Ordinary Portland cement is the most popular binder material for producing concrete. The usage of cement is ever increasing due to its ability to gain early strength as well as prolonged strength gain. However, it is energy intensive and also consumes large amount of natural materials for its production. Further, production of every one ton of cement releases about 1 ton of CO2 in to atmosphere promoting global warming and deterioration of ecosystem [1–3]. It is being noted from the last decade, extensive research works have been carried out in the field of advance growth in inorganic geopolymers because of its extensive scope of prospective applications of these supplementary materials. Different studies are being found in the literature on the performance, synthesis, behaviour and function of geopolymer materials [4–5].
Geopolymer is an inorganic alumino-silicate compound synthesized from geological material or industrial by-products such as fly ash, slag, rice husk ash etc. Further, geopolymers are inorganic in nature and it can be synthesized by means of external activators under alkaline medium and also the proper selection of source materials which are rich in Si and Al content. The chemical mechanism between Si-Al minerals and alkaline liquids consist of destruction of Si and Al atoms from source materials followed by coagulation and condensation of precursor into monomers and finally monomers into polymeric crystalline structures [6–8]. Eventually the polymerization process develops fast accelerated chemical reaction under alkaline environment on Si:Al minerals that forms three dimensional polymeric chain and ring structure consisting Si - Al - O bonds [9].
In general, when the bottom ash was grined finer it will be of more reactive and thereby directly imparts on the high compressive strength in geopolymer specimens [10–11]. The effect of curing mode on the properties of geopolymer mortar made using combustion coal bottom ash shows that dry curing decreases in compressive strength than ambient curing. Also, dry curing exhibits porous microstructure where as ambient cured specimen exhibit homogenous and compact matrix [12]. The geopolymeric reaction depends on the fineness and pore size of material used. It has been observed that fine bottom ash improves the performance of geopolymer in terms of sulphate resistance as well as sorptivity [13]. Subsequently, the effect of molar ratio of Si to NaOH and Na2SiO3 to NaOH on compressive strength of geopolymer mortar. It is noted that mortar with low Si to NaOH attained maximum compressive strength. Further, proper selection of molar ratio can cause high geopolymer reaction at ambient temperature [14]. Further, the durability performance of lignite bottom ash were studied and the test results indicates that finer bottom ash imparts high compressive strength and also exhibit better performance in terms of durability than cement mortar [10 & 15].
Metakaolin which is generally used as pozzolanic micro filler material in terms of high strength and as well as high performance concrete. The characteristics of metakaolin based geopolymer in terms of strength parameters as well as durable properties shows tremendous performance when compare with OPC [16]. Also, when compared with individual score materials performance, blended nature exhibits better results [17–18]. The calcination temperature and period of calcination plays an important role in developing the strength of the matrix [19]. Consequently, the kinetics of geopolymeric reaction of metakaolin and activators shows that silica and alumina content will have direct impact on early strength. Also, increase in molar ratio of Si/Al imparts strength gain at later ages [20]. The increase in fineness of metakaolin enhances the strength of binder material [21]. Also, metakaolin synthesized with sodium hydroxide and sodium silicate imparted the dense texture and high resistant to surface scratching to geopolymer [22].
The performance of geopolymer materials depends on the various factors such as choice of selection of source materials, fineness of the materials used, and mode of curing [19, 23]. Mode of curing has been one of the major issue factors for geopolymer technology. But by taking into account of proper selection of source materials as well as the suitable activators consideration one can able to cast the geopolymer concrete in cast - in – situ under ambient temperature itself [22, 24, 25]. Further, it has been noted that proper selection of molar ratio can cause greater geopolymeric reaction at ambient temperature [14].
In the present experimental investigation, durability properties such as sulphate attack, acid attack and their weight loss and its strength retention were tested at different period of time. Also, sorptivity, rapid chloride penetration test and water absorption tests were also carried out to explore the performance of metakaolin – bottom ash geopolymer concrete activated by sodium based alkaline activators under ambient temperature.
A profuse research work was done earlier in the field of geopolymer concrete by using different alumina- silicate based materials as source materials. Moreover, the performance of geopolymer concrete when they are blended with two or more source materials together under different curing conditions are being investigated by different researchers. But, the synthesis effect on metakaolin and bottom ash geopolymer source materials were not been explored so far. In this background, a venture has been taken up to study the durability performance of MK- BA geopolymer concrete when it is exposed to different period of time under various environment circumstances.