Environmental sustainability is one of the major domains that receives increasing global attention. Environmental sustainability aims to maintain natural resources and ecosystems to ensure the needs and well-being of future generations [1]. Environmental sustainability must be understood not as an option, but as a necessity. Unhealthy environments (avoidable environmental risk factors) are responsible for about a quarter of deaths around the world, as estimated by the World Health Organization [2]. Generations born in 1960 and earlier have clearly witnessed climate changes during the last 50 years, including extreme heat, varying rainfall patterns, and natural disasters. These climate changes are directly related to the human carbon dioxide footprint, mainly from the combustion of fossil fuels and their related industries. It is expected that climate change will be responsible for about 250,000 additional deaths annually between 2030 and 2050 [2]. The cement industry is one of the major carbon dioxide producers, with the production of each ton of cement emitting around 1 ton of greenhouse gases [3]. By observing the global cement production rate, which was around 1.39 billion tons in 1995 and became around 4.1 billion tons in 2020 [4], one can realize the extent to which the construction industry has grown since then. The construction industry has become one of the major consumers of renewable and non-renewable natural resources and environment-polluting industries [5]. The development of new green construction materials has become a global need and a driver for recent research trends [6]. On the other hand, recycling and waste disposal strategies are of great interest in sustainability issues. Geopolymer binders are a promising solution for the aforementioned issues. Geopolymeric binders are initiated by the reaction of a certain alkaline solution with aluminosilicate-rich source materials, such as fly ash or ground-granulated blast furnace sludge [7]. The use of geopolymer binders can reduce the consumption of ordinary cement and utilize several wastes that are usually dumped in landfills, causing several environmental issues.

Geopolymer concrete investigations so far have demonstrated its superior properties as a promising construction material [8,9]. The main parameters that control geopolymeric binder properties, including source material characteristics, alkaline activator properties, mixing proportions of source material and alkaline activator, and curing conditions, have been thoroughly investigated [10,11,12]. These parameters are the main variables controlling the properties of the geopolymeric binder itself. On the other hand, geopolymer concrete should be viewed as a two-phase material involving the binder phase and aggregate phase. The aggregate phase consists of fine and coarse aggregates and occupies the highest percentage of the geopolymer concrete total volume. Yet, limited attention has been paid to the influence of aggregate on geopolymer concrete properties. Parameters including aggregate volume fractions, coarse and fine aggregate grading and their mixing proportions, aggregate type and its reactivity in a geopolymer binder, the bonding of aggregate and geopolymeric matrix, interfacial transition zone density, and microstructure properties should all be widely studied to pave the road for geopolymer concrete acceptance in the real-world construction industry.

In ordinary cement concrete, it is usually assumed that the aggregate is a chemically inert filler that does not interact with the cement hydration reactions [13]. In the case of geopolymer concrete, it has been reported that the reactivity of sand aggregate plays a significant role in the formation of geopolymeric binders and their properties as the geopolymerization processes have different mechanisms to cement hydration reactions [14]. It has been suggested that there is an optimum surface area of sand aggregate at which a higher geopolymerization rate would be achieved by increasing the leachability of the aluminosilicate precursors. Moreover, the finer the sand particles, the higher the strength development rate obtained [15]. The compressive strength increased by more than 65% for the geopolymer with silicate and quartz particles compared to the geopolymer with silicate alone [16]. On the other hand, the overall mixture performance and mechanical properties of the produced concrete are functions of aggregate properties, especially the coarse aggregate. Aggregate gradation and shape mainly specify the packing of the aggregate. The dense packing of the aggregate may reduce the amount of the geopolymer binder layer coating the particles; therefore, the mixture’s workability will be reduced [17]. At the same time, reducing the amount of geopolymer binder is required to reduce the cost of the produced concrete. Hence, there will be an optimum aggregate content and gradation at which the design requirements could be met.

Aggregate may yield different effects on geopolymer concrete’s fresh and hardened properties as compared to ordinary cement concrete. This is due to the nature of the viscous alkaline solution and geopolymeric network structure. Many researchers have highlighted the superior bonding properties between the geopolymer binders and the aggregate. The interfacial transition zone (ITZ) between the geopolymer binder and aggregate was not observed in some types of geopolymer concrete [18,19]. The geopolymer binder can strongly adhere to the aggregate surface, which can be attributed to the unique three-dimensional matrix of the geopolymeric gel [12]. The microstructure and the better physical–chemical properties of the geopolymeric binder (as compared to cement concrete) produce denser and fewer intrinsic defects in the ITZ [20]. This improvement in the ITZ may make geopolymer concrete more affected by the aggregate mechanical properties, which may be similar to the case of high-strength cement concrete [21]. In normal-strength cement concrete, most of the normal-weight aggregate has a strength much greater than the paste strength, and the ITZ strength creates upper limits on the achieved compressive strength [22]. Therefore, the role that aggregate plays in geopolymer concrete should be emphasized to ensure satisfactory performance. At present, there is no rational relationship that permits the prediction of geopolymer concrete strength based on the binder and aggregate variables. In ordinary cement concrete, it was reported that increasing the aggregate volume fraction will reduce the compressive strength of concrete, and a plateau will be reached at a value of 0.5 [23]. The strength of cement concrete is largely influenced by coarse aggregate properties, while fine aggregate shows a smaller effect due to its influence on the amount of required water [22]. This behavior may differ in the case of geopolymer concrete which has different rheological behavior [24]. Over the last two decades, many researchers have attempted to develop standard procedures for geopolymer concrete mix design mainly based on a trial-and-error approach. However, the number of variables associated with geopolymer concrete production complicates this task. Investigating the effect of all parameters at once will be a challenging task. By looking at geopolymer concrete as a two-phase material composed of the geopolymer binder phase and aggregate phase, it would be necessary to first understand the characteristics of each phase and then develop the mixture relationship. In addition, applying the design of experiment techniques may provide an effective tool that could minimize the required efforts [25].

2. Background on the Effect of Aggregate on Geopolymer Concrete