Solid waste management is one among the basic essential services provided by municipal authorities in the country to keep urban centres clean. Due to rapid urbanization and industrialization the production of various types of solid wastes which pose serious problem to the environment have been generated. So the disposal and reuse of solid industrial wastes like phosphogypsum, flurogypsum, flyash, slag and lime sludge, etc. is significant in view of their availability and potential application.
It is estimated that about 300 million tonnes of inorganic waste from industrial and mining sectors are generated every year in India. Advances in solid waste management resulted in alternative construction materials as a substitute to traditional building materials like bricks, blocks, tiles etc. The efforts are being made for recycling different wastes and utilize them in value added applications.
Since almost every natural resources are over exploited and are at the verge of extinction it is the high time for all of us to live aside our traditional conservative approaches and to move forward with new alternatives which are ecofriendly and techniques that leads towards a sustainable development.
Synergistic utilisation of major industrial wastes generated in India, namely fly ash, blast furnace slag and red mud, has been explored to develop novel building components using geopolymerisation. These include: (a) high strength cements (b) self glazed wall tiles, and (c) pavement tiles. Fly ash was used as main source of silico-aluminate for geopolymerisation. Granulated blast furnace slag (GBFS) and red mud were used individually or in combination with fly ash to tailor properties of the developed components. Chemical and mechanical activation have been judiciously incorporated in the processing schemes through an understanding of processing-structure-property relationships. Improvement in the reactivity of fly ash by mechanical activation using highenergy mills was found to results in the formation of a compact microstructure during geopolymerisation leading to high compressive strength (above 100 MPa) in geopolymer cements. The cements also exhibited improved setting time and a very low autoclave expansion. In self-glazed wall tiles, the hard impervious glazed surface was obtained at temperature lower than 150°C by controlling the particle size distribution of solid reactants, viscosity of slurry and reaction atmosphere. The self-glazed surface showed the presence of gismodine (Na-plagioclase) phase which was absent in the main body of the tiles. In pavement tiles, fly ash and granulated blast furnace slag were used to give structural framework,whereas red mud was used to supplement the iron oxide for colouringeffect and alkalis. The setting and hardening occurred due to formation of cementitious A-S-H and C-S-H gel (A =
Al 2 O 3 , S = Si O 2 , C =CaO, H = H 2 O ). The technologies have been developed at bench scale and efforts are underway for scaling up to pilot plant level.
This report aims at studying the recycling and utilisation of industrial wastes in making value added building materials.
The concept of industrial ecology is based on integration of by-productand waste steams across industries leading to production of useful products with near zero flow of material to the environment. Building industry is one of the most dynamic sectors with enormous potential of industrial symbiosis and synergistic utilisation of industrial wastes. With increasing environmentawareness, there is growing concern worldwide for updating production processes, as well as development of green building materials. The green material can be defined as products made from waste, recycled or by-productsto conserve natural resources, circumvent toxic and other emissions, saves energy, and contribute towards a safe and healthy environment. Geopolymers, silico-aluminate materials formed through mimicking natural rock forming process, are fast emerging as new class of greenbuilding construction materials. In the process of geo-synthesis,silicon (Si) and aluminium (Al) atoms react to form molecules that arechemically and structurally comparable to those binding natural rock andallows for novel products synthesis that exhibit the most ideal properties ofrock-forming elements, i.e., hardness, chemical stability and longevity. Flyash, blast furnace slag and red mud are the three major industrial wastes inIndia. Presently over 100 million tonnes of fly ash, 12 million tonnes ofblast furnace slag and nearly 4 million tonnes of red mud are generated. It isestimated that production of these wastes will double in foreseeable futuredue to rapid expansion coal based power generation, and increase in theproduction of iron & steel and aluminium through primary processing. Thesewaste materials contain
SiO 2 and Al 2 O 3 , along with Fe 2 O 3 , CaO, MgO, MnO,etc, and have immense potential as man made raw materials forgeopolymers.
During geopolymerisation process, the alumino-silicate fraction reactswith alkaline media and transform into a solid geopolymer product, via a dissolution-polycondensation-structural reorganisation mechanism, to developstrength. Blast furnace slag behaves differently during geopolymerisationas compared to fly ash and clay. This is attributed to its higher reactivity dueto mostly glassy structure which, leads to faster dissolution of Si and Alduringgeopolymerisation. The CaO portion of the slag particles does not necessarilyparticipate in polycondensation, but reacts with water and may undergo hydrationreaction. It has also been reported that addition of blast furnaceslag in the conventional silico-aluminate geopolymer cement and concreteimproves setting characteristics. Use of red mud in geopolymers appearsto be an attractive proposition from the point of view of its high alkaline content. However, there have very limited attempts in this direction.
This paper is based on recent research on the development ofa wide variety of geopolymeric products using fly ash as the main rawmaterial along with granulated blast furnace slag (GBFS) and red mud. The focus is on: (a) high strength cement, (b) self-glazed tiles, and (c) pavementtiles. Processing, structure and properties of the products are highlighted. The commercialisation prospects of the products are also discussed.
MATERIALS AND METHODS
The fly ash and GBFS used in the study were obtained from a cement grinding unit at Chattisgarh State, India. The red mud was obtained from a Aluminium Plant at Orissa State, India. The chemical analysis and physical properties of the fly ash, GBFS and red mud are given in Table2.1.
Table2.1. Chemical analysis and physical properties of Fly ash, GBFS, Red mud
Si O 2 + Al 2 O 3 + Fe 2 O 3 content of the fly ash was > 70% and it was a Class F fly
ash as per ASTM. The basicity of the slag (CaO/SiO2 ratio) was 1.06.The batch composition of the each product was selected based on chemical, physical and mineralogical characteristic and reactivity. The composition of the products, in terms of the amounts of fly ash, blast furnace slag and red mud used are indicated in the ternary diagram shown in Fig 2.1.
Figure2.1.Composition of geopolymer cement,self glazed tiles and pavement tiles
The geopolymer cements were prepared using the raw as well as pre-processedfly ash. The techniques used for pre-processing include air classification and mechanical activation. Air classification was carried out using an air classifier 50 ATP. Mechanical activation of fly ash was carried out using an attrition mill and a vibratory mill. Sodium hydroxide was dissolved in water atleast 24 hours before use. In case of geopolymer cement, geopolymerisation was carried out by keeping the samples in controlled humidity for 24 hours at 27°C followed by thermal curing at 60°C for 24 hours.
For self glazed tile, fly ash and a ball milled granulated blast furnace slag (GBFS) were used as main materials. The batch composition was thoroughly mixed with alkaline activator and then casted in tile mould. Colour pigments were added into the slurry before casting to get the desired colouring effects. The casted tiles were then edged for 24 hours at 27°C followed by heat treatment in the range of 150-300°C using different thermal cycle.
Inpavement tiles, red mud was intimately mixed with fly ash and ball milled GBFS. The batch was prepared by adding coarse, medium and fine aggregates and alkaline activator. The slurry was casted in rubber moulds and allowed to set at ambient temperature. In all the cases, physical properties were tested as per standard procedures discussed elsewhere. The heat evolution during reactions was monitored using an isothermalconduction calorimeter. XRD patterns of samples were recorded on a Siemens Diffractometer using CoKα radiation. The morphology of the samples was examined using Scanning Electron Microscope (SEM) with EDSattachment for micro-analysis.
GEOPOLYMERISATION OF WASTE
Most proposed mechanisms of geopolymerisation consist of dissolution of aluminosilicate phase, polymerisation and re-precipitation of gel phase, and transformation of the gel phase into geopolymer of varying crystallinity and structure. Depending upon experimental conditions, the different stages of geopolymer formation may overlap and even merge with each other. Isothermal conduction calorimetry was used to study the geopolymerisation of fly ash, mixture of (GBFS+fly ash), and the mixture containing (fly ash+GBFS+red mud). The calorimetry was carried out under following conditions:
(i) 27°C for 24 h (Fig.3.1)
(ii) 60°C for 24 h after keeping the sample at 27°C for 24 h (Fig.3.2)
Figure.3.1 Isothermal Conduction Calorimetry curve showing dissolution of samples at 27°C
Figure.3.2 Isothermal Conduction Calorimetry curve showing geopolymerisation at 60°C after 24 hour dissolution at 27°C
The calorimetric response in Fig.3.1 for the alkali fly ash mixture at 27°C is expected to represent dissolution polymerization and initiation of gel formation. Response in Fig.3.2 signifies the conversion into geopolymer via gel formation step. Based on conduction calorimetric results, the following observations were made:
(i) at 27°C (Fig.3.1), the initial exothermic peak I in all the cases indicates that reaction started as soon as the starting powders was mixed with NaOH solution. In the case of (fly ash + GBFS + red mud) mixture, a minor peak was observed at the start, which after a short induction period of 30 min accelerated to stronger exothermic peak. No such induction and acceleratory period was observed in other samples. In
terms of maximum reaction rate, (fly ash + GBFS + red mud) mixture has shown maximum reaction rate followed by (fly ash + GBFS) and fly ash.
(ii) at 60°C, after edging the sample at 27°C for 24 h (Fig.3.2), two peaks were observed in all the samples. First endothermic peak, observed during 0–250 min, corresponds mainly to dehydration that occurred owing to the evaporation of water formed or water remained unused during gel formation. The area under the endothermic peak for (fly ash + GBFS) sample showed a higher area, indicating greater amount of gel formation. After the endothermic peak, the broad flat exothermic peak, observed during 240–1200 min, corresponds to polycondensation. This peak started and flattens earlier in fly ash. Whereas in other cases, the peak took longer time to flattens possibly due to presence of GBFS,which resulted into formation of C-S-H gel.
Figure 3.3 Conceptual model for geopolymerisation
DEVELOPMENT OF NOVEL BUILDING MATERIALS
4.1. GEOPOLYMER CEMENT
Low reactivity of fly ash has often restricted the use of fly ash for geopolymer cements due to slow strength development. The reactivity of fly ash depends on its vitreous phase content, which participates in geopolymerisation reaction. The remaining constituents takes longer time for reaction due to poor reactivity and leads to slow setting and strength development in geopolymers. Various methods such as chemical activation, mechanical activation and size classification of fly ash hasbeen suggested as a means to improve the reactivity. Recently observations were made by the present authors that use of mechanically activated fly ash leads to high compressive strength in geopolymers. Two different approach were adopted to enhance reactivity of fly ash: (a) air classification to separate finer fractions, and (b) mechanical activation in attrition and vibratory mills. Small size cenosphere cools faster during their formation in coal combustion process and separation of finer fraction by air classification results in increase in the glass contents vis-à-vis raw fly ash. Mechanical activation results due to combined effect of particle breakage (surface area) and other bulk and surface physicochemical changes induced by the process of milling.
The change in reactivity of fly ash due to increased glass content and mechanical activation was assessed vis-à-vis raw fly ash. Fig.4.1 shows typical particle size distribution of raw fly ash (RFA), classified fly ash (CFA), attrition milled fly ash (AMFA) and vibratory milled fly ash (VMFA). Based on median particle size (X50), the samples can be arranged in following descending order: RFA (36.2 μm) > VMFA (5.99 μm) > AMFA (4.85 μm) > CFA (2.79 μm).
Figure.4.1 particle size distribution of RFA, CFA, AMFA and VMFA
Figure.4.2 XRD pattern of RFA, CFA, AMFA and VMFA based geopolymers showing formation of alumina-silicate gel and hydroxysodalite
Figure.4.3 SEM micrograph showing compact structure observed in VMFA based geopolymers
Figure.4.4 Geopolymer cement and concrete cubes
Figure.4.2 shows XRD pattern of RFA, CFA, AMFA and VMFA based geopolymers.Similar nature of XRD patterns suggests formation of same phases in all the samples.The XRD patterns show a broad peak in the range of 10–16° corresponding to aluminosilicate gel. In addition, presence of hydroxyl- sodalite phase is observed indicating geopolymerisation. The most compact microstructure was obtained in VMFA based geopolymers with high proportion of reaction product (Fig.4.3). Based on the relative amount of geopolymer product formed and compactness of microstructure, the reactivity of fly ash samples decreases in following order: VMFA > AMFA > CFA > RFA. In spite of higher particle size of VMFA and AMFA (5-6 μm) as compared to CFA (~ 3 μm), higher reactivity of mechanically activated samples was interesting.
The physical properties of the geopolymer cement are given in Table 2.The higher strength in AMFA and VMFA is attributed to higher reactivity due to mechanical activation that leads to enhanced geopolymerisation and more compact microstructure. Significantly higher strength of samples preparedusing VMFA over corresponding AMFA samples highlights the importance of mechanical activation device. The other properties, such as setting time, autoclave expansion, etc. indicate that the developed geopolymer cements meet the specifications drawn for hydraulic cements like ordinary Portland cement, Portland slag cement and Portland pozzolanacement.
Table 4.1 Properties of geopolymer cement
4.2. SELF GLAZED TILE
Conventionally ceramic tiles are produced by high temperature sintering/ vitrification of aluminosilicate and silicate minerals such as clay, quartz, feldspar, etc. The strength and other properties of tiles are developed due to formation of ceramic bonds. Development of stoneware tiles at 250-400°C by geopolymerisation of alumino-silicate minerals has been reported. The processing involved reaction between aluminosilicate mineral kaolinite and NaOH at 100°C-150°C resulting into the formation of hydro-sodalite
In the alkali activation of fly ash and slag mixture at ambient temperature, fly ash/slag ratio is the most relevant factor on the strength development. The main reaction product is a hydrated calcium silicate with high amount of tetra-coordinated Al in its structure. The additions of calcium content increase the degree of geopolymerisation at elevated temperature and results into higher strength. Beneficial effect of slag on fly ash geopolymerisationwas exploited in the development of self glazed wall tiles.
The glazed surface and the body of tiles showed distinctly different microstructure as revealed by SEM micrographs shown in Fig 4.5a and Fig 4.5b, respectively.
The glazed surface is characterized by well knitted grains giving rise to compact microstructure with very low porosity (Fig.4.5 a). The morphology of the tile body shows the well reacted porous body. XRD studies indicated the presence of gismodine (Na-plagioclase) phase in the glazed surface (Fig.4.5 c).
Figure 4.5 a. Closely knitted microstructure of glazed surface, b. well reacted microstructure of tile body, c. XRD pattern of glazed and body of tile, and d. tiles of different colour
Two type of bulk reaction products were identified by EDX studies: (a) a lowcrystallinecalcium silicate hydrate rich in Al, which includes Na into its structure and results from the alkali activation of slag, and (b) an amorphousalkaline aluminosilicate hydrate resulting from the alkali activation of fly ash.Critical control on particle size distribution, chemical composition, rheology of slurry and reaction environment is necessary for the formation of required phases in the glazed surface.
The properties of glazed geopolymer tile are summarised in Table 4.2.
Table 4.2 Properties of geopolymer tile
The tiles developed conform to the European Nation (EN) specification for wall tiles. The natural colour of the tiles was light grey but different colour and designs were produced using colour pigments. Unlike the fired ceramic tiles, no crazing and other glaze defects were observed. Although the surface of the tile was impervious, the porosity of body was 13-17%, which is good for bonding with cement.
4.3 PAVEMENT TILES
Pavement tiles are small cement structures in geometrical shapes that are usually laid on pathways or on any open ground as a solid platform. As these tiles are not cemented and only laid closely over a bed of loose sand, they can be easily removed, stored and reused as many times as possible. In India, the pavement tiles are mostly vibro-cast and/or pressed cement mortar or concrete hydrated for 28 days. The strength is obtained due to hardening of cement. Earlier research on alkali-slag-red mud-cement (ASRC) has indicated high early and ultimate strength together with excellent resistanceagainst chemical attacks. This was achieved by introduction of solidcomposite alkali activator into slag–red mud mixture system instead of liquid water glass. The hydration products of ASRC cement were mostly C-S-H gel with low Ca/Si ratio in the range of 0.8 to 1.2.
In the present work fly ash, GBFS and red mud was used to develop cementitious phases by alkali activation at ambient temperature. Red mud was used to give colouring effect to the tiles from presence of iron oxides, and also partly supplement alkalies for reaction. Similar to self glazed tiles, two major reaction products were identified in XRD (Fig 4.6) and EDX studies. C-S-H gel with Na in its structure from the activation of GBFS, and amorphous alkaline alumino-silicate hydrate resulted from the activation of fly ash. Also presence of iron oxide and hydroxide resulted from addition of red mud.
Figure 4.6 XRD pattern of pavement tiles
The SEM studies (Fig4.7) have shown a dense microstructure including numerous fibrous structures corresponding to C-S-H gel with Na in structure.
Figure 4.7 Dense microstructure with numerous fibers
Figure 4.8 Pavement tiles
Table 4.3 Properties of pavement tile
The tiles, although produced at ambient temperature (27-35°C), exhibits good compressive and flexural strength.
SYNERGETIC USE OF INDUSTRIAL WASTE
Synergistic use of industrial waste is an emerging concept whereby combination of two or more wastes is used to develop a useful product. The main advantage of the synergy is the deficiency of constituents from one waste is compensated by using second or third waste, which is rich in deficient constituent. Synergistic use also includes industrial symbiosis where physical exchange of waste/by-products between geographically close industries is exploited. In the present work, waste from three industries, fly ash from thermal power plants, fly ash and granulated blast furnace slag from Steel Plants, and fly ash and red mud from Aluminium plants, has been used.
Fly ash was used for the development of geopolymers cement, combination of fly ash and blast furnace slag was used for self-glazed tiles and all three wastes fly ash, GBFS and red mud was used for pavement tiles. From the point of view of zero or minimum flow into environment, geopolymer cement is best suited for thermal power plant, where fly ash is the main by-product. Self-glazed tiles and pavement tiles are more suitable for iron & steel and aluminium industry respectively. Fig.5.1 shows the synergy map of these wastes.
Figure.5.1 Synergy map for the Utilisation of different type of Industrial wastes
ECONOMICS AND COMMERCIAL POTENTIAL
The cost of most industrial production is increasingly influenced by the operations required for the adequate disposal of by-products. The disposal cost as per regulations may add 5-10% of the production cost depending on volume and nature of waste generated. The major benefit of synergistic use of industrial waste is reduction in product cost. The geopolymerisation process is low temperature process and thus the potential for energy savings is substantial. Cement clinker is normally produced at ~1400°C and ceramic tiles are produced at 950-1200°C. Whereas the geopolymers cement and self-glazed tiles require 60-150°C temperature. This significant reduction in temperature is expected to save upto 70% in energy cost. In addition, there will be enormous saving in capital cost as no high temperature processing kilns are required.
Producers of geopolymers products may also benefit from raw-material costs because the main reagents are waste material. In India, fly ash and red mud are free of cost and only transportation cost is involved. The cost of granulated blast furnace slag is around Rs.400-500 per tonne. Based on the various inputs, a techno-economic calculation was carried out as follows(Table.6.1):
Table.6.1 Techno-economics for theproduction of geopolymer products
The techno-economics based on laboratory results indicate high commercialization
potential. Efforts are underway in this direction through setting up of a pilot plant to optimize manufacturing parameters and properties on a larger scale.
Due to their ability to polycondense Silicon and Aluminium into solid monolithic ceramic like structure during alkali activation, geopolymers have the potential of utilization of industrial wastes rich in silico-aluminates such as fly ash, GBFS, red mud, etc. Novel building materials such as high strength geopolymers cement can be developed by additional processing such as mechanical activation, and self-glazed tile and pavement tiles can be developed by synergistic use of industrial waste namely fly ash, GBFS and red mud. The developed geopolymer products qualify as new members in the spectrum of eco-friendly construction materials due to easy and simple processing, low energy requirement and no CO2 emission. The products have good commercialisation potential with significant returns.
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