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Improvement of initial mechanical strength by nanoalumina in belite cements
Materials Letters 61 (2007) 1889 – 1892 www.elsevier.com/locate/matlet
Improvement of initial mechanical strength by nanoalumina in belite cements I. Campillo a,c , A. Guerrero b,c,⁎, J.S. Dolado a,c , A. Porro a,c , J.A. Ibáñez a,c , S. Goñi b,c a
c
Centre for Nanomaterials Application in Construction (NANOC), LABEIN-Tecnalia, Bilbao, Spain b Institute of Construction Science, Eduardo Torroja, CSIC, Madrid, Spain Nanostructured and Eco-efficient Materials for Construction Unit, Associated Unit LABEIN-Tecnalia/CSIC, Spain Received 7 June 2006; accepted 23 July 2006 Available online 15 August 2006
Abstract The development of new environmentally friendly and energy efficient cements such as belite cements are being promoted due the environmental problems related to CO2 emissions in the manufacture of Portland cement. Although long-term hydrated belite cements show comparable and even better mechanical strength to ordinary Portland hydrated cements, they have low initial mechanical strength, which limits their applicability. In this work we analyse the potential of nanomaterials for activation of the initial strength of belite cements. Different nanoparticles are added to belite cement and both the microstructure modification of the resulting paste and the mechanical properties at early ages are studied. Our results show that the addition of nanoparticles can overcome the drawbacks of this type of eco-efficient cements making them competitive to OPC. © 2006 Elsevier B.V. All rights reserved. Keywords: Mortar; Nanomaterials; Mechanical properties; Porosity
1. Introduction In the current scenario brought about by the application of the Kyoto Protocol, the research and development of new clinker production processes and the use of more eco-efficient cement is more necessary than ever in order that the cement industry can positively contribute to fulfil the Kyoto Protocol and sustainable development. The formation of alite (Ca3SiO5), which is the main component of the Portland cement clinker, produces a greater amount of CO2 emissions than the formation of belite (Ca2SiO4). The proportion of alite to belite is about 3 in ordinary Portland clinker. Therefore, by decreasing this proportion less CO2 would be emitted. Furthermore, if industrial byproducts such as fly ash from coal combustion or from incineration of municipal solid wastes are used as raw materials, and alternative hydrothermal–calcination-routes are employed for belite clinker production, CO2 emissions can be strongly reduced [1] or even totally avoided [2–4]. Although long-term hydrated belite cements show comparable and even better mechanical strength to ordinary Portland ⁎ Corresponding author. Institute of Construction Science, Eduardo Torroja, CSIC, Madrid, Spain. E-mail address: [email protected] (A. Guerrero). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.07.150
hydrated cements, they have low initial mechanical strength, which limits their applicability. However, the slowest hydration kinetics avoids retraction problems and makes them suitable for the production of big concrete elements. Pure Ca2SiO4 exhibits 5 polymorphic forms, depending on the temperature and pressure during formation [5,6]. The two high-temperature polymorphs, α and α′ forms (ranging from ∼ 1400 °C to ∼ 1200 °C), are the most reactive ones. The β polymorph (∼ 800 °C) is the usual form present in ordinary Portland clinker, and it does not contribute to the gain of strength during the first 28 days of hydration owing to its low hydraulic activity. Therefore a great effort has been devoted to the production of active belite cements in which the most reactive high-temperature α′ polymorphs are stabilised during clinker production. This has been mainly achieved by rapid quenching of the clinker and the introduction of impurity ions during formation (alkalis, sulphate, etc.) into the crystal structure [6]. By exploiting the properties of nanomaterials, new higher performance materials for construction can be obtained [7,8]. In this paper we analyse the potential application of nanoparticles for activation of the initial strength of belite cements. The addition of silica (SiO2) nanoparticles to ordinary Portland cements has proved to notably increase the mechanical properties of cement-based materials [9–12]. Besides, it has been shown that
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Fig. 1. X-ray diffraction pattern of the belite cement. α: α′-Ca2SiO4, β: β-Ca2SiO4, C: CaCO3, 1: CaO, M: C12A7, and ♣: C3A.
small additions of nanosilica increase the durability against the Ca-leaching [13,14]. It is also remarkable that the addition of small quantities of nanoalumina (nanosized particles of alumina, Al2O3) also increases the mechanical properties of both ordinary Portland and belite cement pastes [15]. 2. Experimental 2.1. Raw materials 2.1.1. Belite cement The belite cement used in this work has been synthesized in our laboratory by a hydrothermal–calcination-route and using coal fly ash (FA) as raw materials. As afore-mentioned, this is the most eco-efficient method for the production of belite cements. The fabrication process included a pre-hydrothermal treatment of the coal fly ash and CaO in which the hydrated precursors of the cement were obtained. During this hydrothermal treatment (200 °C for 4 h and 1.24 MPa of pressure) the fly ash pozzolanic reaction is strongly activated leading to hydraulic products: katoite (C3ASH4) (Ca3Al2(SiO4)(OH)8) together with C–S–H gel (Ca1.5SiO3.5·xH2O) and α-C2SH, which are the precursors of the cement. The subsequent dehydration of these phases, by controlled heating up to 800 °C, gave rise to a mixture of β- and α′-C2S, mayenite C12A7 and CaCO3 [4,7]. Fig. 1 displays the X-
ray diffraction pattern of the so obtained belite cement. The BETN2 surface area of cement was 4.4 m2/g [5]. 2.1.2. Nanomaterials In this paper two types of nanoalumina have been used. The first one is an agglomerated dry alumina (ADA) with an average grain size ranging from 0.1 μm (100 nm) to 1 μm. The second nanoalumina type is a colloidal alumina (CA) composed of 50 nm alumina nanoparticles dispersed in water. The content of alumina in this colloidal dispersion is 20%. The two of them are products that can be obtained commercially. 2.2. Experimental Mortar samples were prepared at normalised (α-quartz) sand to a belite cement ratio of 3 and demineralised water to cement ratio of 0.8, with addition of 0, 3 and 9% by weight of cement of the two different nano-Al2O3: ADA and CA. The high w/c was necessary due to high finesses of the cement powder. When CA was used, the quantity of water was corrected taken into account the water of the dispersion in such a way that the w/c is 0.8 always. In the case of CA, the suspension was directly added to the mixing water, while the precipitated alumina was added to the cement powder and homogenised. After mixing all the components, the resulting mortar was moulded in prism-shaped
Table 1 Compressive strength and total porosity of belite cement mortar modified by nanoalumina Addition (%) a
7 days
28 days Pt (wt.%)
Rc (MPa)
Increase (%)
Agglomerated dry alumina 0% 4.0 ± 0.3 – 3% 7.9 ± 0.4 96 9% 9.7 ± 0.1 142
13.24 13.16 12.98
8.5 ± 0.3 15.7 ± 0.2 18.5 ± 0.7
– 85 119
Colloidal alumina 0% 4.0 ± 0.3 0.6% (3%) 6.3 ± 0.2 1.8% (9%) 7.4 ± 0.3
13.24 14.14 12.05
8.5 ± 0.3 16.0 ± 0.9 18.0 ± 1
– 89 113
a
Rc (MPa)
By weight of cement.
Increase (%)
– 56 84
Fig. 2. Influence of nanoadditons in percentage of pores diameter at 7 days.
I. Campillo et al. / Materials Letters 61 (2007) 1889–1892
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Fig. 3. X-ray diffractions patterns at 7 days: a) plain belite cement paste; b) with 9% of ADA and c) with 1.8% of CA.
specimens (1 × 1 × 6 cm) and compacted by vibration. The samples were stored at a temperature of 21 ± 2 °C and N90% relative humidity for 7 and 28 days. At each age the porosity and compressive strength were measured. X-ray was performed over all the samples in order to study the belite consumption. 3. Results and discussion 3.1. Mechanical properties Table 1 displays the results of the compressive strength for the different samples; in the case of CA, the quantity of product has been written between brackets, since the dispersion is in a 20% concentration. It is clear that the addition of the two types of nanoalumina increases the compressive strength at both 7 and 28 days. It can be noticed that the CA is much more effective than ADA, because similar strengths are obtained with five times less quantity of alumina. This is due to the fact that CA is composed of truly nanosized particles (∅ ∼ 50 nm), which enhances their reactivity.
3.3. X-ray diffraction In order to understand the different behaviour of CA and ADA, X-ray diffraction (Fig. 3) was performed over these samples at 7 days. From Fig. 2 it can be seen that the addition of both nanoalumina types leads to a different behaviour. In the two cases, the peaks of α′and β-C2S reflections decrease (note the X-ray reflections of intensity 100 appearing at 2θ ∼33° angular zone from belite cement pastes). But the CA addition leads to the formation of C–S–H gel (Ca1.5SiO3.5·xH2O) phase (whose X-ray reflections at 2θ ∼29° are overlapped to the CaCO3 phase) as principal hydration product; in turn, the ADA reacts with belite phases to form vertumnite (Ca4Al4Si4O6 (OH)24·3H2O) and stratlingite (Ca2Al2SiO7· 8H2O) whose reflections increase from those of the plain cement paste. Furthermore, by comparing the X-ray diffraction patterns of ADA (grey) and CA (white), it is clear that CA is much more reactive with belite to form C–S–H gel: the 2θ ∼33° peaks are reduced by a factor of ∼2 for 1.8% of CA, whereas for a 9% of ADA the reduction is about 1.35. It is the nanosize of the particles of the CA that determines this higher reactivity and therefore high mechanical properties. Therefore, contrary to what is stated in Ref. [12], CA does not work as a filler, but it dissolves and reacts with the belite phase to form C– S–H gel. Further experiments will clarify how this process happens.
3.2. Porosity The porosity, as measured by Hg intrusion. The value of this parameter of the samples with and without nano-Al2O3 appeared in Table 1. The pore-size distribution of the samples is also presented in Fig. 2. The main pore-size distribution change is the refinement of pores, whose diameters are shifted to lower values. This effect has been produced during the first days of study (7 days), thereafter; the values remain more or less constant. The total porosity almost does not change with the addition of nanoalumina, and achieves values of about 13% by weight. Therefore, in this case, the increase of the strength does not correspond with a decrease of total porosity. However, the good mechanical properties are related to a refinement of the microstructure, where we can see an important shift from pores N1 μm to b 0.1 μm. More investigations are needed to clarify this point, which will be the subject of future work.
4. Conclusions From these results it can be stated that: 1. The compressive strength at an early age (7 days) increases notably by the addition of two types of nanoalumina. The colloidal alumina behaves much better than the agglomerated alumina, since with lower weight of alumina a similar strength increase is achieved. 2. According to this result, special attention could be made to the composition of nanoparticles: agglomerated dry alumina comes from precipitation after drying colloidal alumina. Therefore it cannot exhibit the whole specific surface area of the primary particles of which it is made.
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3. The nanoalumina can be used as a reactive agent to increase the hydraulic activity of the slowly reactive belite phase present in cements and refine the microstructure of paste leading to the improvement of the mechanical strength at early ages. Further studies on the activation mechanisms and microstructure changes brought about the nanoalumina, as well as extensive durability assessment, are under way. References [1] A. Guerrero, S. Goñi, A. Macías, M.P. Luxán, Journal of Materials Research 14 (1999) 2680–2687. [2] S. Goñi, A. Guerrero, M.A. Macías, R. Peña, E. Escalante, Materiales de Construcción 51 (2001) 263–264. [3] S. Goñi, A. Guerrero, A. Moragues, M.F. Tallafigo, I. Campillo, J.S. Dolado, A. Porro, Spanish Patent Publication Number ES 2223275A1. [4] A. Guerrero, S. Goñi, I. Campillo, A. Moragues, Environmental Science and Technology 38 (2004) 3209–3213. [5] H.F. Taylor, Cement. [6] C. Peter Hewlett (Ed.), Lea's Chemistry of Cement and Concrete, Elsevier, Oxford, 2004.
[7] Second International Symposium on Nanotechnology in Construction, edited by Y. R. De Miguel, A. Porro, P. J. M. Bartos, RILEM, Bagneux (in press). [8] I. Campillo, J.S. Dolado, A. Porro, in: P.J.M. Bartos, J.J. Hughes, P. Trtik, W. Zhu (Eds.), Nanotechnology in Construction, The Royal Society of Chemistry, Cambridge, 2004, pp. 215–225. [9] A. Porro, J.S. Dolado, I. Campillo, E. Erkizia, Y.R. de Miguel, Y. Sáez de Ibarra, A. Ayuela, in: R.K. Dhir, M.D. Newlands, L.J. Csetenyi (Eds.), Applications of Nanotechnology in Concrete Design, Thomas Telford, London, 2005, pp. 87–98. [10] J.J. Gaitero, Y. Sáez de Ibarra, E. Erkizia, I. Campillo, Physica Status Solidi. A, Applied Research 203 (2006) 1313–1318. [11] G. Li, Properties of high-volume fly ash concrete incorporating nano-SiO2, Cement and Concrete Research 34 86 (2004) 1043–1049. [12] Z. Li, H. Wang, S. He, Y. Lu, M. Wang, Investigations on the preparation and mechanical properties of the nano-alumina reinforced cement composite, Materials Letters 60 (2006) 356–359. [13] J.J. Gaitero, Y. Sáez de Ibarra, E. Erkizia, I. Campillo, in Second International Symposium on Nanotechnology in Construction, edited by Y. R. De Miguel, A. Porro, P. J. M. Bartos, RILEM, Bagneux (in press). [14] J.J. Gaitero, I. Campillo, E. Erkizia, A. Guerrero, S. Goñi (in press). [15] A. Porro, I. Campillo, J.S. Dolado, S. Goñi, A. Guerrero, International Patent Publication Number WO 2005082802A1.
Improvement of initial mechanical strength by nanoalumina in belite cements I. Campillo a,c , A. Guerrero b,c,⁎, J.S. Dolado a,c , A. Porro a,c , J.A. Ibáñez a,c , S. Goñi b,c a
c
Centre for Nanomaterials Application in Construction (NANOC), LABEIN-Tecnalia, Bilbao, Spain b Institute of Construction Science, Eduardo Torroja, CSIC, Madrid, Spain Nanostructured and Eco-efficient Materials for Construction Unit, Associated Unit LABEIN-Tecnalia/CSIC, Spain Received 7 June 2006; accepted 23 July 2006 Available online 15 August 2006
Abstract The development of new environmentally friendly and energy efficient cements such as belite cements are being promoted due the environmental problems related to CO2 emissions in the manufacture of Portland cement. Although long-term hydrated belite cements show comparable and even better mechanical strength to ordinary Portland hydrated cements, they have low initial mechanical strength, which limits their applicability. In this work we analyse the potential of nanomaterials for activation of the initial strength of belite cements. Different nanoparticles are added to belite cement and both the microstructure modification of the resulting paste and the mechanical properties at early ages are studied. Our results show that the addition of nanoparticles can overcome the drawbacks of this type of eco-efficient cements making them competitive to OPC. © 2006 Elsevier B.V. All rights reserved. Keywords: Mortar; Nanomaterials; Mechanical properties; Porosity
1. Introduction In the current scenario brought about by the application of the Kyoto Protocol, the research and development of new clinker production processes and the use of more eco-efficient cement is more necessary than ever in order that the cement industry can positively contribute to fulfil the Kyoto Protocol and sustainable development. The formation of alite (Ca3SiO5), which is the main component of the Portland cement clinker, produces a greater amount of CO2 emissions than the formation of belite (Ca2SiO4). The proportion of alite to belite is about 3 in ordinary Portland clinker. Therefore, by decreasing this proportion less CO2 would be emitted. Furthermore, if industrial byproducts such as fly ash from coal combustion or from incineration of municipal solid wastes are used as raw materials, and alternative hydrothermal–calcination-routes are employed for belite clinker production, CO2 emissions can be strongly reduced [1] or even totally avoided [2–4]. Although long-term hydrated belite cements show comparable and even better mechanical strength to ordinary Portland ⁎ Corresponding author. Institute of Construction Science, Eduardo Torroja, CSIC, Madrid, Spain. E-mail address: [email protected] (A. Guerrero). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.07.150
hydrated cements, they have low initial mechanical strength, which limits their applicability. However, the slowest hydration kinetics avoids retraction problems and makes them suitable for the production of big concrete elements. Pure Ca2SiO4 exhibits 5 polymorphic forms, depending on the temperature and pressure during formation [5,6]. The two high-temperature polymorphs, α and α′ forms (ranging from ∼ 1400 °C to ∼ 1200 °C), are the most reactive ones. The β polymorph (∼ 800 °C) is the usual form present in ordinary Portland clinker, and it does not contribute to the gain of strength during the first 28 days of hydration owing to its low hydraulic activity. Therefore a great effort has been devoted to the production of active belite cements in which the most reactive high-temperature α′ polymorphs are stabilised during clinker production. This has been mainly achieved by rapid quenching of the clinker and the introduction of impurity ions during formation (alkalis, sulphate, etc.) into the crystal structure [6]. By exploiting the properties of nanomaterials, new higher performance materials for construction can be obtained [7,8]. In this paper we analyse the potential application of nanoparticles for activation of the initial strength of belite cements. The addition of silica (SiO2) nanoparticles to ordinary Portland cements has proved to notably increase the mechanical properties of cement-based materials [9–12]. Besides, it has been shown that
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I. Campillo et al. / Materials Letters 61 (2007) 1889–1892
Fig. 1. X-ray diffraction pattern of the belite cement. α: α′-Ca2SiO4, β: β-Ca2SiO4, C: CaCO3, 1: CaO, M: C12A7, and ♣: C3A.
small additions of nanosilica increase the durability against the Ca-leaching [13,14]. It is also remarkable that the addition of small quantities of nanoalumina (nanosized particles of alumina, Al2O3) also increases the mechanical properties of both ordinary Portland and belite cement pastes [15]. 2. Experimental 2.1. Raw materials 2.1.1. Belite cement The belite cement used in this work has been synthesized in our laboratory by a hydrothermal–calcination-route and using coal fly ash (FA) as raw materials. As afore-mentioned, this is the most eco-efficient method for the production of belite cements. The fabrication process included a pre-hydrothermal treatment of the coal fly ash and CaO in which the hydrated precursors of the cement were obtained. During this hydrothermal treatment (200 °C for 4 h and 1.24 MPa of pressure) the fly ash pozzolanic reaction is strongly activated leading to hydraulic products: katoite (C3ASH4) (Ca3Al2(SiO4)(OH)8) together with C–S–H gel (Ca1.5SiO3.5·xH2O) and α-C2SH, which are the precursors of the cement. The subsequent dehydration of these phases, by controlled heating up to 800 °C, gave rise to a mixture of β- and α′-C2S, mayenite C12A7 and CaCO3 [4,7]. Fig. 1 displays the X-
ray diffraction pattern of the so obtained belite cement. The BETN2 surface area of cement was 4.4 m2/g [5]. 2.1.2. Nanomaterials In this paper two types of nanoalumina have been used. The first one is an agglomerated dry alumina (ADA) with an average grain size ranging from 0.1 μm (100 nm) to 1 μm. The second nanoalumina type is a colloidal alumina (CA) composed of 50 nm alumina nanoparticles dispersed in water. The content of alumina in this colloidal dispersion is 20%. The two of them are products that can be obtained commercially. 2.2. Experimental Mortar samples were prepared at normalised (α-quartz) sand to a belite cement ratio of 3 and demineralised water to cement ratio of 0.8, with addition of 0, 3 and 9% by weight of cement of the two different nano-Al2O3: ADA and CA. The high w/c was necessary due to high finesses of the cement powder. When CA was used, the quantity of water was corrected taken into account the water of the dispersion in such a way that the w/c is 0.8 always. In the case of CA, the suspension was directly added to the mixing water, while the precipitated alumina was added to the cement powder and homogenised. After mixing all the components, the resulting mortar was moulded in prism-shaped
Table 1 Compressive strength and total porosity of belite cement mortar modified by nanoalumina Addition (%) a
7 days
28 days Pt (wt.%)
Rc (MPa)
Increase (%)
Agglomerated dry alumina 0% 4.0 ± 0.3 – 3% 7.9 ± 0.4 96 9% 9.7 ± 0.1 142
13.24 13.16 12.98
8.5 ± 0.3 15.7 ± 0.2 18.5 ± 0.7
– 85 119
Colloidal alumina 0% 4.0 ± 0.3 0.6% (3%) 6.3 ± 0.2 1.8% (9%) 7.4 ± 0.3
13.24 14.14 12.05
8.5 ± 0.3 16.0 ± 0.9 18.0 ± 1
– 89 113
a
Rc (MPa)
By weight of cement.
Increase (%)
– 56 84
Fig. 2. Influence of nanoadditons in percentage of pores diameter at 7 days.
I. Campillo et al. / Materials Letters 61 (2007) 1889–1892
1891
Fig. 3. X-ray diffractions patterns at 7 days: a) plain belite cement paste; b) with 9% of ADA and c) with 1.8% of CA.
specimens (1 × 1 × 6 cm) and compacted by vibration. The samples were stored at a temperature of 21 ± 2 °C and N90% relative humidity for 7 and 28 days. At each age the porosity and compressive strength were measured. X-ray was performed over all the samples in order to study the belite consumption. 3. Results and discussion 3.1. Mechanical properties Table 1 displays the results of the compressive strength for the different samples; in the case of CA, the quantity of product has been written between brackets, since the dispersion is in a 20% concentration. It is clear that the addition of the two types of nanoalumina increases the compressive strength at both 7 and 28 days. It can be noticed that the CA is much more effective than ADA, because similar strengths are obtained with five times less quantity of alumina. This is due to the fact that CA is composed of truly nanosized particles (∅ ∼ 50 nm), which enhances their reactivity.
3.3. X-ray diffraction In order to understand the different behaviour of CA and ADA, X-ray diffraction (Fig. 3) was performed over these samples at 7 days. From Fig. 2 it can be seen that the addition of both nanoalumina types leads to a different behaviour. In the two cases, the peaks of α′and β-C2S reflections decrease (note the X-ray reflections of intensity 100 appearing at 2θ ∼33° angular zone from belite cement pastes). But the CA addition leads to the formation of C–S–H gel (Ca1.5SiO3.5·xH2O) phase (whose X-ray reflections at 2θ ∼29° are overlapped to the CaCO3 phase) as principal hydration product; in turn, the ADA reacts with belite phases to form vertumnite (Ca4Al4Si4O6 (OH)24·3H2O) and stratlingite (Ca2Al2SiO7· 8H2O) whose reflections increase from those of the plain cement paste. Furthermore, by comparing the X-ray diffraction patterns of ADA (grey) and CA (white), it is clear that CA is much more reactive with belite to form C–S–H gel: the 2θ ∼33° peaks are reduced by a factor of ∼2 for 1.8% of CA, whereas for a 9% of ADA the reduction is about 1.35. It is the nanosize of the particles of the CA that determines this higher reactivity and therefore high mechanical properties. Therefore, contrary to what is stated in Ref. [12], CA does not work as a filler, but it dissolves and reacts with the belite phase to form C– S–H gel. Further experiments will clarify how this process happens.
3.2. Porosity The porosity, as measured by Hg intrusion. The value of this parameter of the samples with and without nano-Al2O3 appeared in Table 1. The pore-size distribution of the samples is also presented in Fig. 2. The main pore-size distribution change is the refinement of pores, whose diameters are shifted to lower values. This effect has been produced during the first days of study (7 days), thereafter; the values remain more or less constant. The total porosity almost does not change with the addition of nanoalumina, and achieves values of about 13% by weight. Therefore, in this case, the increase of the strength does not correspond with a decrease of total porosity. However, the good mechanical properties are related to a refinement of the microstructure, where we can see an important shift from pores N1 μm to b 0.1 μm. More investigations are needed to clarify this point, which will be the subject of future work.
4. Conclusions From these results it can be stated that: 1. The compressive strength at an early age (7 days) increases notably by the addition of two types of nanoalumina. The colloidal alumina behaves much better than the agglomerated alumina, since with lower weight of alumina a similar strength increase is achieved. 2. According to this result, special attention could be made to the composition of nanoparticles: agglomerated dry alumina comes from precipitation after drying colloidal alumina. Therefore it cannot exhibit the whole specific surface area of the primary particles of which it is made.
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3. The nanoalumina can be used as a reactive agent to increase the hydraulic activity of the slowly reactive belite phase present in cements and refine the microstructure of paste leading to the improvement of the mechanical strength at early ages. Further studies on the activation mechanisms and microstructure changes brought about the nanoalumina, as well as extensive durability assessment, are under way. References [1] A. Guerrero, S. Goñi, A. Macías, M.P. Luxán, Journal of Materials Research 14 (1999) 2680–2687. [2] S. Goñi, A. Guerrero, M.A. Macías, R. Peña, E. Escalante, Materiales de Construcción 51 (2001) 263–264. [3] S. Goñi, A. Guerrero, A. Moragues, M.F. Tallafigo, I. Campillo, J.S. Dolado, A. Porro, Spanish Patent Publication Number ES 2223275A1. [4] A. Guerrero, S. Goñi, I. Campillo, A. Moragues, Environmental Science and Technology 38 (2004) 3209–3213. [5] H.F. Taylor, Cement. [6] C. Peter Hewlett (Ed.), Lea's Chemistry of Cement and Concrete, Elsevier, Oxford, 2004.
[7] Second International Symposium on Nanotechnology in Construction, edited by Y. R. De Miguel, A. Porro, P. J. M. Bartos, RILEM, Bagneux (in press). [8] I. Campillo, J.S. Dolado, A. Porro, in: P.J.M. Bartos, J.J. Hughes, P. Trtik, W. Zhu (Eds.), Nanotechnology in Construction, The Royal Society of Chemistry, Cambridge, 2004, pp. 215–225. [9] A. Porro, J.S. Dolado, I. Campillo, E. Erkizia, Y.R. de Miguel, Y. Sáez de Ibarra, A. Ayuela, in: R.K. Dhir, M.D. Newlands, L.J. Csetenyi (Eds.), Applications of Nanotechnology in Concrete Design, Thomas Telford, London, 2005, pp. 87–98. [10] J.J. Gaitero, Y. Sáez de Ibarra, E. Erkizia, I. Campillo, Physica Status Solidi. A, Applied Research 203 (2006) 1313–1318. [11] G. Li, Properties of high-volume fly ash concrete incorporating nano-SiO2, Cement and Concrete Research 34 86 (2004) 1043–1049. [12] Z. Li, H. Wang, S. He, Y. Lu, M. Wang, Investigations on the preparation and mechanical properties of the nano-alumina reinforced cement composite, Materials Letters 60 (2006) 356–359. [13] J.J. Gaitero, Y. Sáez de Ibarra, E. Erkizia, I. Campillo, in Second International Symposium on Nanotechnology in Construction, edited by Y. R. De Miguel, A. Porro, P. J. M. Bartos, RILEM, Bagneux (in press). [14] J.J. Gaitero, I. Campillo, E. Erkizia, A. Guerrero, S. Goñi (in press). [15] A. Porro, I. Campillo, J.S. Dolado, S. Goñi, A. Guerrero, International Patent Publication Number WO 2005082802A1.