Physical ^ mechanical and microscopic characterization of cold bonded fly ash lightweight aggregates

El estudio presenta los resultados obtenidos de ensayos realizados sobre áridos ligeros de cenizas volantes fabricados con tres tipos de conglomerantes: cal, cemento Portland puro y cemento portland puzolánico. El proceso de fabricación empleado para la elaboración de los áridos consistió en un sistema de aglomeración por agitación bajo condiciones de endurecimiento enfrío. Los ensayos efectuados comprenden la caracterización física de los áridos, la determinación de las propiedades mecánicas por medio de un ensayo de trituración y, finalmente, el estudio de la microestructura de los áridos con técnicas de microscopía de barrido (SEM) y energía dispersiva de rayos X (EDX). Desde el punto de vista de resistencia mecánica los resultados indican que el mejor tipo de árido es el fabricado con un 5% de cemento portland puzolánico. Este resultado se confirma con el análisis de la microestructura que demuestra que este tipo de árido presenta la mayor velocidad de hidratación. SUMMARY

two subjects have been studied in depth (1,2,3); however, though the lightweight aggregate production has been developed in a theoretical and practical way (4,5), the literature available with regard to innovations and/or microstructure, and its relationship with the properties of the final product, is scanty.On the other hand, the most common technologies for the production of the fly ash lightweight aggregates (FALA), are those based on hydrothermcd and sintering processes (4,7), which call for a high consumption of energy.Therefore, it is necessary to investigate the proportions and design of mixtures and procedures of manufacture of the aggregates, relating these variables with the properties and structure of the aggregates obtained and their possible interaction in concrete.This information would allow to obtain a product that not only meets the technical requirements (physical and mechanical properties), but also the current demands for clean technologies, with low energy consumption and minimum environmental impact which favor sustainable development (6).
In order to know and optimize mixtures for the production of FALA, the present investigation was carried out using clean technologies and three types of binders.
The physical characterization of the aggregates included determining specific gravity and absorption.The mechanical properties were evaluated through the Crushing Value at a specific load.
Finally, in order to characterize the microstructure of the aggregates, scanning electron microscopy (SEM) cmd energy dispersive X ray (EDX) were carried out, from the point of view of the hydration state of the aggregate at 7 and 28 days of age.

EXPERIMENTAL PROCEDURE 2,1. Materials
The Fly Ash (FA) used in the investigation was obtained from the combustion of bituminous coal generated in the thermoelectric plant of Electroandina, Chile.This material is collected by means of electrostatic precipitators.The chemical composition and physical characterization of the fly ash are shown in Tables 1  and 2, respectively.According to ASTM C6J8-93 standards, the fly ash can be classified as Class F X-ray Diffraction (XRD) analysis was carried out to., determine the mineralogical composition of fly ash.Los conglomerantes empleados fueron: Cal (L), cemento Portland (P) y cemento Portland Puzolánico (PP) con un 19% de puzolana natural.En la Tabla 2 se presentan las propiedades físicas de estos materiales.

Study of mixtures
The literature reports the percentage of binders used in fly ash mixtures for the production of aggregates ranging from 5% to 10% of the total weight of cementitious material (4,8,9), The mixtures design for the present study considered binders in proportions of 3, 5 and 7% in weight of total weight of cementitious material (FA + binder).

Two tests were developed with the purpose of selecting the best aggregate according to the type of binder: Setting Time (ST) and Modified Pozzolanic Activity Index (MPAI). Each test was carried out on 3 twin specimens obtained of correspond mixtures.
The objective of the setting time test was to determine the lowest setting time of the designed mixtures.This test was carried out in accordance to ASTM CI91-99.
On the other hand, the Pozzolanic Activity Index (PAI) test specified in ASTM C311-92 aims to determine the fly ash capacity to fix calcium hydroxide (Ca(OH)J and therefore to harden and develop strength when mixed with Lime.Because new binders were considered for the aggregate manufacture, a Modified Pozzolanic Activity Index (MPAI) test was proposed to represent de áridos, se propuso un ensayo denominado índice Modificado de Actividad Puzolánica (MPAI) con el objeto de representar el comportamiento de las mezclas a usar.El procedimiento normalizado fue modificado en los siguientes aspectos: materiales constituyentes y sus proporciones, y forma y dimensión de las probetas.No se usó arena en las mezclas por no considerarse este material en la futura fabricación de áridos.Los ensayos a compresión se realizaron sobre probetas cilindricas de 50 X 100 mm.Tanto el almacenamiento de las probetas como el procedimiento de ensayo obedecieron a las especificaciones entregadas en la norma.El requerimiento de agua de cada mezcla se determinó según procedimiento ASTM Cl 87-98.
the actual behavior of the mixes to be used.The standard procedure was modified in the following aspects: materials constituents and proportions, and specimen shape and dimension.Sand was not used in the mixes.Compressive strength tests were carried out on 50 X J 00 mm cylinder specimens.Storage and testing of the specimens was peiformed according to the specifications in the standard test.The fly ashbinder blend considered the following binders content: 3, 5 and 7% by weight of binder The water requirement for each mix was determined according to ASTM C187-98.

Fly ash lightweight aggregates
The production ofFALA was made using a pan disc for fly ash agglomeration by the agitation method.The ''green pellets'' were submitted to a hardening process denominated ''cold bonded'', in other words, in outdoors exposure and temperature, subjecting it to a system of curing that consisted in spraying water at regular intervals.This was done in order to reproduce the real conditions at an industrial scale production without necessity of excessive consumption of energy.
The following tests were carried out to determine the mechanical and physical properties of the aggregates: • Physical properties.Specific gravity and absorption.

' Mechanical properties. The Aggregate Crushing Value test (ACV^.) was performed according to BS 8]2-75 in what respects equipment and rate of load application. However, due to the weakness of the aggregates, a modified load of 25 KN was considered. This decision was taken in order to allow performing the test after the first day of production of the aggregates. The load of 25 KN was adopted based on results of previous investigations on lightweight aggregates (JO). The ACV^. test was carried out at the following ages: 7 and 28 days. These tests were peiformed on two twin samples by type of aggregate.
Additionally, scanning electron microscopy (SEM) analyses were also carried out to determine the degree of hydration of the aggregates as a function of binder type and age, complementing the observation with the EDX analysis.Both tests were made at 7 and 28 days of age.

Fly ash
The results of XRD studies (Table 3) show that the mineralogical composition of FA presents the main necessary' compound for the formation of the hydration products, principally Calcium Silicate Hydrate (C-S-H), a fundamental compound in developing the mechanical strength of the aggregate.

Efecto del tipo y dosis de conglomerante sobre el comportamiento de las mezclas de cenizas volantes
La Figura 1 muestra el efecto del tipo y dosis de conglomerante sobre la actividad puzolánica de las cenizas volantes.Se observa que, independiente del tipo de conglomerante, a mayor proporción de éste mayor es la resistencia a compresión de las mezclas de cenizas.Además, al considerar la cal como agente enlazante, se tiene que a mayor contenido de cal menor es el tiempo de fraguado.En cambio, al considerar el efecto de los cementos, los resultados indicarían que existe una dosis óptima para la cual se minimiza el tiempo de fraguado.

Effect of the type and content of binder on the behavior of the fly ash mixtures
Figure 1 shows the effect of the type and proportion of the binder on the Pozzolanic activity^ of FA.It is observed that, independent of binder type, the larger the binder content, the larger the compressive strength of the fly ash mixes.Also, in the case of Lime, the larger the Lime content, the smaller the setting time, while for binders based on cements the results appear to indicate that there is an optimum dosage that minimizes the setting time.
To select the best mix for aggregate manufacture the setting time was defined as the main variable because it showed a higher sensibility than the results determined by the MPAI test.For lo tanto, de la Figura 1 se puede concluir que las dosis óptimas de conglomerante, de acuerdo al tipo considerado y tomando como referencia el menor tiempo de fraguado, son: -RA. + Cal = 7% en peso de cal (L7).-F.A. + cemento Portland = 5% en peso de cemento Portland (P5).-F.A. + cemento Portland Puzolánico = 5% en peso de cemento Portland Puzolánico (PP5).

Propiedades físicas y mecánicas de los FALA
En la Tabla 4 se indican las características físicas de los FALA.Se observa que el peso específico se encuentra en el rango comprendido entre 1,222 y 1,274, Therefore, from Figure 2 it can be concluded that the best binder dosage according to binder type, taking into account the smallest ST, are: -Fly ash + Lime = 7% wt of Lime (L7) -Fly ash + Portland cement = 5% wt of Portland cement (P5) -Fly ash + Portland Pozzolan cement = 5% wt of Portland Pozzolan cement (PP5) It must also be noted that the L7 mix presents the desired conditions simultaneously, i.e., a smaller the ST and a larger crushing strength of the mixture.However, mixes manufactured with Lime present excessively longer setting time.Mixes P5 and PP5 do not show an optimum binder content that simultaneously satisfies both conditions.Finally, the results suggest that the optimum binder to be used with the available fly ash from a technical and economical point of view should be Portland Pozzolan cement.This, because apart from displaying similar properties to the mixes with Portland cement, it has other additional advantages, namely, a lower cost than others binders and minimum environmental impact in the fabrication process.existiendo una diferencia despreciable de un 4% entre el valor máximo y mínimo.Los valores obtenidos corresponden aproximadamente al 50% del peso específico de los áridos normales.
between the maximum and minimum value.The results correspond to 50% of the specific g ravit}' of the normal aggregate approximately.
Considering the measured absorption value, there is an oscillation between 29,6% and 32,5%, presenting a difference of 10% between them.These values are very similar to the other lightweight aggregates (pumice and expanded clay), but much higher than the normal aggregates.(7 days), the crushing value for L7 and P5 is the same one with PP5 being the more resistant aggregate, with a 24% value higher than the other two samples.At 28 days of hardening these differences increase, determining a 50% higher in the capacity to resist crushing between the strongest aggregate (PP5) and the weakest (L7), a considerable value from the mechanical point of view.
On the other hand, the Strength Index (I^), that indicates the increasing speed of the strength to each aggregate type, shows that the aggregate which increases its crushing strength at a greater speed is, also, the denominated PP5, presenting an increase of 31% between 7 and 28 days of age.

Scanning Electronic Microscopy (SEM) and Energy dispersive X-Ray (EDX) on FALA
The microscopy studies allowed establishing clear differences in the hydration state of each one of the aggregates manufactured.
Los áridos fabricados con cemento portland puzolánico (PP5), presentan una estructura más homogénea a los 7 días, sin una presencia tan evidente de vacíos, aspecto beneficioso desde el punto de vista de la resistencia mecánica.En la zona As shown in Figure 2, the aggregate type P5 presents at 7 days a superficial area with crystalline mac ros true tu re that resembles the crystals of portlandite structure.( 3) shows the appearance of hydration products with ciliary morphology, characteristic of the structures of C-S-H (calcium silicate hydrate) (13,14).The hydration products present in these images are rich in Ca, Si and A I, that confirm the presence of C-S-H around the particles of FA and possibly the portlandite formation.In both cases FA particles that are not hydrated are observed, identified by their characteristic morphology constituted by spherical particles (3,13).It is worth mentioning that the SEM analysis has allowed to evidence a great quantity of voids among the hydration compound (capillary voids), of the order of 1 to 5 ¡im, and even of 10 ¡im.

l], 12), an hypothesis that is confirmed by means of the EDX analysis evidencing an important presence of Ca, with low proportions of Si and AL On the other hand, an interior analysis of the aggregate (Figure
At 28 days an increase in the density of the crystalline macrostructure present in the superficial area is observed; they are rich in Ca, Si and Al, the latter two elements in larger proportion that the previous one.However, there still appear considerable voids of 3 ¡im size (Figure 4).Towards the center of the aggregate there is a notorious increase in the density of the hydration product, and the presence of glassy structures in the form of needles, or acicular structure, is observed that would indicate the presence of ettringite (12,13)

. The EDX analysis reveals Al, Ca, Si and S in smaller amounts, which confirms the presence of ettringite, accompanied by other hydration products, in this case C-S-H. A more homogeneous distribution of the hydration products is observed, with these being greater in the external zone and decreasing to the interior of the aggregate. The EDX analysis indicates the presence of Si, Al and Ca as predominant elements in the center of the aggregates, presenting a morphology similar to the one indicated in the external zone at the age of 7 days (Figure 5). These observations allow concluding that the state of hydration advances from the exterior to the interior of the aggregate, as a product of the curing system that consists of a supeificial water spraying at regulars intervals of times. This contribution of humidity favors the hydration reactions on the surface.
The aggregates manufactured with Portland Pozzolan cement (PP5), present a more homogeneous structure at 7 days, without an obvious presence of voids, a beneficial aspect from the point of view of mechanical strength.A network rich in Si, Al and Ca (figure 6) can externa se observan redes ricas en Si, Al y Ca (Figura 6).Estas estructuras son la base para los cristales de C-S-H, principales compuestos que otorgan resistencia a las pastas de cemento y que, en este caso, estarían otorgando resistencia a la pasta de cenizas volantes.Se observan cristales tanto de forma hexagonal como acicular los que se componen de elementos de Ca, Al, Si y S en menor cantidad, lo que indica la presencia de cristales de portlandita y, posiblemente, monosulfatos (AFJ o ettringita (AF ).Hacia el centro del árido se observan estados de hidratación similares, con gran presencia de partículas de cenizas volantes que actúan como centros de nucleación para productos ciliares ricos en Si, Al y Ca, permitiendo establecer nuevamente la formación de estructuras de C-S-H (Figura 7).Vale destacar que se observan espacios vacíos, los que sin embargo son menores a los detectados en los áridos P5, situación que, como se mencionó anteriormente, refuerza la mayor resistencia a la trituración ofrecida por este tipo de árido.
;\-^ ^^ ^m.be observed in the external zone.Those structures are the basis for the crystal of C-S-H, principal compounds that give the strength of the cement paste, and in this case would be giving strength to the FA paste.Hexagonal and acicular forms of glasses are observed, composed of elements ofCa, Al, Si and S to a lesser degree, indicating the presence of portlandite crystal and possibly monosulfates (AFJ or ettringite (AF).Toward the center of the aggregate, a similar hydration state is observed, with a great presence of FA particles acting as nucleation centers for ciliar products rich in Si, Al and Ca that allow to establish once again the formation of the structure of C-S-H (Figure 7).It is worth mentioning that empty spaces are observed which, however, are smaller that those detected in the P5 aggregates, a situation which, as mentioned previously, further adds to the greater compressive strength offered by this kind of aggregate.A los 28 días de edad la densidad de los productos de hidratación de los áridos PP5 es considerablemente superior a la observada para las estructuras de P5 a igual edad (comparar Figura 4 y Figura 8).En la Figura 8 se observan densas estructuras cristalinas de morfología hexagonal, acompañadas de macroestructuras cristalinas.Según análisis de EDX estos productos de hidratación estarían compuestos por Ca, S, Al y Si, siendo el Ca el elemento más abundante, lo que nos lleva a confirmar la presencia importante de portlandita (CaCOH)^) y C-S-H, con un alto nivel de reacción y aportando de manera considerable a la resistencia superficial del árido.Finalmente, se estima la presencia de monosulfato hidratado (AF^^^), cuya morfología también corresponde a cristales hexagonales (13) y que posee entre sus constituyentes azufre.En Figura 9 se observa la zona interna del árido, que presenta gran cantidad de productos de hidratación con cristales de portlandita, ricos en Ca, y estructuras ciliares rodeando las partículas de cenizas volantes.Dichas estructuras corresponderían a C-S-H debido a la abundante presencia de Si detectada por medio del análisis de EDX.Por otro lado, se detectan cristales en forma de aguja que indicarían la existencia de estructuras de ettringita, hecho concordante con la presencia de Al y S arrojada del análisis.Nuevamente es posible concluir que los estados de hidratación más avanzados se presentan en las regiones exteriores, aspecto muy conveniente desde el punto de vista de eficacia de los áridos, es decir, resistencia a la solicitación de cargas externas.
Las estructuras cristalinas presentes en los áridos tipo L7, evidencian la menor velocidad de hidratación con respecto a los otros dos tipos de muestras consideradas.En la Figura 10 se muestran productos hidratados similares a los observados en P5 a igual edad (Figura 2), es decir, 7 días.Se aprecian numerosas partículas de cenizas volantes que mantienen su morfología esférica, lo que indica que la reacción puzolánica a esta edad es aún muy baja, situación observada además en la zona interior del árido (Figura 11).Se siguen observando vacíos, aunque inferiores a At 28 days of age the density of the hydration products of the PP5 aggregate is considerably higher than the one observed for the P5 structures at the same age (compare Figure 4 and Figure 8).Figure 8 shows dense crystal structures of hexagonal morphology, accompanied by ciystalline macrostructure.
According to the EDX analysis these hydrated products would be made up by Ca, S, Al and Si, where Ca is the most abundant element, leading us to infer the important presence of portlandite (Ca(OH)J and C-S H, with a high reaction level and contributing to a considerable degree to the superficial strength of the aggregate.Finally, the presence of mono sulfate hydrate (AFJ is assumed, whose morphology also corresponds to hexagonal glasses (13) and that has sulfur among its constituents.In Figure 9 the internal area of the aggregate is obsen^ed, presenting a great quantity of hydrated products with portlandite crystal rich in Ca and ciliar structures surrounding the particles of FA.These structures would correspond to C-S-H due to the abundant presence of Si detected by means of the EDX analysis.On the other hand, ciystal in the form of needles are detected that would indicate the existence of ettringite structures which agrees with the presence ofAl and S. Again it is possible to conclude that the most advanced hydration states are present in the external regions, a very convenient aspect from the point of view of the effectiveness of the aggregate,i.e.compressive strength of external loads.
The crystalline structures present in type LVaggregates evidence the lower hydration speed with respect to the other 2 types of samples considered.Figure 10 shows hydrated products similar to those observed in P5 at the same age (Figure 2), that is, 7 days.A great number of FA can be seen retaining their spherical morphology, which indicates that the Pozzolan reaction at this age is still very low, a situation also observed in the interior zone of the aggregate (Figure 11 ).It keeps showing voids, but smaller that those seen before in P5.Ciliar structures made up by Si, Al and Ca surrounding the FA          A los 28 días aparecen productos de hidratación más densos pero aún se identifican numerosas partículas de cenizas volantes sin reaccionar y vacíos del orden de 5 \im entre partículas (Figuras 12 y 13).Destacar que la microestructura de este tipo de árido presenta un estado de hidratación muy homogéneo a través de toda su sección, sin presentar diferencias notorias entre el exterior e interior.Esto podría explicar el menor I^^ mencionado en el apartado 3.3, ya que no se cuenta con la influencia favorable del estado de hidratación más avanzado de la zona exterior que otorgaría mayores resistencias superficiales.
Finalmente, al observar y comparar el borde exterior de los áridos, en un corte transversal (Figuras 14 y 15), es posible identificar una especie de costra externa, de mayor densidad o menor índice de poros, que los recubre.En la Figura 14 se muestra una ampliación de esta zona característica, a los 7 días de particles can he appreciated; these structures correspond to C-S-H compounds in an initial state of reaction.
At 28 days more dense hydration products appears hut still many FA particles without any reaction and voids in the order of 5 mm between particles are still identified (Figure 12 and 13).It is worth mentioning that the microstructure of this type of aggregate presents a very homogeneous state of hydration through all its section, without presenting notorious differences between the exterior and interior This could explain the lower I^^ reported in item 3.3, because it does not have the favorable influence of the most advanced hydration state in the external area that would afford greater superficial strength.
Finally, when we observe and compare the external border of the aggregates, in cross-section (Figure 14 and 15), it is possible to identify a kind of an external coat, of bigger density or smaller pores index that covers it.Figure 14 shows an amplification a/this characteristic area at 7 days of age, with a thickness of   edad, determinándose un espesor de la costra de aproximadamente 150 |im.En la Figura 15 se muestra la misma zona pero a 28 días de edad, observándose que el espesor de esta especie de recubrimiento alcanza los 1.000 |im aproximadamente.Esta observación rjsafirma la conclusión de que el aporte de humedad, proporcionado por el sistema de curado empleado, favorece la hidratación de la superficie de los áridos, generando un sello superficial más resistente.
the coat of approximately 150 ^m. Figure 15 shows the same area but at 28 days of age, and the thickness of this kind of coal reaches 1.000 lim approximately.This observation reaffirms the conclusion that the contribution of humidity, provided by the system of curing resorted to, favors the hydration of the aggregates surface, generating a stronger superficial coat.
Summing up, the hydration mechanisms are similar in each one of the aggregates analyzed, but there are differences generated in the reaction speeds as in the content of pores.The aggregates manufactured with Pozzolan Portland cement are characterized by a larger hydration speed with greater presence ofCa and Si, indicating the presence of C-S-H, compound fundamental for the gain of mechanical strength of the pastes, in this case of FA.The authors posit that the greatest degree of hydration presented in this type of aggregates is due to a great extent to two main factors.First, to the largest fineness that Pozzolan Portland cement binder presents, represented by the Blaine specific surface (Table 2).And second, to the fact that the Pozzolan reactions, i.e., fixing of calcium hydroxide, are faster when they derive from the process of hydrolysis of the cement.
On the other hand, a lower content of pores was detected in the PP5 aggregates, which also presents smaller sizes.This would also be due to the largest degree of hydration reached with the pozzolan portland cement binder, the one that generates hydration products that fill the capillary void.It is worth mentioning that all the aggregates are characterized by presenting a kind of an external coat, which would originate due to the system of curing designed for the product.
• De las observaciones realizadas por medio de SEM, es posible concluir que dado el estado de hidratación y la morfología y composición de los productos de hidratación, el tipo de árido que ofrece las mayores ventajas para la elaboración de áridos, en condiciones de endurecimiento en frío, son los fabricados con cemento portland puzolánico.

4, CONCLUSIONS
From the tests and results obtained on the aggregates produced with FA it is possible to conclude the following: • The test of physical characterization allows classifying the product developed as a lightweight aggregate.
• The aggregates have a high absorption value, similar to other lightweight aggregates, such as pumice and expanded clay.
• The mechanical properties of the FALA indicate that the aggregate type that meets the requirements of crushing strength best is the PP5 type, produced with 5% wt of Pozzolan Portland cement They also offer a higher crushing strength both at 7 days and at 28 days of age.
• At 28 days, the crushing strength of the PP5 aggregate is higher by 35% and 50% with regard to the manufactured aggregate with Portland cement and Lime, respectively.
• The higher crushing strength of the PP5 aggregate is associated with a slight increase of its specific gravity, difference that however is negligible.
• From the observations carried out by means of SEM, it is possible to conclude that, given the hydration state and the morphology and composition of the hydration products, the type of aggregate that offers the best advantages for the production of aggregates, under cold bonded hardening conditions, are those manufactured with Pozzolan Portland cement.
• Given the characteristics of the Pozzolan reactions, it can be expected that the use of aggregates discussed in the paper would permit obtaining a concrete of more homogeneous characteristics than the one manufactured with natural aggregates.The potential reactions that would occur in the transition zone, or interface, could substantially reduce the calcium hydroxide, a compound that weakens this area, being replaced by compounds of C-S-H, a product that would give a resistant and homogeneous characteristic to the area.
• The main advantage of using cement as a binding agent, instead of lime, would be the greater speed in the fixation of the calcium hydroxide product of the hydrolysis of the cement, bringing about a quicker hydration of the FA paste.On the other hand, the best microstructural as well as mechanic behavior of the aggregates produced with pozzolan portland cement, as regards those with portland fabricados con cemento portland puzolánico, respecto a ios con cemento portland, se debería a la mayor finura del primero.Esta propiedad otorgaría mayor capacidad de reacción y, por tanto, mayor capacidad de desarrollo de resistencias mecánicas originadas por la microestructura de los áridos.
cement, would he due to the greater fineness in the former This property would give a larger reaction capacity, and therefore, a larger capacity to develop mechanical strength originated by the microstructure of the aggregates.
• Finally, in the process of aggregates production, under cold bonded hardening conditions, it is technically feasible to obtain a product with clear advantages for its use as a lightweight aggregate for concrete.

Figure
Figure L-Setting Time (ST) and Modified Activity Index (MPAI) for the FA mixes.

Table 4
shows the physical characteristics of FALA.It can be seen that the specific gravity is in range of 1,222 and 1,274, with a negligible difference of 4%

Table 4 indicates
, besides, the result of the Crushing Value test for a 25 KN load (ACV^^) developed at 7 and 28 days.The aggregate with larger crushing strength both at 7 days as well at 28 days of age is the PP5 type.The table shows that at an early age