Abstract
The geopolymer concept has gained wide international attention during the last two
decades and is now seen as a potential alternative to ordinary Portland cement; however, before
full implementation in the national and international standards, the geopolymer concept requires
clarity on the commonly used definitions and mix design methodologies. The lack of a common
definition and methodology has led to inconsistency and confusion across disciplines. This review
aims to clarify the most existing geopolymer definitions and the diverse procedures on geopolymer
methodologies to attain a good understanding of both the unary and binary geopolymer systems.
This review puts into perspective the most crucial facets to facilitate the sustainable development and
adoption of geopolymer design standards. A systematic review protocol was developed based on the
Preferred Reporting of Items for Systematic Reviews and Meta-Analyses (PRISMA) checklist and
applied to the Scopus database to retrieve articles. Geopolymer is a product of a polycondensation
reaction that yields a three-dimensional tecto-aluminosilicate matrix. Compared to unary geopolymer
systems, binary geopolymer systems contain complex hydrated gel structures and polymerized
networks that influence workability, strength, and durability. The optimum utilization of high
calcium industrial by-products such as ground granulated blast furnace slag, Class-C fly ash, and
phosphogypsum in unary or binary geopolymer systems give C-S-H or C-A-S-H gels with dense
polymerized networks that enhance strength gains and setting times. As there is no geopolymer mix
design standard, most geopolymer mix designs apply the trial-and-error approach, and a few apply
the Taguchi approach, particle packing fraction method, and response surface methodology. The
adopted mix designs require the optimization of certain mixture variables whilst keeping constant
other nominal material factors. The production of NaOH gives less CO2 emission compared to
Na2SiO3, which requires higher calcination temperatures for Na2CO3 and SiO2. However, their usage
is considered unsustainable due to their caustic nature, high energy demand, and cost. Besides the
blending of fly ash with other industrial by-products, phosphogypsum also has the potential for
use as an ingredient in blended geopolymer systems. The parameters identified in this review can
help foster the robust adoption of geopolymer as a potential “go-to” alternative to ordinary Portland
cement for construction. Furthermore, the proposed future research areas will help address the
various innovation gaps observed in current literature with a view of the environment and society.