3.4.2 Microstructure of RM-based GP

There are three parent materials contributing in the synthesis of RHA/RM geopolymers: RM, RHA, and NaOH solution. However, only amorphous phases in raw materials contribute in geopolymerization reaction [36]. Among the three raw materials, the red mud provides NaOH, A12O3, and NaAlO2; rice husk ash provides amorphous SiO2; sodium hydroxide solution provides NaOH.

Figure 10. SEM micrograph of RHA.

Figure 11. XRD pattern of red mud.

The SEM [36] of the investigated samples shows that the microstructure of RHA and RM geopolymer comprises non-dissolved particles of RM, which are bonded in an extent gel phase and the formation of gel silicate as shows Figure 12. The reaction with the alkaline solution to form a particulate gel network took place at the border of particles then involving the entire surface.

3.5 Mechanical properties

XRD pattern of RM-RHA based GP [36].

DOI: http://dx.doi.org/10.5772/intechopen.82413

Figure 13.

talline zeolites at extended curing time.

3.5.1 Durability

tural concrete [33].

91

Generally, a strong alkaline medium is necessary to increase the surface hydro-

Geopolymers durability is the most important issue in determining the success of these new materials. The fact that samples that have been exposed to service conditions for in excess of 30 years show little degradation means that geopolymers do therefore appear to stand the test of time [12]. Since those samples were of the (Si + Ca) type conclusion cannot be extended to geopolymers defined as alkali aluminosilicate gel, with aluminum and silicon linked in a tetrahedral gel framework. One of the key unsolved questions in the development and application of alkali activation technology is the issue of durability. Whether geopolymer concretes are durable remains the major obstacle to recognition in standards for struc-

Efflorescence is originated by the fact that alkaline and/or soluble silicates that are added during processing cannot be totally consumed during geopolymerization [38]. It is the presence of water that weakens the bond of sodium in the aluminosilicate polymers. In the crystalline zeolites the leaching of sodium is negligible, contrary to what happens in the geopolymers. Na and/or K ions in geopolymers are

lysis of the aluminosilicate [6]. Particles present in the raw material while the concentration of the chemical activator has a pronounced effect on the mechanical properties of the geopolymers; on the compressive strength in particular. Compressive strength is an indication of the compactness and strength of a construction material. Most of the inorganic polymer work reported is consisted upon an extensive discussion of compressive strength and its relation with time, temperature, chemical composition, source material and activating alkaline solutions. Besides compressive strength, flexural strength was also reported in the recent papers. It was observed that aluminosilicate based GP produced higher compressive strength compared to other activating systems. Most of the GPs attained maximum compressive strength within 7 days and only partial increase has been recorded in certain cases. Few papers also reported the descending trend in compressive strength with increasing time. This trend was explained on the formation of crys-

Survey of Bauxite Resources, Alumina Industry and the Prospects of the Production…

The XRD diagram (Figure 13) shows that the product is not clean geopolymer matrix, but a composite consisting of the geopolymer structure and crystalline phases from parent materials. It show a broad reflection related to the high amorphous content. However, the center of this reflection is shifted to 2Ɵ = 29° due to changes in composition and structure when RHA is activated by NaOH and NaSiO2 solutions. This is in conformity with the microstructure RM and MK-based GP of red mud and metakaolin based-geopolymers comprising undissolved particles of red mud, which are bonded in an extent gel phase [37]. It indicates a Fe content higher than 65% assigned to the existence of undissolved red mud. While, microanalysis of the area presented in Figure 13 indicated the formation of gel silicate phase (Si > 40% and Fe < 7%). Therefore, red mud may participate within the geopolymeric structures as active and not as reactive material.

Figure 12. SEM of RM-RHA based GP [36].

Survey of Bauxite Resources, Alumina Industry and the Prospects of the Production… DOI: http://dx.doi.org/10.5772/intechopen.82413

Figure 13. XRD pattern of RM-RHA based GP [36].

#### 3.5 Mechanical properties

The SEM [36] of the investigated samples shows that the microstructure of RHA and RM geopolymer comprises non-dissolved particles of RM, which are bonded in an extent gel phase and the formation of gel silicate as shows Figure 12. The reaction with the alkaline solution to form a particulate gel network took place at

The XRD diagram (Figure 13) shows that the product is not clean geopolymer matrix, but a composite consisting of the geopolymer structure and crystalline phases from parent materials. It show a broad reflection related to the high amorphous content. However, the center of this reflection is shifted to 2Ɵ = 29° due to changes in composition and structure when RHA is activated by NaOH and NaSiO2 solutions. This is in conformity with the microstructure RM and MK-based GP of red mud and metakaolin based-geopolymers comprising undissolved particles of red mud, which are bonded in an extent gel phase [37]. It indicates a Fe content higher than 65% assigned to the existence of undissolved red mud. While, microanalysis of the area presented in Figure 13 indicated the formation of gel silicate phase (Si > 40% and Fe < 7%). Therefore, red mud may participate within the

the border of particles then involving the entire surface.

Figure 11.

Figure 12.

90

SEM of RM-RHA based GP [36].

XRD pattern of red mud.

Geopolymers and Other Geosynthetics

geopolymeric structures as active and not as reactive material.

Generally, a strong alkaline medium is necessary to increase the surface hydrolysis of the aluminosilicate [6]. Particles present in the raw material while the concentration of the chemical activator has a pronounced effect on the mechanical properties of the geopolymers; on the compressive strength in particular. Compressive strength is an indication of the compactness and strength of a construction material. Most of the inorganic polymer work reported is consisted upon an extensive discussion of compressive strength and its relation with time, temperature, chemical composition, source material and activating alkaline solutions. Besides compressive strength, flexural strength was also reported in the recent papers. It was observed that aluminosilicate based GP produced higher compressive strength compared to other activating systems. Most of the GPs attained maximum compressive strength within 7 days and only partial increase has been recorded in certain cases. Few papers also reported the descending trend in compressive strength with increasing time. This trend was explained on the formation of crystalline zeolites at extended curing time.

#### 3.5.1 Durability

Geopolymers durability is the most important issue in determining the success of these new materials. The fact that samples that have been exposed to service conditions for in excess of 30 years show little degradation means that geopolymers do therefore appear to stand the test of time [12]. Since those samples were of the (Si + Ca) type conclusion cannot be extended to geopolymers defined as alkali aluminosilicate gel, with aluminum and silicon linked in a tetrahedral gel framework. One of the key unsolved questions in the development and application of alkali activation technology is the issue of durability. Whether geopolymer concretes are durable remains the major obstacle to recognition in standards for structural concrete [33].

Efflorescence is originated by the fact that alkaline and/or soluble silicates that are added during processing cannot be totally consumed during geopolymerization [38]. It is the presence of water that weakens the bond of sodium in the aluminosilicate polymers. In the crystalline zeolites the leaching of sodium is negligible, contrary to what happens in the geopolymers. Na and/or K ions in geopolymers are bounded only weakly in the nanostructure of the geopolymer gel and are therefore almost completely leachable.

their potential damage to the environment. Lastly the elimination of OPC usage can

Survey of Bauxite Resources, Alumina Industry and the Prospects of the Production…

Silicate-based geopolymer binders have been utilized in applications such as pathways, pavers, mine backfill, railway sleepers, sewerage pipes, and earth retaining [40]. For red mud-based geopolymers to penetrate similar markets, in situ and ambient temperature curing are required. This has been achieved in laboratory

Geopolymer production tackles two main issues of the present time. On one hand, with the increasing amount of waste generation from different processes, there has been a growing interest in the use of waste in producing sustainable building materials to achieve potential benefits. Cleaner production is a pressing issue of our time. Residues or byproducts resulting from different industrial processes requires proper management to ensure a sounder environment [14]. The use of recycled materials in new sustainable materials production is very attractive due to the low-cost related to the waste materials in addition to saving required space for landfill purposes and the development as well as improvement in the materials

On the other, the global warming is one of the most pronounced terms in the present time. Thus reducing the greenhouse gas emissions, which is the main reason behind global warming, is the need of the present time and the future. Efforts are underway to develop environmentally sustainable construction materials, which make minimum utility of fast dwindling natural resources and help to reduce greenhouse gas emissions [41–43]. It is an established fact that the greenhouse gas emissions are reduced by 80% in GP concrete vis-a-vis the conventional OPC manufacturing, as it does not involve carbonate burns. In this connection, GP are showing great potential and several researchers have critically examined the various aspects of their viability as binder system. For manufacturing each tone of the OPC as the primary component of concrete about 1.5 tons of raw materials is needed. Furthermore; in this process about one tone of carbon dioxide will be released into the atmosphere. Geopolymers generate just 0.184 t of CO2 per ton of binder. Although the CO2 emissions generated during the production of Na2O are very high, still the production of alkali-activated binders is associated to a level of carbon dioxide emissions lower than the emissions generated in the production of OPC.

In the light of the above it appears clearly that extensive research has been carried out into the field of geopolymers for better understanding the chemical mechanism conducting to their formation and to develop the ability to design geopolymers with specific applications. In particular the variance of source material makes difficult a standard mix design. Nowadays the most investigations agree that source material, mixing ratio, alkali activator, curation time are key factors in the geopolymerization process. With regard to the immense bauxite resources of Guinea, the development of a subsequent alumina industry is expected and implies two questions: (i) a highly qualified manpower is required to manage the very complex issue of the alumina production and the management of the resulting waste; (ii) a worldwide integrated approach will be required to Takle the management of industrial wastes an environmental challenge and a technological opportunity to develop promising engineering

application. The valorization of the red mud is at the same ns.

reduce the CO2 emission caused by firing carbonates.

DOI: http://dx.doi.org/10.5772/intechopen.82413

properties.

5. Conclusion

93

trials while targeting a compressive strength above 20 MPa [37].

4.1 Environment considerations on Geopolymers applications
