Published on Nov 30, 2023
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 SiO2 and Al2O3 , along with Fe2O3 , 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 Al duringgeopolymerisation. 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.
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).
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.
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.
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
Si2O5 , Al2(OH)4 + NaOH⇒Na(-Si-O-Al-O)n
kaolinitehydro-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.
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.
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.
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