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Alternative cements based on alkali-activated red clay brick waste
Construction and Building Materials 128 (2016) 163–169
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Alternative cements based on alkali-activated red clay brick waste Rafael A. Robayo, Alexandra Mulford, Jorge Munera, Ruby Mejía de Gutiérrez ⇑ Composites Materials Group (CENM), School of Materials Engineering, Calle 13 # 100-00, Edif. 349, 2° piso, Universidad del Valle, Cali, Colombia
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The alkaline activation of red clay
brick waste (RCBW) was investigated. The effect of the addition (620%) of
Portland Cement (OPC) was studied. The addition of Na2SiO3 and OPC
contributed in the better mechanical behaviour. Compressive strength of up to 102 MPa was obtained at room temperature (25 °C). The feasibility of obtain hybrid binders using RCBW is demonstrated.
a r t i c l e
i n f o
Article history: Received 3 July 2016 Received in revised form 13 September 2016 Accepted 6 October 2016
Keywords: Red clay brick waste Ceramic wastes Alkaline activation Alkali-activated cement Hybrid cement Compressive strength
a b s t r a c t The synthesis of alkali-activated and hybrid cements based on red clay brick waste was investigated using sodium hydroxide and sodium silicate solution as alkaline activators. The effect on the compressive strength of Na2O/SiO2 and SiO2/Al2O3 molar ratios and type of curing was evaluated. The combined presence of Portland cement (20%) and sodium silicate yielded a maximum compressive strength of 102.6 MPa at 28 days and 25 °C; this value is two times higher than the strength obtained in mixtures without Portland cement and 7.3 times higher than when sodium hydroxide is used. The results demonstrated the feasibility of using these materials to produce structural and non-structural construction elements. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Komnitsas [1] mentions that the sustainable cities of the future apart from having low energy consumption and greenhouse gas emissions should also adopt the ‘‘zero waste” principle, which contribute to sustainable development and reduction of carbon footprint. Several wastes, including those from mining, metallurgy, municipal, construction and demolition, which are produced today ⇑ Corresponding author. E-mail addresses: [email protected] (R.A. Robayo), material@ univalle.edu.co (A. Mulford), [email protected] (J. Munera), [email protected] (R. Mejía de Gutiérrez). http://dx.doi.org/10.1016/j.conbuildmat.2016.10.023 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
in huge quantities in each country, can be used as raw materials of other industries [1]. The main barriers for recycling are the quality of these wastes, specially its chemical and physical characteristics (which are very heterogeneous), and the low cost or high availability of some virgin raw materials [2]. Therefore, alkaline activation and geopolymerization are technologies that allows us to use waste (with reactive SiO2 and Al2O3 species) that are unsuitable in other industries. It is noteworthy that the application of these technologies in the production of binders, in addition to promoting the use of waste and / or industrial by-products as raw materials, have additional benefits such as lower energy consumption and
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reduced level of emissions, which it has been shown in several studies to compare these to traditional Portland cements [3]. Ceramic wastes are generated by construction and demolition processes in the industrial sector and as a result of the ceramic processing. Approximately 45% of construction and demolition wastes (CDW) is attributed to ceramic products such as bricks, tiles, and porcelain [4]. According to Pacheco-Torgal and Jalali [5], the European ceramics industry generates a volume of waste that is equivalent to 3–7% of total production, which indicates that millions of tons of clay per year are disposed in landfills as the reused volume is minimal. Currently, the countries that generate the largest amount of CDW include China, the United States of America and the countries in the European Union, accounting for approximately 605.5 million tons per year; however, this number may be larger due to natural disasters in recent years [6–8]. In some countries, such as Germany, Denmark and the Netherlands, reuse approaches 80%, whereas the average in other countries is 30% [9]. Although a national statistical study of CDW in Colombia is not available, some studies indicate that 12 million tons per year are generated and remain unused in some cities with higher demographic growth, such as the city of Bogota [10,11]. In the capital of Valle del Cauca, Cali, an average volume of 2480 cubic meters of CDW is generated daily. Of this volume, approximately 76.6% (1900 cubic meters) is generated by construction companies and public construction, which is known as ‘‘formal generation”; the remaining 23.4% (580 cubic meters) is generated by private construction and remodelling, which is referred as the ‘‘informal sector” [11,12]. These large generated CDW volumes have motivated the search for alternatives to enable better use of different component materials (brick, glass, ceramics, and concrete). Alkaline activation technology has been recently considered to be an important option for the reuse of inorganic materials in CDW [3,13–17]. Note that some of these materials, such as red clay brick waste (RCBW), have been subjected to high temperatures for bricks manufacture and in some cases without adequate control, therefore, their reactivity can be reduced [18,19]. Puertas et al. [20] performed alkaline activation of different ceramic wastes with a mixture of sodium hydroxide (NaOH) and sodium silicate (Na2SiO3) at a concentration of 6 M and reported a compressive strength after 8 days of curing of 13 MPa. They attribute this relatively low strength to the semi-crystalline nature of this type of raw materials . Allahverdi and Kani [21] carried out alkaline activation of construction brick waste with a mixture of NaOH and Na2SiO3 (silica modulus 0.6) at a ratio of 8% Na2O with respect to the precursor and obtained a compressive strength of 40 MPa after 28 days of curing. In a subsequent study, the author reported a maximum strength of 50 MPa using a mixture of 60% concrete waste and 40% brick waste activated with NaOH and Na2SiO3 solution (solution ratio: 1.4) as a precursor and the same percentage of Na2O in the mixture. The authors emphasise the importance of taking into account the efflorescence phenomena, which suggests the addition of additives that are abundant in alumina to achieve better crosslinking of the geopolymer and lower mobility of the alkali [19]. Zaharaki et al. [16] obtained similar results (39.4 MPa) using NaOH 10 M and thermal treatment at 80 °C. Sun et al. [22] obtained a compressive strength of 71.1 MPa with urban ceramic waste activated with a mixture of KOH and NaOH and subjected to a treatment process at 60 °C for 28 days. Reig et al. [4,23] produced RCBW geopolymer pastes and mortar with a precursor:sand ratio of 1:3 activated with NaOH (7 M) and sodium silicate with a SiO2/Na2O ratio of 1.60. The authors report a compressive strength of 30 MPa at 7 days of curing at 65 °C and relative humidity above 90%. According to the authors, the optimization of variables such as the liquid/solid ratio can significantly increase the strength, with a maximum value of 50 MPa. Reig et al. [24] with the addition of calcium aluminate
cement (40%) accelerate the reaction and to obtain, at room temperature and 3 days of curing, 50 MPa. Recently, Komnitsas et al. [25] investigated the potential for geopolymerization of different construction wastes; bricks, tiles and concrete, and reported maximum compressive strengths of 49.5 MPa and 57.8 MPa for brick and tile waste, respectively. However, the reported value for concrete waste was only 13 MPa with the use of NaOH (14 M) and a curing treatment at 90 °C for seven days, which is 43% higher than the value reported by Pathak and Kumar [26]. This study presents the evaluation of the mechanical and microstructural properties of RCBW when is activated with NaOH and Na2SiO3 to obtain an alkali-activated cement (100% RCBW) and a hybrid cement (RCBW + ordinary Portland cement (OPC)). RCBW was supplied by a bricks factory. The effect on the compressive strength of the following synthesis variables was analysed: Na2O/SiO2 and SiO2/Al2O3 molar ratios, type of curing (room temperature curing at 25 °C and thermal curing at 70 °C for 24 and 48 h) and quantity of added OPC (between 5 and 20% by weight with respect to the RCBW content).
2. Materials and experimental methodology 2.1. Materials The raw materials employed in the production of alkaliactivated and hybrid cements were RCBW and OPC, respectively. The RCBW was selected from a pile of debris from a brick factory in the region (Cali, Colombia). This waste is generated during the production process, because the bricks are broken during the firing and transportation steps. The comminution of RCBW required the use of a ball mill. The chemical compositions of these materials, as presented in Table 1, were determined by X-ray fluorescence (XRF) using a MagiX-Pro PW–2440 Phillips spectrometer equipped with a Rhodium tube with a maximum power of 4 kW. In RCBW, the high molar ratio of SiO2/Al2O3 (5.58) is significant. The particle size analysis was performed by laser granulometry in a Mastersizer-2000 device by Malvern Instruments, coupled with a Hydro2000MU dispersion unit, in which distilled water was applied as a dispersing medium. The average particle sizes D [4;3] for RCBW and OPC were 24.25 lm and 21.65 lm, respectively. Fig. 1 shows the X-ray diffractogram of the RCBW, which indicates the semi-crystalline nature of the precursor and the presence of quartz (SiO2) (Ref. Pattern: 01-079-1910) as the main phase. The following crystalline minor components were identified: hematite (Fe2O3) (Ref. Pattern: 01-085-0987), muscovite (KAl2 (AlSi3O10) (OH)2) (Ref. Pattern: 00-002-1019) and plagioclases, albite NaAlSi3O8 (Ref. Pattern: 01-076-0898) and anorthite (CaAl2Si2O8) (Ref. Pattern: 01-086-1706).
Table 1 Chemical composition of the raw materials (% oxides). Element
SiO2 Al2O3 Fe2O3 CaO Na2O MgO K2O TiO2 Others LOI Molar ratio SiO2/Al2O3
Material OPC
RCBW
21.13 4.92 4.88 64.27 0.26 1.61 0.25 0.24 4.42 4.14 7.88
65.92 20.08 9.10 0.73 0.44 0.86 0.97 1.09 0.81 – 5.58
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Fig. 1. X-ray diffractogram (XRD) of RCBW.
2.2. Preparation of mixtures and trials The simple (100% RCBW) and hybrid (RCBW + OPC) alkaliactivated pastes were prepared in equivalent conditions. The OPC was incorporated in concentrations between 5 and 20% by weight with respect to the RCBW. As alkaline activators, NaOH (98.22% purity) and commercial sodium silicate (Na2SiO3xH2O) (SiO2 = 32.09%, Na2O = 11.92%, and H2O = 55.99%) were employed. The effect of the Na2O (2-10%) concentration of the activator solution and the SiO2/Al2O3 and Na2O/SiO2 ratios were investigated, which varied from 5.8–7.1 and from 0.06–0.18, respectively. These relations were calculated taking in account the proportions of each component present in the precursor and the activator. The liquid/solid ratio (L/S) was 0.25 ensuring a proper workability. The mixing was carried out in several stages, as follows: (1) preparation of the activating solution in the required proportions, (2) mixture of the activator solution with the solid (RCBW) or mixture of solids (OPC + RCBW) previously homogenized and then addition of the remaining water to complete the L/S ratio specified. The alkaliactivated pastes were obtained in a Hobart mixer with a mixing time of five minutes. The pastes in their fresh states were modelled in cubes with 20-mm sides and were vibrated for 30 s in an electrical vibrating table to remove trapped air. Then, the moulds were covered with a polyethylene film that prevents the evaporation of the free mixing water and cured at room temperature (25 °C) and 70 °C for 24 and 48 h. After this period, the test cubes were unmoulded and taken to a curing chamber that ensures a relative humidity near 80% until the test age was reached. The compressive strength was evaluated in an INSTRON 3369 universal test device with a capacity of 50 kN and a strain rate of 1 mm/min. In each case three specimens was employed. For the microstructural study of the optimal mixtures, the following instrumental techniques were employed: &
&
Fourier transform infrared spectroscopy (FTIR) using the IR-100 spectrometer by Perkin Elmer in transmittance mode with a frequency range between 4000 and 450 cm 1. The samples were evaluated using the compressed KBr pellet methodology. Scanning electron microscopy (SEM) using the JSM 6490LV JOEL electronic microscope with an acceleration voltage of 20 kV. The samples were evaluated in vacuum mode. Coupled with the microscope, the Link-Isis X-ray spectrometer by Oxford Instruments was employed.
3. Results and discussion 3.1. Effect of Na2O content and curing temperature on the compressive strength of 100% RCBW pastes activated with NaOH Fig. 2 shows the effect of the Na2O content and the curing temperature on the compressive strength after 28 days of curing for the 100% RCBW paste activated with NaOH. The highest strength values at 25 °C were achieved with Na2O concentrations between
Fig. 2. Effect of the Na2O concentration and the curing treatment on the compressive strength at 28 days (100% RCBW) (activator: NaOH).
8 and 10%, with a maximum of 7.49 MPa for the mixture with 10% of Na2O. This optimal activator concentration is sufficient for balancing the charges of the Si and Al tetrahedrons. Low concentration of NaOH (<6%) do not provide a sufficient alkalinity affecting the solubility of the precursor, but an excess of NaOH would cause the formation of carbonate salts – as the result of atmospheric carbonation – and generate efflorescence [4,19,21]. Generally, there is an optimum concentration of the alkaline activator, which depends on the particle size and the mineralogy of the raw materials as well as on the synthesis conditions, curing temperature and ageing period [25]. It is well known that the rate of a chemical reaction is increased with the temperature, and an adequate curing is required to achieve advanced mechanical and durability performance in geopolymeric materials. So, an increase in the curing temperature accelerates the reaction kinetics geopolymerization because it favors the dissolution of the active species and thus the material can achieve better mechanical resistance. Generally temperatures between 30 °C and 90 °C are acceptable, on the contrary high temperatures, above 150 °C, and prolonged exposure times can cause negative effects due to the possibility of contractions and microcracks in the matrix by rapid dehydroxylation in the geopolymer gel. As seen in the study, any increase in both time (24 and 48 h) and temperature (25 °C and 70 °C) of hydrothermal curing causes an increase in compressive strength confirming the positive effect of curing at 70 °C. At 70 °C and 48 h, a maximum compressive strength of 16.36 MPa is obtained in the mixture activated with 8% Na2O, which is higher than twice the results at room temperature and 1.5 times more than the result obtained at 70 °C and 24 h. This result corroborates that obtained by Komnitsas et al. [25], the application of thermal energy is necessary to increase the mechanical strength of this type of precursor due to its predominantly crystalline nature, as it was shown in Fig. 1. It is clear that, regardless of the temperature and curing time studied, the proportion of activator would be between 8% and 10% Na2O (Fig. 2). 3.2. Effect of molar ratios SiO2/Al2O3 and Na2O/SiO2 on the compressive strength of 100% RCBW pastes activated with NaOH+Na2SiO3 The effect of the molar ratios SiO2/Al2O3 and Na2O/SiO2 and the curing temperature on the compressive strength of the pastes can be observed using the NaOH + Na2SiO3 mixture as an activator. The results for different curing systems are shown in Figs. 3 and 4. The results indicate that the presence of Na2SiO3 causes a significant increase in the compressive strength of the 100% RCBW pastes.
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R.A. Robayo et al. / Construction and Building Materials 128 (2016) 163–169 Table 2 Optimal synthesis parameters for the 100% RCBW paste activated with NaOH + Na2SiO3. Variable
25 °C
70 °C 24 h
Na2O/SiO2 Ratio SiO2/Al2O3 Ratio Liquid/solid ratio (L/S) CS (MPa) Standard deviation (MPa)
0.12 6.60 0.25 54.38 7.44
0.12 7.10 0.25 66.56 2.5
CS: compressive strength at 28 days of curing.
Fig. 3. Effect of the SiO2/Al2O3 and Na2O/SiO2 ratios on the compressive strength at 28 days of curing (100% RCBW) (activator: NaOH + Na2SiO3) (25 °C).
achieved with the application of 70 °C for 24 h and SiO2/Al2O3 and Na2O/SiO2 ratios of 7.10 and 0.12, respectively. Note that the optimal strength at 28 curing days of the 100% RCBW mixtures, which were activated with NaOH + Na2SiO3 and cured at room temperature (25 °C), is 7.3 times higher than the optimal strength obtained in mixtures activated with NaOH. These results are in agreement with previous studies [4,21,25] where a significant improvement in compressive strength has been observed using sodium silicate solution as activator, although some of the studies have used curing temperatures up to 90 °C. 3.3. Effect of the addition of OPC on the compressive strength of the pastes
Fig. 4. Effect of the SiO2/Al2O3 and Na2O/SiO2 ratios on the compressive strength at 28 days (curing treatment at 70 °C during 24 h) (100% RCBW) (activator: NaOH + Na2SiO3).
This behaviour, according to Duxson et al. [27] and Pacheco-Torgal et al. [28], is attributed to the presence of soluble silica (Si-O-), which is provided by the activator; it provides proper conditions for the formation of a geopolymer structure. Sodium silicate contributes to a better interface between the unreactive particles and the gel matrix and, consequently, to the higher mechanical strength of the aluminosilicate gel [19]. According to the characteristics of the precursor, soluble silicate (SiO2/Al2O3) and sodium (Na2O/SiO2) have optimal quantities that may significantly affect the material properties such as setting time and compressive strength [4]. The optimal SiO2/Al2O3 and Na2O/SiO2 ratios for each curing treatment are listed in Table 2. The best alkali-activated mixture at 25 °C (54.38 MPa) was obtained using an activator solution with a silica modulus (SiO2/Na2O) of 1.29, which is equivalent to SiO2/ Al2O3 and Na2O/SiO2 ratios of 6.6 and 0.12 respectively. Generally, the application of temperature to the pastes promoted an increase in the mechanical strength. However, the optimal synthesis conditions vary according to the applied thermal treatment [25]. The maximum compressive strength (28 days) of 66.56 MPa was
The influence of the addition of OPC on the compressive strength of the optimal mixtures cured at room temperature (25 °C) and activated with NaOH (8% Na2O) and NaOH + Na2SiO3 (SiO2/Al2O3 ratio of 6.60 and Na2O/SiO2 ratio of 0.12) is shown in Figs. 5 and 6, respectively. The addition of OPC to the pastes, regardless of the type of activator, contributes to the increase in the compressive strength over time. In the case of the NaOH activator, the maximum strength was achieved with 10% OPC (41.39 MPa), with the possibility of improving this property by 555% after 28 days of curing compared with the 100% RCBW mixture activated with NaOH. The maximum compressive strength in the pastes activated with NaOH + Na2SiO3 was achieved with 20% OPC, reaching values up to 102.59 MPa after 28 days of curing at room temperature. Similarly, the addition of 5% OPC resulted in 36.61 MPa after seven days of curing. The positive effect of the incorporation of OPC can be attributed to the contribution of CaO and the coexistence of C-S-H gels (product of the hydration of OPC) and N-A-S-H (product of the alkaline activation of aluminosilicate) [17,29–31]. Also, the presence of OPC
Fig. 5. Effect of OPC concentration on the compressive strength at 7 and 28 days (activator: NaOH) (25 °C).
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Fig. 6. Effect of OPC concentration on the compressive strength at 7 and 28 days (activator: NaOH + Na2SiO3) (25 °C).
accelerates the activation of the aluminosilicate, this has been explained by the contribution of heat released in the cement hydration process [31,32]. It should be noted that the reaction products formed during the alkali activation of mixtures of portland cement and aluminosilicates (hybrid cements) are still under study [32]. In the hybrid cements, some authors have demonstrated the possibility of coprecipitation of the two gels N-A-SH and C-S-H, however other studies have concluded that these gels interact, resulting microstructural and compositional changes [32]. Studies conducted by Garcia-Lodeiro et al. [31,32] showed that the stability of N-A-S-H structure in the presence of calcium depends of the pH. At low alkalinity, sodium is partially replaced by calcium to form (N,C)-A-S-H gel. On the contrary, in presence of sufficient calcium and high alkalinity (pH over 12), the formation of C-A-S-H is favored [17,31,32]. This is in accordance with the higher compressive strength obtained in this work (Figs. 5 and 6).
3.4. Microstructure of the alkali-activated pastes For the microstructural analysis, three mixtures were selected: the alkali-activated mixture (100% RCBW) and two hybrid mixtures containing 10% OPC. Fig. 7 presents the FTIR spectra of the pastes with 100% RCBW (NaOH + Na2SiO3), 10% OPC (NaOH) and
Fig. 7. FTIR of the alkali-activated pastes: a) RCBW, b) 100% RCBW (NaOH + Na2SiO3), c) 10% OPC (NaOH) and d) 10% OPC (NaOH + Na2SiO3).
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10% OPC (NaOH + Na2SiO3) cured at room temperature for 28 days. In general, attention is drawn to the reduction in the intensity of some peaks between 475 and 797 cm 1, which correspond to the stretching vibrations of the Si-O-Al bonds and the bonds from the SiO4 tetrahedrons (Si-O stretching), suggesting the dissolution of some phases that are abundant in aluminosilicate in the RCBW [20]. This finding corroborates an increase in the compressive strength of these systems [25]. Similarly, the displacement of the main band of RCBW (1087 cm 1), which is attributed to the stretching vibration of the bonds from the SiO4 (Si-O) tetrahedrons, to higher values between 1011 and 1019 cm 1 suggests the formation of C-A-S-H and/or N-A-S-H gels during the alkaline activation process of the hybrid pastes (RCBW + OPC) [33,34]. Additionally, the position and amplitude of this main band may also be related to the Si-O bonds in the N-A-S-H and C-S-H gels and the coexistence of products associated with Si-O-Si (gel C-S-H) and Si-O-T (gel N-A-S-H) bonds [35]. The band located between 1661 and 1651 cm 1 is characteristic of the vibration for the deformation of the molecular bonds of the study water. Similarly, the detected signal between 3441 and 3464 cm 1 corresponds to the stretching vibration of the –OH bonds. The signals at 694 and 779 cm 1 can be associated with the presence of quartz. For the different systems, a band was identified between 1428 and 1485 cm 1, which may be associated with the asymmetrical stretching vibration of the O–C–O bond produced by the atmospheric carbonatation phenomena of the pastes [19]. Fig. 8 presents the SEM images for the systems with 100% RCBW (NaOH + Na2SiO3), 10% OPC (NaOH) and 10% OPC (NaOH + Na2SiO3) cured at room temperature for 28 days. They indicate that the pastes activated with NaOH + Na2SiO3 presented a denser and more homogeneous gel compared with the paste activated with NaOH, in which the RCBW particles are partially dissolved by the alkaline solution and exhibit low cohesion. According to Yunsheng et al. [36], these unreactive particles reduce the mechanical strength of the produced paste [4]. The average pore size in the sample activated with NaOH (Fig 8b) was 0.679 ± 0.135 lm, which is significantly higher than in the sample activated with NaOH + Na2SiO3 (Fig. 8c). These results corroborate the mechanical behaviour exhibited by these systems.
4. Conclusions The feasibility of RCBW as a precursor for the production of cement using the alkaline activation technique was demonstrated. The NaOH content incorporated with the proportion of 10% Na2O, in relation to the mass of RCBW in the paste, is optimal for achieving the best compressive strength. The compressive strength of this mixture, which was cured at 25 °C and 70 °C for 48 h, was 7.49 MPa and 15.57 MPa, respectively, at 28 days. The addition of soluble silica to the alkaline activator (NaOH + Na2SiO3) had a positive effect on the compressive strength of 100% RCBW alkaliactivated, with the possibility of achieving a maximum compressive strength of 54.38 MPa when cured at room temperature (25 °C), which represents a 626% improvement in strength in relation to NaOH. The combined presence of 20% OPC and sodium silicate in the alkaline activator (NaOH + Na2SiO3) yielded a maximum compressive strength of 102.6 MPa at 28 days of curing at room temperature (25 °C). The addition of 10% OPC to the paste activated with NaOH produced a strength of 41.1 MPa for the same curing conditions. These results reflect the positive effect of the addition of a CaO source (OPC) and the presence of soluble silicate (Na2SiO3) in the sample, which corroborates the microstructural analysis.
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Fig. 8. SEM of the alkali-activated pastes; a) 100% RCBW (NaOH + Na2SiO3), b) 10% OPC (NaOH) and c) 10% OPC (NaOH + Na2SiO3).
This study focused on the optimisation of the synthesis variables to obtain the maximum compressive strength. The microstructural analysis indicated that the addition of OPC and Na2SiO3 to the mixture promoted the dissolution of some phases in the RCBW and an increase in their participation in the alkaliactivating processes, which contributed to the improved mechanical behaviour observed in the hybrid systems (RCBW + OPC). Acknowledgements This study was funded by the Colombian Institute for the Development of Science, Technology, and Innovation COLCIENCIAS (Project Hybricement (Contract N° 0638-2013). The authors, who are members of the Composite Materials Group (GMC) from the Centre of Excellence in New Materials (CENM), would like to thanks to Universidad del Valle (Cali, Colombia), in which the experimental work was carried out. References [1] K. Komnitsas, Potential of geopolymer technology towards green buildings and sustainable cities, Procedia Eng. 21 (2011) 1023–1032, http://dx.doi.org/ 10.1016/j.proeng.2011.11.2108. [2] H. Dahlbo, J. Bacher, K. Lahtinen, T. Jouttijarvi, P. Suoheimo, T. Mattila, S. Sironen, T. Myllymaa, K. Saramaki, Construction and demolition waste management e a holistic evaluation of environmental performance, J. Cleaner Prod. 107 (2015) 333–341, http://dx.doi.org/10.1016/j. jclepro.2015.02.073. [3] G. Habert, J.B. d’Espinose de Lacaillerie, N. Roussel, An environmental evaluation of geopolymer based concrete production: reviewing current research trends, J. Cleaner Prod. 19 (2011) 1229–1238, http://dx.doi.org/ 10.1016/j.jclepro.2011.03.012. [4] L. Reig, M. Tashima, M. Borrachero, J. Monzó, C. Cheeseman, J. Payá, Properties and microstructure of alkali-activated red clay brick waste, Constr. Build. Mater. 43 (2013) 98–106, http://dx.doi.org/10.1016/ j.conbuildmat.2013.01.031. [5] F. Pacheco-Torgal, S. Jalali, Reusing ceramics waste in concrete, Constr. Build. Mater. 24 (5) (2010) 832–838, http://dx.doi.org/10.1016/ j.conbuildmat.2009.10.023. [6] J. Xiao, W. Li, Y. Fan, X. Huang, An overview of study on recycled aggregate concrete in China (1996–2011), Constr. Build. Mater. 31 (2012) 364–383, http://dx.doi.org/10.1016/j.conbuildmat.2011.12.074. [7] H. Yuan, A.R. Choni, Y. Lu, L. Shen, A dynamic model for assessing the effects of management strategies on the reduction of construction and demolition waste, Waste Manage. 32 (2012) 521–531, http://dx.doi.org/10.1016/j. wasman.2011.11.006. [8] European Commission, 2015, Resource Efficient Use of Mixed Wastes, Environment, Waste, Studies, Available from: . [9] M. Bravo, J. de Brito, J. Pontes, L. Evangelista, Mechanical performance of concrete made with aggregates from construction and demolition waste recycling plants, J. Cleaner Prod. 99 (2015) 59–74, http://dx.doi.org/10.1016/j. jclepro.2015.03.012. [10] J. Castaño, R. Rodríguez, L. Lasso, A. Goméz, S. Ocampo, Waste management from construction and demolition (RCD) in Bogota: prospects and limitations. Gestión de residuos de construcción y demolición (RCD) en Bogotá: Perspectivas y limitantes, Revista Tecnura 17 (38) (2013) 121–129.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Alternative cements based on alkali-activated red clay brick waste Rafael A. Robayo, Alexandra Mulford, Jorge Munera, Ruby Mejía de Gutiérrez ⇑ Composites Materials Group (CENM), School of Materials Engineering, Calle 13 # 100-00, Edif. 349, 2° piso, Universidad del Valle, Cali, Colombia
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The alkaline activation of red clay
brick waste (RCBW) was investigated. The effect of the addition (620%) of
Portland Cement (OPC) was studied. The addition of Na2SiO3 and OPC
contributed in the better mechanical behaviour. Compressive strength of up to 102 MPa was obtained at room temperature (25 °C). The feasibility of obtain hybrid binders using RCBW is demonstrated.
a r t i c l e
i n f o
Article history: Received 3 July 2016 Received in revised form 13 September 2016 Accepted 6 October 2016
Keywords: Red clay brick waste Ceramic wastes Alkaline activation Alkali-activated cement Hybrid cement Compressive strength
a b s t r a c t The synthesis of alkali-activated and hybrid cements based on red clay brick waste was investigated using sodium hydroxide and sodium silicate solution as alkaline activators. The effect on the compressive strength of Na2O/SiO2 and SiO2/Al2O3 molar ratios and type of curing was evaluated. The combined presence of Portland cement (20%) and sodium silicate yielded a maximum compressive strength of 102.6 MPa at 28 days and 25 °C; this value is two times higher than the strength obtained in mixtures without Portland cement and 7.3 times higher than when sodium hydroxide is used. The results demonstrated the feasibility of using these materials to produce structural and non-structural construction elements. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Komnitsas [1] mentions that the sustainable cities of the future apart from having low energy consumption and greenhouse gas emissions should also adopt the ‘‘zero waste” principle, which contribute to sustainable development and reduction of carbon footprint. Several wastes, including those from mining, metallurgy, municipal, construction and demolition, which are produced today ⇑ Corresponding author. E-mail addresses: [email protected] (R.A. Robayo), material@ univalle.edu.co (A. Mulford), [email protected] (J. Munera), [email protected] (R. Mejía de Gutiérrez). http://dx.doi.org/10.1016/j.conbuildmat.2016.10.023 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
in huge quantities in each country, can be used as raw materials of other industries [1]. The main barriers for recycling are the quality of these wastes, specially its chemical and physical characteristics (which are very heterogeneous), and the low cost or high availability of some virgin raw materials [2]. Therefore, alkaline activation and geopolymerization are technologies that allows us to use waste (with reactive SiO2 and Al2O3 species) that are unsuitable in other industries. It is noteworthy that the application of these technologies in the production of binders, in addition to promoting the use of waste and / or industrial by-products as raw materials, have additional benefits such as lower energy consumption and
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reduced level of emissions, which it has been shown in several studies to compare these to traditional Portland cements [3]. Ceramic wastes are generated by construction and demolition processes in the industrial sector and as a result of the ceramic processing. Approximately 45% of construction and demolition wastes (CDW) is attributed to ceramic products such as bricks, tiles, and porcelain [4]. According to Pacheco-Torgal and Jalali [5], the European ceramics industry generates a volume of waste that is equivalent to 3–7% of total production, which indicates that millions of tons of clay per year are disposed in landfills as the reused volume is minimal. Currently, the countries that generate the largest amount of CDW include China, the United States of America and the countries in the European Union, accounting for approximately 605.5 million tons per year; however, this number may be larger due to natural disasters in recent years [6–8]. In some countries, such as Germany, Denmark and the Netherlands, reuse approaches 80%, whereas the average in other countries is 30% [9]. Although a national statistical study of CDW in Colombia is not available, some studies indicate that 12 million tons per year are generated and remain unused in some cities with higher demographic growth, such as the city of Bogota [10,11]. In the capital of Valle del Cauca, Cali, an average volume of 2480 cubic meters of CDW is generated daily. Of this volume, approximately 76.6% (1900 cubic meters) is generated by construction companies and public construction, which is known as ‘‘formal generation”; the remaining 23.4% (580 cubic meters) is generated by private construction and remodelling, which is referred as the ‘‘informal sector” [11,12]. These large generated CDW volumes have motivated the search for alternatives to enable better use of different component materials (brick, glass, ceramics, and concrete). Alkaline activation technology has been recently considered to be an important option for the reuse of inorganic materials in CDW [3,13–17]. Note that some of these materials, such as red clay brick waste (RCBW), have been subjected to high temperatures for bricks manufacture and in some cases without adequate control, therefore, their reactivity can be reduced [18,19]. Puertas et al. [20] performed alkaline activation of different ceramic wastes with a mixture of sodium hydroxide (NaOH) and sodium silicate (Na2SiO3) at a concentration of 6 M and reported a compressive strength after 8 days of curing of 13 MPa. They attribute this relatively low strength to the semi-crystalline nature of this type of raw materials . Allahverdi and Kani [21] carried out alkaline activation of construction brick waste with a mixture of NaOH and Na2SiO3 (silica modulus 0.6) at a ratio of 8% Na2O with respect to the precursor and obtained a compressive strength of 40 MPa after 28 days of curing. In a subsequent study, the author reported a maximum strength of 50 MPa using a mixture of 60% concrete waste and 40% brick waste activated with NaOH and Na2SiO3 solution (solution ratio: 1.4) as a precursor and the same percentage of Na2O in the mixture. The authors emphasise the importance of taking into account the efflorescence phenomena, which suggests the addition of additives that are abundant in alumina to achieve better crosslinking of the geopolymer and lower mobility of the alkali [19]. Zaharaki et al. [16] obtained similar results (39.4 MPa) using NaOH 10 M and thermal treatment at 80 °C. Sun et al. [22] obtained a compressive strength of 71.1 MPa with urban ceramic waste activated with a mixture of KOH and NaOH and subjected to a treatment process at 60 °C for 28 days. Reig et al. [4,23] produced RCBW geopolymer pastes and mortar with a precursor:sand ratio of 1:3 activated with NaOH (7 M) and sodium silicate with a SiO2/Na2O ratio of 1.60. The authors report a compressive strength of 30 MPa at 7 days of curing at 65 °C and relative humidity above 90%. According to the authors, the optimization of variables such as the liquid/solid ratio can significantly increase the strength, with a maximum value of 50 MPa. Reig et al. [24] with the addition of calcium aluminate
cement (40%) accelerate the reaction and to obtain, at room temperature and 3 days of curing, 50 MPa. Recently, Komnitsas et al. [25] investigated the potential for geopolymerization of different construction wastes; bricks, tiles and concrete, and reported maximum compressive strengths of 49.5 MPa and 57.8 MPa for brick and tile waste, respectively. However, the reported value for concrete waste was only 13 MPa with the use of NaOH (14 M) and a curing treatment at 90 °C for seven days, which is 43% higher than the value reported by Pathak and Kumar [26]. This study presents the evaluation of the mechanical and microstructural properties of RCBW when is activated with NaOH and Na2SiO3 to obtain an alkali-activated cement (100% RCBW) and a hybrid cement (RCBW + ordinary Portland cement (OPC)). RCBW was supplied by a bricks factory. The effect on the compressive strength of the following synthesis variables was analysed: Na2O/SiO2 and SiO2/Al2O3 molar ratios, type of curing (room temperature curing at 25 °C and thermal curing at 70 °C for 24 and 48 h) and quantity of added OPC (between 5 and 20% by weight with respect to the RCBW content).
2. Materials and experimental methodology 2.1. Materials The raw materials employed in the production of alkaliactivated and hybrid cements were RCBW and OPC, respectively. The RCBW was selected from a pile of debris from a brick factory in the region (Cali, Colombia). This waste is generated during the production process, because the bricks are broken during the firing and transportation steps. The comminution of RCBW required the use of a ball mill. The chemical compositions of these materials, as presented in Table 1, were determined by X-ray fluorescence (XRF) using a MagiX-Pro PW–2440 Phillips spectrometer equipped with a Rhodium tube with a maximum power of 4 kW. In RCBW, the high molar ratio of SiO2/Al2O3 (5.58) is significant. The particle size analysis was performed by laser granulometry in a Mastersizer-2000 device by Malvern Instruments, coupled with a Hydro2000MU dispersion unit, in which distilled water was applied as a dispersing medium. The average particle sizes D [4;3] for RCBW and OPC were 24.25 lm and 21.65 lm, respectively. Fig. 1 shows the X-ray diffractogram of the RCBW, which indicates the semi-crystalline nature of the precursor and the presence of quartz (SiO2) (Ref. Pattern: 01-079-1910) as the main phase. The following crystalline minor components were identified: hematite (Fe2O3) (Ref. Pattern: 01-085-0987), muscovite (KAl2 (AlSi3O10) (OH)2) (Ref. Pattern: 00-002-1019) and plagioclases, albite NaAlSi3O8 (Ref. Pattern: 01-076-0898) and anorthite (CaAl2Si2O8) (Ref. Pattern: 01-086-1706).
Table 1 Chemical composition of the raw materials (% oxides). Element
SiO2 Al2O3 Fe2O3 CaO Na2O MgO K2O TiO2 Others LOI Molar ratio SiO2/Al2O3
Material OPC
RCBW
21.13 4.92 4.88 64.27 0.26 1.61 0.25 0.24 4.42 4.14 7.88
65.92 20.08 9.10 0.73 0.44 0.86 0.97 1.09 0.81 – 5.58
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Fig. 1. X-ray diffractogram (XRD) of RCBW.
2.2. Preparation of mixtures and trials The simple (100% RCBW) and hybrid (RCBW + OPC) alkaliactivated pastes were prepared in equivalent conditions. The OPC was incorporated in concentrations between 5 and 20% by weight with respect to the RCBW. As alkaline activators, NaOH (98.22% purity) and commercial sodium silicate (Na2SiO3xH2O) (SiO2 = 32.09%, Na2O = 11.92%, and H2O = 55.99%) were employed. The effect of the Na2O (2-10%) concentration of the activator solution and the SiO2/Al2O3 and Na2O/SiO2 ratios were investigated, which varied from 5.8–7.1 and from 0.06–0.18, respectively. These relations were calculated taking in account the proportions of each component present in the precursor and the activator. The liquid/solid ratio (L/S) was 0.25 ensuring a proper workability. The mixing was carried out in several stages, as follows: (1) preparation of the activating solution in the required proportions, (2) mixture of the activator solution with the solid (RCBW) or mixture of solids (OPC + RCBW) previously homogenized and then addition of the remaining water to complete the L/S ratio specified. The alkaliactivated pastes were obtained in a Hobart mixer with a mixing time of five minutes. The pastes in their fresh states were modelled in cubes with 20-mm sides and were vibrated for 30 s in an electrical vibrating table to remove trapped air. Then, the moulds were covered with a polyethylene film that prevents the evaporation of the free mixing water and cured at room temperature (25 °C) and 70 °C for 24 and 48 h. After this period, the test cubes were unmoulded and taken to a curing chamber that ensures a relative humidity near 80% until the test age was reached. The compressive strength was evaluated in an INSTRON 3369 universal test device with a capacity of 50 kN and a strain rate of 1 mm/min. In each case three specimens was employed. For the microstructural study of the optimal mixtures, the following instrumental techniques were employed: &
&
Fourier transform infrared spectroscopy (FTIR) using the IR-100 spectrometer by Perkin Elmer in transmittance mode with a frequency range between 4000 and 450 cm 1. The samples were evaluated using the compressed KBr pellet methodology. Scanning electron microscopy (SEM) using the JSM 6490LV JOEL electronic microscope with an acceleration voltage of 20 kV. The samples were evaluated in vacuum mode. Coupled with the microscope, the Link-Isis X-ray spectrometer by Oxford Instruments was employed.
3. Results and discussion 3.1. Effect of Na2O content and curing temperature on the compressive strength of 100% RCBW pastes activated with NaOH Fig. 2 shows the effect of the Na2O content and the curing temperature on the compressive strength after 28 days of curing for the 100% RCBW paste activated with NaOH. The highest strength values at 25 °C were achieved with Na2O concentrations between
Fig. 2. Effect of the Na2O concentration and the curing treatment on the compressive strength at 28 days (100% RCBW) (activator: NaOH).
8 and 10%, with a maximum of 7.49 MPa for the mixture with 10% of Na2O. This optimal activator concentration is sufficient for balancing the charges of the Si and Al tetrahedrons. Low concentration of NaOH (<6%) do not provide a sufficient alkalinity affecting the solubility of the precursor, but an excess of NaOH would cause the formation of carbonate salts – as the result of atmospheric carbonation – and generate efflorescence [4,19,21]. Generally, there is an optimum concentration of the alkaline activator, which depends on the particle size and the mineralogy of the raw materials as well as on the synthesis conditions, curing temperature and ageing period [25]. It is well known that the rate of a chemical reaction is increased with the temperature, and an adequate curing is required to achieve advanced mechanical and durability performance in geopolymeric materials. So, an increase in the curing temperature accelerates the reaction kinetics geopolymerization because it favors the dissolution of the active species and thus the material can achieve better mechanical resistance. Generally temperatures between 30 °C and 90 °C are acceptable, on the contrary high temperatures, above 150 °C, and prolonged exposure times can cause negative effects due to the possibility of contractions and microcracks in the matrix by rapid dehydroxylation in the geopolymer gel. As seen in the study, any increase in both time (24 and 48 h) and temperature (25 °C and 70 °C) of hydrothermal curing causes an increase in compressive strength confirming the positive effect of curing at 70 °C. At 70 °C and 48 h, a maximum compressive strength of 16.36 MPa is obtained in the mixture activated with 8% Na2O, which is higher than twice the results at room temperature and 1.5 times more than the result obtained at 70 °C and 24 h. This result corroborates that obtained by Komnitsas et al. [25], the application of thermal energy is necessary to increase the mechanical strength of this type of precursor due to its predominantly crystalline nature, as it was shown in Fig. 1. It is clear that, regardless of the temperature and curing time studied, the proportion of activator would be between 8% and 10% Na2O (Fig. 2). 3.2. Effect of molar ratios SiO2/Al2O3 and Na2O/SiO2 on the compressive strength of 100% RCBW pastes activated with NaOH+Na2SiO3 The effect of the molar ratios SiO2/Al2O3 and Na2O/SiO2 and the curing temperature on the compressive strength of the pastes can be observed using the NaOH + Na2SiO3 mixture as an activator. The results for different curing systems are shown in Figs. 3 and 4. The results indicate that the presence of Na2SiO3 causes a significant increase in the compressive strength of the 100% RCBW pastes.
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R.A. Robayo et al. / Construction and Building Materials 128 (2016) 163–169 Table 2 Optimal synthesis parameters for the 100% RCBW paste activated with NaOH + Na2SiO3. Variable
25 °C
70 °C 24 h
Na2O/SiO2 Ratio SiO2/Al2O3 Ratio Liquid/solid ratio (L/S) CS (MPa) Standard deviation (MPa)
0.12 6.60 0.25 54.38 7.44
0.12 7.10 0.25 66.56 2.5
CS: compressive strength at 28 days of curing.
Fig. 3. Effect of the SiO2/Al2O3 and Na2O/SiO2 ratios on the compressive strength at 28 days of curing (100% RCBW) (activator: NaOH + Na2SiO3) (25 °C).
achieved with the application of 70 °C for 24 h and SiO2/Al2O3 and Na2O/SiO2 ratios of 7.10 and 0.12, respectively. Note that the optimal strength at 28 curing days of the 100% RCBW mixtures, which were activated with NaOH + Na2SiO3 and cured at room temperature (25 °C), is 7.3 times higher than the optimal strength obtained in mixtures activated with NaOH. These results are in agreement with previous studies [4,21,25] where a significant improvement in compressive strength has been observed using sodium silicate solution as activator, although some of the studies have used curing temperatures up to 90 °C. 3.3. Effect of the addition of OPC on the compressive strength of the pastes
Fig. 4. Effect of the SiO2/Al2O3 and Na2O/SiO2 ratios on the compressive strength at 28 days (curing treatment at 70 °C during 24 h) (100% RCBW) (activator: NaOH + Na2SiO3).
This behaviour, according to Duxson et al. [27] and Pacheco-Torgal et al. [28], is attributed to the presence of soluble silica (Si-O-), which is provided by the activator; it provides proper conditions for the formation of a geopolymer structure. Sodium silicate contributes to a better interface between the unreactive particles and the gel matrix and, consequently, to the higher mechanical strength of the aluminosilicate gel [19]. According to the characteristics of the precursor, soluble silicate (SiO2/Al2O3) and sodium (Na2O/SiO2) have optimal quantities that may significantly affect the material properties such as setting time and compressive strength [4]. The optimal SiO2/Al2O3 and Na2O/SiO2 ratios for each curing treatment are listed in Table 2. The best alkali-activated mixture at 25 °C (54.38 MPa) was obtained using an activator solution with a silica modulus (SiO2/Na2O) of 1.29, which is equivalent to SiO2/ Al2O3 and Na2O/SiO2 ratios of 6.6 and 0.12 respectively. Generally, the application of temperature to the pastes promoted an increase in the mechanical strength. However, the optimal synthesis conditions vary according to the applied thermal treatment [25]. The maximum compressive strength (28 days) of 66.56 MPa was
The influence of the addition of OPC on the compressive strength of the optimal mixtures cured at room temperature (25 °C) and activated with NaOH (8% Na2O) and NaOH + Na2SiO3 (SiO2/Al2O3 ratio of 6.60 and Na2O/SiO2 ratio of 0.12) is shown in Figs. 5 and 6, respectively. The addition of OPC to the pastes, regardless of the type of activator, contributes to the increase in the compressive strength over time. In the case of the NaOH activator, the maximum strength was achieved with 10% OPC (41.39 MPa), with the possibility of improving this property by 555% after 28 days of curing compared with the 100% RCBW mixture activated with NaOH. The maximum compressive strength in the pastes activated with NaOH + Na2SiO3 was achieved with 20% OPC, reaching values up to 102.59 MPa after 28 days of curing at room temperature. Similarly, the addition of 5% OPC resulted in 36.61 MPa after seven days of curing. The positive effect of the incorporation of OPC can be attributed to the contribution of CaO and the coexistence of C-S-H gels (product of the hydration of OPC) and N-A-S-H (product of the alkaline activation of aluminosilicate) [17,29–31]. Also, the presence of OPC
Fig. 5. Effect of OPC concentration on the compressive strength at 7 and 28 days (activator: NaOH) (25 °C).
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Fig. 6. Effect of OPC concentration on the compressive strength at 7 and 28 days (activator: NaOH + Na2SiO3) (25 °C).
accelerates the activation of the aluminosilicate, this has been explained by the contribution of heat released in the cement hydration process [31,32]. It should be noted that the reaction products formed during the alkali activation of mixtures of portland cement and aluminosilicates (hybrid cements) are still under study [32]. In the hybrid cements, some authors have demonstrated the possibility of coprecipitation of the two gels N-A-SH and C-S-H, however other studies have concluded that these gels interact, resulting microstructural and compositional changes [32]. Studies conducted by Garcia-Lodeiro et al. [31,32] showed that the stability of N-A-S-H structure in the presence of calcium depends of the pH. At low alkalinity, sodium is partially replaced by calcium to form (N,C)-A-S-H gel. On the contrary, in presence of sufficient calcium and high alkalinity (pH over 12), the formation of C-A-S-H is favored [17,31,32]. This is in accordance with the higher compressive strength obtained in this work (Figs. 5 and 6).
3.4. Microstructure of the alkali-activated pastes For the microstructural analysis, three mixtures were selected: the alkali-activated mixture (100% RCBW) and two hybrid mixtures containing 10% OPC. Fig. 7 presents the FTIR spectra of the pastes with 100% RCBW (NaOH + Na2SiO3), 10% OPC (NaOH) and
Fig. 7. FTIR of the alkali-activated pastes: a) RCBW, b) 100% RCBW (NaOH + Na2SiO3), c) 10% OPC (NaOH) and d) 10% OPC (NaOH + Na2SiO3).
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10% OPC (NaOH + Na2SiO3) cured at room temperature for 28 days. In general, attention is drawn to the reduction in the intensity of some peaks between 475 and 797 cm 1, which correspond to the stretching vibrations of the Si-O-Al bonds and the bonds from the SiO4 tetrahedrons (Si-O stretching), suggesting the dissolution of some phases that are abundant in aluminosilicate in the RCBW [20]. This finding corroborates an increase in the compressive strength of these systems [25]. Similarly, the displacement of the main band of RCBW (1087 cm 1), which is attributed to the stretching vibration of the bonds from the SiO4 (Si-O) tetrahedrons, to higher values between 1011 and 1019 cm 1 suggests the formation of C-A-S-H and/or N-A-S-H gels during the alkaline activation process of the hybrid pastes (RCBW + OPC) [33,34]. Additionally, the position and amplitude of this main band may also be related to the Si-O bonds in the N-A-S-H and C-S-H gels and the coexistence of products associated with Si-O-Si (gel C-S-H) and Si-O-T (gel N-A-S-H) bonds [35]. The band located between 1661 and 1651 cm 1 is characteristic of the vibration for the deformation of the molecular bonds of the study water. Similarly, the detected signal between 3441 and 3464 cm 1 corresponds to the stretching vibration of the –OH bonds. The signals at 694 and 779 cm 1 can be associated with the presence of quartz. For the different systems, a band was identified between 1428 and 1485 cm 1, which may be associated with the asymmetrical stretching vibration of the O–C–O bond produced by the atmospheric carbonatation phenomena of the pastes [19]. Fig. 8 presents the SEM images for the systems with 100% RCBW (NaOH + Na2SiO3), 10% OPC (NaOH) and 10% OPC (NaOH + Na2SiO3) cured at room temperature for 28 days. They indicate that the pastes activated with NaOH + Na2SiO3 presented a denser and more homogeneous gel compared with the paste activated with NaOH, in which the RCBW particles are partially dissolved by the alkaline solution and exhibit low cohesion. According to Yunsheng et al. [36], these unreactive particles reduce the mechanical strength of the produced paste [4]. The average pore size in the sample activated with NaOH (Fig 8b) was 0.679 ± 0.135 lm, which is significantly higher than in the sample activated with NaOH + Na2SiO3 (Fig. 8c). These results corroborate the mechanical behaviour exhibited by these systems.
4. Conclusions The feasibility of RCBW as a precursor for the production of cement using the alkaline activation technique was demonstrated. The NaOH content incorporated with the proportion of 10% Na2O, in relation to the mass of RCBW in the paste, is optimal for achieving the best compressive strength. The compressive strength of this mixture, which was cured at 25 °C and 70 °C for 48 h, was 7.49 MPa and 15.57 MPa, respectively, at 28 days. The addition of soluble silica to the alkaline activator (NaOH + Na2SiO3) had a positive effect on the compressive strength of 100% RCBW alkaliactivated, with the possibility of achieving a maximum compressive strength of 54.38 MPa when cured at room temperature (25 °C), which represents a 626% improvement in strength in relation to NaOH. The combined presence of 20% OPC and sodium silicate in the alkaline activator (NaOH + Na2SiO3) yielded a maximum compressive strength of 102.6 MPa at 28 days of curing at room temperature (25 °C). The addition of 10% OPC to the paste activated with NaOH produced a strength of 41.1 MPa for the same curing conditions. These results reflect the positive effect of the addition of a CaO source (OPC) and the presence of soluble silicate (Na2SiO3) in the sample, which corroborates the microstructural analysis.
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Fig. 8. SEM of the alkali-activated pastes; a) 100% RCBW (NaOH + Na2SiO3), b) 10% OPC (NaOH) and c) 10% OPC (NaOH + Na2SiO3).
This study focused on the optimisation of the synthesis variables to obtain the maximum compressive strength. The microstructural analysis indicated that the addition of OPC and Na2SiO3 to the mixture promoted the dissolution of some phases in the RCBW and an increase in their participation in the alkaliactivating processes, which contributed to the improved mechanical behaviour observed in the hybrid systems (RCBW + OPC). Acknowledgements This study was funded by the Colombian Institute for the Development of Science, Technology, and Innovation COLCIENCIAS (Project Hybricement (Contract N° 0638-2013). The authors, who are members of the Composite Materials Group (GMC) from the Centre of Excellence in New Materials (CENM), would like to thanks to Universidad del Valle (Cali, Colombia), in which the experimental work was carried out. References [1] K. Komnitsas, Potential of geopolymer technology towards green buildings and sustainable cities, Procedia Eng. 21 (2011) 1023–1032, http://dx.doi.org/ 10.1016/j.proeng.2011.11.2108. [2] H. Dahlbo, J. Bacher, K. Lahtinen, T. Jouttijarvi, P. Suoheimo, T. Mattila, S. Sironen, T. Myllymaa, K. Saramaki, Construction and demolition waste management e a holistic evaluation of environmental performance, J. Cleaner Prod. 107 (2015) 333–341, http://dx.doi.org/10.1016/j. jclepro.2015.02.073. [3] G. Habert, J.B. d’Espinose de Lacaillerie, N. Roussel, An environmental evaluation of geopolymer based concrete production: reviewing current research trends, J. Cleaner Prod. 19 (2011) 1229–1238, http://dx.doi.org/ 10.1016/j.jclepro.2011.03.012. [4] L. Reig, M. Tashima, M. Borrachero, J. Monzó, C. Cheeseman, J. Payá, Properties and microstructure of alkali-activated red clay brick waste, Constr. Build. Mater. 43 (2013) 98–106, http://dx.doi.org/10.1016/ j.conbuildmat.2013.01.031. [5] F. Pacheco-Torgal, S. Jalali, Reusing ceramics waste in concrete, Constr. Build. Mater. 24 (5) (2010) 832–838, http://dx.doi.org/10.1016/ j.conbuildmat.2009.10.023. [6] J. Xiao, W. Li, Y. Fan, X. Huang, An overview of study on recycled aggregate concrete in China (1996–2011), Constr. Build. Mater. 31 (2012) 364–383, http://dx.doi.org/10.1016/j.conbuildmat.2011.12.074. [7] H. Yuan, A.R. Choni, Y. Lu, L. Shen, A dynamic model for assessing the effects of management strategies on the reduction of construction and demolition waste, Waste Manage. 32 (2012) 521–531, http://dx.doi.org/10.1016/j. wasman.2011.11.006. [8] European Commission, 2015, Resource Efficient Use of Mixed Wastes, Environment, Waste, Studies, Available from:
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