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Sustainability of alkali-activated cementitious materials and geopolymers
Sustainability of alkali-activated cementitious materials and geopolymers
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P. Mangat, P. Lambert Sheffield Hallam University, Sheffield, United Kingdom
18.1 Introduction Alkali-activated cementitious materials (AACMs) and geopolymers represent a family of alternative binders for the full or partial replacement of Portland cements. An AACM is essentially a material that gains strength by means of a chemical reaction between a source of alkali, commonly an alkali silicate solution, and an aluminate-rich material. As well as being highly sustainable, largely composed of industrial by-products already familiar to the concrete producer, such as fly ash and blast furnace slag, they can exhibit performance characteristics beyond those of conventional Portland cement-based materials. Enhanced chemical, wear, and temperature resistance have all been shown to be achievable. Bearing more resemblance to a natural stone than a cement, these materials promise low-carbon, high-performance alternatives to current binder technology. The term ‘geopolymer’, first used in 1979 although the process is clearly much older, describes binders produced by the polycondensation of alumina-silicates such as fly ash and slag with alkali activators to produce inorganic polymers (Davidovits, 1979). The large-scale production and use of geopolymer-type materials can be traced to the immediate post–World War II Soviet Union where ‘soil cements’ were employed in the construction of high-rise apartment buildings, which have subsequently proved to be very durable. While there are distinctions between AACMs and geopolymers, for end-users the terms are often interchangeable. Wider take-up of such materials has to date been restricted by the lack of track record and a dearth of nationally and internationally recognised standards to define their characteristics and performance. Practical applications with associated evidence of performance, plus standards and guidance on their use, are starting to become available (RILEM TC 224, 2014; BSI, 2016). These should allow such materials to be more widely adopted in larger scale commercial applications. Many formulations remain alkaline in nature and capable of working well with conventional reinforcement, although it may be some time before sufficient data are available to predict the long-term corrosion behaviour of embedded steel in more aggressive environments, such as marine applications or where exposed to deicing salts. Sustainability of Construction Materials. http://dx.doi.org/10.1016/B978-0-08-100370-1.00018-4 © 2016 Elsevier Ltd. All rights reserved.
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The sustainability credentials of such materials may be expected to be of the greatest interest to potential adopters of the technology. For the main component in the binder, the energy commitment has already been made. Relatively little additional energy is required to convert them to useful materials and significant savings of energy and carbon equivalent, up to 80%, are often claimed over Portland cement equivalents (RILEM TC 244). However, as most modern conventional concrete contains a large proportion of the same components, such as blast furnace slag (BFS) and fly ash, as cement replacements, simple energy input comparisons can sometimes be misleading. The chemical resistance of certain of these materials can be superior to their conventional counterparts so their use in wastewater and other potentially aggressive applications should prove attractive. Unlike traditional Portland cement-based materials, the structure of the hardened geopolymer does not rely on hydrates, making it much more stable at high temperatures and potentially imparting other advantages over their conventional counterparts. Unlike some earlier versions that required heating to cure, many AACM formulations are ambient curing, allowing their use in most if not all forms of construction. They can be placed, poured, or sprayed in a similar manner to any conventional material and have been successfully trialled for precast applications (see Section 18.5). The technology of AACMs and geopolymers promises low-energy/low-carbon materials with enhanced temperature and chemical resistance properties. They provide most of the attributes required from conventional Portland cement-based materials, potentially delivering the levels of durability and sustainability that modern construction practices increasingly demand.
18.2 The manufacture of inorganic polymers 18.2.1 Raw materials The raw materials of AACMs and geopolymers fall into three categories. The first category produces low-calcium, alkali-activated systems, which include materials often described as geopolymers. A common precursor in this category is fly ash, which has relatively high alumina (Al2O3) and silica (SiO2) content. The second category is based on high-calcium precursors such as BFSs that have higher calcium oxide (CaO) content than fly ash but less than Portland cement. The third category is hybrid binders that use mixtures of precursors.
18.2.1.1 Low-calcium precursors Low-calcium, alkali-activated binders constitute raw materials rich in aluminosilicates, alumina (Al2O3), and silica (SiO2). A prime example is lower calcium fly ash of Class F, which are highly pozzolanic. The chemical composition of a typical Class F material constitutes over 70% of the pozzolanic compounds SiO2, Al2O3, and Fe2O3. However, it is not only the fly ashes derived from traditional black coal that are suitable for alkali activation to produce geopolymers. Low-calcium ashes produced from
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the combustion of brown coal and fluidised bed ash from coal combustion are also suitable for this purpose. Rice husk combustion ashes, which are rich in SiO2, are also potential raw materials (RILEM TC 224, 2014). Although low-calcium fly ashes are generally considered to produce superior geopolymer performance, the role of Ca content is not straightforward (Oh et al., 2010). The calcium content of Class C fly ashes, usually produced from lignite and subbituminous coals, is greater than Class F fly ashes and exceeds 20%. In some studies, high-calcium ashes have shown greater strength development while others have shown the opposite (RILEM TC 224, 2014).
18.2.1.2 High-calcium precursors Ground granulated BFS from the steel industry is a commonly used precursor for alkali-activated materials (Shi et al., 2006). The high calcium level is provided by the CaO component that typically ranges between 31% and 46% by weight. Other major compounds and their typical contents are SiO2 (30–40%), Al2O3 (6–18%) and MgO (2–18%) (EUROSLAG, 2003). The optimum fineness of BFS for use in alkali-activated materials ranges between 400 and 550 m2/kg (RILEM TC 224, 2014). The glass content of slag used in blended Portland cement is 90–100%. Glassy BFS with CaO/SiO2 ratios between 0.5 and 2.0 and Al2O3/SiO2 ratios of 0.1–0.6 are suitable precursors for alkali activation (Talling and Brandstetr, 1989).
18.2.1.3 Hybrid binders The development of hybrid binders to combine the individual benefits of low-calcium and high-calcium alkali-activated materials has gained momentum in recent years. High strength and durability together with low cost and sustainability benefits are providing the incentive for developing these binders. The blending of fly ash with BFS produces a suitable hybrid binder where the calcium from the BFS and both alumina and silica from the fly ash aid the alkali-activated reactions of the binder. The nature of the reactions is not currently fully understood. The replacement levels of BFS with fly ash in the blended binder can be optimised for different chemical compositions of each precursor to achieve best performance. Some other constituents that can be used in hybrid binders include metakaolin, silica fume, calcined clays, and calcium aluminate cement (RILEM TC 224, 2014). The performance of BFS-based, alkali-activated materials under high-temperature exposure is improved by adding metakaolin to the system. Calcium aluminate cement can be blended with calcined clays, natural zeolites, or pozzolans (Fernández-Jiménez et al., 2008) to improve strength and to control efflorescence (RILEM TC 224, 2014). Metakaolin plays an important role in hybrid blends with various fly ashes and slags by supplying Al to the reaction process. The properties of alkali-activated fluidised bed combustion fly ash can be improved by blending with silica fume and Al(OH)3. The high silica content of silica fume reacts with the alkali solutions and CaO to form additional calcium silicate hydrate.
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18.2.2 Activators When mixed with water, BFS undergoes a low rate of hydration and develops strength slowly. Exposure to an alkaline activator creates a high pH environment and accelerates the hydration reactions. Commonly used activators are industrial products whose manufacturing technology is well established and are manufactured in large quantities and therefore easily available for supporting new applications. Sodium silicate (Na2SiO3) and sodium hydroxide (NaOH) are commonly used activators. Sodium silicate solution is available in different grades defined by its silica modulus or SiO2/Na2O ratio. Sodium hydroxide (NaOH) is produced as a solid by the electrolysis of sodium chloride solution, which produces NaOH, hydrogen and chlorine. Solid NaOH is dissolved in water to provide a solution of the required molarity. Sodium carbonate and sodium sulphate have also been used as activators for BFS. They are less reactive than the silicate-based activators and strength development is slow. Other silica-rich sources can also form suitable activators. For example, silica fume combined with NaOH activator when used with BFS binders produces a high- performance, alkali-activated material that is comparable to sodium silicate activated formulations (Živica, 2006). Other high-silica sources that form potential activators are rice husk ash and nanosilica (RILEM TC 224, 2014). The use of an alkali silicate with finely ground limestone as activator for a BFS binder has the potential of making an economical alkali-activated material of moderate performance (Sakulich et al., 2009).
18.2.3 Alkali activation reactions The two general cases of low-calcium precursors, such as fly ash, and high-calcium precursors, such as BFS, are considered. BFS is more reactive than fly ash at moderate alkaline pH levels and, therefore, can be used with a greater range of activators in addition to the hydroxides and silicates that are suitable for fly ash. Low-calcium precursors, typically fly ashes, are rich in SiO2 and Al2O3 compounds, whereas high CaO content of high-calcium precursors dominates the hydration reactions. Alkali activation of a low-calcium precursor produces an amorphous aluminosilicate gel, which resembles the structure of a zeolite. The gel also contains small amounts of minerals such as quartz present in the original fly ash. Aluminium and silicon atoms are arranged in the cross-linked framework of the tetrahedral structure of the gel. The negative charge of the SiO4 and AlO4 groups is balanced by alkali ions such as sodium. The water in the low-calcium, alkali-activated fly ash does not take part in the hydration reactions since there is no formation of Ca(OH)2 to activate pozzolanic reactions. The products of alkali activation of BFS are influenced by the chemical constituents of the slag and the activator, such as Ca, Mg, Si, Al, Na, S, in addition to the pH, liquid/binder ratio, and curing. The dominant hydration phase is the calcium silicate hydrate (C-S-H) broadly similar to the tobermorite gel formed in hydrated Portland cement. The lower Ca content of slag compared to Portland cement and the greater Al substitution in the tetrahedral structure leads to cross-linking between tobermorite
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chains (Provis, 2014). The complex hydration system of alkali-activated slags can be simplified into the following categories: N-A-S-H gel is formed in alkali-activated slags with low Ca + Mg content. A secondary phase of crystalline zeolites is also present. C-A-S-H gel is formed in alkali-activated slags with high Ca content. Na ions are also present in the gel at charge-balancing sites and the gel may be defined as C-(N)-A-S-H. AFm (monosulphate) in crystalline form appears as a secondary phase if sufficient Al is available. N-A-S-H gel exists as a secondary product with the C-A-S-H gel in most chemical compositions of slags and in hybrid binders involving high and low Ca precursors.
The main difference in the above hydration reactions of alkali-activated binders and ordinary Portland cement is that in the latter case chemical reactions of hydration are activated by water. The main hydration products are the C-S-H tobermorite gel and calcium hydroxide, with AFm (monosulphate) and AFt (ettringite) as secondary phases.
18.3 Fresh properties One of the challenges for this technology has been to achieve scaled-up production of alkali-activated concrete with sufficient workability and working time. AACMs generally tend to be more cohesive than Portland cement concrete and stiffen rapidly. The numerous workability and plasticising admixtures available for normal concrete are not specifically designed for compatibility with the AACM chemistry. The efficiency of the few that can be used with AACMs is less than in Portland cement-based binders. However, commercial compositions of AACMs suitable for large applications are available, and there are many examples of their use. Working time has been managed and there are examples of compositions developed for structural applications including repair (Science Daily, 2009; Concrete Repair Bulletin, 2015; Lambert et al., 2015). The future scope for developing admixtures tailored for AACMs is promising and, in turn, will open up many more opportunities for the use of alkali-activated materials. The density of the fresh and hardened AACM concrete is influenced by the specific gravity of binder materials, the aggregates, and their mix proportions. The lower specific gravity of precursors such as fly ash can reduce the density to some extent but it remains in a similar range to Portland cement-based concrete.
18.4 Mechanical properties 18.4.1 Strength and stiffness Compressive strengths greater than 50 MPa have been reported in many laboratory evaluations of alkali-activated, BFS high-calcium binders and fly ash-based low- calcium binders. Higher strengths up to 110 MPa have also been achieved (Talling and Krivenko, 1997). Typically, high-temperature curing for relatively long periods
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has been required to activate the hydration process and achieve such values. Steam curing has also been used to develop good compressive strength in precast members (Sumajouw et al., 2007; Talling and Krivenko, 1997). However, a well-designed AACM can cure at ambient temperature and develop high compressive strength (Science Daily, 2009). Such materials cure at ambient temperature to give 28-day compressive strengths exceeding 65 MPa. A typical modulus of rupture value for this material is 9.5 MPa, which is over 10% of its compressive strength. The relationship between compressive strength and age of the AACM concrete cured in air (20°C, 60 RH) is plotted in Fig. 18.1 for two compositions with different aggregate grading and content. The compressive strength under wet curing can be somewhat lower but occasionally it is higher. In practice, wet curing in the early period followed by air curing is desirable to prevent cracking, control shrinkage, and optimise strength development. Fig. 18.2 shows the shear-bond strength of the hybrid AACM mortar (Science Daily, 2009) used to bond a sandstone façade to a medium-strength breeze block. The bond tests were conducted after different durability exposures given in the figure. A comparison of Figs. 18.2 and 18.3 shows the significantly superior shear-bond capacity of the AACM mortar relative to a common proprietary multipurpose epoxy adhesive particularly at very high temperature exposure. The behaviour of AACM concrete under compression is similar to conventional Portland cement concrete giving comparable values for strain at failure and the relationship between elastic modulus and strength. However, more detailed relationships between compressive strength and such parameters need to be developed for the many variations of potential compositions of AACM structural concrete. Some contradictory results have been reported on the influence of Si/Al ratio on elastic modulus (RILEM TC 224, 2014). The Poisson's ratio of geopolymer concrete under 80
Compressive strength (MPa)
70 60 50 40 30 20 10 0
0
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10
15 20 Time (days)
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Fig. 18.1 Relationship between compressive strength and age of hybrid AACM concrete (Mangat, Unpublished data).
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3.5 Failure stress (MPa)
3 2.5 2 1.5 1 0.5 0
Control
Heat ageing Freeze–thaw
Water immersion
High temp 200°C
High temp 400°C
High temp 600°C
Fig. 18.2 Shear-bond strength of a hybrid AACM applied to a medium-strength breeze block (Mangat, Unpublished data).
Failure stress (MPa)
3 2.5 2 1.5 1 0.5 0
Control
Heat ageing
Freeze–thaw
Water immersion
High temp 200°C
High temp 400°C
Fig. 18.3 Shear-bond strength of multipurpose epoxy adhesive applied to a medium-strength breeze block (Mangat, Unpublished data).
c ompression is s imilar to Portland cement-based concrete with compressive strengths from 40 to 90 MPa. Typical values range between 0.12 and 0.16. The relationship between modulus of rupture and compressive strength of AACM concretes shows a higher flexural strength relative to Portland cement-based concretes. This is possibly an indication of the stronger bond at the aggregate–matrix interface due to improved pore structure of the interface zone.
18.4.2 Shrinkage AACMs are typically prone to higher drying shrinkage, relative to Portland cement-based binders, when manufactured and cured under ambient temperatures. The shrinkage reduces by an order of magnitude under high-temperature curing conditions such as are commonly adopted for AACMs. The higher shrinkage is due to the lower degree of chemical binding of water in AACM hydration gels compared with Portland cement hydration products. The shrinkage trends are variable with different precursors and activators. It generally increases with increasing activator content and increasing activator silica modulus.
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The factors that help to control shrinkage are good mix design of the AACM concrete, resulting in high aggregate content that restrains shrinkage, adequate early age curing preventing rapid drying, plus the use of shrinkage-reducing admixtures (Mangat and Ojedokun, 2016; Dunster, 2013). Fibre reinforcement of AACMs also offers a mechanism for reducing shrinkage and providing crack control (Mangat & Bordeian, in press). The use of lightweight aggregate and other porous aggregates is also beneficial for shrinkage, while providing a strong chemical interaction with the binder at the interface. Fig. 18.4 shows the shrinkage development with age of two hybrid AACM concrete compositions (Science Daily, 2009), giving a typical range of shrinkage values from tests conducted according to BS ISO 1920-8:2009, Part 8. AACM compositions are also prone to microcracking at early ages, before adequate tensile strength has been developed. The problem is aggravated with high paste (binder) content and dry curing, as would also be the case with Portland cement-based matrices. High activator content and high modulus of activator increase the risk and extent of microcracking.
18.4.3 Creep The creep behaviour of AACMs is different from Portland cement-based concrete, and is caused by differences in microstructure, water held in the gel structure, and the rate of strength development. The longer-term creep deformation of AACM concrete, beyond 90 days, continues to increase at a greater rate than Portland cement concrete. The limited data available in the literature indicate that creep of BFS-based AACM concrete is lower with silicate activation compared with hydroxide activation and is highest with carbonate activation (Talling and Krivenko, 1997). Similarly, limited data available on low-calcium (fly ash)-based alkali-activated concrete show little influence of curing temperature (oven or steam curing) on creep. The specific creep
Shrinkage (microstrain)
1200 1000 800 600 400 200 0
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60 Time (days)
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Fig. 18.4 Relationship between drying shrinkage and age of hybrid AACM concrete (Mangat, Unpublished data).
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of AACM concrete is reported to be lower than Portland cement concrete of similar grade and falls within acceptable limits defined by the design codes (Rangan, 2014; Sagoe-Crentsil et al., 2013).
18.5 Physical and durability properties 18.5.1 Permeability and porosity The distribution of capillary pores in the AACM matrix is important since it controls the mass transport mechanisms of deleterious substances such as chlorides and sulphates. While mercury intrusion porosity can provide useful insights to this property, relatively limited information on the pore structure of AACMs is available in the literature. The current state of knowledge indicates that the pore size distribution of AACMs falls into two separate zones of pore size >1 μm and <20 nm range, with insignificant porosity in between. In contrast, similar grades of Portland cement-based matrices have a unimodal pore size distribution of 10–100 nm. The impact of this difference in the pore structure of AACMs relative to normal concrete may be significant with respect to the durability properties of AACMs and require more research. The type of activator also has an effect, with potassium-based activators reducing the median pore diameter more effectively than sodium-based activators. As with good practice in concrete technology, porosity is reduced by high-quality curing, such as sealed conditions and longer periods of hydration. Generally, the sorptivity of AACMs is within a comparable range with similar grade concretes (RILEM TC 224, 2014). The capillary sorptivity is reduced by employing a lower water content and a higher silica modulus activator. One potential advantage of AACMs relative to normal concrete is likely to be in the aggregate–matrix interfacial zone. This zone in Portland cement matrices tends to be more porous and weaker than the bulk matrix remote from the aggregate surface. The alkali activator in AACMs, however, may be expected to interact chemically with the aggregate particles in addition to the binder. There is already evidence available of this effect but further research is required for validation.
18.5.2 Chloride, carbonation, and corrosion The mechanisms of chloride diffusion, carbonation, and steel reinforcement corrosion in normal concrete form the starting point for understanding such mechanisms in AACMs. The basic difference is the pore structure and the chemistry of the pore fluid in AACMs relative to normal concrete. Chloride diffusion test methods that measure the progress of chloride penetration directly, such as the BS EN 12390-11 (BSI, 2015) and the older Nordtest (Nordtest, 1995), give a more reliable comparison of chloride diffusion between AACMs and concrete. Such test methods that analyse powder samples to determine soluble chlorides (water or acid) are relatively independent of the pore fluid chemistry of the AACM binder. The pore fluid chemistry of AACMs leads to inconclusive results on chloride diffusion and requires more research. Chloride
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diffusion tests based on these standards have been conducted on the hybrid AACM formulations, which were cured under ambient conditions (Science Daily, 2009). The results show superior chloride resistance of the AACM concrete compared with a similar grade of normal Portland cement-based concrete (Mangat and Ojedokun, 2016). Pore fluid provides the electrolyte for both the protection and the corrosion processes of steel in AACMs and it is important to understand its chemical composition. AACMs can be based on a wide range of precursors and activators each of which will vary the pore fluid chemistry. The presence of threshold levels of chloride concentration at the steel interface initiates corrosion in normal concrete. The chemistry and, in particular, the buffered high pH of the pore fluid provides passivation to steel corrosion until chlorides exceeding the threshold level cause the initiation of corrosion. The Ca(OH)2 buffer is limited in AACM concrete and its extent depends on the hydration products of the particular precursor used. The presence of sulphide in AACM binders using BFS influences steel corrosion differently to the corrosion mechanism in normal concrete (RILEM TC 224, 2014). Service life prediction models and accelerated durability test methods used for concrete will not be directly valid for AACM concrete due to these differences. AACMs based on BFS and blends with metakaolin show higher degrees of carbonation relative to conventional cement and mortar due to the lack of Ca(OH)2 in the hydration products. However, a suitable choice of activator and its concentration can mitigate this effect. Mix design of AACM concrete is an important factor in controlling carbonation. Carbonation depths comparable to similar grade concrete can be achieved by providing higher binder (paste) contents in AACM concrete mixes. Carbonation results of AACMs under accelerated carbonation testing often show inferior performance to Portland cement binders, which contradict data obtained under long-term natural exposure. There are some field data on carbonation of AACMs available for structures built in Russia, Ukraine, and Poland under a service life of over 40 years. Carbonation rates of under 0.5 mm/year are indicated for these climates. Carbonation rates of under 1 mm/year were obtained with the standard phenolphthalein test that is considered unsuitable for AACMs (RILEM TC 224, 2014). The phenolphthalein test for measuring carbonation depth was developed for normal concrete and it is compatible with the chemistry of ordinary Portland cement hydration products (CaOH2). The application and interpretation of this test to AACMs and to modern blended cements requires investigation, and alternative test methods will need to be developed. Carbonation induced reinforcement corrosion in concrete is considered to occur when the carbonation front reaches the steel surface, reducing the high alkalinity of the pore fluid through the conversion of calcium hydroxide (CaOH2) to calcium carbonate (CaCO3). The reduction in pH depassivates the steel and can allow the initiation of corrosion. This process is unlikely to be replicated in AACMs due to the different hydration products, notably to absence of CaOH2, and other differences in the chemical composition of the pore fluid. In addition to reinforcement corrosion, the other deleterious effect of carbonation in Portland cement concrete is carbonation shrinkage. The carbonation shrinkage property of AACMs is currently unknown and requires further investigation.
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The effect of accelerated exposure to carbonation is more severe on AACMs than on normal concrete. The correlation of accelerated test data with natural carbonation under service conditions is also not clear. This is related to the difference in the CO2 diffusion process and the chemical composition of the pore fluid relative to Portland cement binders where the Ca(OH)2 concentration is high. The rate of carbonation in AACMs is strongly affected by the CO2 concentration of the exposure environment. The rate of pore fluid carbonation and degradation of the gel pore structure change significantly at high CO2 exposure levels and the ratio of carbonate to bicarbonate is affected (Puertas and Palacios, 2007; RILEM TC 224, 2014). Accelerated carbonation testing at CO2 concentrations above 1% is not recommended.
18.5.3 Efflorescence Efflorescence in AACMs is due to three factors: porous microstructure of some AACM compositions, high alkali concentration in the pore fluid such as in sodium aluminosilicate binders activated with high Na2O/Al2O3 solutions, and relatively weak binding of Na in the aluminosilicate gel structure. Replacement of sodium with potassium improves binding with aluminosilicate gel, and increasing the reactive aluminium content provides more alkali-binding capacity. However, low permeability is an important requirement for a durable material and an appropriately formulated and cured AACM will be more resistant to efflorescence (RILEM TC 224, 2014).
18.5.4 Freeze–thaw resistance The freeze–thaw resistance of Portland cement concrete is related to physical characteristics such as pore structure, pore saturation, strength of the matrix, and voids provided by air entrainment. The chemical composition of the hydration products is less significant. The same factors are likely to control the freeze–thaw resistance of AACMs. The freezing temperature of the pore fluid, however, will be affected by the differences in ionic composition of the pore fluid and by the relative degree of its confinement within the pore structure of Portland cement and AACM binders. The freezing point of pore fluid in a BFS-based AACM has been reported as significantly lowered to below −50°C due to its high ionic concentration, resulting in higher freeze–thaw resistance than a Portland cement-based binder. Many studies showing satisfactory frost resistance and resistance to frost-salt attack of AACMs have been reported. Data from in-service structures have shown better frost resistance of AACM concrete compared with similar grade conventional concretes exposed to the same conditions (Davidovits, 2008; Rostovskaya et al., 2007). Early age freezing and thawing is less damaging to AACM concrete than to Portland cement concrete, and freezing is detrimental only before the AACM concrete has reached 5 MPa strength. The freeze–thaw resistance of fly ash-based AACMs also meets the specifications for cold weather applications. Some studies have shown worse freeze–thaw resistance of AACM concrete than similar grade Portland cement concrete (Bilek and Szklorzova, 2009). This is
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a ttributed to the higher amount of free water present in the pores of AACMs due to the lack of crystal phases, such as CaOH2 and ettringite, which are present in Portland cement hydration products. These crystal phases contain the water in chemically and physically bound form and prevent it from freezing. This effect is aggravated in AACMs by the presaturation of test specimens as required by testing standards before exposing the specimens to freeze–thaw conditions. Such binders can thus contain more water in the pore network within the gel than Portland cement binders, which then undergo freeze–thaw cycles. This can result in more freeze–thaw damage if the tests are conducted near the water saturation level of the specimen. The freeze– thaw test methods given in most standards are expected to be suitable for AACMs but any precuring phase that requires presaturation of the test specimens needs to be reviewed. The freeze–thaw and scaling resistance of AACMs can be reduced by carbonation and microcracking due to restrained shrinkage. Carbonation of the matrix makes it more brittle and carbonation shrinkage can lead to cracking and scaling.
18.5.5 Sulphate and acid resistance Low-calcium binder-based alkali-activated material cured at 60°C and exposed to sodium sulphate solution showed excellent sulphate resistance. Portland cement binders can suffer sulphate attack due to chemical reactions with the hydration products resulting in the formation of gypsum and subsequently ettringite that is expansive and can result in disintegration of the concrete. The hydration products of alkali-activated, low-calcium precursors such as fly ash are relatively free from C-S-H, and sufficient expansive ettringite is not formed to cause damage. High-calcium precursors such as BFS can produce more expansive ettringite. The acid resistance of alkali-activated, low-calcium binders is similarly superior to Portland cement concrete due to the difference in hydration products and ettringite formation.
18.5.6 Fire resistance The high-temperature resistance of AACMs based on low-calcium binders (geopolymers), with fly ash as the precursor, has been widely reported in the literature (RILEM TC 224, 2014). Their amorphous gel structure is considered to provide fire resistance by preventing the buildup of internal pressure, which leads to violent splitting and spalling of Portland cement concrete when exposed to fire. A hybrid AACM composition has passed the Eurocode (EN 1991-1-2:2002) fire test at temperatures over 1100°C (Science Daily, 2009). In addition, a fireplug test demonstrated satisfactory insulation to services while satisfying the design load capacity of 5 kN/m2 (Mangat, 2009). Fig. 18.5 shows a furnace test sample of the same AACM composition, displaying its typical response to high temperature. Comparative fire tests against similar strength Portland cement concretes demonstrate the release of steam from the gel structure of the AACM concrete, preventing spalling, while the Portland cement concrete spalls explosively.
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Fig. 18.5 Heat resistance of AACM (Mangat, Unpublished data).
Comparative tests on fly ash and metakaolin-based AACMs exposed to high temperature have shown that the strength of the metakaolin-based AACM decreased after heating, whereas the fly ash-based AACM had higher strength (Kong and Sanjayan, 2008; Kong et al., 2007). This was attributed to the differences in pore distribution in the two materials that allowed the steam to escape more easily and without damaging the microstructure in the fly ash-based geopolymer. Fire tests up to 800°C on metakaolin-based AACMs and blends with BFS showed a higher residual strength (after firing) for the blended materials (RILEM TC 224, 2014). The residual strength of AACM mortar and concrete increases due to further geopolymer reaction while the matrix-aggregate thermal incompatibility causes strength reduction. The balance between the two controls the final strength after fire testing.
18.6 Structural applications There are many examples of applications of AACM concretes since the 1950s in Europe, the former Soviet Union, and more recently in Asia, North America, and Australia (RILEM TC 224, 2014). These applications have demonstrated the performance of these materials in rigorous environmental and service conditions. In general, their durability after decades of exposure to conditions such as carbonation, freeze– thaw attack, acid attack, and corrosion of the reinforcement has been judged to be satisfactory and the strength of the matrix has increased with time. Some limitations, such as rapid loss of workability, have also been experienced, which are being addressed in current research and development work. Development of AACMs has progressed to the stage where well-defined formulation of the binder composition together with good mix design to optimise aggregate content, and appropriate liquid/binder ratios, can produce mixes suitable for structural application. Standard structural elements such as beams and columns have been
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Fig. 18.6 Load-deflection curve of the reinforced AACM lintel (Mangat, Unpublished data).
manufactured and cured at either elevated temperature (steam curing) or at ambient temperature. Test results on these elements show that the strength and deformation characteristics of AACM concrete structural elements are similar to Portland cement concrete elements of the same nominal grade. Reinforced concrete structural design codes can therefore be considered relevant to AACM concrete structures. A reinforced concrete lintel was manufactured with an AACM concrete composition with similar properties to a structural grade concrete (Science Daily, 2009). The beam was manufactured at a precast plant where the quality control was relatively poor and the reinforcement cage suffered some displacement during casting. A second similarly reinforced concrete beam with Portland cement concrete was manufactured as a control element under better quality control. The beam dimensions were 140 × 213 mm cross-sectional width and depth and a length of 2.7 m. Both beams were manufactured and cured at ambient temperature. Both were tested under four-point bending (span 2.12 m) using the procedure of BS8110. The load-deflection curve of the reinforced AACM concrete beam is shown in Fig. 18.6 and their relative performance is further analysed in the next paragraphs.
18.6.1 Serviceability limit state 18.6.1.1 AACM concrete beam Instantaneous deflection at serviceability load of (34.5 + 0.455) kN = 11.51 mm Residual instantaneous deflection upon unloading = 3.15 mm % recovery after first loading =
11.51 - 3.15 ´ 100 = 72.6% 11.51
18.6.1.2 Portland cement concrete beam Instantaneous deflection at serviceability load of (34.5 + 0.455) kN = 9.62 mm
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Residual instantaneous deflection upon unloading = 1.83 mm % recovery after first loading =
9.62 - 1.83 ´ 100 = 81% 9.62
Hairline flexural cracks were observed in the both beams at serviceability load.
18.6.2 Ultimate limit state Both beams failed at a load of 60 kN with an ultimate moment of resistance of 30.3 kNm. In comparison, the design ultimate moment of resistance of the beam was 26.1 kNm. The deflection at ultimate load of the AACM and Portland cement concrete beams was 26.0 and 22.3 mm respectively. The maximum deflection permitted by BS 8110-2: 1985 at the ultimate design lo ad = span/40 = 1/40 × 2120 mm = 53 mm These results are summarised in Table 18.1. Similar conclusions have been reported for heat-cured (steam-cured) beams and columns of low-calcium-based geopolymer binders (Rangan, 2014). Beam sections of 200 × 300 mm and 3.3 m length were manufactured with tensile reinforcement ratios between 0.64% and 2.69%. The flexural, shear, and bond characteristics of the beams were subsequently investigated. Columns of effective length 1.7 m and 175 mm square cross section were made with 1.47% and 2.95% longitudinal reinforcement. The compressive strength of the steam-cured geopolymer concrete was 40–60 MPa. The structural performance and failure mode of the reinforced geopolymer concrete beams and columns was similar to Portland cement concrete elements of similar grade concrete.
18.7 Future trends Based on the evidence that structural performance can be equivalent or superior to Portland cement concretes, and a growing understanding of the benefits and limitations with respect to durability and the emergence of standards for their specification, one may expect AACMs and geopolymers to play an increasing role in providing durable and sustainable alternatives to the current products based on traditional Portland-based binders. The use of secondary wastes such as flue gas desulphurisation (FGD) products within the precursors are also potential future developments (Mangat et al., 2006; Khatib et al., in press). There is already significant interest in their use in challenging environments where resistance to high temperatures or aggressive chemicals is required. The potential savings in, for example, a sewage treatment environment could be considerable if the alternative is to employ conventional concrete with an expensive, chemical-resistant protective coating. Similarly, a resistance to high-temperature exposure resulting in little or no loss of properties, is particularly valuable when selecting materials for tunnel linings. A major limitation to the more widespread use of AACMs and geopolymers is their lack of track record, especially with respect to the durability of embedded steel reinforcement. While this may restrict the direct replacement of Portland cement binders in conventional reinforced concrete designs, it should also encourage the use of alternative methods of reinforcement. Where the maintenance of a high-alkalinity
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Table 18.1 Flexural test results of reinforced beams of AACM and Portland cement concrete, according to BS8110 (Mangat, Unpublished data) Residual deflection (mm)
Recovery (%)
Moment of resistance (kNm)
Design moment of resistance (kNm)
Ultimate deflection (mm)
Permissible deflection (mm)
AACM beam Control concrete beam
11.51 9.62
3.15 1.83
72.6 81
30.3 30.3
26.1 26.1
26.0 22.3
53 53
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Beam
Instant deflection (mm)
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e nvironment is not a necessity for durable service, nonmetallic fibres should offer viable and more sustainable alternatives.
References Bilek, V., Szklorzova, H., 2009. Freezing and thawing resistance of alkali-activated concretes for the production of building elements. In: Malhotra, V.M. (Ed.), Proceedings of 10th CANMET/ACI Conference on Recent Advances in Concrete Technology, Supplementary Papers, Seville, Spain, pp. 661–670. BSI, 2015. BS EN 12390-11, Testing hardened concrete. Determination of the chloride resistance of concrete, unidirectional diffusion, UK. BSI, 2016. PAS 8820, Alkali-activated cementitious material and concrete—specification, UK. Concrete Repair Bulletin, 2015. Commerce Trust Building Entrance Repair, vol. 28, No. 6. International Concrete Repair Institute, St. Paul, MN, p. 42, November/December. Davidovits, J., 1979. Synthesis of New High-Temperature Geopolymers. PACTEC IV, Society of Plastic Engineers, Bethel, CT, pp. 151–155. Davidovits, J., 2008. Geopolymer Chemistry and Applications. Institut Géopolymère, SaintQuentin, France. Dunster, A., 2013. Durability of alkali activated binder concretes. BRE Information Paper, IP 5/13 March. EUROSLAG, 2003. Granulated blast furnace slag. Technical leaflet no. 1. Fernández-Jiménez, A., Palomo, A., Vazquez, T., Vallepu, R., Terai, T., Ikeda, K., 2008. Alkaline activation of blends of metakaolin and calcium aluminate cement. Part I: strength and microstructural development. J. Am. Ceram. Soc. 91 (4), 1231–1236. Khatib, J.M., Mangat, P.S., Wright, L., In Press. Mechanical and Physical Properties of Concrete Containing FGD Waste. Mag. Concr. Res. http://dx.doi.org/10.1680/macr.15.00092 Article number: MACR-D-15-00092. Kong, D.L.Y., Sanjayan, J.G., 2008. Damage behaviour of geopolymer composites exposed to elevated temperatures. Cem. Concr. Compos. 30 (10), 986–991. Kong, D.L.Y., Sanjayan, J.G., Sagoe-Crentsil, K., 2007. Compararative performance of geopolymers made with metakaolin and fly ash after exposure to elevated temperatures. Cem. Concr. Res. 37, 1583–1589. Lambert, P., Mangat, P., O'Flaherty, F., Nguyen, V.C., Jones, G.A., 2015. Combined solution for the strengthening and corrosion protection of reinforced concrete structures. Concr.: Concr. Soc. 49 (01), 15–18. Mangat, P.S., 2009. Fire resistance of liquid granite. Fire Protection Association Magazine, Article R1. Mangat, P. S., Unpublished data. Mangat, P.S., Khatib, J.M., Wright, L., 2006. Optimum utilisation of flue gas desulphurisation (FGD) waste in blended binder for concrete. Constr. Mater. J. Proc. Inst. Civil Eng. 1 (2), 60–68. ISNN 1747-650X. Mangat, P.S., Bordeian, S. Shrinkage of fibre reinforced alkali activated composites. Cem. Concr. Compos., in press. Mangat, P.S., Ojedokun, O., 2016. Chloride diffusion in alkali activated concrete. In: Second International Conference on Concrete Sustainability, ICCS Madrid 16 September. Nordtest, 1995. Concrete, hardened: accelerated chloride penetration (NT BUILD 443). Nordtest, Denmark.
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Oh, J.E., Monteiro, P.J.M., Jun, S.S., Choi, S., Clark, S.M., 2010. The evolution of strength and crystalline phases for alkali-activated ground blast furnace slag and fly ash-based geopolymers. Cem. Concr. Res. 40 (2), 189–196. Provis, J., 2014. Geopolymers and other alkali activated materials: why, how and what? Mater. Struct. 47, 11–25. Provis, J.L., Van Deventer, J.S.J. (Eds.), 2014. RILEM TC 224-AAM, Alkali-Activated Materials: State of the Art Report. Springer/RILEM TC 224, Dordrecht. Puertas, F., Palacios, M., 2007. Changes in C-S-H of alkali-activated slag and cement pastes after accelerated carbonation. In: Beaudoin, J.J., Makar, J.M., Raki, L. (Eds.), 12th International Congress on the Chemistry of Cement, Montreal, Canada, CD-ROM Proceedings. Rangan, B.V., 2014. Geopolymer concrete for environmental protection. Indian Concr. J. 88 (4), 41–59. Rostovskaya, G., Ilyin, V., Blazhis, A., 2007. The service properties of the slag alkaline concretes. In: Ertl, Z. (Ed.), Proceedings of the International Conference on Alkali Activated Materials—Research, Production and Utilization, Prague, Czech Republic, Ceská rozvojová agentura, pp. 593–610. Sagoe-Crentsil, K., Brown, T., Taylor, A., 2013. Drying shrinkage and creep performance of geopolymer concrete. J. Sustain. Cem. Based Mater. 2 (1), 35–42. Sakulich, A.R., Anderson, E., Schauer, C., Barsoum, M.W., 2009. Mechanical and microstructural characterization of an alkali-activated slag/limestone fine aggregate concrete. Constr. Build. Mater. 23, 2951–2959. Science Daily, 2009. Liquid Granite: Building Material of the Future Unveiled. www.sciencedaily. com/releases/2009/10/091029161253.htm. 4 November. Shi, C., Krivenko, P.V., Roy, D.M., 2006. Alkali-Activated Cements and Concretes. Taylor & Francis, Abingdon, UK. Sumajouw, D.M.J., Hardjito, D., Wallah, S.E., Rangan, B.V., 2007. Fly ash-based geopolymer concrete: study of slender reinforced columns. J. Mater. Sci. 42 (9), 3124–3130. Talling, B., Brandstetr, J., 1989. Present state and future of alkali-activated slag concretes. In: Malhotra, V.M. (Ed.), 3rd International Conference on Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete, ACI SP114, Trondheim, Norway, Vol. 2. American Concrete Institute, Farmington Hills, MI, pp. 1519–1546. Talling, B., Krivenko, P.V., 1997. Blast furnace slag—the ultimate binder. In: Chandra, S. (Ed.), Waste Materials Used in Concrete Manufacturing. Noyes Publications, Park Ridge, NJ, pp. 235–289. Živica, V., 2006. Effectiveness of new silica fume alkali activator. Cem. Concr. Comp. 28 (1), 21–25.
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P. Mangat, P. Lambert Sheffield Hallam University, Sheffield, United Kingdom
18.1 Introduction Alkali-activated cementitious materials (AACMs) and geopolymers represent a family of alternative binders for the full or partial replacement of Portland cements. An AACM is essentially a material that gains strength by means of a chemical reaction between a source of alkali, commonly an alkali silicate solution, and an aluminate-rich material. As well as being highly sustainable, largely composed of industrial by-products already familiar to the concrete producer, such as fly ash and blast furnace slag, they can exhibit performance characteristics beyond those of conventional Portland cement-based materials. Enhanced chemical, wear, and temperature resistance have all been shown to be achievable. Bearing more resemblance to a natural stone than a cement, these materials promise low-carbon, high-performance alternatives to current binder technology. The term ‘geopolymer’, first used in 1979 although the process is clearly much older, describes binders produced by the polycondensation of alumina-silicates such as fly ash and slag with alkali activators to produce inorganic polymers (Davidovits, 1979). The large-scale production and use of geopolymer-type materials can be traced to the immediate post–World War II Soviet Union where ‘soil cements’ were employed in the construction of high-rise apartment buildings, which have subsequently proved to be very durable. While there are distinctions between AACMs and geopolymers, for end-users the terms are often interchangeable. Wider take-up of such materials has to date been restricted by the lack of track record and a dearth of nationally and internationally recognised standards to define their characteristics and performance. Practical applications with associated evidence of performance, plus standards and guidance on their use, are starting to become available (RILEM TC 224, 2014; BSI, 2016). These should allow such materials to be more widely adopted in larger scale commercial applications. Many formulations remain alkaline in nature and capable of working well with conventional reinforcement, although it may be some time before sufficient data are available to predict the long-term corrosion behaviour of embedded steel in more aggressive environments, such as marine applications or where exposed to deicing salts. Sustainability of Construction Materials. http://dx.doi.org/10.1016/B978-0-08-100370-1.00018-4 © 2016 Elsevier Ltd. All rights reserved.
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The sustainability credentials of such materials may be expected to be of the greatest interest to potential adopters of the technology. For the main component in the binder, the energy commitment has already been made. Relatively little additional energy is required to convert them to useful materials and significant savings of energy and carbon equivalent, up to 80%, are often claimed over Portland cement equivalents (RILEM TC 244). However, as most modern conventional concrete contains a large proportion of the same components, such as blast furnace slag (BFS) and fly ash, as cement replacements, simple energy input comparisons can sometimes be misleading. The chemical resistance of certain of these materials can be superior to their conventional counterparts so their use in wastewater and other potentially aggressive applications should prove attractive. Unlike traditional Portland cement-based materials, the structure of the hardened geopolymer does not rely on hydrates, making it much more stable at high temperatures and potentially imparting other advantages over their conventional counterparts. Unlike some earlier versions that required heating to cure, many AACM formulations are ambient curing, allowing their use in most if not all forms of construction. They can be placed, poured, or sprayed in a similar manner to any conventional material and have been successfully trialled for precast applications (see Section 18.5). The technology of AACMs and geopolymers promises low-energy/low-carbon materials with enhanced temperature and chemical resistance properties. They provide most of the attributes required from conventional Portland cement-based materials, potentially delivering the levels of durability and sustainability that modern construction practices increasingly demand.
18.2 The manufacture of inorganic polymers 18.2.1 Raw materials The raw materials of AACMs and geopolymers fall into three categories. The first category produces low-calcium, alkali-activated systems, which include materials often described as geopolymers. A common precursor in this category is fly ash, which has relatively high alumina (Al2O3) and silica (SiO2) content. The second category is based on high-calcium precursors such as BFSs that have higher calcium oxide (CaO) content than fly ash but less than Portland cement. The third category is hybrid binders that use mixtures of precursors.
18.2.1.1 Low-calcium precursors Low-calcium, alkali-activated binders constitute raw materials rich in aluminosilicates, alumina (Al2O3), and silica (SiO2). A prime example is lower calcium fly ash of Class F, which are highly pozzolanic. The chemical composition of a typical Class F material constitutes over 70% of the pozzolanic compounds SiO2, Al2O3, and Fe2O3. However, it is not only the fly ashes derived from traditional black coal that are suitable for alkali activation to produce geopolymers. Low-calcium ashes produced from
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the combustion of brown coal and fluidised bed ash from coal combustion are also suitable for this purpose. Rice husk combustion ashes, which are rich in SiO2, are also potential raw materials (RILEM TC 224, 2014). Although low-calcium fly ashes are generally considered to produce superior geopolymer performance, the role of Ca content is not straightforward (Oh et al., 2010). The calcium content of Class C fly ashes, usually produced from lignite and subbituminous coals, is greater than Class F fly ashes and exceeds 20%. In some studies, high-calcium ashes have shown greater strength development while others have shown the opposite (RILEM TC 224, 2014).
18.2.1.2 High-calcium precursors Ground granulated BFS from the steel industry is a commonly used precursor for alkali-activated materials (Shi et al., 2006). The high calcium level is provided by the CaO component that typically ranges between 31% and 46% by weight. Other major compounds and their typical contents are SiO2 (30–40%), Al2O3 (6–18%) and MgO (2–18%) (EUROSLAG, 2003). The optimum fineness of BFS for use in alkali-activated materials ranges between 400 and 550 m2/kg (RILEM TC 224, 2014). The glass content of slag used in blended Portland cement is 90–100%. Glassy BFS with CaO/SiO2 ratios between 0.5 and 2.0 and Al2O3/SiO2 ratios of 0.1–0.6 are suitable precursors for alkali activation (Talling and Brandstetr, 1989).
18.2.1.3 Hybrid binders The development of hybrid binders to combine the individual benefits of low-calcium and high-calcium alkali-activated materials has gained momentum in recent years. High strength and durability together with low cost and sustainability benefits are providing the incentive for developing these binders. The blending of fly ash with BFS produces a suitable hybrid binder where the calcium from the BFS and both alumina and silica from the fly ash aid the alkali-activated reactions of the binder. The nature of the reactions is not currently fully understood. The replacement levels of BFS with fly ash in the blended binder can be optimised for different chemical compositions of each precursor to achieve best performance. Some other constituents that can be used in hybrid binders include metakaolin, silica fume, calcined clays, and calcium aluminate cement (RILEM TC 224, 2014). The performance of BFS-based, alkali-activated materials under high-temperature exposure is improved by adding metakaolin to the system. Calcium aluminate cement can be blended with calcined clays, natural zeolites, or pozzolans (Fernández-Jiménez et al., 2008) to improve strength and to control efflorescence (RILEM TC 224, 2014). Metakaolin plays an important role in hybrid blends with various fly ashes and slags by supplying Al to the reaction process. The properties of alkali-activated fluidised bed combustion fly ash can be improved by blending with silica fume and Al(OH)3. The high silica content of silica fume reacts with the alkali solutions and CaO to form additional calcium silicate hydrate.
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18.2.2 Activators When mixed with water, BFS undergoes a low rate of hydration and develops strength slowly. Exposure to an alkaline activator creates a high pH environment and accelerates the hydration reactions. Commonly used activators are industrial products whose manufacturing technology is well established and are manufactured in large quantities and therefore easily available for supporting new applications. Sodium silicate (Na2SiO3) and sodium hydroxide (NaOH) are commonly used activators. Sodium silicate solution is available in different grades defined by its silica modulus or SiO2/Na2O ratio. Sodium hydroxide (NaOH) is produced as a solid by the electrolysis of sodium chloride solution, which produces NaOH, hydrogen and chlorine. Solid NaOH is dissolved in water to provide a solution of the required molarity. Sodium carbonate and sodium sulphate have also been used as activators for BFS. They are less reactive than the silicate-based activators and strength development is slow. Other silica-rich sources can also form suitable activators. For example, silica fume combined with NaOH activator when used with BFS binders produces a high- performance, alkali-activated material that is comparable to sodium silicate activated formulations (Živica, 2006). Other high-silica sources that form potential activators are rice husk ash and nanosilica (RILEM TC 224, 2014). The use of an alkali silicate with finely ground limestone as activator for a BFS binder has the potential of making an economical alkali-activated material of moderate performance (Sakulich et al., 2009).
18.2.3 Alkali activation reactions The two general cases of low-calcium precursors, such as fly ash, and high-calcium precursors, such as BFS, are considered. BFS is more reactive than fly ash at moderate alkaline pH levels and, therefore, can be used with a greater range of activators in addition to the hydroxides and silicates that are suitable for fly ash. Low-calcium precursors, typically fly ashes, are rich in SiO2 and Al2O3 compounds, whereas high CaO content of high-calcium precursors dominates the hydration reactions. Alkali activation of a low-calcium precursor produces an amorphous aluminosilicate gel, which resembles the structure of a zeolite. The gel also contains small amounts of minerals such as quartz present in the original fly ash. Aluminium and silicon atoms are arranged in the cross-linked framework of the tetrahedral structure of the gel. The negative charge of the SiO4 and AlO4 groups is balanced by alkali ions such as sodium. The water in the low-calcium, alkali-activated fly ash does not take part in the hydration reactions since there is no formation of Ca(OH)2 to activate pozzolanic reactions. The products of alkali activation of BFS are influenced by the chemical constituents of the slag and the activator, such as Ca, Mg, Si, Al, Na, S, in addition to the pH, liquid/binder ratio, and curing. The dominant hydration phase is the calcium silicate hydrate (C-S-H) broadly similar to the tobermorite gel formed in hydrated Portland cement. The lower Ca content of slag compared to Portland cement and the greater Al substitution in the tetrahedral structure leads to cross-linking between tobermorite
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chains (Provis, 2014). The complex hydration system of alkali-activated slags can be simplified into the following categories: N-A-S-H gel is formed in alkali-activated slags with low Ca + Mg content. A secondary phase of crystalline zeolites is also present. C-A-S-H gel is formed in alkali-activated slags with high Ca content. Na ions are also present in the gel at charge-balancing sites and the gel may be defined as C-(N)-A-S-H. AFm (monosulphate) in crystalline form appears as a secondary phase if sufficient Al is available. N-A-S-H gel exists as a secondary product with the C-A-S-H gel in most chemical compositions of slags and in hybrid binders involving high and low Ca precursors.
The main difference in the above hydration reactions of alkali-activated binders and ordinary Portland cement is that in the latter case chemical reactions of hydration are activated by water. The main hydration products are the C-S-H tobermorite gel and calcium hydroxide, with AFm (monosulphate) and AFt (ettringite) as secondary phases.
18.3 Fresh properties One of the challenges for this technology has been to achieve scaled-up production of alkali-activated concrete with sufficient workability and working time. AACMs generally tend to be more cohesive than Portland cement concrete and stiffen rapidly. The numerous workability and plasticising admixtures available for normal concrete are not specifically designed for compatibility with the AACM chemistry. The efficiency of the few that can be used with AACMs is less than in Portland cement-based binders. However, commercial compositions of AACMs suitable for large applications are available, and there are many examples of their use. Working time has been managed and there are examples of compositions developed for structural applications including repair (Science Daily, 2009; Concrete Repair Bulletin, 2015; Lambert et al., 2015). The future scope for developing admixtures tailored for AACMs is promising and, in turn, will open up many more opportunities for the use of alkali-activated materials. The density of the fresh and hardened AACM concrete is influenced by the specific gravity of binder materials, the aggregates, and their mix proportions. The lower specific gravity of precursors such as fly ash can reduce the density to some extent but it remains in a similar range to Portland cement-based concrete.
18.4 Mechanical properties 18.4.1 Strength and stiffness Compressive strengths greater than 50 MPa have been reported in many laboratory evaluations of alkali-activated, BFS high-calcium binders and fly ash-based low- calcium binders. Higher strengths up to 110 MPa have also been achieved (Talling and Krivenko, 1997). Typically, high-temperature curing for relatively long periods
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has been required to activate the hydration process and achieve such values. Steam curing has also been used to develop good compressive strength in precast members (Sumajouw et al., 2007; Talling and Krivenko, 1997). However, a well-designed AACM can cure at ambient temperature and develop high compressive strength (Science Daily, 2009). Such materials cure at ambient temperature to give 28-day compressive strengths exceeding 65 MPa. A typical modulus of rupture value for this material is 9.5 MPa, which is over 10% of its compressive strength. The relationship between compressive strength and age of the AACM concrete cured in air (20°C, 60 RH) is plotted in Fig. 18.1 for two compositions with different aggregate grading and content. The compressive strength under wet curing can be somewhat lower but occasionally it is higher. In practice, wet curing in the early period followed by air curing is desirable to prevent cracking, control shrinkage, and optimise strength development. Fig. 18.2 shows the shear-bond strength of the hybrid AACM mortar (Science Daily, 2009) used to bond a sandstone façade to a medium-strength breeze block. The bond tests were conducted after different durability exposures given in the figure. A comparison of Figs. 18.2 and 18.3 shows the significantly superior shear-bond capacity of the AACM mortar relative to a common proprietary multipurpose epoxy adhesive particularly at very high temperature exposure. The behaviour of AACM concrete under compression is similar to conventional Portland cement concrete giving comparable values for strain at failure and the relationship between elastic modulus and strength. However, more detailed relationships between compressive strength and such parameters need to be developed for the many variations of potential compositions of AACM structural concrete. Some contradictory results have been reported on the influence of Si/Al ratio on elastic modulus (RILEM TC 224, 2014). The Poisson's ratio of geopolymer concrete under 80
Compressive strength (MPa)
70 60 50 40 30 20 10 0
0
5
10
15 20 Time (days)
25
30
Fig. 18.1 Relationship between compressive strength and age of hybrid AACM concrete (Mangat, Unpublished data).
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3.5 Failure stress (MPa)
3 2.5 2 1.5 1 0.5 0
Control
Heat ageing Freeze–thaw
Water immersion
High temp 200°C
High temp 400°C
High temp 600°C
Fig. 18.2 Shear-bond strength of a hybrid AACM applied to a medium-strength breeze block (Mangat, Unpublished data).
Failure stress (MPa)
3 2.5 2 1.5 1 0.5 0
Control
Heat ageing
Freeze–thaw
Water immersion
High temp 200°C
High temp 400°C
Fig. 18.3 Shear-bond strength of multipurpose epoxy adhesive applied to a medium-strength breeze block (Mangat, Unpublished data).
c ompression is s imilar to Portland cement-based concrete with compressive strengths from 40 to 90 MPa. Typical values range between 0.12 and 0.16. The relationship between modulus of rupture and compressive strength of AACM concretes shows a higher flexural strength relative to Portland cement-based concretes. This is possibly an indication of the stronger bond at the aggregate–matrix interface due to improved pore structure of the interface zone.
18.4.2 Shrinkage AACMs are typically prone to higher drying shrinkage, relative to Portland cement-based binders, when manufactured and cured under ambient temperatures. The shrinkage reduces by an order of magnitude under high-temperature curing conditions such as are commonly adopted for AACMs. The higher shrinkage is due to the lower degree of chemical binding of water in AACM hydration gels compared with Portland cement hydration products. The shrinkage trends are variable with different precursors and activators. It generally increases with increasing activator content and increasing activator silica modulus.
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The factors that help to control shrinkage are good mix design of the AACM concrete, resulting in high aggregate content that restrains shrinkage, adequate early age curing preventing rapid drying, plus the use of shrinkage-reducing admixtures (Mangat and Ojedokun, 2016; Dunster, 2013). Fibre reinforcement of AACMs also offers a mechanism for reducing shrinkage and providing crack control (Mangat & Bordeian, in press). The use of lightweight aggregate and other porous aggregates is also beneficial for shrinkage, while providing a strong chemical interaction with the binder at the interface. Fig. 18.4 shows the shrinkage development with age of two hybrid AACM concrete compositions (Science Daily, 2009), giving a typical range of shrinkage values from tests conducted according to BS ISO 1920-8:2009, Part 8. AACM compositions are also prone to microcracking at early ages, before adequate tensile strength has been developed. The problem is aggravated with high paste (binder) content and dry curing, as would also be the case with Portland cement-based matrices. High activator content and high modulus of activator increase the risk and extent of microcracking.
18.4.3 Creep The creep behaviour of AACMs is different from Portland cement-based concrete, and is caused by differences in microstructure, water held in the gel structure, and the rate of strength development. The longer-term creep deformation of AACM concrete, beyond 90 days, continues to increase at a greater rate than Portland cement concrete. The limited data available in the literature indicate that creep of BFS-based AACM concrete is lower with silicate activation compared with hydroxide activation and is highest with carbonate activation (Talling and Krivenko, 1997). Similarly, limited data available on low-calcium (fly ash)-based alkali-activated concrete show little influence of curing temperature (oven or steam curing) on creep. The specific creep
Shrinkage (microstrain)
1200 1000 800 600 400 200 0
0
20
40
60 Time (days)
80
100
120
Fig. 18.4 Relationship between drying shrinkage and age of hybrid AACM concrete (Mangat, Unpublished data).
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of AACM concrete is reported to be lower than Portland cement concrete of similar grade and falls within acceptable limits defined by the design codes (Rangan, 2014; Sagoe-Crentsil et al., 2013).
18.5 Physical and durability properties 18.5.1 Permeability and porosity The distribution of capillary pores in the AACM matrix is important since it controls the mass transport mechanisms of deleterious substances such as chlorides and sulphates. While mercury intrusion porosity can provide useful insights to this property, relatively limited information on the pore structure of AACMs is available in the literature. The current state of knowledge indicates that the pore size distribution of AACMs falls into two separate zones of pore size >1 μm and <20 nm range, with insignificant porosity in between. In contrast, similar grades of Portland cement-based matrices have a unimodal pore size distribution of 10–100 nm. The impact of this difference in the pore structure of AACMs relative to normal concrete may be significant with respect to the durability properties of AACMs and require more research. The type of activator also has an effect, with potassium-based activators reducing the median pore diameter more effectively than sodium-based activators. As with good practice in concrete technology, porosity is reduced by high-quality curing, such as sealed conditions and longer periods of hydration. Generally, the sorptivity of AACMs is within a comparable range with similar grade concretes (RILEM TC 224, 2014). The capillary sorptivity is reduced by employing a lower water content and a higher silica modulus activator. One potential advantage of AACMs relative to normal concrete is likely to be in the aggregate–matrix interfacial zone. This zone in Portland cement matrices tends to be more porous and weaker than the bulk matrix remote from the aggregate surface. The alkali activator in AACMs, however, may be expected to interact chemically with the aggregate particles in addition to the binder. There is already evidence available of this effect but further research is required for validation.
18.5.2 Chloride, carbonation, and corrosion The mechanisms of chloride diffusion, carbonation, and steel reinforcement corrosion in normal concrete form the starting point for understanding such mechanisms in AACMs. The basic difference is the pore structure and the chemistry of the pore fluid in AACMs relative to normal concrete. Chloride diffusion test methods that measure the progress of chloride penetration directly, such as the BS EN 12390-11 (BSI, 2015) and the older Nordtest (Nordtest, 1995), give a more reliable comparison of chloride diffusion between AACMs and concrete. Such test methods that analyse powder samples to determine soluble chlorides (water or acid) are relatively independent of the pore fluid chemistry of the AACM binder. The pore fluid chemistry of AACMs leads to inconclusive results on chloride diffusion and requires more research. Chloride
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diffusion tests based on these standards have been conducted on the hybrid AACM formulations, which were cured under ambient conditions (Science Daily, 2009). The results show superior chloride resistance of the AACM concrete compared with a similar grade of normal Portland cement-based concrete (Mangat and Ojedokun, 2016). Pore fluid provides the electrolyte for both the protection and the corrosion processes of steel in AACMs and it is important to understand its chemical composition. AACMs can be based on a wide range of precursors and activators each of which will vary the pore fluid chemistry. The presence of threshold levels of chloride concentration at the steel interface initiates corrosion in normal concrete. The chemistry and, in particular, the buffered high pH of the pore fluid provides passivation to steel corrosion until chlorides exceeding the threshold level cause the initiation of corrosion. The Ca(OH)2 buffer is limited in AACM concrete and its extent depends on the hydration products of the particular precursor used. The presence of sulphide in AACM binders using BFS influences steel corrosion differently to the corrosion mechanism in normal concrete (RILEM TC 224, 2014). Service life prediction models and accelerated durability test methods used for concrete will not be directly valid for AACM concrete due to these differences. AACMs based on BFS and blends with metakaolin show higher degrees of carbonation relative to conventional cement and mortar due to the lack of Ca(OH)2 in the hydration products. However, a suitable choice of activator and its concentration can mitigate this effect. Mix design of AACM concrete is an important factor in controlling carbonation. Carbonation depths comparable to similar grade concrete can be achieved by providing higher binder (paste) contents in AACM concrete mixes. Carbonation results of AACMs under accelerated carbonation testing often show inferior performance to Portland cement binders, which contradict data obtained under long-term natural exposure. There are some field data on carbonation of AACMs available for structures built in Russia, Ukraine, and Poland under a service life of over 40 years. Carbonation rates of under 0.5 mm/year are indicated for these climates. Carbonation rates of under 1 mm/year were obtained with the standard phenolphthalein test that is considered unsuitable for AACMs (RILEM TC 224, 2014). The phenolphthalein test for measuring carbonation depth was developed for normal concrete and it is compatible with the chemistry of ordinary Portland cement hydration products (CaOH2). The application and interpretation of this test to AACMs and to modern blended cements requires investigation, and alternative test methods will need to be developed. Carbonation induced reinforcement corrosion in concrete is considered to occur when the carbonation front reaches the steel surface, reducing the high alkalinity of the pore fluid through the conversion of calcium hydroxide (CaOH2) to calcium carbonate (CaCO3). The reduction in pH depassivates the steel and can allow the initiation of corrosion. This process is unlikely to be replicated in AACMs due to the different hydration products, notably to absence of CaOH2, and other differences in the chemical composition of the pore fluid. In addition to reinforcement corrosion, the other deleterious effect of carbonation in Portland cement concrete is carbonation shrinkage. The carbonation shrinkage property of AACMs is currently unknown and requires further investigation.
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The effect of accelerated exposure to carbonation is more severe on AACMs than on normal concrete. The correlation of accelerated test data with natural carbonation under service conditions is also not clear. This is related to the difference in the CO2 diffusion process and the chemical composition of the pore fluid relative to Portland cement binders where the Ca(OH)2 concentration is high. The rate of carbonation in AACMs is strongly affected by the CO2 concentration of the exposure environment. The rate of pore fluid carbonation and degradation of the gel pore structure change significantly at high CO2 exposure levels and the ratio of carbonate to bicarbonate is affected (Puertas and Palacios, 2007; RILEM TC 224, 2014). Accelerated carbonation testing at CO2 concentrations above 1% is not recommended.
18.5.3 Efflorescence Efflorescence in AACMs is due to three factors: porous microstructure of some AACM compositions, high alkali concentration in the pore fluid such as in sodium aluminosilicate binders activated with high Na2O/Al2O3 solutions, and relatively weak binding of Na in the aluminosilicate gel structure. Replacement of sodium with potassium improves binding with aluminosilicate gel, and increasing the reactive aluminium content provides more alkali-binding capacity. However, low permeability is an important requirement for a durable material and an appropriately formulated and cured AACM will be more resistant to efflorescence (RILEM TC 224, 2014).
18.5.4 Freeze–thaw resistance The freeze–thaw resistance of Portland cement concrete is related to physical characteristics such as pore structure, pore saturation, strength of the matrix, and voids provided by air entrainment. The chemical composition of the hydration products is less significant. The same factors are likely to control the freeze–thaw resistance of AACMs. The freezing temperature of the pore fluid, however, will be affected by the differences in ionic composition of the pore fluid and by the relative degree of its confinement within the pore structure of Portland cement and AACM binders. The freezing point of pore fluid in a BFS-based AACM has been reported as significantly lowered to below −50°C due to its high ionic concentration, resulting in higher freeze–thaw resistance than a Portland cement-based binder. Many studies showing satisfactory frost resistance and resistance to frost-salt attack of AACMs have been reported. Data from in-service structures have shown better frost resistance of AACM concrete compared with similar grade conventional concretes exposed to the same conditions (Davidovits, 2008; Rostovskaya et al., 2007). Early age freezing and thawing is less damaging to AACM concrete than to Portland cement concrete, and freezing is detrimental only before the AACM concrete has reached 5 MPa strength. The freeze–thaw resistance of fly ash-based AACMs also meets the specifications for cold weather applications. Some studies have shown worse freeze–thaw resistance of AACM concrete than similar grade Portland cement concrete (Bilek and Szklorzova, 2009). This is
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a ttributed to the higher amount of free water present in the pores of AACMs due to the lack of crystal phases, such as CaOH2 and ettringite, which are present in Portland cement hydration products. These crystal phases contain the water in chemically and physically bound form and prevent it from freezing. This effect is aggravated in AACMs by the presaturation of test specimens as required by testing standards before exposing the specimens to freeze–thaw conditions. Such binders can thus contain more water in the pore network within the gel than Portland cement binders, which then undergo freeze–thaw cycles. This can result in more freeze–thaw damage if the tests are conducted near the water saturation level of the specimen. The freeze– thaw test methods given in most standards are expected to be suitable for AACMs but any precuring phase that requires presaturation of the test specimens needs to be reviewed. The freeze–thaw and scaling resistance of AACMs can be reduced by carbonation and microcracking due to restrained shrinkage. Carbonation of the matrix makes it more brittle and carbonation shrinkage can lead to cracking and scaling.
18.5.5 Sulphate and acid resistance Low-calcium binder-based alkali-activated material cured at 60°C and exposed to sodium sulphate solution showed excellent sulphate resistance. Portland cement binders can suffer sulphate attack due to chemical reactions with the hydration products resulting in the formation of gypsum and subsequently ettringite that is expansive and can result in disintegration of the concrete. The hydration products of alkali-activated, low-calcium precursors such as fly ash are relatively free from C-S-H, and sufficient expansive ettringite is not formed to cause damage. High-calcium precursors such as BFS can produce more expansive ettringite. The acid resistance of alkali-activated, low-calcium binders is similarly superior to Portland cement concrete due to the difference in hydration products and ettringite formation.
18.5.6 Fire resistance The high-temperature resistance of AACMs based on low-calcium binders (geopolymers), with fly ash as the precursor, has been widely reported in the literature (RILEM TC 224, 2014). Their amorphous gel structure is considered to provide fire resistance by preventing the buildup of internal pressure, which leads to violent splitting and spalling of Portland cement concrete when exposed to fire. A hybrid AACM composition has passed the Eurocode (EN 1991-1-2:2002) fire test at temperatures over 1100°C (Science Daily, 2009). In addition, a fireplug test demonstrated satisfactory insulation to services while satisfying the design load capacity of 5 kN/m2 (Mangat, 2009). Fig. 18.5 shows a furnace test sample of the same AACM composition, displaying its typical response to high temperature. Comparative fire tests against similar strength Portland cement concretes demonstrate the release of steam from the gel structure of the AACM concrete, preventing spalling, while the Portland cement concrete spalls explosively.
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Fig. 18.5 Heat resistance of AACM (Mangat, Unpublished data).
Comparative tests on fly ash and metakaolin-based AACMs exposed to high temperature have shown that the strength of the metakaolin-based AACM decreased after heating, whereas the fly ash-based AACM had higher strength (Kong and Sanjayan, 2008; Kong et al., 2007). This was attributed to the differences in pore distribution in the two materials that allowed the steam to escape more easily and without damaging the microstructure in the fly ash-based geopolymer. Fire tests up to 800°C on metakaolin-based AACMs and blends with BFS showed a higher residual strength (after firing) for the blended materials (RILEM TC 224, 2014). The residual strength of AACM mortar and concrete increases due to further geopolymer reaction while the matrix-aggregate thermal incompatibility causes strength reduction. The balance between the two controls the final strength after fire testing.
18.6 Structural applications There are many examples of applications of AACM concretes since the 1950s in Europe, the former Soviet Union, and more recently in Asia, North America, and Australia (RILEM TC 224, 2014). These applications have demonstrated the performance of these materials in rigorous environmental and service conditions. In general, their durability after decades of exposure to conditions such as carbonation, freeze– thaw attack, acid attack, and corrosion of the reinforcement has been judged to be satisfactory and the strength of the matrix has increased with time. Some limitations, such as rapid loss of workability, have also been experienced, which are being addressed in current research and development work. Development of AACMs has progressed to the stage where well-defined formulation of the binder composition together with good mix design to optimise aggregate content, and appropriate liquid/binder ratios, can produce mixes suitable for structural application. Standard structural elements such as beams and columns have been
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Fig. 18.6 Load-deflection curve of the reinforced AACM lintel (Mangat, Unpublished data).
manufactured and cured at either elevated temperature (steam curing) or at ambient temperature. Test results on these elements show that the strength and deformation characteristics of AACM concrete structural elements are similar to Portland cement concrete elements of the same nominal grade. Reinforced concrete structural design codes can therefore be considered relevant to AACM concrete structures. A reinforced concrete lintel was manufactured with an AACM concrete composition with similar properties to a structural grade concrete (Science Daily, 2009). The beam was manufactured at a precast plant where the quality control was relatively poor and the reinforcement cage suffered some displacement during casting. A second similarly reinforced concrete beam with Portland cement concrete was manufactured as a control element under better quality control. The beam dimensions were 140 × 213 mm cross-sectional width and depth and a length of 2.7 m. Both beams were manufactured and cured at ambient temperature. Both were tested under four-point bending (span 2.12 m) using the procedure of BS8110. The load-deflection curve of the reinforced AACM concrete beam is shown in Fig. 18.6 and their relative performance is further analysed in the next paragraphs.
18.6.1 Serviceability limit state 18.6.1.1 AACM concrete beam Instantaneous deflection at serviceability load of (34.5 + 0.455) kN = 11.51 mm Residual instantaneous deflection upon unloading = 3.15 mm % recovery after first loading =
11.51 - 3.15 ´ 100 = 72.6% 11.51
18.6.1.2 Portland cement concrete beam Instantaneous deflection at serviceability load of (34.5 + 0.455) kN = 9.62 mm
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Residual instantaneous deflection upon unloading = 1.83 mm % recovery after first loading =
9.62 - 1.83 ´ 100 = 81% 9.62
Hairline flexural cracks were observed in the both beams at serviceability load.
18.6.2 Ultimate limit state Both beams failed at a load of 60 kN with an ultimate moment of resistance of 30.3 kNm. In comparison, the design ultimate moment of resistance of the beam was 26.1 kNm. The deflection at ultimate load of the AACM and Portland cement concrete beams was 26.0 and 22.3 mm respectively. The maximum deflection permitted by BS 8110-2: 1985 at the ultimate design lo ad = span/40 = 1/40 × 2120 mm = 53 mm These results are summarised in Table 18.1. Similar conclusions have been reported for heat-cured (steam-cured) beams and columns of low-calcium-based geopolymer binders (Rangan, 2014). Beam sections of 200 × 300 mm and 3.3 m length were manufactured with tensile reinforcement ratios between 0.64% and 2.69%. The flexural, shear, and bond characteristics of the beams were subsequently investigated. Columns of effective length 1.7 m and 175 mm square cross section were made with 1.47% and 2.95% longitudinal reinforcement. The compressive strength of the steam-cured geopolymer concrete was 40–60 MPa. The structural performance and failure mode of the reinforced geopolymer concrete beams and columns was similar to Portland cement concrete elements of similar grade concrete.
18.7 Future trends Based on the evidence that structural performance can be equivalent or superior to Portland cement concretes, and a growing understanding of the benefits and limitations with respect to durability and the emergence of standards for their specification, one may expect AACMs and geopolymers to play an increasing role in providing durable and sustainable alternatives to the current products based on traditional Portland-based binders. The use of secondary wastes such as flue gas desulphurisation (FGD) products within the precursors are also potential future developments (Mangat et al., 2006; Khatib et al., in press). There is already significant interest in their use in challenging environments where resistance to high temperatures or aggressive chemicals is required. The potential savings in, for example, a sewage treatment environment could be considerable if the alternative is to employ conventional concrete with an expensive, chemical-resistant protective coating. Similarly, a resistance to high-temperature exposure resulting in little or no loss of properties, is particularly valuable when selecting materials for tunnel linings. A major limitation to the more widespread use of AACMs and geopolymers is their lack of track record, especially with respect to the durability of embedded steel reinforcement. While this may restrict the direct replacement of Portland cement binders in conventional reinforced concrete designs, it should also encourage the use of alternative methods of reinforcement. Where the maintenance of a high-alkalinity
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Table 18.1 Flexural test results of reinforced beams of AACM and Portland cement concrete, according to BS8110 (Mangat, Unpublished data) Residual deflection (mm)
Recovery (%)
Moment of resistance (kNm)
Design moment of resistance (kNm)
Ultimate deflection (mm)
Permissible deflection (mm)
AACM beam Control concrete beam
11.51 9.62
3.15 1.83
72.6 81
30.3 30.3
26.1 26.1
26.0 22.3
53 53
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Beam
Instant deflection (mm)
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e nvironment is not a necessity for durable service, nonmetallic fibres should offer viable and more sustainable alternatives.
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Oh, J.E., Monteiro, P.J.M., Jun, S.S., Choi, S., Clark, S.M., 2010. The evolution of strength and crystalline phases for alkali-activated ground blast furnace slag and fly ash-based geopolymers. Cem. Concr. Res. 40 (2), 189–196. Provis, J., 2014. Geopolymers and other alkali activated materials: why, how and what? Mater. Struct. 47, 11–25. Provis, J.L., Van Deventer, J.S.J. (Eds.), 2014. RILEM TC 224-AAM, Alkali-Activated Materials: State of the Art Report. Springer/RILEM TC 224, Dordrecht. Puertas, F., Palacios, M., 2007. Changes in C-S-H of alkali-activated slag and cement pastes after accelerated carbonation. In: Beaudoin, J.J., Makar, J.M., Raki, L. (Eds.), 12th International Congress on the Chemistry of Cement, Montreal, Canada, CD-ROM Proceedings. Rangan, B.V., 2014. Geopolymer concrete for environmental protection. Indian Concr. J. 88 (4), 41–59. Rostovskaya, G., Ilyin, V., Blazhis, A., 2007. The service properties of the slag alkaline concretes. In: Ertl, Z. (Ed.), Proceedings of the International Conference on Alkali Activated Materials—Research, Production and Utilization, Prague, Czech Republic, Ceská rozvojová agentura, pp. 593–610. Sagoe-Crentsil, K., Brown, T., Taylor, A., 2013. Drying shrinkage and creep performance of geopolymer concrete. J. Sustain. Cem. Based Mater. 2 (1), 35–42. Sakulich, A.R., Anderson, E., Schauer, C., Barsoum, M.W., 2009. Mechanical and microstructural characterization of an alkali-activated slag/limestone fine aggregate concrete. Constr. Build. Mater. 23, 2951–2959. Science Daily, 2009. Liquid Granite: Building Material of the Future Unveiled. www.sciencedaily. com/releases/2009/10/091029161253.htm. 4 November. Shi, C., Krivenko, P.V., Roy, D.M., 2006. Alkali-Activated Cements and Concretes. Taylor & Francis, Abingdon, UK. Sumajouw, D.M.J., Hardjito, D., Wallah, S.E., Rangan, B.V., 2007. Fly ash-based geopolymer concrete: study of slender reinforced columns. J. Mater. Sci. 42 (9), 3124–3130. Talling, B., Brandstetr, J., 1989. Present state and future of alkali-activated slag concretes. In: Malhotra, V.M. (Ed.), 3rd International Conference on Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete, ACI SP114, Trondheim, Norway, Vol. 2. American Concrete Institute, Farmington Hills, MI, pp. 1519–1546. Talling, B., Krivenko, P.V., 1997. Blast furnace slag—the ultimate binder. In: Chandra, S. (Ed.), Waste Materials Used in Concrete Manufacturing. Noyes Publications, Park Ridge, NJ, pp. 235–289. Živica, V., 2006. Effectiveness of new silica fume alkali activator. Cem. Concr. Comp. 28 (1), 21–25.