri-calcium aluminate (C 3 A) on anhydrous cement is close to 80 ppm, followed by displacement of lower values during the progress of hydration process, mainly as a consequence of Al(IV) in the tetra-calcium ferro-aluminate (C 4 AF).This behavior was observed in all analyzed spectrums (Brunet et al., 2010). 1 día (Cem-HS) k).7 días (Cem-HS) l).días

X-ray diffraction analysis
Figure 6 present the results of X-ray diffraction technique.This Figure shows that all diffraction patterns are similar; however the intensity of blended products is different.
Silvia Izquierdo, Jherson Diaz, Ruby Mejía, Janneth Torres In relation with curing ages, the same behavior was observed for the portlandite peak, located at a value of 2 around 17.8.At early ages, a marked decrease is observed for FCC and MK blended pastes, which behavior is due to similarities of chemical composition.At 90 curing days, blended samples show a significant portlandite decrease comparison to pattern pastes, thus matching the results reported by Hsu et al. (2001).Reduced peak intensity, located at a value of 2 around 8.6, corresponds to the ettringite phase.

Scanning electron microscopy analysis
Figure 7 presents the SEM micrographs of reference cement pastes and FCC, MK and FS blended pastes cured at 3 days.Such micrographs confirm the presence of hydration products found by the X-ray diffraction analysis.Small needles are observed in the reference cement pastes and SF blended pastes, thus indicating the formation of ettringite; however, FCC and MK blended pastes have a higher amount of ettringite and they contain calcium aluminate hydrates of hexagon shape, due to a high content of Al 2 O 3 .In general all pastes show the formation of calcium silicate hydrate gel.

Conclusions Based on the results of the current research:
• The potential of FCC as pozzolanic material was proven in Portland cement matrixes, showing the highest reactivity in 10% blended samples, with a proportion of affixed lime of 61% at 360 curing days.This value exceeds the ones reported by FS and MK blended pastes at the same percentage.
• By means of different characterization techniques, it was determined that the main hydration products for FCC blended cement are: calcium silicate hydrate (CSH), ettringite, aluminate hydrates (CAH) and hydrated calcium aluminosilicates (CASH).
It is worth mentioning the possibility of using to a 20% FCC in replacement of Portland cement, as this mixture presents a similar behavior as 10%MK and 10%SF blended pastes.

Acknowlegments
The authors of this study -members of Composite Material Group at Universidad del Valle, and Materials and Environment at the Universidad Nacional, Palmira -wish to express their gratitude to their respective universities, to the Excellence Center of New Materials (CENM-Univalle) and to the Administrative Department of Science and Technologies (Colciencias) for the support provided for the development of this research.

Table 1 .
They were used as reference material, for comparison purposes only.
Tabla 1. Características químicas y físicas de los materiales utilizados Table 1.Chemical and Physical Characteristics of employed material

Table 2 .
Porcentaje de cal fijada para cada tipo de adición Percentage of affixed lime for each blended material