Robustness of Self-Compacting Geopolymer Concrete (SCGC)

Warren Mc Kenzie, Technical Manager, BASF Construction Chemicals, Dubai, U.A.E

It’s widely known and has been reported on numerous occasions and in multiple journals that concrete is one of the most used construction materials in the world. With current cement production in excess of 2.8 billion tonnes globally it is forecast to exceed four billion tonnes year on year. [1] The production of OPC contributes to greenhouse gasses, in the form of CO₂. Latest reports indicate the 0.6 to 0.8 kg of CO₂ is emitted per 1 kg of OPC produced. The net result: CO₂ contributes to approximately 65% of global warming. The cement industry is the third largest contributor, responsible for approximately 5 - 6% of CO₂. [1][2]

Despite the numerous uses and benefits of concrete OPC remains the primary source of binder in conventional concrete. The production of which consumes vast quantities of energy while simultaneously depleting quarried resources. However, these concerns remain a secondary concern compared to the volume of CO₂ produced as a result of the calcination process. Through the implementation of sustainable developments within the civil engineering society, the use of materials with lower environmental impact have gained traction. This has led to the acceptance, use and specification of materials such as PFa, GGBS, MS, etc.

Based on their characteristic properties, pozzolanic and/or hydraulic, these materials are considered to be either secondary or supplementary materials. Replacement of OPC with PFa and GGBS are generally minimal, however research has shown that high levels of replacement are possible. Recently it has been reported that environmentally friendly concrete can be produced by replacing OPC with materials such as PFa and GGBS and activating them using a high alkali solution(s). This process is affectionately referred to/identified as AAM or Geopolymer concrete, a term coined by French Prof. Davidovits. [3][4]

Geopolymer concrete provides a sustainable, environmentally friendly alternative to OPC based concrete by reducing CO₂ emission by up to 45% while remaining economically competitive. [1] These reductions are often looked at only as operational CO₂ (oCO₂) and not as the embodied carbon dioxide (eCO₂), which can be defined as the emissions derived from the construction and disposal phase in terms of the element’s life cycle. Geopolymer concrete has been successfully deployed in countries such as Australia. The next progressional step in the evolution of concrete technology is a truly revolutionary material, Self-Compacting Geopolymer Concrete (SCGC). [5][6] SCGS is in a situation similar to what conventional Portland cement based self-compacting concrete (SCC) was, where despite the numerous benefits associated with SCC such as improved durability, improved production rate/output, reduction of repairs reduced or reallocation of labour etc. the sensitivity and robustness of the mix remains questionable and unproven. [7][8][9]

Scope
Most studies on geopolymer concrete have focused on the reaction kinetics of the polymerization process rather than the development and understanding of a concrete that can be introduced into the industry, with a limited number of studies focusing on concrete as opposed to mortar mixes. Review of literature shows that the focus has been on molarity of the activator and the Al/Si ratio with little regard to the complexities of preparing a mix design, one that is robust enough to tolerate variation in water content. The term “robustness” is open to interpretation in the industry as there are various definitions of robustness for concrete. For example, RILEM TC 288 MPS classifies robustness as “the characteristic of a mixture that encompasses its tolerance to variation in constituent characteristics and quantities, variations during concrete mixing, transport, and placement, as well as environmental conditions”. [10] While the European Guidelines defines robustness as “the capacity of concrete to retain its fresh properties when small variations in the property’s quantities of the constituent materials occur”. [11] To investigate the robustness the mixing water was decreased by 5 l/m³ (3.57%) and 10 l/m³ (7.14%) or increase by 5 l/m³ (3.57%) and 10 l/ m³ (7.14%), presenting with a total variation of 20 l/m³ (14.28%).

Fly Ash Based Geopolymer
The reaction between highly concentrated aqueous alkali solutions and binders, consisting predominantly of silicon (Si) and aluminium (Al), produces a synthetic alkali aluminosilicate material often referred to as geopolymer, a term Davidovits used to emphasize greater polymerization. [3][4][12] However, this reaction of an inorganic polymer has also been referred to as low-temperature aluminosilicate glass [13], inorganic polymer concrete [14], alkali-activated cement [15], and alkali-activated material, resulting in confusion globally as to how we identify this breakthrough technology correctly.

This process is affectionately referred to/identified as geopolymer concrete or alkali-activated material (AAM). Going forward the term geopolymer will be used as it is preferred by most researchers and the industry. The typical geopolymerization process of Figure 1, [14] provides a very general and linear illustration of the different phases in the polymerization process while Figure 2, illustrates different poly(sialate) structures formed as a result of the different Si:Al ratios. [15] The term poly(sialate) is a chemical designation of geopolymer based on silica-aluminates, where the sialate is the abbreviation for silicon-oxo-aluminate and the alkali may be either sodium (Na+), potassium (K+), lithium (Li+) or calcium (Ca²⁺). [12] Polysialates range from amorphous to semi-crystalline three-dimensional silica-aluminate structures and are chain and ring polymers with Si⁴⁺ and Al³⁺ in a IV fold coordination with oxygen. The accepted formula of polysiliates is; Mn(-(SiO₂)z-AlO₂)n.wH₂O, where (M) is a cation and (n) is a degree of poly condensation.

To have successful polymerization, the formation of a (Mz(AlO₂)x(SiO₂)y.nMOH.H₂O) gel is critical and a regarded as a dominant step in the geopolymerization process.

Ground-Granulated Blastfurnace Slag Based Geopolymer
Factors effecting the strength development of geopolymer with GGBS as the primary source of binder are: the type and dosage of alkaline activator, the means of adding said activator, the type and fineness of binder, ratio of SiO₂:Na₂O, curing temperatures and water:binder ratio. [16] The reaction of GGBS based geopolymer is different from that of PFa. The latent hydraulic GGBS main component is the development of calcium silicate hydrate (C-S-H) gel along with aluminium resulting in calcium silicate hydrate with aluminium gel (C-A-S-H), similar to the C-S-H formed by OPC based concretes. [17] The C-A-S-H formed through alkali activation has a Ca:Si ratio below 1.5 whereas the Ca:Si ratio of OPC based concrete is approximately 2.0. [12][18][19]

The high calcium content along with the low aluminium content of GGBS changes the mechanism of alkali activation compared to PFa, where the cation (R+) acts as a catalyst in the initial phases of hydration via cationic exchange with the Ca²⁺ ions during the early stages of hydration and strength development as per Figure 3. They are then combined into the structure forming zeolite-like phases. [12][18] The use of alkali hydroxides, sodium being the most common, play multiple roles in the hydration process. During the hydration phase of GGBS, in both OPC and geopolymer systems, a coating forms on the slag particles which in turn prevents further hydration. In cement based systems the hydration of the Ca(OH)₂ maintains an alkaline environment which is required to break this coating allowing continuous hydration. Similarly, the alkali hydroxide provides and maintains the supply of OH- anions in the system ensuring the coating breaks down. [12][20]

The hydration process is effected by the sodium content and the Ca:Si ratio and begins with the destruction of the slag bonds Ca-O, Mg-O, Si-O-Si, Al-O-A and Al-O-Si before a Si-Al layer is formed of the binder prior to the formation of the hydration products as per Figure 3.


The nature and composition of products formed during the activation of GGBS remain controversial and require continuous research. Most research conclude and accept the following assertions: [18]

• The main hydration product is a composition of calcium silicate hydrate with aluminium (C-A-S-H gel) with a Ca:Si ratio less than 1.5;
• The structure and composition of the C-A-S-H gel is dependent on the type and amount of activator used.

The presence of aluminium results in a gel forming long linear chains, as well as, the possible existence of sporadic inter-chain Si-O-Al bonds consequently forming two-dimensional cross-linked structures. Figure 4 shows that under these conditions the C-S-H gel would become a C-(A)-S-H gel. The type of activators used, being either silicate or hydroxide based, has a direct effect on the degree of silicate species condensation in the gel structure, with sodium silicate presenting with highly condensed structures. It was noted that the use of sodium hydroxide to activate the GGBS results in tetrahedral aluminium in bridging positions of the silicate chains. [18]


Sodium Hydroxide
NaOH more commonly known as caustic soda or lye is produced via electrolysis. Three types of electrolytic cells are used, Castner-Kellner cells (Mercury process), Nelson diaphragm cells and membrane cells. The preferred method is shown in Figure 2.30 and discussed further below. [21] The membrane cell method is favoured for the production of NaOH; it has a low energy demand to produce pure NaOH with no hazardous waste. The cell consists of a two-compartment system with a titanium anode, nickel cathode and an ion-exchange membrane separating the anode from the cathode. Saturated brine solution enters the anode compartment where oxidation takes places releasing chlorine gas, 2Cl^-(aq) => Cl_2(g) + 2e^-. The depleted brine is removed and replaced in a continuous process. [21]

The membrane selectively allows Na+ and water to migrate from the anode compartment to the cathode compartment where the solution reacting with the cathode releases hydrogen, 2H₂O(i) + 2e => H_2(g) + 2OH^-(aq).

The Na+ combines with the OH- to form a concentrated NaOH, 2H₂O_(i) + 2C_l-(aq) + 2Na_+(aq) => 2Na⁺_(aq) + 2OH^-_(aq) + H_2(g) + Cl_2(g). Solid NaOH is obtained by the evaporation of this solution.[68]

Molarity
The concentration of the solution is referred to as molarity and is defined as the number of moles of solute per liter of solution. The molarity of NaOH is calculated where molarity is compared to moles in solute. The relationship between activator molarity and the performance (compressive strength) of geopolymer concrete has been studied. Geopolymer produced using a molarity of 14 showed significant increase in compressive strength while mixes produced with 18 mole showed marginal decrease in compressive strength. It has been observed that increasing the molarity of the activator leads to better results. [22][23][24]

Several reports have been made regarding the dosage of activator; however, they remain inconsistent. For example, geopolymer prepared with NaOH within the range of 1-10% by mass of binder reported that the higher the dosage, the better the strength; other reports indicate the variation of dosage within the range of 3-11% had little effect on the strength. [25]

Wang reports that to increase the molarity in combination with the dosage of activator, defined as the ratio between Na₂O in the activator and binder as a percentage, has a significant influence on the compressive strength. [25][26]

It’s reported that the optimum Na₂O dosage for GGBS based geopolymer is between 3-6%, based on early strength requirements and the type of GGBS used. However, the Na₂O value varies according to total binder volumes as illustrated in Table 2.9 whereby the activator dosage remains constant at 25l/m³ and the total binder ranges from 300 kg/m³ to 500 kg/m³.

Molarity in the case of NaOH is limited to a maximum of approximately 19 mole per litre as NaOH solidifies at 20°C at a concentration of 52% by mass. [85] Therefore a solution of approximately 18 mole (M) NaOH was prepared by dissolving NaOH flakes in water to achieve an approximate 50% mass fraction (w/w). Several trials were conducted using various levels of NaOH before settling on a value as per the proposed mix design. Laboratory trials indicated a strong correlation to results found in research, showing that there appears to be an optimum dosage for the activator.

Literature has revealed that increasing the molarity of activators has a negative effect on plastic properties such as slump-flow, v-funnel and slump-flow t-500 times; yet a positive effect on compressive strength. [6][27][28][29][30][31][12][13]

With the primary focus being on the molarity of the activator(s), the effect of free water has been erroneously disregarded. Numbers, in the first row of Table 1, taken from [29] illustrate the focus on molarity; not taking into account the change of free water as the dosage of activator remains consistent, depicted in the rows that follow. Table 1 provides a breakdown of total water by calculating the solid content of NaOH at various molarities from a theoretical mix while Figure 5 illustrates the effect of free water on compressive strength.

Table 1, Breakdown of total water by calculating NaOH solid content at various molarities from a theoretical mix design
Binder	Coarse Aggregate	Fine Aggregate	Molarity	NaOH Activator	Active NaOH (kg/m3)	Free Water (kg/m3)	Water (12% of Binder)	Total Water in Mix (kg/m3)
425	935	830		210			51
			1		6.4	201.6		252.6
			2		16.8	193.2		244.2
			4		33.6	176.4		227.4
			8		67.2	142.8		193.8
			16		134.4	75.6		126.6

Super Plasticizers
Commercial superplasticizers such as lignosulphonate, sulphonate naphthalene formaldehyde, sulphonate melamine formaldehyde and polycarboxylate ether, used in OPC based concretes, have been well documented in literature. The use of these constituents has become common practice as they adhere onto the cement surface, often building up multiple layers on individual grains, whereby they deflocculate and disperse the agglomerated cement particles through the following mechanisms: electrostatic repulsion, reduction of the surface tension of water and steric hinderance, releasing the entrapped water and increasing fluidification. [32][33]

The charge of both cement and superplasticizers is recorded as zeta potentials, which is the function of the charge of the particles with absorbed polymers. Therefore, it should be noted that OPC is positively charged to approximately +10 mV as a result of the multi-phase dissolution of the silanol phases. Superplasticizers, with a negative zeta potential, endeavour to reduce the positive zeta potential in the system. By cancelling one another out, the superplasticizers become absorbed onto the cement grain. Despite the latest generation superplasticizers having a zeta potential of -5 to -10 mV they are rendered useless in the high alkaline environment of geopolymer and provide little to no improvement in workability; however, they may compromise the set time and compressive strength by as much as 50%. [34][35]

Conventional PCE’s consist of an anionic backbone which attracts counter ions resulting in the neutralization of their anionic charge and therefore influencing their adsorption characteristics. The presence of cations and the amount found within the pore solution blocks the negative charges resulting in a reduction of adsorption onto the binder and therefore provides minimal plasticizing properties. [15]

BASF Construction Chemicals have developed a superplasticizing admixture for use in geopolymer concrete. However, the intellectual property of the admixture is protected by patents and other commercialization processes therefore the properties of the admixture will not be discussed or disclosed.

Mix Design
Multiple studies have shown particle packing principles (PPP), also known as particle packing models (PPMs), which take into account particle shape, size and distribution to improve the performance of concrete by reducing porosity and is an essential step in the mix design phase. PPP can be applied to most concretes, however, there are a few notable differences between conventional vibrated concrete (CVC) and SCC, some more important than others, while SCGC is not dissimilar to SCC.

Both SCC and SCGC require a higher paste content compared CVC and is achieved by the manipulation of the aggregate proportions, binder content and total water content. Figure 6 shows the progression of mix proportions form CVC to SCC to SCGC. Reducing the ratio of coarse aggregate to fine aggregate adds stability to the mix, although it may be necessary to substitute the largest aggregate for one size down; i.e., 20 mm to 10 mm.

A well-designed thixotropic SCC/SCGC should be able to resist dynamic segregation in the fluid state yet reduce the lateral stress on the formwork i.e. hydrostatic pressure. Once the concrete comes to a rest, the fluid structure can restore the yield stress and prevents further dynamic, segregation a phenomenon resulting from flocculation. [36][37]

Based on PPP and the conceptual models shown, a theoretical mix design, Table 2, has been developed to meet the guidelines, as set out by BS EN 206:2013 and EFNARC. [38][39]

Table 2, Theoretically proposed mix design
Raw Material	Kg/m3	% m3	eCO2	Dry vs Liquid %	Aggregate vs Mix %	Coarse vs Fine vs Liquid %
20mm Stone	380	11.76	3.53	91.60	73.95	29.62
10mm Stone	425	7.85
Crusher Sand	680	28.57	3.78			61.97
Dune Sand	375	15.76
GGBS	375	15.76	19.50		26.05
MS	45	1.89	0.63
Water	140	5.88	0.14	8.40		8.40
Admixture	35	1.47	0.35
NaOH	25	1.05	28.75
Theoretical Total	2 380	100.00	0.024	100.00	100.00	100.00

Curing
It is thought that geopolymer concrete produced utilizing conventional and well-established curing procedures will add further confirmation regarding the robustness of the mix.

Curing of conventional concrete has been described as “the maintenance of appropriate moisture and temperature conditions to permit the continuation of the hydration or pozzolanic reaction”. The “hydration” phase of geopolymer concrete is more of a polymerization phase than an actual hydration phase as is the case in conventional concrete where CSH is formed. It is thought that geopolymer concrete produced with a basic GGBS will have a minimal calcium content and could be considered as self-curing, where no water curing is required.

Evaluation of Plastic Properties
In an effort to ensure a harmonious and representative evaluation of each test characteristic, concrete used for a single test is returned to the pan where it was manually mixed by hand prior to moving onto the next test method, i.e. following the completion of the mixing sequence the slump-flow and slump-flow t500 were recorded. The concrete was then returned to the pan and using a scoop, the concrete was mixed by hand to form a representative sample before conducting the J-ring test. The procedure followed for evaluation of the plastic properties was based on the ergonomics of the laboratory and were: slump flow and slump-flow t500, J-ring, V-funnel, plastic viscosity and finally sieve segregation. After which all concrete was returned to the mixer and covered with polythene to measure slump retention

Slump Flow
The slump-flow values of the various mixes highlight the direct effect water has on workability. The slump-flow decreases in a near linear trajectory as the mixing water is reduced; while slump-flow increases, as water content increases, in a more parabolic trajectory, indicating that further increases in water content would cause segregation and thus reduce the slump-flow values as depicted in Figure 7 and Figure 8. As per BS EN 206:2013 the concrete is classed as SF3.



Plastic Viscosity
The results recorded from the plastic viscosity measurement are reported as Pa.S in Table 3. Analysing the box plot Figure 9, the histogram Figure 10, it is noted the that the plastic viscosity increases as the water content decreases and visa versa as the plastic viscosity decreases as the total water content is increased as per figure 11. These results follow the notion that variation in water has a direct effect on plastic viscosity as a result of the relationship between stress and rate of strain.

Table 3, Plastic viscosity values (Pa.S) reference mix with water contents decreased and increased by 5 and 10 litres respectively
Plastic Viscosity	Reference Mix	-5 Ltr Water Mix	-10 Ltr Water Mix	+5 Ltr Water Mix	+10 Ltr Water Mix
Repeat 1	38.9	55.5	54.8	26.7	24.4
Repeat 2	38.4	54.2	63.7	39.5	29.5
Repeat 3	58.8	39.5	52.3	27.7	28.6
Repeat 4	38.5	38.1	41.4	29.0	28.5
Repeat 5	40.4	45.0	62.7	41.2	32.8
Repeat 6	33.3	62.6	71.9	35.1	28.1
Repeat 7	27.5	55.2	68.2	34.6	28.0

Compressive Strength
The strength development of test specimens remain consistent despite being subjected to different curing methods. The strength development between mixes is shown in Figure 12, providing a clear illustration of the strength development trend. Figures 1, 2, 3, 4, 5 in Appendix 3 provide a direct comparison between water cured and air cured specimens per mix design. These results are also shown in Tables 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10 in Appendix 3. Specimens cured in the stability chamber demonstrated that the water curing was not a requirement to ensure the geopolymerization process continues. Furthermore, the continuous strength development indicates that this process had not concluded and that higher strengths could be expected at later ages.


The influence of the water content on the compressive strength suggests the water:binder ratio is a critical factor in the production of geopolymer concrete. Following extensive research into activation of geopolymer concrete, molarity can be regarded as a critical factor regarding the success of the mix. It has also been noted that molarity is only ever considered as a value of the activator and not of the total mix design. It is therefore proposed that the active molarity of activator and subsequently the active Na₂O be used for a more accurate calculation in the development of geopolymer concrete. In other words, a mix design using 250 l/ m³ water and 50 l/ m³ of 18 M sodium hydroxide has an active / effective molarity of 0.126 in a m³. Using a superplasticizer to reduce the water content by 100 litres increases the active/effective molarity to 0.189 in a m³, an improvement of more than 65%.

Summary
The results of the investigation into both plastic and hardened properties, as well as curing, can be summarized as follows:

1. Geopolymer produced using GGBS blended with MS presents with acceptable performance properties and therefore validates the replacement of 100% OPC.
2. The use of exciting test methods were used in accordance with the appropriate method except where highlighted and discussed.
3. Changes in water content had a direct effect on the plastic properties. However, evidence suggests that the changes in plastic properties were subtle, proving the concrete was able to tolerate the variance without segregation.
4. The decrease in water content had the greatest effect on the pass-ability of the v-funnel and the plastic viscosity.
5. The strength development of the various mixes was not affected by the curing procedures/methodologies. However, the compressive strength is related to the water content of the mix and not the Na₂O as previously reported.
6. While the methods of curing had minimal effect on compressive strength, the specimens subjected to air curing presented with improved performance properties with regard to RCPT and water absorption. The low water absorption can be attributed to a dense matrix as a result of the fineness of the GGBS and thus compliments the chloride resistance properties of geopolymer producing a longer service life.
7. The porosity of the concrete represented by the low water absorption values are attributed to the high fineness and irregular shape of the GGBS. Both the high fineness and irregular shape increase the surface area and therefore increased the reaction between the binder and activator.
8. The improved chloride diffusion rate results from the low porosity of the concrete. However, it is noted that the concentration of NaOH increases the chloride binding capacity of the concrete thereby providing a dual action system for the reduction of chloride ingress.
9. Figure 13: The saturated capillaries result in polymerization around the capillaries
   While all initial results indicate geopolymer produced with GGBS and MS provide acceptable compressive strength and improved durability results, the long-term performance properties require further examination in order to be validated.
10. It is theorized that the compromised performance properties of hardened properties subjected to water curing is a result of excess water interfering with the polymerization process as illustrated in Figure 13, with saturated capillaries forcing the polymerization to wrap around the capillary; while Figure 14 illustrates how the C-A-S-H gel may cross through the capillary thereby changing and limiting the absorption properties.
11. Although this interference is negligible for compressive strength, it is thought that in the absence of excess water, the C-A-S-H gel is allowed to form through the capillaries thereby providing a denser matrix and improving the overall durability of the concrete.

Conclusion
This chapter presents the main conclusions which can be drawn following the investigation into the robustness of SCGC to changes in water content and the effect curing has on the hardened properties of the various mixes.

Analysis of the results from this study present the following conclusions:

1. The results confirm SCGC is robust enough to tolerate changes in water content without segregation, contrary to its reputation of being sensitive to change in moisture.
2. The study found that SCGC is repeatable across a range of tests, in both the plastic and hardened states.
3. Changes in plastic properties remain intuitive, whereby the increase or decrease of water has the desired/expected result.
4. The results derived from air curing are counter intuitive and produced improved durability results. This finding introduces the industry to a unique advantage over conventional concrete, with further cost saving possibility as a result of/in a reduction of curing cost.
5. SCGC may be regarded as state of the art in the continuous progression to improve concrete quality and extend service life of structures. However, there remains much work needed till we achieve an understanding similar to that of conventional concrete.
6. The primary hydration product is amorphous and comprised of C-A-S-H gel.
7. All mix designs remained within the limits set out by EN BS 206:2013, despite some variation from one classification to another.
8. The mix design proportions and understanding of how they interact in a holistic manner is a primary step to ensure a robust mix design is produced.
9. The strength development of geopolymer is a complex multi-stage process, which is affected by several components such as water:binder ratio, Si:Al ratios, activator types and dosage, the percentage of Na₂O per binder, as well as, the total free molarity.
10. Despite changes to the curing methodology geopolymer demonstrates suitable performance characteristics for harsh conditions, such as marine environments where there is a need to resist the ingress of chlorides.
11. In general, both plastic and hardened properties have successfully demonstrated acceptable performance in the short term. The evaluation of these properties needs to be evaluated to understand the long-term characteristics, insuring there is no relapse of performance at a later stage.
12. The use of standard/common test methods proved suitable for the evaluation of the selected properties, however there is a requirement to standardize test procedures for geopolymer concrete, particularly if the specimens are air cured; i.e. drying shrinkage, the specimens may have a higher shrinkage rate initially, however as the capillaries are filled as a result of polymerization the shrinkage value may be lower than that of a water cured specimen.

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About author
Warren Mc Kenzie is a Concrete Technologist and R&D specialist with 10 years’ experience in the industry. He graduated from the University of Leeds, UK, with MSc in civil engineering – specializing in advanced concrete technology. He has focused on topics such as 3D printing and Geopolymer concrete, becoming internationally recognized for his work on the later. He is currently the Technical Manager for BASF Construction Chemical for the Gulf States.