RSC Advances PAPER
Cite this: RSC Adv., 2016, 6, 39106
Mechanical activation of volcanic ash for geopolymer synthesis: effect on reaction kinetics, gel characteristics, physical and mechanical properties ab a a Jean Noe ¨l Yankwa Djobo,* Antoine Elimbi, Herve´ Kouamo Tchakoute´ b and Sanjay Kumar*
This paper looks at the possibility of using low reactive volcanic ash for making geopolymer cement. The research is directed towards (a) alteration of the reactivity of volcanic ash by mechanical activation, and (b) use of mechanically activated volcanic ash for the synthesis of a geopolymer. The effect of mechanical activation was quite visible on particle size distribution and the degree of crystallinity. The disappearance of some anorthite peaks and appearance of quartz peaks in volcanic ashes milled for 120 min demonstrate the change in mineralogy. The appearance of an intense carbonate band with milling time could be related to sorption of atmospheric CO2 on the grains surface during mechanical activation. The manifestation of mechanical activation of volcanic ash was prominent on (a) the reaction kinetics, (b) microstructural development, and (c) physico-mechanical properties of the geopolymer product. The rate constant and extent of geopolymerization increased with milling time but decreased with curing temperature. This decrease is in non-conformity with other alumina-silicate materials used for geopolymerization such as metakaolin and fly ash. FEG-SEM and EDAX results revealed that the geopolymer gel obtained is mixture of poly(ferro-sialate-siloxo) and poly(ferro-sialate-disiloxo) binder type with a formula close to [Ca,Na,K,Mg]–[–Fe–O–]x–[Si–O–Al–O–]1x–[–Si–O–]y. The physicomechanical properties changed significantly. Setting time reduced by >95% in samples milled for 60 min Received 9th February 2016 Accepted 14th April 2016
or more. The compressive strength which was negligible for 0–30 min milled volcanic ash reached 29– 54 MPa after 60–120 min of milling time. Heat curing influenced the early age (7 and 28 days)
DOI: 10.1039/c6ra03667h
compressive strength but the 90 day compressive strength of both ambient and heat cured samples
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were comparable.
1. Introduction Volcanic ashes are natural waste materials which consist of fragments of pulverized rocks during volcanic eruptions. The size of the ash particles, which fall on the ground decreases exponentially with increasing distance from the volcano and differs from one volcano to another.1 Volcanoes with a huge amount of volcanic ash surrounding them are available throughout the world, for example, Mount St. Helens in the USA, Bezymianny and Klyuchevskoy in Russia, Fuji in Japan, Etna in Italy, Pinatubo in the Philippines, Hekla in Iceland, Awu in Indonesia, and Nyiragongo in the Democratic Republic of Congo etc. In Cameroon volcanic ashes are also available along the Cameroon volcanic line which is oriented at N30 E and a
Laboratory of Applied Inorganic Chemistry, Department of Inorganic Chemistry, Faculty of Science, University of Yaound´e I, P.O. Box 812, Yaound´e, Cameroon. E-mail:
[email protected]; Tel: +237 675 97 87 70
b
CSIR-National Metallurgical Laboratory, Jamshedpur 831007, India. E-mail: sunju@ nmlindia.org; Tel: +91 9939326346
39106 | RSC Adv., 2016, 6, 39106–39117
spreads for around 1600 km.2,3 The chemical composition of volcanic ashes is closely related to the composition of source magma. Due to their physical, chemical and mineralogical properties, they are used in cement industry as pozzolanic materials to enhance the durability of cement,4–7 as lightweight aggregates for road construction or for making lightweight concretes8 etc. Recent works have shown that volcanic ashes can also be used as feedstock for geopolymer cement synthesis.2,9–12 Geopolymers are a new class of three-dimensional inorganic polymer obtained by reaction of an aluminosilicate material with an alkaline solution.13 Geopolymer materials have drawn more attention in past decade, mostly because of low CO2 emission and also due to its good physical and mechanical properties and excellent durability.14 These properties depend on many parameters including the type of starting materials, the mineralogical and the chemical composition of the aluminosilicate sources, the particle size distribution of the raw material, the curing temperature and regime, the composition of the alkaline solution and the liquid to solid ratio.15 Use of
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Paper
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volcanic ash for synthesis of geopolymer are reported in literature.2,9,10,12,16–18 However, its application as a building material is restricted due to limitations like long setting time, high shrinkage and excess efflorescence.9 These limitations are due to the poor reactivity of volcanic ashes with alkali activators, which results into the inconsistent and incomplete reaction. Attempts were made to ameliorate these properties by using mineral additives,16,19 or optimizing the composition of the alkaline solution and curing regime.10,17 Tchakout´ e et al.20,21 have recently proposed alkali fusion process as a mean to increase the reactivity of volcanic ashes for geopolymer synthesis. But this process is energy-intensive. One of the promising processes used for increasing reactivity of aluminosilicate materials is mechanical activation.22 Mechanical activation (MA) is a process of mechanical breakdown (by grinding) of solids into smaller particles without changing their state of aggregation.22 The rst step of MA consists of increasing the reactivity of the substance (an increase of internal and surface energy, an increase of surface area and change in the structural disordering). The second and nal step is aggregation, adsorption, and/or recrystallization, which takes place spontaneously in the activated systems aer some time of milling.22 Mechanical activation was successfully applied to y ash and slag for geopolymers synthesis and has shown signicant increase in nal properties of result products.23–27 The present work was carried out with an objective to alter the reactivity of volcanic ash using mechanical activation. The effect of MA upon the degree of crystallinity, reaction kinetic of geopolymerization and changes that occurred on reaction products at ambient and elevated temperatures was studied. The reaction kinetic of geopolymerization was monitored by isothermal conduction calorimetry, Fourier transformed infrared spectroscopy, X-ray diffractometry and eld emission gun-scanning electron microscopy (FEG-SEM) coupled with energy dispersive X-ray spectrometer were used for assessing changes occurred in both mechanically activated volcanic ash and resulting geopolymer products and its effects on physical and mechanical properties were measured. An attempt has been made to correlate the reaction, structure, and properties.
2.
Experimental methods
2.1. Materials and synthesis process Volcanic ash used in this work originated from the volcanic deposit of Loum located N 04 430 210 500 and altitude 482 meters (Littoral Region of Cameroon). The chemical composition of volcanic ash carried out by X-ray uorescence spectrometer (XRF) is reported in Table 1. The mineralogical composition of volcanic ash determined by XRD (as described in following
Table 1
Oxides Wt%
Fig. 1 XRD patterns of initial (0 min), 90 min and 120 min milled volcanic ash Ano (anorthite); F (feldspar-Na); A (augite); H (hematite); Ds (diopside sodian); Da (diopside aluminian); Fs (forsterite syn); Q (quartz).
section), is shown in Fig. 1 and includes: anorthite (Ano) Ca(Al2Si2O8), PDF#89-1459; feldspar-Na (F) NaAlSi3O8, PDF # 898575; hematite (H) Fe2O3, PDF #03-0812; forsterite syn (Fs) Mg2SiO4,PDF #85-1357; diopside aluminian (Da) (Ca (Mg, Fe, Al) (Si, Al)2O6), PDF # 38-0466; diopside sodian (Ds) (Ca0.52Na0.29Fe0.10Mg0.09)(Mg0.057Fe0.14Al0.27Mn0.01Ti0.01)(Si2O6), PDF # 851692; augite (A) Ca0.61Mg0.76Fe0.49(SiO3)2, PDF #76-0544. Volcanic ash was rst ground in a ball mill in order to get powder of
