Scheme of the Portland Cement Hydration with

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The direct stimulation would be due to the mixing water that initially moistens the particles, whereas the non-direct ... would be extensive to any type of cement: one, with a high ... progress of the hydration reaction, assuming that both the ... PC1/C 60/40. 55.0. 10.5. 126.4. 230.3. 0.158. PC2 100/00. 42.5. 21.5. 100.0. 100.0.

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Scheme of the Portland Cement Hydration with Crystalline Mineral Admixtures and Other AspectsS V. Rahhal*, V. Bonavetti*, A. Delgado**, R. Talero** *Civil Engineering Department – Faculty of Engineering – U.N.C.P.B.A Avda. del Valle 5737 – (B7400JWI) Olavarría – Argentina; E-mail: [email protected]fio.unicen.edu.ar **”Eduardo Torroja” Institute for Construction Science; C/. Serrano Galvache, 4 – 28033 Madrid – Spain; E-mail: [email protected] ABSTRACT The effects of including mineral admixtures in Portland cement on cement hydration are ultimately reflected in the mechanical behaviour and durability of the resulting concrete. The physical and chemical interactions involved may expedite or retard the hydration rate. The present paper describes the hydration mechanism in Portland cements containing crystalline mineral admixtures (fillers, non-pozzolanic) based on the reaction rates and amount of product formed. The mineralogical composition of the Portland cements used determines their differential behaviour when exposed to sulphate or chloride attack, separately. Ground quartz and limestone were the admixtures chosen. The results show that direct and non-direct stimulation of the hydration reactions increase with the replacement ratio and, obviously, dilution effect. The direct stimulation would be due to the mixing water that initially moistens the particles, whereas the non-direct stimulation would be promoted very to the beginning of the hydration, for the positive and negative electrostatic charge acquired by the particles during grinding, and later on, for the zeta potential specially, originated when the hydration moves forward. Consequently, overall system behaviour varies depending on the crystalline mineral admixture, the type of Portland cement and the replacement ratio. Keywords: Portland cements, crystalline mineral admixtures, hydration.

INTRODUCTION Although perfections continue to be introduced in the existing qualitative and quantitative models for Portland cement paste hydration mechanisms, the main outlines are in place [1]. The hydration mechanisms for tricalcium silicate and tricalcium aluminate in the presence of pozzolanic admixtures have likewise been modelled [2-3]. Pozzolanic admixtures are natural or artificial materials with properties such that the chemical interactions in which they are involved, due to their pozzolanic activity, lead to the capillary pore network segmentation and particle size reduction of the calcium hydroxide generated [4]. The use of such admixtures has been driven by the enhancements introduced in concrete mechanical properties and, as appropriate, durability when exposed to aggressive environments, as well as by the lesser consumption of natural resources entailed, in particular the lower amount of energy needed to manufacture blended cements. The development of the mass manufacture of cements with pozzolanic (i.e., amorphous and/or vitreous) admixtures has led to the gradual inclusion of crystalline mineral admixtures known as fillers, some of which interact physically and/or chemically with Portland cement or its reaction

products, while others do not [5,6]. Initially, such materials were used in response to the need to control bleeding in concretes with low cement content. Their effect on early age hydration of the Portland cement to which they were added was subsequently analyzed and studied [7]. The main conclusion drawn from these studies is that they stimulate hydration in the initial stages when their particles are moistened by the mixing water. This hydration mechanism has been termed direct stimulation [7]. Finally, the use of fillers has become so widespread that they and the cements containing them have now been standardized [8]. Research has been conducted to determine the optimal replacement ratio for fillers depending on Portland cement, mortar or concrete requirements. The present paper describes a possible mechanism for their interaction with Portland cement based on the many previous studies that support its validity [10, 11]. OBJECTIVE The present paper aims to describe and propose a hydration mechanism for portland cements containing filler by fitting their behaviour to quantitative findings.

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MATERIALS AND METHODOLOGY Two Portland cements with widely different mineralogical compositions were chosen to ensure that the results would be extensive to any type of cement: one, with a high C3A content, denominated PC1, had 51% C3S, 16.5% C2S, 14% C3A and 5.5% C4AF, a density of 3.08, a Blaine specific surface, BSS, of 319 m2/kg and a loss on ignition of 1.6%; and the other, PC2, had a low C3A and a high C3S content, 79.5 C3S, 2.5% C2S, ≈0% C3A and 10% C4AF, a density of 3.21 m2/kg, a BSS of 329 m2/kg and a loss on ignition of 1.1%. The crystalline, i.e., non-pozzolanic, mineral admixtures chosen as fillers were: a siliceous addition, Q (ground ASTM C 778-92a sand [12]), with a SiO2 content of over 99%, a density of 2.70, a BSS of 395 m2/kg and a loss on ignition of 0.0%; and the other a limestone, C (ground Spanish limestone routinely used to make cement concrete and mortar) with a CaCO3 content of over 95%, a density of 2.71, a BSS of 362 m2/kg and a loss on ignition of 42.5%. Since three replacement ratios –20, 30 and 40 were used-, a total of twelve blended cements were prepared and denominated as follows: 80/20, 70/30 or 60/40 PC1/ or PC2/Q or C. A 100/00 ratio denotes the two pure Portland cements used as a control. The trials and analyses conducted in this study included hydraulicity [13] and chemically combined water [14] tests and X-ray diffraction. The amount of chemically combined water [14] was determined on the basis of loss of mass in heated samples whose hydration was arrested at the age of two days. The specimens were heated from 40 to 925 ºC and the value obtained was used to estimate the progress of the hydration reaction, assuming that both the ground limestone and the ground quartz were hydraulically inactive. X-ray diffraction analyses were conducted on a Philips PW 40 Cu Kα20 diffractometer fitted with a graphite monochromator and set for 40 kV and 20 mA. RESULTS AND DISCUSSION The results of the two-day hydraulicity and chemically combined water trials are given in Table 1. Note (Table 1) that the two-day hydraulicity trial shows that as the replacement ratio grew, more calcium hydroxide CH (= portlandite) per gram of portland cement was generated. The inference is that the particles of the two crystalline mineral admixtures directly stimulated the hydration reactions [7] while at the same time acting as nucleation centres for calcium hydroxide. Thus, this latter behaviour should constitute an additional stimulus due initially, to the static electricity acquired by the PC and filler particles during grinding. In other words, the particle surfaces are positively

or negatively charged during its grinding, respectively attracting OH- and Ca2+ ions [15,16] very to the beginning of the hydration. This first layer of anions or cations in turn attracts a second cluster of Ca++ or OH- ions, respectively. Table 1 Hydraulicity and chemically combined water tests. Age: 2 days Concentration Cements

[CaO]

[OH-]

[CaO]

Chemically combined water (gwater/gPC)

Absolute (mM/l) [OH-]

per gram of PC (%)

PC1 100/00

72.5

7.6

100.0

100.0

0.139

PC1/Q 80/20

65.0

9.5

112.1

156.3

0.152

PC1/Q 70/30

60.0

10.3

118.2

193.6

0.164

PC 1/Q 60/40

57.0

11.6

131.0

254.4

0.180

PC1/C 80/20

65.0

9.0

112.1

148.0

0.144

PC1/C 70/30

61.0

9.8

120.2

184.2

0.151

PC1/C 60/40

55.0

10.5

126.4

230.3

0.158 0.093

PC2 100/00

42.5

21.5

100.0

100.0

PC2/Q 80/20

44.5

21.9

130.9

127.3

0.110

PC2/Q 70/30

44.0

22.2

147.9

147.5

0.128

PC2/Q 60/40

43.5

22.3

170.6

172.9

0.145

PC2/C 80/20

42.5

20.1

125.0

116.9

0.101

PC2/C 70/30

43.5

20.3

146.2

134.9

0.106

PC2/C 60/40

44.0

20.6

172.5

159.7

0.113

As the ionic layer thickens, the electrostatic force of the particles declines [17]. But besides this, the following much more important considerations have to be also taken into account above all, when the hydration moves forward: all inorganic particles assume a charge when dispersed in water. In the case of crystalline silica (filler Q, in this work), this is due to surface silanol, Si-OH, groups losing a proton. The aqueous phase becomes slightly acidic (since it receives protons) whilst the silica surface becomes negative (due to the formation of Si-O-). The charged particle surface then attracts a layer of counter-ions (ions of the opposite charge) from the aqueous phase. In the case of silica, positive ions (Ca2+ mainly, in this work) will crowd the surface. Due to ionic radii considerations, the strongly adsorbed counter ions will not fully offset the surface charge. A second layer of more loosely held counter ions then forms. At a certain distance from the particle surface, the surface charge will be fully balanced by counters ions. Beyond this point, a bulk suspension with a balance of negative and positive electrolyte exists. The size of the double layer will depend firstly on the amount of charge on the particle surface. A large charge, whether positive or negative, will result in a large double layer that stops particles getting close to each other because of the electrostatic repulsion between particles carrying the same electrical charge. This situation is typical of stable (deflocculated) suspensions having a low viscosity.

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Conversely, a low surface charge requires fewer counter ions and so smaller double layers. Accordingly, particles then tend to flocculate which leads to high viscosity suspension. The zeta potential is measured in mV and can be related to the energy needed to shear the particle and its inner layer of counter-ions away from the outer layer / bulk medium. It is possible to alter the zeta potential by changing the concentration and valence of soluble ions in the suspension. For example, if we return to our silica example, a single ca2+ ion in suspension will do a much better job of cancelling out the negative surface charge on the silica than a Na+ or K+ ion. The result is a more compressed double layer and increased flocculation. Often suspension state intermediate of flocculation and deflocculation is required. For example, a casting slip that is too deflocculated will take too long to cast and result in a “brittle” cast piece that is difficult to fettle. Conversely, a flocculated slip will cast quickly but retain too much water in the cast to create a cast piece liable to distort before drying. On the other hand and with regard to the calcite, CaCO3, (ground limestone or fíller C used also in this work) which is crystalline too, it is necessary to say that has three polymorphous (18): calcite (solubility product: log Ksp = 8.48 at 25°c) is the thermodinamically most stable form and most calcium carbonate occurs in the form of calcite in nature. Less stable form, aragonite (log Ksp = -8.34), is mainly found in the biosynthetic CaCO3 such as shells and corals. The most unstable polymorph, vaterite (log Ksp = -7.91), rarely occurs in natural fields, but plays an important role in the calcium carbonate formation from solution. K. Sawada has demonstrated by different concentrations of phosphate (19), that the chemical species adsorbed on the vaterite surface are negatively charged one, [Ca2+•PO4-3], whereas, the chemical species adsorbed on the calcite surface are electrically neutral one: low phosphate concentration… [(Ca2+)3(PO43-)2], and high phosphate concentration…[(Ca2+)(PO42-)], i.e., the chemical species adsorbed are entirely different between vaterite and calcite, that is, negatively charged on vaterite, and non-charged species on calcite (note that the portlandite is also non-charged specie one, i.e., electrically neutral specie one). This difference is predominantly caused by the difference of the surface charge of the crystals. Surface charge of the calcium carbonates was determined by a streaming potential method. The zeta potentials of calcite and vaterite in solution are plotted as a function of the logaritmic activity of calcium ion, log {Ca2+}. As the solution is equilibrated with crystals, the concentration of carbonate ions is correlated to log {CO32-} = log Ksp – log {Ca2+}. The surface charge of the crystals shifts to the positive side with the increase in the calcium ion activity in both systems. The potential zeta curves are totally different between calcite and vaterite. Un-

der the usual experimental conditions (the order of Ca2+ concentration is 10-3 mol.dm-3), the surface of calcite is negatively charged. Consequently, calcite requires the adsorption of electrically neutralized species. Conversely, the vaterite surface is positive and thus the adsorption of negatively charged species is reasonably explained. Furthermore, the surface charge of calcium carbonates is scarcely changed by the change in pH under the conditions that the activity of CO32- or Ca2+ is kept constant. Consequently, the hydrogen ion is not the surface charge controlling ion. It has been reported that the surface charge of the calcium carbonate shifts to positive side with the decrease in pH. These experiments were conducted under the conditions that the equilibrium concentrations of lattice ions are not kept constant. Thus, the change in the potential is the result of the decrease in the carbonate ion concentration, [CO32-], caused by its protonation, i.e., the increase in the [Ca2+] concentration. In addition, the surface properties of crystals are significantly altered by the conditions of the way of preparation and history of the crystals. Particularly, the crystal surface created by crush is very rough and active (both fíllers, the Q and the C specially, in this work). The rate of crystal growth on rough surface is much higher than that on the smooth surface and that surface is more favorable for adsorption. Consequently, attention is called for the comparison of the thermodynamic and kinetic data between the crystals from different origins and treatment even if they have the same polymorph. In short, this second mechanism when the hydration moves forward (19) would have to be added in an underhand way to the first one, and thus both would also explain why, when the proportion of filler particles rises, hydration is stimulated more actively: the reason is that the larger number of nucleation centres would strengthen initially the overall electrostatic force, but over all later on, the zeta potential originated, resulting finally in the generation of more calcium hydroxide, CH, per gram of Portland cement. This effect was observed in the hydraulicity trial. The above findings were corroborated by the amount of chemically combined water found, which increased with the replacement ratio, regardless of which of the two crystalline mineral admixtures was used. Table 1 gives the amount of chemically combined water per gram of Portland cement, which was calculated assuming that the fillers behaved inertly and taking account of the calcination loss in anhydrous materials. As the temperature rose, water was lost first from the AFt phase and then from the AFm phase; at higher temperatures, the water and then the calcium hydroxide decom-

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posed; finally, the water combined with the hydrated calcium silicates was eliminated [20]. All the mass lost was attributed to the elimination of chemically combined water.

Figure 1: XRD patterns at 2 days-age. a) PC1, b) 60/40 PC1/Q, c) 60/40 PC1/C, d) PC2, e) 60/40 PC2/Q, and f) 60/40 PC2/C. Note: AFt phase: Ettringite, HC: Calcium hydroxide, Q: Quartz, C: Calcite.

As in the preceding trial, XRD analysis was conducted on the hydrated cements up to the age of two days (Figure 1). Used to monitor the calcium hydroxide, CH, this technique can explain the new pathway proposed for stimulating hydration, which to differentiate it from direct stimulation might be simply denominated non-direct stimulation. It also shows that at 2 days-age, neither of the two types of filler was chemically involved in the formation of the hydration products contributing to system hydraulicity. In indirect stimulation, by contrast, the very reactive pozzolanic admixtures (for instance, SF, nanosilica, metakaolin, some natural pozzolans and fly ashes etc.) are deeply involved [21-24] forming also new hydration products contributing to system hydraulicity [21-24]. The XRD traces in Figures 1a to 1d show that the 2θ = 18.02º calcium hydroxide, CH, peak was more intense in PC1 than in PC2 (433 and 135 counts, respectively), despite the higher C3S content in the latter. This may be explained by the mineralogical composition of the two Portland cements, as well as by the small w/c ratio = 0.5 used. With that

amount of mixing water, the C3A (14%) in PC1 (and the 51% C3S) must have used more water for hydration than the C3S (79.5%) in PC2 (and the ≈ 0% of C3A), since C3A is known to have a higher hydration rate than C3S.

The foregoing hypothesis is supported by the behaviour observed in both 60/40 PC blended cements with both of the crystalline mineral admixtures, Q and C. Since neither crystalline mineral addition, naturally, is pozzolanic, the mixing water that initially moistens their particles must prompt greater direct stimulation of the respective PC fractions, an effect that would be enhanced by the non–direct stimulation described above. Comparing the two pure PC (with no mineral admixtures), the PC2 fraction must benefit more than the C3A from the mixing water that initially moistens the particles of the two crystalline mineral admixtures. Support for this assertion is provided by a comparison of Figures 1a to 1d and 1b to 1f, which shows that both crystalline mineral admixtures, when blended with PC2, consistently generated more calcium hydroxide, CH, (369 and 454 counts for 40% Q and C, respectively) than when blended with the respective PC1 (253 and 259 counts for 40% Q and C, respectively). Further proof lies in the observation that the more effectively the crystalline mineral admixture stimulates hydration of the 60% PC2 directly and/or non-directly, the greater is the amount of calcium hydroxide generated by that fraction. In the present study, that behaviour was in fact observed in crystalline mineral admixture C, whose randomly granulated texture, a result of its geochemical origin [25, 26], enabled it to absorb more moisture at laboratory temperature than Q (whose texture is compact), when ≈ 1.0 g of each was separately stored in porcelain vials and placed in a desiccator (with water distilled at bottom instead of silica gel) at 23±1.7°C and RH ≥ 95% (Table 2). This would mean that from the outset, when it was mixed with the Portland cement to form a 60/40 blend, the crystalline mineral admixture C comprising the 40% fraction must have adsorbed more of the initial mixing water (w/c = 0.5) than Q, from which the 60% PC2 fraction would have benefited more than the 60% PC1 fraction. Hence as noted earlier, when these two crystalline mineral admixtures were mixed separately with each Portland cement, the 2θ = 18.02º calcium hydroxide peak was less intense in the PC1 than in the PC2 blends. Table 2 Humidity absorption for Q y C Humidity Absorption (%)

Crystalli ne Mineral Admixtu res

1

3

7

14

21

28

Q

0.41

0.49

1.38

1.76

2.75

2.75

C

2.07

2.53

3.35

3.90

6.67

6.67

Age (days)

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studies conducted. Note in particular the decline in anhydrous PC to the benefit of calcium hydroxide (= portlandite), hydrated calcium silicate and phases AFt and AFm. At 2 days-age, neither portlandite nor cement paste exhibit chemical interaction with each crystalline mineral admixture, Q or C. CONCLUSIONS With the materials chosen and the analytical and test methodology adopted, a proposal has been put forward for the hydration behaviour of the Portland cement—crystalline mineral admixture–water system, as shown in Figure 2, and the reaction mechanism suggested is described below. The particles of both crystalline mineral admixtures stimulate hydration reactions directly [7], due to the mixing water that moistens the surface of their particles during initial mixing. At the same time, they also stimulate hydration by constituting nucleation centres for the calcium hydroxide (= portlandite) generated after the 60% PC fraction is hydrated: i.e., they act, in addition, like “seed crystals”. This other type of hydration stimulation may take place in an underhand way, -

due, very to the beginning, to the positive and negative electrostatic charge acquired by the particles during grinding, which would attract the OH- and Ca++ ions in the calcium hydroxide molecule until their entire surface is covered, and this first layer of anions or cations in turn attracts a second cluster of Ca++ and OH- ions, respectively, and so on until the electrostatic force of the original “pole of attraction” (each particle of the crystalline mineral admixture) is depleted, and later on (19),

-

due to the zeta potential specially, originated when the hydration moves forward (19).

Figure 2: Scheme for Portland cement hydration with crystalline mineral admixtures

In short, the non-direct stimulation of hydration in the 60% Portland cement fraction by the 40% crystalline mineral admixture fractions, Q and C, must have been more intense in PC2 than in PC1 (to which the effect of direct stimulation would have to be added [7]). And conversely, the dilution effect was greater in the 60% Portland cement fraction in PC1 than in PC2. Finally, opposite relation has been found between the [CaO] values of the hydraulicity test (in its liquid phase, Table 1) for Q and C fillers, and the CH (= portlandite) content present in its respective cement paste at 2 days-age, determined by XRD analysis (Figures. 1b to 1f). This point out that the C particles must have behaved more and better like “seed crystals” than the Q particles. In addition, C particles can never react chemically with portlandite, but in contrast, Q particles can, even though for much later [27]. A flow chart depicting the physical-chemical interactions in the Portland cement-crystalline mineral admixture-water system is proposed in Figure 2, based on the analyses and

Thus, raising the replacement ratio naturally increases the number of “poles of attraction”, but above all, the zeta potential originated, and with it the rate of precipitation of portlandite crystals, further stimulating the hydration reactions To summarize, early hydration of the component minerals of Portland cement is stimulated both directly and non-directly, as discussed above, although the crystalline mineral admixtures do not interact chemically at 2 days-age, in the formation of the hydration products that contribute to enhance system hydraulicity. By contrast, such interaction is observed enough in the indirect stimulation of the pozzolanic mineral admixtures very active only [21-24].

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ACKNOWLEDGMENTS We would like to thank the Fundación Rotaria, the Universidad Nacional del Centro de la Provincia de Buenos Aires, for financial support, and the Instituto de C.C. “Eduardo Torroja”-CSIC from Spain as well, for having provided the authors with necessary cementing materials and some analytical and experimental techniques. REFERENCES [1] S.Mindess, J.Young: “Concrete” - Prentice-Hall, Inc., Englewood Cliffs, New Jersey, 1981. [2] K. Takemoto, H. Uchikawa: Hydratation des ciments pouzzolaniques.- 7th International Congress on Chemistry of Cement, Vol. IV, Paris 1980, pp. II-1-II-29. [3] H. Uchikawa, S. Uchida: Influence of pozzolan on the hydration of C3A.- 7th International Congress on Chemistry of Cement, Vol. III, Paris 1980, pp. IV-24-IV-35. [4] P.K.Mehta, P Monteiro: Concrete: Structure, Properties and Materials - Prentice Hall, Inc, New Jersey. USA., 1993. [5] W.A. Gutteridge, J.A. Dalziel: Filler Cement: The Effect of the Secondary Component on the Hydration of Portland Cement. Par 1. A Fine Non-hydraulic Filler.- Cem. Concr. Res., 20 pp.778-782, 1990. [6] V.F. Rahhal, V.L. Bonavetti, R. Talero: Early hydration of cements with medium and hight content of crystaline mineral admixtures.- Revista de la Construcción, Escuela de Construcción Civil Pontifícia Universidad Católica de Chile. Vol. 4, N° 2, pp.13-24, 2005.

[14] T.C. Power: The non evaporable water content of hardened portland cement paste.- ASTM Bull. 158, pp. 68-76, 1949. [15] R. Talero: Patologías y terapias preventivas del hormigón: II. La capacidad de cambio de las adiciones puzolánicas (puzolanas naturales, cenizas volantes, arcillas activadas y similares): ¿Patología o terapia para su hormigón en masa o armado correspondiente?.- Monografía N° 406, Instituto de Ciencias de la Construcción “Eduardo Torroja”. Consejo Superior de Investigaciones Científicas, 68 pp., 1991. [16] R. Talero : Is the clay exchange capacity concept wholly applicable to pozzolans?.- Mater. Construcc., 54 (276) (2004) 17-36 and 55 (277) (2005) 82. [17] J.P. Bombled: Etude rhéologique des pâtes crues de cimenterie.- Revue de Matériaux de Construction, 609, pp. 229-238, 1970. [18] L. N. Plummer and E. Bunsengerg: Geochim. Cosmochim. Acta, 46, 1011, (1982). [19] Kiyoshi Sawada: “The mechanism of crystallization and transformtion of calcium carbonates”.- Pure & Appl. Chem., Vol. 69, No. 5, pp. 921-928, 1997. [20] H.F.W. Taylor: La Química de los Cementos (Chemistry of Cements).- Ed. Urmo, Calle Espartero, 10, Bilbao-Spain, 1978 (“in Spanish”). [21] V. Rahhal, O. Cabrera, R. Talero and A. Delgado: Calorimetry of portland cement with silica fume and gypsum admixtures.- J. Therm.Ana.Cal., 87 (2) (2007) 331-336.

V. Rahhal, R. Talero: Early hydration of portland cement with crystalline mineral admixtures.- Cem. Concr. Res., 35 (7) (2005) 1285-1291.

[22] R. Talero and V. Rahhal: Influence of aluminic pozzolans, quartz and gypsum admixtures on portland cement hydration.- 12th Intern. Congress on Chemistry of Cement, Proceedings, Ref.No.: 299, Montreal – Canada, 8-13 july 2007.

[8] Norma EN 197-1, 2000. “Cemento: Composición, especificaciones y criterios de conformidad para cementos comunes” - AENOR, Calle Génova nº 6, 28004-MADRID-Spain.

[23] V. Rahhal and R. Talero: Calorimetry of portland cements with metakaolins, quartz and gypsum admixtures.- J. Therm. Anal. Cal., 91 (3) (2008) 825-834.

[7]

[9] V.L. Bonavetti G. Menéndez, V. Rahhal, H. Donza, E. Irassar: Hormigones con cemento con material calcáreo. Parte I: Estado fresco y propiedades mecánicas.- Revista Internacional Construlink, N° 9 Vol. 3 pp.12-18, 2005. [10] V.L. Bonavetti G. Menéndez, V. Rahhal, H. Donza, E. Irassar: Hormigones con cemento con material calcáreo. Parte I: Comportamiento durable.- Revista Internacional Construlink, N° 10 Vol. 3 pp.1-7, 2005.

[24] Talero and V. Rahhal: “Calorimetric comparison of portland cement containing silica fume and metakaolin: Is silica fume, like metakaolin, characterized by pozzolanic activity that is more specific than generic? J. Ttherm. Anal. Cal., , Ref. No.: 9096 (accepted for publication: 30-09-2008; in press).

[11] V.F. Rahhal: Characterization of Pozzolanic Admixtures by Conduction Calorimetry.- PhD Thesis, Universidad Politécnica de MadridSpain, Dec.12th 2002 (“in Spanish”).

[25] ATLAS DE ASOCIACIONES MINERALES EN LÁMINA DELGADA.- Editor: Joan Carles Melgarejo, Vols. I and II. Universidad de Barcelona, Fundación Folch, Reedición: Publicaciones UB, 2003. Publicaciones de la Univ. De Barcelona, Adolfo Florensa, s/n, 08028– Barcelona.

[12] ASTM C 778 – 92a Standard: Standard Specification of Standard Sand.- ANNUAL BOOK OF ASTM STANDARDS, Section 4 Construction Vol 0.4.01 Cement; Lime; Gypsum, 1995.

[26] F. Bastida: Geología: Una Visión Moderna de las Ciencias de la Tierra, Vol. I.- EDICIONES TREA, S.L., María González la Pondala, 98, ; Nave D, 33393 – Gijón (Asturias).

[13] UNE EN 196-5-96 Standard: Métodos de ensayos de cementos. Parte 5: Ensayo de puzolanicidad para cementos puzolánicos (Pozzolanicity test for POZC) - AENOR, Calle Génova nº 6, 28004-MADRIDSpain.

[27] A. Delgado: “A contribution to the analysis and study of the aggregate-portland cement paste interface”.- PhD Thesis, next upcoming defence at the Chemical Engineering Department of the University of Castilla-La Mancha, Spain (‘in Spanish’).

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