Effect of Aggregate on Geopolymer Concrete: Comparison
Please note this is a comparison between Version 2 by Sirius Huang and Version 1 by Ahmad B. Malkawi.

The construction industry has become one of the major consumers of renewable and non-renewable natural resources and environment-polluting industries. The development of new green construction materials has become a global need and a driver for recent research trends. 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
  • aggregate
  • compressive strength
  • workability
  • air content
  • porosity
  • response surface
  • optimization

1. Introduction

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][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][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][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

Geopolymer binders can be produced using any rich aluminosilicate source material [12]. Fly-ash- and slag-based geopolymers are the most common in use due to their high aluminosilicate content and availability [26,27][26][27]. For the alkaline activator, sodium silicate and sodium hydroxide solutions have been regularly used in geopolymer binder production due to their low cost compared to other possible alkaline activators [12,26][12][26]. The following sections consider the production of geopolymer concrete based on fly ash as the source material and a mixture of sodium silicate and sodium hydroxide solutions as the activating solution unless specified otherwise. The discussion mainly focuses on the effect of aggregate on geopolymer concrete properties. The literature was collected from two databases: Web of Science and ScienceDirect. Among the collected literature, few studies have solely considered the effect of aggregate parameters on the properties of geopolymer concrete. Previous evaluations of the aggregate influence on geopolymer concrete properties have indirectly considered the aggregate effect (not as the main variable), and only a few aggregate parameters have been involved. By searching for the keywords “Geopolymer” and “Aggregate” or “Mix Design” in the Title field, most found publications were seen to have considered the use of recycled or manufactured geopolymer aggregate [28[28][29][30],29,30], the effects of aggregate type [31], biomass aggregate [32], lightweight aggregate [33[33][34][35],34,35], the alkali–silica reaction of aggregate [36], and mix design studies that have considered the factors affecting the geopolymer paste [37], geopolymer mortars [38], or rigid pavement mix design [39]. Other studies investigated the different types of geopolymer concrete, including lightweight [40], foam [41], and fiber-reinforced [42]. Some studies considered the effect of aggregate on compressive strength and neglected its effect on workability. Nevertheless, a good mix design cannot overlook the workability requirements. The effect of fine aggregate content and grading on the mechanical properties of geopolymer mortars has been investigated by many researchers. For the metakaolin-based geopolymer, the compressive strength decreased slightly when the content of silica sand (size in the range of 0.3–0.6 mm) increased up to 50 V.%. Any further increment resulted in a sharp strength reduction [43]. Similar observations were reported by Arellano-Aguilar et al. [44] where the compressive strength decreased by more than 60% when the fine limestone content increased from 75 to 88 V.%. The addition of fine aggregate induces the formation of the pores, and a high content will increase the pore radius dramatically which can correspond to the strength reduction [43]. However, the slight strength reduction at a lower content can be attributed to the formation of combinations on the surface of particles which prompts their incorporation within the geopolymeric matrix. NMR measurements show that more tetrahedral silicon forms when sand or limestone is added to the geopolymeric binder. This is due to the surface dissolution of the small particle sizes less than 75 µm [43,44][43][44]. Large-sized aggregate particles will not show significant combination formation. Similar observations were reported by Isabella et al. [15], where a higher rate of silicon dissolution from the sand particles was reported for the particles of higher surface areas. The grading of fine particles also affects the strength and workability of geopolymer mortars. Larger fine particles (in the range of 2–4 mm) tend to provide higher mortar flowability and strength [45]. A recent investigation devoted to studying the effect of sand gradation and content on geopolymer mortar properties revealed that good mechanical performance can be achieved when the water–binder ratio and sand–binder ratio are in the range of 0.35–0.4 and 0.5–0.65, respectively. The flowability of mortars was directly proportionated to the fineness modulus of the fine aggregate in a linear fashion. The same effect was observed for compressive and flexural strengths. On the other hand, an optimum fineness modulus in the range of 2.2–2.6 resulted in the highest brittleness index and tensile strength [46]. Another study by Furkan et al. focused on the effect of the fine aggregate type, whereby six types of aggregate were investigated. The highest flexural and compressive strengths were obtained using basalt sand followed by silica sand, sandstone, river sand, Rilem sand, and waste concrete aggregate, respectively. Parallel results were obtained in terms of workability [47]. Some researchers were also interested in finding the optimum blend of different types of fine aggregates, such as granite slurry and sand [48], and limestone and sand [45]. Good efforts have been made to understand the physical and chemical characteristics of the interfacial transition zone between the geopolymer binder and aggregate. A uniform and dense interfacial zone has been reported by many researchers independently on the type of aggregate [8,49][8][49]. No obvious distinction was noticed between the ITZ microstructure and the bulk of the geopolymeric matrix. This can be attributed to the better chemical interaction between the aggregate and the geopolymeric binders [50,51][50][51]. The fly ash content was reported to insignificantly affect the compressive strength of the geopolymer concrete if reasonable content was used [52]. It was suggested that a weak bond exists between unreacted fly ash particles and the binder matrix itself. Hence, the greater amounts of fly ash reduced the geopolymer concrete strength. This could be reasonable only if the fly ash content increased without increasing the alkaline solution. However, at a constant alkaline solution to fly ash ratio, the amount of the produced gel will increase [53]. A study by Olivia and Nikraz [26] investigated the effect of total aggregate content which varied between 75 and 79 Wt.% on the durability of geopolymer concrete in a sea water environment. The results showed that the aggregate content had the highest response index as compared to the other investigated parameters, including the ratio of the alkaline solution to fly ash, the sodium silicate/sodium hydroxide ratio, and the curing conditions. The highest compressive strength was also achieved at the highest aggregate content. An opposite trend was also reported in the literature [54]. In this study, the investigated range was between 70 and 85 V.% which may be considered to be high aggregate content in the case of geopolymer concrete. Peng Y. and Unluer C. [55] conducted a study using machine learning techniques in data collected from several publications to study the effect of geopolymer concrete variables on its performance. Their study reported that the content of the fine and coarse aggregate had an insignificant influence on compressive strength. Another trend was also found in the literature in the case of the flash metakaolin-based geopolymer, where the total aggregate content yielded an insignificant effect on geopolymer concrete performance in the range of 71 to 83 Wt.%. This was attributed to the strongly developed ITZ [56]. Nevertheless, it was reported that both the total aggregate content and fine aggregate to total aggregate content ratio interacted and played a significant role in determining the performance of geopolymer concrete [57,58][57][58]. In these two studies, the total aggregate content varied between 60 and 75 V.% and the fine aggregate ratio varied between 20 and 40 Wt.%. Optimum compressive strength was obtained at a total aggregate content of 70 V.% and a fine aggregate ratio of 35 Wt.%. Similar results were obtained by Joseph and Mathew [59] where almost similar parameters and ranges were considered. The total aggregate content was also reported to affect the geopolymer concrete slump. The higher the aggregate content, the lower the slump value. A zero-slump value was reported for an aggregate content of 80% [60]. The results showed that even with high aggregate content, geopolymer concrete can provide good workability if the binder proportions are controlled. According to Raphaëlle and Martin [56], the higher aggregate content will reduce the total porosity of geopolymer concrete. Their results showed that the total porosity dropped by about 9% as the aggregate content increased from 72% to 83%. Interestingly, such a high aggregate content is expected to increase the total porosity. The coarse aggregate size was reported to provide a slight strength increment as it increased in the range from 4.75 to 18.75 mm. Beyond this range, a remarkable reduction occurred [61]. In contrast, it was reported that a sharp strength reduction occurred as the mean aggregate size increased from 7.5 to 18 mm in the case of metakaolin-based geopolymer concrete [62]. This can be attributed to the ambient curing that was applied. Ambient curing conditions are known to produce a weak geopolymer binder in the case of metakaolin-based geopolymers [12]. This will enlarge the difference between the elasticity modulus of the binder and the aggregate; hence, strength will be reduced. The aggregate size is also found to be an influencing parameter on the spalling of geopolymer concrete subjected to elevated temperature [63]. In this study, the aggregate size was the only investigated parameter. Three size ranges of coarse aggregate, 2.36–4.75, 4.75–10, and 10–14 mm, were used to produce different geopolymer concrete mixtures. The mixtures’ compressive strength was in the range of 44.1–72.4 MPa. The highest strength was obtained for the mixture with the largest size range. The specimens of the mixture with the largest aggregate size did not show explosive spalling at elevated temperatures as compared to mixtures with medium and small aggregate sizes. This was attributed to the reduction in pore pressure due to the increase in the length of the fracture zone and microcracking for larger aggregate particles. The effect of coarse aggregate size in the range of 5–20 mm on the bond strength between CFRP sheets and geopolymer concrete based on Metakaolin was also investigated. The results revealed that the failure load was reduced by about 15% as the maximum aggregate size increased to 20 mm, while the effective bond length was increased by more than 40%. It was suggested that the aggregate size should be included in the equations while predicting the bond strength and load capacity [62]. A comprehensive review of the mix design of geopolymer concrete reported that the required volume of coarse aggregate increases as the maximum coarse aggregate size increases and reduces as the fineness modulus of fine aggregate increases [64]. According to Osuji and Inerhunwa [65], the optimum bulk density of different types of aggregate blends can be achieved when the coarse aggregate content is in the range of 50–60 V.% for both rodded and loose aggregate conditions. A higher value for the rodded condition was achieved. This study has only considered the aggregate packing properties and no test was conducted on concrete mixtures. In summary, the aggregate in the case of geopolymer concrete may provide different behavior from that known for cement concrete. Geopolymer concrete has different reaction mechanisms and different rheological behavior. In the available literature, it was difficult to find an integrated study that was devoted to investigating the aggregate influencing parameters. Additionally, contradictory behavior is reported in the literature. A thorough understanding of the aggregate role is required to develop a clear mixture relationship.


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