Original Research Paper
Effect of synthesis parameters on the quality of construction and demolition wastes (CDW) geopolymers

https://doi.org/10.1016/j.apt.2014.11.012Get rights and content

Highlights

  • Tiles and bricks are well geopolymerized; their compressive strength exceeds 49 MPa.

  • Concrete shows low geopolymerization potential (max. compressive strength 13 MPa).

  • The optimum conditions are: 8–10 M NaOH, temperature 80–90 °C and ageing for 7 days.

  • SEM–BSI images show increased homogeneity of the geopolymeric structure.

  • FTIR analysis identifies the major fingerprints of a geopolymeric matrix.

Abstract

In the present study, the geopolymerization potential of construction and demolition wastes (CDW) as well as the effects of the molarity of the alkaline activating solution, the curing temperature, the ageing period and the particle size of the raw materials on the compressive strength of the final products have been studied. For the synthesis of geopolymers, concrete, bricks and tiles collected from various demolished buildings were mixed with the activating solution (NaOH and Na2SiO3). Various synthesis conditions (curing at 60–90 °C, 8–14 M NaOH molarity, particle size) have been considered. Results have shown that tiles and bricks are well geopolymerized, reaching a compressive strength of 49.5 and 57.8 MPa, respectively, while concrete shows limited geopolymerization potential since it reaches a compressive strength of only 13 MPa. The effects on the compressive strength of the specimens were also assessed by considering various molar ratios of the oxides present in the initial paste including SiO2/Al2O3 and H2O/(Na2O + K2O). CDW geopolymers synthesized under the optimum conditions were also subjected to high temperature heating for one hour, freeze–thaw cycles and immersed in distilled water for one and two months to assess changes in their structural integrity. Analytical techniques, X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscopy (SEM) were used for the identification of the morphology and structure of the final products.

Graphical abstract

Evolution of the compressive strength of geopolymers produced from (a) tiles, (b) bricks and (c) concrete vs. NaOH concentration and heating temperature.

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Introduction

Geopolymers are inorganic materials formed by the chemical reaction between alumino-silicate oxides and alkali-metal silicate solutions under alkaline conditions, with partially or fully amorphous polymeric structures consisting of Si–O–Al bonds. The tetrahedral AlO4 and SiO4 units are built in three dimensional structures [1], [2], [3].

The structure and mechanical properties of geopolymers are affected by several parameters such as chemical composition and particle size of the raw materials, nature and concentration of the activating solution, curing temperature and ageing period. Most studies have shown that the alkali activator type and its concentration play the most important role in the strength of geopolymers [4], [5]. Normally, increased molarity of the activating solution has a beneficial effect on the compressive strength of the geopolymers [6], [7], [8]. Other studies indicate that very high molarities of the activating solution have an adverse effect on the compressive strength [9], [10], [11].

Komnitsas et al. [12] and Görhan and Kürklü [13] mention that there is an optimum concentration of the alkaline activator, which depends on the particle size and the mineralogy of the starting materials as well as on the synthesis conditions, beyond which compressive strength may substantially decrease. Curing temperature and ageing period are also key factors that determine the final properties of geopolymers [14].

During the last years, various wastes such as fly ash and slag have been extensively investigated as potential raw materials for the synthesis of geopolymers [15], [16], [17]. Geopolymerisation of other wastes and especially construction and demolition wastes (CDW) still remains a great challenge. Allahverdi and Kani [18] investigated the possibility of utilizing bricks and concrete in different mix proportions to produce geopolymer cement. Their results indicate that waste bricks are more suitable than waste concrete for geopolymeric reactions. Ahmari et al. [19] focused on the synthesis of geopolymeric binder using mixtures of waste concrete powder and fly ash, showing that increased NaOH molarity (10 M) results in higher compressive strength, reaching almost 30 MPa. They also mention that addition of sodium silicate solution improves compressive strength.

Reig et al. [20] aimed to optimize alkali-activation of red clay brick waste investigating the effects of the type and concentration of alkali activator used on the mechanical strength and microstructure of geopolymers. The best alkali activated products were obtained using 5 mol/kg NaOH after curing for 7 days and acquired compressive strength close to 30 MPa which was further increased to 50 MPa by optimizing the water to binder, binder to sand and SiO2/Na2O ratios. Sun et al. [21] studied the potential of using waste ceramic for the synthesis of geopolymers and specimens with a compressive strength of 71 MPa were produced.

The present experimental study aims at the synthesis and characterization of geopolymers produced using the main components of CDW, namely concrete, bricks and tiles as source material. It also attempts to assess their behavior after high temperature heating for one hour, freeze–thaw cycles and immersion tests in distilled water for a period of two months. XRD, FTIR and SEM were used for the elucidation of the morphology of the final products.

Section snippets

Materials

CDW including concrete, bricks and tiles were used as raw materials in this study. CDW were collected from various demolished buildings, cleaned, dried, pulverized and homogenized. The materials were pulverized using a Bico Pulverizer Type UA and a FRITSCH-BICO Pulverizer. Two different particle size fractions were obtained, as shown in Table 1, in order to study the effect of the particle size of raw materials on the compressive strength of the final products.

Table 2 shows the chemical

Effect of NaOH concentration and curing temperature

Fig. 1 shows the evolution of the compressive strength of geopolymers produced from tiles, bricks and concrete vs. the molarity of NaOH solution and curing temperature after an ageing period of 7 days.

It is shown that tiles (Fig. 1a) are the components of CDW that exhibit the best geopolymerisation potential. Under the optimum synthesis conditions (10 M NaOH, curing temperature 80 °C) their compressive strength reached 57.8 MPa after 7 days of ageing. It is also shown that lower or higher NaOH

Conclusions

Two of the main components of construction and demolition wastes, namely bricks and tiles can be successfully geopolymerized reaching a compressive strength of 49.5 and 57.8 MPa, respectively. The optimum synthesis conditions are 8–10 M NaOH, curing temperature 80–90 °C and ageing period 7 days. Lower strength (13 MPa) was obtained for concrete-based geopolymers. Bricks and tiles are geopolymerized more easily since they have high SiO2 and Al2O3 and low CaO content. Also, the grain size of the raw

Acknowledgements

The present study has been co-funded by the European Commission (European Regional Development Fund) and by national funds through the Operational Programme “Competitiveness and Entrepreneurship” (OPCE II 2007–2013), National Strategic Reference Framework – Research funded project: “Recycling of quarry dust and construction and demolition wastes for the production of novel ecological building elements”, DURECOBEL 11SYN_8_584, in the framework of the Action COOPERATION 2011– Partnerships of

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