Volcanic Ash as a Sustainable Binder Material

Abstract

The construction industry is constantly challenged by the increasing demands of urbanization. Traditional cement production poses significant environmental challenges, thereby necessitating the exploration of alternative sustainable solutions. Among these alternatives, volcanic ash (VA) emerges as an abundant, low-cost material with unique properties, due to its chemical composition and amorphous structure. Over the past decade, there has been a growing interest in utilizing alkali-activated VA as a viable replacement for Portland cement clinker in binding applications. By doing so, we can transform a largely under-utilized natural resource into a valuable product. Moreover, the mechanical properties, low porosity, and favorable densification behavior of alkali-activated VA materials enhance their suitability for construction applications. This article reviews significant global research findings regarding the chemical composition and mineralogy of VA, their reactivity during alkali activation, as well as the impact of various synthesis factors, such as alkaline activator concentration, solution-to-binder ratios, and curing conditions, on the properties of these inventive materials.

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Keywords: volcanic ash, treatments for reactivity, alkali-activated materials, curing method, strength properties, durability properties

1. Introduction

Ordinary Portland cement (OPC) is one of the most commonly used building materials for construction worldwide; it is also the most extensively manufactured product by mass on Earth. OPC is used in various mixtures combined with water and aggregates, sometimes including chemical admixtures to form cement-based materials like concrete [1,2].

As the global urban population continues to rise, the demand for cement is escalating. The idea of constructing infrastructures without OPC seems almost unattainable. Its advantages of affordability and adaptability for various construction requirements further solidify its demand [3]. However, the production of each ton of OPC emits approximately 0.55 tons of carbon dioxide (CO2). Additionally, activities such as baking and grinding yield another 0.39 tons of CO2 emissions. Thus, the cumulative emissions from cement production can reach 0.94 tons of CO2 for just one ton of cement produced [4,5]. Cement stands as the second most consumed material globally, following water. Projections indicate a staggering 200% increase in global OPC demand by [6,7].

In light of such challenges, the scientific community is focused on developing a deeper understanding of supplementary cementitious materials that can partially or fully replace OPC. Pozzolanic materials rich in silica (SiO2) and alumina (Al2O3) lack hydraulic properties. However, when mixed with lime or an alkaline product and introduced to water, these components can effectively capture calcium hydroxide, yielding stable compounds with hydraulic cementitious properties [8,9]. Evidence of pozzolanic materials dates back to the Greek and Roman civilizations, which adeptly employed them in lime mortars to construct bridges and temples; their application saw a resurgence during the early 20th century in the United States [10]. Introducing pozzolanic materials as supplementary cementitious agents significantly enhances properties when compared to using 100% OPC [11,12]. For instance, in the fresh state, cementitious materials enhance workability, mitigate water retention, and lower exothermic heat during hydration. Once hardened, they exhibit increased resistance to sulfate and mitigate the alkali-aggregate reaction, ultimately bolstering long-term strength.

Pozzolanic materials can be broadly classified based on their origin into artificial or natural types. The American Society for Testing and Materials (ASTM) International categorizes artificial pozzolanic materials as Class F and Class C materials, predominantly composed of fly ash sourced from coal combustion. Natural pozzolanic materials, or natural pozzolans (NPs), fall under Class N; these include diatomaceous soil, cherts, calcined or uncalcined tuffs, volcanic ashes (VAs), as well as certain clays that necessitate calcination to exhibit satisfactory performance properties.

Natural pozzolanic materials, such as VAs, hold potential as supplementary cementitious agents, requiring minimal pre-treatment (like grinding, calcination, or chemical activation) before use as alkali-activated materials (AAM) [13]. While many natural resources are subjected to a clinkerization process at temperatures exceeding 1,450 °C to produce OPC, AAM materials typically form at lower temperatures ranging from 25–100 °C, or 600–700 °C if calcination is required. This discrepancy significantly reduces CO2 emissions associated with the production process [5]. Moreover, natural pozzolanic materials are abundantly available in unexploited deposits throughout the world [12,14].

One of the most widely utilized VA types as a supplementary cementitious material is pyroclastic ash. Diagenetic processes can alter VA into zeolite-rich tuffs. The weathering process may lead to zeolitization or argillation of the ash, transforming pozzolan glass into zeolitic or clay minerals. While zeolitization can enhance pozzolanic properties, argillation typically detracts from them [8].

The alkali activation of VAs can be divided into two primary categories. The first comprises alkali-activated binders devoid of OPC, recognized as geopolymers or inorganic alkaline polymers. This category primarily yields a three-dimensional aluminosilicate network consisting of sodium aluminosilicate hydrate (N-A-S-H) gel or potassium aluminosilicate hydrate (K-A-S-H) gel along with zeolitic by-products [15,16,17]. The second category, which remains lesser-known and researched, involves blended cement integrating OPC and alkali-activated aluminosilicate binders. Typically, this blended cement comprises a minimal OPC fraction and an excess of VA (typically over 70%), serving as an aluminosilicate precursor. The reaction products within this group are notably complex, consisting mainly of various cementitious gels. In highly alkaline conditions—such as a mixture containing 10 M NaOH (NH)—sodium and calcium aluminosilicate hydrate (N-(C-)A-S-H) gels can be observed. In slightly alkaline conditions (2 M NH), calcium silicate hydrate (C-S-H) predominantly prevails. However, the reaction kinetics involving such hybrid alkaline activation remains insufficiently comprehended, especially during the preliminary phases [18,19,20].

The application of AAM in mortar or concrete production presents numerous advantages: it effectively converts waste and by-products into useful supplementary cementitious materials, lessens greenhouse gas emissions during the concrete production phase, and enhances mechanical properties and durability in comparison to conventional concrete [12,20,21,22]. Nonetheless, further research is essential for large-scale industrial applications of AAM and there is no definitive agreement that alkali-activated cements provide a lower carbon footprint than OPC materials, largely due to dependencies on mix designs—especially the number of used activators. Studies indicate that Na2SiO3 (NS) possesses a higher carbon footprint, as does NaOH [23,24].

For a thorough life-cycle assessment of AAM materials, raw material transport and manufacturing processes must be accounted for; however, only a scant number of studies have attempted this [2,25]. Hence, extensive research and development within this area of study is crucial.

This report focuses on the applications of natural pozzolans derived from volcanic sources in construction and civil engineering. The following sections describe the discoveries from recent studies on using VA as a supplementary cementitious material. Specific attention is given to publications highlighting the design and evaluation of VA-based AAM materials, which aim to advance scientific knowledge and contribute to practical applications within this domain.

3. Treatments to Increase the Reactivity of Natural Volcanic Ashes

This section could be structured using subheadings, offering a concise overview of experimental findings, their interpretations, and conclusions derived from the research.

Enhancing the pozzolanic activity of VA can address the significant challenge of low early strength gains in VA-containing cement mixtures [66,70]. Consequently, diverse pre-treatment techniques have been investigated to enhance the reactivity potential of VAs [43,49,50,51,71,72]. Three main pre-treatment categories exist: mechanical activation (MA), thermal methods through calcination, and chemical activation techniques. The MA process addresses all the mineral components in the VA while the thermal calcination predominantly modifies the clay minerals within the material [73]. The importance of chemical activation treatment methods has surged in recent years, becoming vital in the synthesis of alkali-activated cements (AACs) since they serve as the sole source of the reactive aluminosilicate precursor. Special emphasis will be placed on this method in Section 4.

3.1. Mechanical Activation

MA involves grinding processes to reduce VA material particle sizes. This increase in surface area exposes imperfections and active centers to reactions, thereby boosting pozzolanic material reactivity. Active centers possess higher energy states compared to normal structures, and by increasing the quantity of these centers, VA material reactivity can be amplified [66]. Additionally, the grinding process contributes to the loss of crystallinity and promotes greater amorphization among minerals. Consequently, this leads to augmented mineral reactivity due to the disruption of valence bonds and crystal structure distortions associated with the partial or complete amorphization of the ground substance [74].

When observed in their natural form, VA materials comprise particles typically less than 2 mm in diameter; some materials may even include particles with diameters under 1 µm [21]. However, in alignment with ASTM C618, approximately 2/3 of the VA particles should measure less than 45 µm to be recognized as suitable binder material for concrete production.

Several studies [38,50,75,76,77] have implemented milling processes on VA materials as supplementary cementitious agents, revealing that finer VA particle sizes yield greater amounts of C-S-H and C-A-S-H gel phases, responsible for early compressive strength in cement paste. Moreover, reduced VA particle sizes create a denser pore structure, thus yielding enhanced compressive strength. Furthermore, Ardoğa et al. [77] reported that blended cements with finer VA particles exhibited increased hydration heat due to a nucleation effect.

However, Djobo et al. [38] postulated that an extended milling time may diminish VA reactivity. The properties of VA materials are summarized in Tables 1 and 2. The MA milling experiment was performed for intervals of 30, 60, 90, and 120 minutes using a vibratory mill. Results indicated no significant changes in reactivity following 30 minutes of milling. Notable structural modifications commenced after 60 minutes, while significant alterations in WA crystallinity were established at the 90-minute mark. XRD analysis indicated that the crystallinity degree decreased from 60% to 38%, attributed to enhanced amorphization through milling. Conversely, the crystallinity degree increased to 60% following 120 minutes of milling, resulting from partial recrystallization and quartz formation. The crystallinity degree (Xc) was assessed to measure the effectiveness of mechanical activation in altering volcanic ash crystallinity, calculated as the proportion of crystalline peaks' area (Ac) against the combination of the amorphous phase area (Aa) and the crystalline peaks' area (Ac). The crystalline peaks' area (Ac) was determined through integration of the upper region of a smooth curve linking peak baselines, while the amorphous area (Aa) was derived as the integral of the lower zone, amid a smooth curve and the linear baseline, connecting two points at 22° and 37° (2θ).

Research by Shi and Day [75] confirmed that prolonged grinding of VA material produced varying Blaine fineness values between 259–554 m²/kg. Furthermore, an observed increase in compressive strength in mixtures composed of 20 wt.% hydrated lime and 80 wt.% pozzolan directly correlated with higher Blaine fineness values of VA particles (reference Figure 2). Kunal et al. [50] also identified that cement paste produced with finer VA particles generated more C-S-H and C-A-S-H gel phases in comparison to pastes produced from coarser particles.

Figure 2.

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Relationship between the compressive strength of lime-natural pozzolan (NP) pastes and the Blaine fineness values of the NP [75].

Kupwade-Patil et al. [76] experimented with cement pastes blending 50% OPC with two types of milled VA: one category with an average particle size of 6 µm, and the other with particle sizes averaging 17 µm. The compressive strength in samples containing 6 µm VA particles increased by 15% compared to those composed of 17 µm VA particles. However, results indicated that the samples containing 6 µm VA particles showed a 4% rise in embodied energy values relative to those made with 17 µm VA particles, attributed to the extensive energy requirement during milling. Notably, the embodied energy of concrete made with 6 µm VA particles was 17% lower than corresponding values derived from conventional OPC, although it did experience a 30% reduction in compressive strength in comparison to OPC-based materials. Consequently, it is feasible to design energy-efficient VA-OPC concrete mixtures to achieve compressive strength meeting specific structural and non-structural expectations.

3.2. Thermal Activation Via Calcination

Thermal activation through calcination of aluminosilicate materials induces the loss of volatile components while restructuring atomic configurations and disrupting crystalline phases; these effects result in enhanced amorphous and reactive VA materials [78]. Shi's research [66] suggested that calcination or preheating processes at elevated temperatures would enhance VA density and reactivity.

Commonly, VA material containing approximately 40% amorphous components is deemed soluble in alkaline environments [37,47]. However, if the semi-crystalline nature of the VA prevails, resulting in a deficiency of reactive components like CaO and Al2O3, enhancing reactivity in alkali-activated binders and producing cementitious construction materials with improved mechanical properties typically necessitates calcination processes exceeding 700 °C [8,49,72,79]. Bondar et al. [43] indicated that calcination sequences at 700, 800, and 900 °C positively influenced the mechanical properties of geopolymer pastes. Tables 1 and 2 summarize VA materials' physical and chemical properties. Raw VA-type 1 contained high zeolitic mineral percentages: 40% clinoptilolite, 14% albite, 11% calcite, and 32% quartz. Following calcination at 700 and 800 °C, the VA microstructure was modified, containing 26% mordenite and 38% opal. The calcination process caused the VA to become exceedingly reactive in alkaline solutions. Furthermore, mixtures crafted from this VA material displayed a commendable compressive strength of 68.5 MPa during curing at 20 °C. VA-type 2, lacking amorphous phases, while possessing minimal soluble silica, witnessed a reactivity increase post-calcination at 900 °C. Meanwhile, VA-type 3 and VA-type 4, comprising 33% and 25% amorphous phases respectively and an augmented presence of altered minerals (like montmorillonite clay minerals), indicated that the reactivity of VA-type 3 remained unchanged post-calcination, while the VA-type 4 displayed marginal activation when treated at 800 °C. A rapid chemical assessment was performed to assess the presence of amorphous phases within VA materials, utilizing 200 mL of boiling 0.5 M NaOH poured into a beaker housing 0.15 g of dried VA, swiftly returning the resulting suspension to boiling. Filter processes coupled with washing residue in cold water followed; drying occurred overnight at 105 °C. The weight of undissolved material was noted post-weighing, establishing the percentage of material dissolved, termed alkali-solubility. Subsequent dissolution produced a solution for element analysis, utilizing Inductively Coupled Plasma (ICP) methods.

Findings from Kılıç and Sertabipoglu [51] supported that the pozzolanic activity of volcanic pumice improved following calcination at 600 °C. Mortar incorporating 20% raw volcanic pumice in place of OPC yielded 73% of the strength index (SAI) compared to conventional mortars with 100% OPC. In contrast, calcined volcanic pumice mortar achieved 107% of the SAI benchmark associated with conventional mortar. Kani and Allahverdi [52] acknowledged potential alterations in the specific surface area (SSA) of VA materials post-calcination, though they did not observe direct correlations among SSA, thermal processing, and reactivity.

VA material's mineral composition can profoundly impact the calcination process's effectiveness. Research by Hamidi et al., alongside Askarinejad et al. [49,67], determined that thermally treated andesite VA enhanced calcium fixation at a swifter rate when mixed with a Ca(OH)2 solution compared to andesite VA calcined at 900 °C. Moreover, when underpinned largely by analcite and phillipsite as principal mineral phases, VA materials presented a decline in pozzolanic activity post-calcination between 500–800 °C. Analcite retained its crystal structure despite water loss; while phillipsite transitioned into metaphillipsite—an ultrareactive product—upon being calcined at 500 °C, it turned into a stable feldspar phase reminiscent of diminished pozzolanic activity by 800 °C [71,80]. Robayo-Salazar et al. [42] applied a calcination procedure at 700 °C using Colombian VA rich in montmorillonite, characterized by its profound water-adsorption capacity and limited reactivity. This calcination procedure proved essential in lessening water demand and amplifying reactivity for alkali-activated cements.

Contradicting conclusions concerning the calcination process's influence on VA materials' properties were drawn by several researchers [79,81,82]. They suggested that while enhanced pozzolanic activity of vitreous, zeolitic, and clay phases is attributable to calcination, a reactivation could manifest through increases in the specific gravity of VA materials when subjected to higher thermal processes. Additionally, reductions in porosity and increases in crystallinity were observed as materials underwent sintering at elevated temperatures.

4. Alkaline Activation of Natural Volcanic Ashes

VA material composed of aluminosilicate solids may develop effective binding properties when subjected to alkaline environments prompted by an alkaline activator that exhibits moderate to high alkaline characteristics. The resultant alkali-activated binders are commonly referred to as geopolymers [83].

Research efforts directed towards the synthesis of VA geopolymers have correlated with increased scholarly interest in recent years. This reaction mechanism initiates with hydroxyl (OH-) ions from alkali activators, disrupting siloxane (Si–O–Si) linkages to create silanol (-Si–OH) and sialate (-Si–O-) entities. Alkali cations help to normalize the negative charges, while complex species, such as Si–O–Na+ and Si–O–Al form, might complicate the reversion to siloxane [19]. Typically, VA particles exhibiting high fineness [84] coupled with an alkaline solution of pH ranging from 12–14 [85] enhance the dissolution of various constituents alongside the generation of monomeric aluminate and silicate units. Monomer units join to form dimers, subsequently creating oligomers that reorganize, generating three-dimensional polymeric chains and ring structures containing Si–O–Al–O bonds. The reaction processes can persist for extended periods, and prolonged curing under controlled hydrothermal settings encourages zeolitic crystallite growth [18,86,87].

Numerous studies [31,37,38] have corroborated the low reactivity associated with alkali-activated VA materials. For instance, Ndjock et al. [31] highlighted the importance of both amorphous phase composition and content within five VA samples, revealing that suitable geometric characterization arose from specific SiO2/Al2O3 ratios. They determined that VA material was appropriate for geopolymer synthesis when the SiO2/Al2O3 ratio stayed below 3.9 and suitable as filler when ratios exceeded 3.9. Optimal alkaline activation for VA materials possessing minimal amorphous phase content, elevated SiO2/Al2O3 molecular ratios, alongside low reactivity when subjected to NaOH with 77.5 wt.% Na2O and a curing temperature of 80 °C was recorded (as illustrated in Figure 3). Various researchers [87,88] have identified that ideal SiO2/Al2O3 ratios for AAM might range from 3.3 to 4.5; this range heavily depends on Si and Al component availability in precursor materials. Other scholars [32,33] maintained that the amorphous proportion must reach at least 36% to yield alkali-activated binders exhibiting desirable characteristics.

Figure 3.

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The optimal conditions explored by Ndjock et al. [31] for alkaline activation of VAs, considering the structural qualities related to amorphous phases, composition of alkaline solutions, and curing temperature.

The predominant alkaline activators utilized for VA activation are often NaOH and KOH combined with sodium waterglass (nSiO2Na2O) or potassium waterglass (nSiO2K2O) [17,86]. These activators contribute alkali-metal cations, elevate the mixture's pH, and facilitate the dissolution of solid precursors that ultimately accelerate strength development in AAM [89]. Furthermore, the alkaline solution performs a dual function, dissolving both the aluminate and silicate precursors while hydrolyzing particle surfaces, thereby enabling chemical reactions between the dissolved silicate and aluminate ions alongside the particle surface itself. Surface interactions often contribute to binding undissolved particles to the final structure of the alkali-activated products [83].

Table 3 compiles significant mixtures formed from alkaline-activated VA materials [32,38,41,43,44,47,52]. The synthesis parameters, including SiO2/Al2O3 ratios, activator types and dosages, along with curing conditions are comprehensively detailed, elucidating their impacts on the properties of alkaline-activated VA materials as paste, mortar, and concrete.

Table 3.

Summary of the most relevant synthesis parameters on alkali-activated volcanic ash (VA)-based pastes.

Raw Material Types of Activators/Alkaline Solution Curing Conditions Compressive Strength Ref. VA 1 (NS 2, NH 3, KH 4) SiO2/Na2O Na2O/SiO2 L/S 5 VA (ZD), <80 µm, '2.3 m2/g, SiO2/Al2O3 4.90 NS + NH (12 M) 0.7, 0.9, 1.1, 1.3 and 1.4 0.37 Ambient temp. 24 ± 3 °C for 28 days. 19 MPa at 28 days Tchakouté et al. [32] VA (ZG), <80 µm, '15.7 m2/g, SiO2/Al2O3 4.55 0.49 50 MPa at 28 days Vas < 75 µm: Type 1 (High soluble CaO) KH (2.5, 5, 7.5, 10 M)
NH (2.5, 5, 7.5, 10 M)
NS 2.1, 2.4, and 3.1 3.33 Sealed and cured at 40 °C and 60 °C and Autoclave at 2 MPa and 150 °C for 3 h 44 MPa at 28 days Bondar et al. [44] Type 2 (High soluble silicates) Sealed and cured at 40 °C and 60 °C 56.2 MPa at 60 °C at 28 days 49.7 MPa at 90 days (Ms, 3.1) Type 3 (Type 2 but calcined at 800 °C) Sealed and cured at 40 °C and 60 °C 39.8 MPa at 90 days (Ms, 3.1) VA ' 200 µm NH (1'16.5 M) 0.3 50 °C and 80% RH for 3 days 80.1 MPa at 3 days Takeda et al. [47] Volcanic pumice ground '305 m2/kg, '22.63 µm NH + NS 0.6 and 0.75 Na2O/Al2O30.92, 1.08, 1.23 Precuring: 95% RH at 25 °C
Hydrothermal treatment in steam-saturated atmosphere at 45, 65, 85 °C for 5, 10, 15, and 20 h after 1 and 7 days of precuring 37.5 MPa at 28 days after 1-day of precuring Kani & Allahverdi [52] 57.5 MPa at 28 days after 7-day of precuring NH +NS 0.6 Na2O/Al2O31.08 Autoclave curing at 125, 150, 180, and 210 °C for time periods of 20, 30, 40, and 50 h 108.75 MPa VA < 75 µm NH (10 M) 3.22 0.45 Oven at 80 °C and 100% RH cured for 1, 3, 7, and 28 days 33 MPa at 28 days Moon et al. [41] 80% NH (10 M) + 20% NS 47 MPa at 28 days VA < 75 µm
VAs untreated and calcined at 700, 800, 900 °C Type 1 KH (5'7.5 M) + NS 2.1 Curing temperature of 20, 40, 60 and 80 °C for 27 days. 68.5 MPa calcined at 800 °C and cured at 20 °C
81.55 MPa untreated and cured at 80 °C Bondar et al. [43] Type 2 32.9 MPa calcined at 900 °C and cured at 80 °C
8.0 MPa untreated and cured at 80 °C Type 3 13 MPa calcined at 800 °C and cured at 80 °C
29 MPa untreated and cured at 60 °C Type 4 42.4 MPa calcined at 800 °C and cured at 80 °C
22.3 MPa untreated and cured at 60 °C Type 5 65 MPa calcined at 700 °C cured at 80 °C
53 MPa cured at 60 °C VA < 200 µm and mechanical activation,
milling time 60 min NS + NH (12 M)
NS/NH (mass ratio) = 2.4 1.45 0.40 Cured at 27, 45, and 60 °C for 24 h then demolded and cured at room temperature 27 °C: 32.1 MPa; 45 °C: 34.5 MPa; 60 °C: 29.4 Djobo et al. [38] 90 min 27 °C: 37 MPa; 45 °C: 52.5 MPa; 60 °C: 48.3 120 min 27 °C: 45.8 MPa; 45 °C: 53.6 MPa; 60 °C: 46.8 Open in a new tab

4.1. Type and Dosage of Alkaline Activators

NaOH serves as the primary alkaline activator, promoting rapid reactions with augmented dissolution of aluminosilicate solid precursors and silicate monomers. KOH, however, enhances the degree of polycondensation, often yielding stronger matrices [44,90]. The optimal concentration of these activators hinges on their functional properties and economic factors. Generally, KOH and NaOH molarities range from 5 to 12 M for VA activation. Furthermore, excessive alkaline concentration may lead to adverse effects, including efflorescence and brittleness due to elevated levels of free OH- present in final products. Thus, it is discouraged to raise alkali dosage above 12 M.

The silica modulus also holds sway over the mechanical characteristics of mixtures. Commercially produced silicates maintain SiO2/Na2O ratios ranging from 1.5 to 3.2. Typically, silicates featuring high-ratio moduli are more amenable to chemical bonding as their siliceous component reacts with the cations present. Using a low-ratio modulus (i.e., <2.0) is recommendable only when alkaline activation proves lacking in the mixture [91].

According to Bondar et al. [43,44], the activation of VAs utilizing KOH yielded superior compressive strength than its NaOH counterpart. Nonetheless, from a cost perspective, NaOH activators might be favored over KOH, particularly under expedited curing conditions. Mixtures produced using VA-type 1 material with high soluble CaO alongside both 5 M NaOH and 7.5 M KOH activators achieved equal compressive strength readings of 44.0 MPa at 28 days, identical to those manufactured under autoclave curing parameters. However, the VA-type 2 material, characterized by elevated soluble silicates, displayed the best compressive strength of 56.2 MPa at 28 days when activated with 7.5 M KOH and cured at 60 °C. Additionally, integrating sodium water-glass (i.e., sodium silicate in solution) into a 7.5 M KOH solution augmented activator effectiveness regarding strength. The compressive strengths recorded for geopolymers derived from untreated VA-type 2 and calcined VA-type 3 at 90 days measured 49.7 and 39.8 MPa, linked to a SiO2/Na2O ratio of 3.1. Water-glass containing a silica modulus of 3.1 proved apt as an activator for VA dense in CaO or if previously calcined [44].

Moon et al. [41] substantiated that mixtures possessing 80 wt.% NH (10 M NH) and 20 wt.% NS yielded favorable outcomes, improving compressive strength and leading to denser microstructures versus mixtures solely containing NH activator. Microstructural analysis uncovered the existence of phillipsite zeolite and C-S-H-like crystals within the samples. In this experiment, the C-S-H production was noticeable in both samples despite the limited calcium presence in raw VA (8.8 wt.% CaO; reference Table 1). Importantly, sodium silicate's addition accelerated C-S-H-like crystal formation. The compressive strengths at the 28-day mark were recorded at 33 MPa for the VA-NH blend and 47 MPa for the VA-NH + NS blends, both subjected to curing conditions involving 80 °C and 100% relative humidity.

4.2. Conditions of the Curing Regime

High curing temperatures (40–90 °C) and extended time periods can significantly expedite early rates of dissolution and enhance mechanical and physical characteristics in alkali-activated VA-based cements [89]. Elevated curing temperatures promote reactivity and condense setting times for alkali-activated VA-based cements. One investigation found that curing temperatures capping at 100 °C under atmospheric conditions produced optimal compressive strength in material [52].

Takeda et al. [47] formulated VA hardened compounds holding amorphous phase content below 30%. The VA was mixed with NaOH solutions (10 mL) at concentrations spanning from 1 M to 16.5 M after undergoing 3 days of curing at 50 °C with 80% relative humidity. The apex compressive strength of the hardened entities reached 80.1 MPa with a 13.5 M NaOH solution. Furthermore, findings indicated that a 3-day curing window sufficed, as compressive strength did not witness significant improvement beyond this duration. Prolonged periods at 50 °C warranted caution to mitigate moisture loss and mitigate shrinkage crack propagation associated with water evaporation [58].

Kani and Allahverdi et al. [52] produced various alkali-activated VA-based paste mixtures, modifying activators and adjusting molar ratios SiO2/Na2O to 0.6 and 0.75 and Na2O/Al2O3 ratios of the binder mixture ranged from 0.92 to 1.23. These mixtures necessitated a preconditioning protocol at 25 °C with 95% relative humidity lasting 7 days prior to implementing the hydrothermal process or autoclave curing to secure significant increases in compressive strength. Alkali-activated mixture samples attained compressive strengths peaking at 37.5 MPa, while samples subjected to hydrothermal treatment at 85 °C for 20 hours registered peak compressive strengths hitting 57.5 MPa. Noteworthy durability enhancements were observed with greater quantities of alkali-aluminosilicate gels formulated and the mitigation of structural micro-cracks. The autoclaved samples at 210 °C for 30 hours achieved a staggering compressive strength of 108.7 MPa.

To qualify VA material as suitable for alkali-activated pastes, it should exhibit a high specific surface area and at least 36 wt.% of amorphous phases (i.e., SiO2 + Al2O3 content). Five alkaline solutions with variable SiO2/Na2O molar ratios of 0.7, 0.9, 1.1, 1.3, and 1.4 were formulated and mixed with Na2SiO3 (modulus of silica, Ms = 1.40) alongside NaOH (12 M). The VA samples were subsequently covered in a thin polyethylene film to inhibit moisture evaporation and cured at 24 ± 3 °C for 24 hours pre-demolding. The VA sample exhibiting the highest specific surface area of 15.7 m2/g and significant amorphous phase content (37 wt.%) was activated at a SiO2/Na2O molar ratio of 1.4, leading to an impressive compressive strength of 50 MPa [32].

Bondar et al. [43] determined that untreated VA samples high in soluble silicate (i.e., VA-type 1; please reference synthesis parameters in Table 3) activated with combined solutions of 4 mL of 7.5 M KOH and 0.5 mL of NS—exhibiting a solid content of 2.1%—performed as intended with a 28-day compressive strength of 81.55 MPa, under heat curing conditions of 80 °C. Comparatively, samples of the same VA material—post-calcination at 800 °C—activated with identical alkaline solutions at 20 °C exhibited a compressive strength of 68.5 MPa. Moreover, researchers established that untreated VA material with low loss on ignition (LOI), high CaO content, and a soluble SiO2/Al2O3 ratio of 4.65 (VA-type 5; as indicated in Table 3), proved most suitable for activation. Under the same alkaline conditions, the untreated VA material achieved a paste strength of 53 MPa when cured at 60 °C. Caution should be heeded, however; due to variations in the SiO2/Al2O3 molar ratio, the likelihood of complete utilization of SiO2 and Al2O3 in the synthesis reaction remains improbable.

4.3. Effect of Mineral Additives or Correctors

Evidence indicates that VA materials generally exhibit lower reactivity relative to alternative aluminosilicate materials (i.e., mineral additives) such as metakaolin (MK), fly ash, and granulated blast furnace slag (GBFS). Adding reactive materials abundant in SiO2, Al2O3, and CaO has the potential to redress this deficiency by amplifying reactivity and significantly improving the overall properties of the resulting products [33,35,45,54]. Additionally, the incorporation of mineral additives may mitigate the need for thermal curing and calcination processes applied to raw VA materials [27]. Table 4 outlines compressive strengths of various alkali-activated mixtures comprising VA materials and mineral additives. Despite the recognized benefits of mineral additives in VA mixtures, such materials may not always be readily accessible, as highlighted by Djobo et al. [64]. This factor can contribute to higher logistical costs and potentially undermine the sustainability of VA material's usage in industrial settings. Consequently, alternative, locally available mineral additives should be evaluated.

Table 4.

Summary of the effects of including mineral additives and correctors to activate VA.

Raw Material
(VA 1) Product Types of Activators/
Alkaline Solution
(KH 2, NS 3, NH 4, Ms 5) Additive Curing Conditions Compressive Strength
(28 Days) Optimal Conditions Ref. Iran VA Paste KH (7.5 M) + NS
SiO2/Al2O3 = 3.3 & 6.5 for final products Kaolinite Autoclave 45.61 MPa 20% kaolinite Bondar et al. [45] Sealed cured at 25 °C 19.26 MPa Calcined VA (at 800 °C for 12 h) Autoclave 45.56 MPa 16.7% calcined VA Sealed cured at 25 °C 25.28 MPa Burnt Lime Autoclave 27.8 MPa 3.4% Burnt lime Sealed cured at 25 °C 19.6 MPa Cameroon VA Paste NS + NH (12 M) Alumina (Al2O3) Sealed 24 h. Ambient temperature (24 ± 3 °C) 47.8 MPA 40% Alumina Tchakouté et al. [35] Fused-Cameroon VA with NaOH Mortar NS Metakaolin Sealed 24 h. Ambient temperature (24 ± 3 °C) 68.8 MPA 60% Metakaolin Tchakouté et al. [34] Colombian VA Paste NS + NH
SiO2/Al2O3 molar ratios 6.0–8.0
Na2O/SiO2 molar ratios 0.05–0.20 GBFS 6 Sealed 24 h. Humidity 90% at 25 °C until the test age 125 MPa 6.5–7.0 SiO2/Al2O3,
30% GBFS Robayo-Salazar et al. [27] Iran VA Mortar NS + NH
Ms = 1, 1.5, 2
SiO2/Al2O3 = 2.35 CAC 7 24 h at 23.0 ± 2.0 °C and a RH > 95%
hydrothermal conditions at 95 °C for 20 h 65 MPa 24% CAC Vafaei & Allahverdi [54] Cameroon VA Paste NH (12 M) + NS (Ms = 1.1 & 1.4) Metakaolin Sealed 24 h. Ambient temperature (24 ± 3 °C) 69 MPa 25% Metakaolin Djobo et al. [84] Cameroon VA Paste NH + NS
(SiO2/Na2O =1.6) GBFS Stored at 25 °C until the 3, 7, and 28-days compressive strength tests. 85 MPa 50% Slag Lemougna et al. [92] Open in a new tab

Bondar et al. [45] studied the impact of active mineral additives, such as kaolinite, burnt lime, and calcined VA, on the initial composition of raw VA preceding alkaline activation. The addition of mineral additives enriched Alumina (Al), Silica (Si), and Calcium (Ca)—imperative for enhancing the composition before activation. The resultant mixtures contained as much as 40% kaolinite, 100% calcined VA, and 7% burnt lime; all mixtures utilized a 7.5 M KOH solution for activation (refer to synthesis parameters in Table 4). The compressive strengths of alkali-activated VA-based pastes lacking mineral additives recorded values of 44 and 19.48 MPa, corresponding to autoclaved conditions at 2 MPa and 150 °C for 3 hours, and sealed conditions maintained at 25 °C respectively. Noteworthily, compressive strength for the mixture incorporating 20% kaolinite improved up to 13% and 4.5% upon autoclave and sealed curing at 25 °C respectively, when contrasted with VA without additives. The incorporation of 16.7% calcined VA enhanced compressive strength of the mixture by 11% and 10.5% for autoclaved and sealed curing, relative to the VA without additives. The AAM materials cured by autoclaves displayed enhanced strength compared to those core samples sealed without additives. The average molar oxide ratios for the final products remained between 3.3 and 6.5, within acceptable parameters for promoting alkaline activation.

Tchakouté et al. [34,35] analyzed how Al2O3 additions influences the properties of alkali-activated pastes synthesized from VA. The findings indicated integrating up to 40% Al2O3 elevated compressive strength by 32.4% (47.8 MPa) against the strength of pastes produced with pure VA (36.1 MPa). That said, excess Al2O3 (i.e., >40%) yielded a heterogeneous microstructure riddled with cracks. In addition, Tchakouté et al. [34,35] employed an alkali-fusion technique to bolster VA reactivity, wherein VA material was thoroughly combined with NaOH pellets at a low alkali/VA mass ratio of 0.7 prior to incorporating reactive MK to mitigate excess alkali surplus. The presence of the reactive phase in the VA following the alkali-fusion exceeded 76% than in raw forms of VA (26%). Mortars derived from alkali activation mixtures of fused VA (f-VA), multi-varying MK proportions, and river sand produced optimal outcomes, achieving compressive strengths of 68.8 MPa at 28 days for an f-VA/MK ratio of 40/60.

Djobo et al. [84] gathered related insights, employing assorted MK percentages (5 wt.% to 25 wt.%) to compensate for Al2O3 deficiencies and bolster the amorphous phase of VA. Notably, the introduction of 25% MK in the VA enhanced the quantity of amorphous Al2O3 and SiO2 within the system. High-alkaline solution (SiO2/Na2O ratio of 1.4) conditions favored dissolution of these components, thus delivering alkali-activated paste with a compressive strength of 69 MPa, evolving into shorter setting times.

Robayo-Salazar et al. [27] reported relative findings suggesting the low reactivity encapsulated within VA (25.5% amorphous phase content) required the application of thermal curing at 70 °C for hardening to develop strength in early curing stages. However, the inclusion of GBFS to the mixture up to 30 wt.% of VA obviated the need for thermal curing during the binder synthesis. An alkaline activator combination of NaOH and Na2SiO3 with SiO2/Al2O3 ratios of 6.5–7.0 were deployed in synthesizing this binder (see synthesis parameters in Table 4). After 28 days of curing at a consistent room temperature of 25 °C, the binder exhibited a compressive strength of 125 MPa. Comparatively, utilizing NaOH (5% Na2O) without Na2SiO3 as the activator demanded a 2-day curing period at 70 °C to secure a compressive strength at 28 days of 30 MPa. Scanning Electron Microscopy-Energy Dispersive X-ray Spectroscopy (SEM/EDS) analyses confirmed that C-S-H gels with high calcium content were produced along with N-A-S-H gels possessing low calcium content; over the passage of time, interactions between N-A-S-H and C-S-H gels resulted in intermediate composition-type gels (C, N)-A-S-H and C-A-S-H with enhanced calcium and aluminum concentrations [42]. This phase gel formation contributed to a reduction in both pore sizes and total pore volumes, declining from 12.6% (100% VA mixture) down to 6.2% (70% VA:30% GBFS mixture). These gels exemplified superior mechanical performance compared to the 100% VA paste.

Vafei and Allahverdi [54] explored calcium aluminate cement (CAC) as a mineral addition; they substantiated that mixtures compounded with CAC at 24% of total binder proportions with a silica modulus of 1.5 and 10% Na2O registered compressive strengths reaching up to 65 MPa under hydrothermal curing conditions. Under analogous conditions, a 100% VA mixture's compressive strength remained beneath 20 MPa. The workability and set duration of the produced mixture diminished with the augmented CAC composition.

5. Fresh and Hardened Properties of Alkali-Activated Volcanic Ash-Based Pastes and Mortars

The deployment of alkali-activated VA-based materials during mortar production offers marked improvements to fresh state properties, enhancing workability, mobility, and water retention compared to OPC-based mortars. Moreover, utilizing alkali-activated VAs mitigates mortar permeability issues, enhancing durability properties and resistance to sulfate and chloride attacks, often surpassing the capabilities of OPC mortars [22,36,46,55]. Table 5 encapsulates key studies on alkali-activated VA-based mortars composed of an aggregate/binder ratio of 2 [22,46,55].

Table 5.

Summary of representative studies on alkali-activated VA-based mortars.

Binder Types of Activators/
Alkaline Solution Aggregates (A) Curing Conditions Compressive Strength Ref. VA 1 NH 2, NS 3, Ms 4 S/B 5 A/B 6 Volcanic Tuff < 45 µm, specific gravity = 2.38 NH (10, 12, 14, 16 M) 0.35 0.45 River aggregates (0'2, 2'4 and 4'8 mm), A/B = 2 Curing in covered molds at 90, 120, 150 °C for 72 h, then samples were kept in laboratory conditions until the test day. 25.83 MPa at 90 days Kantarci et al.
[55] NS (Ms = 0.6, 0.7, 0.8, 0.9, 1.0) + NH (10 M) Curing in covered molds at 90, 105, 120 °C for 72 h then samples were kept in laboratory conditions until the test day. 37.09 MPa at 90 days VA < 200 µm, specific gravity = 2.62 NS + NH (12 M)
Mass ratio of NS/NH = 2.4 0.45 Sand (specific gravity 2.55) A/B = 2 Room temperature (27 ± 3 °C) for 7-days
80 °C for 24 h
The specimens after casting were demolded and kept at ambient temp until test performed. 37.9 MPa at 90 days Djobo et al. [22] VA, specific gravity = 2.29
SSA 7 = cm2/g NH (2.5, 5, 7.5, 10, 12.5 M) 0.50, 0.54, 0.58 Fine aggregate (specific gravity = 2.76)
A/B = 2.0 Pre-curing: 60 °C for 3 h in an oven before demolding
Then oven at 80 °C (exposed, sealed, and moist cured; 1, 3, and 7 days) 31.8 MPa at 7 days (exposed) Ghafoori et al. [46] 26.6 MPa at 7 days (wet) 37.7 MPa at 7 days (sealed) VA < 440 µm NH
Na2O/SiO2 *
(0.15'0.30) 0.21 Sand (density 2.56 g/cm3) (40% by wt.) Dry curing in the open air at 90 °C for 5 days 30 MPa at 28 days for a sample with Na2O/SiO2 molar ratio of 0.30 Lemougna et al. [36] Open in a new tab

5.1. Fresh Properties

Setting Time and Heat Hydration

The setting time of alkali-activated pastes is influenced by the raw VA material’s particle size distribution and chemical composition. Low specific surface area (SSA) combined with elevated free CaO levels may result in more extended setting times within alkali-activated blends [19]. When investigating alkaline pastes formulated from VA exhibiting high CaO (5.11 wt.%) and high SSA (15.7 m2/g), findings revealed a marked reduction in setting time, from 490 min to 180 min, as the silica modulus of the alkaline solution was modified from 0.7 to 1.4 [32]. Djobo et al. [84] observed that blends containing VA together with MK and a highly siliceous alkaline solution upheld diminished setting times within the produced paste. A mixture derived from an alkali solution featuring a silica modulus of SiO2/Na2O of 1.4 resulted in a rapid setting time of just 180 min.

Geopolymer pastes activated via MA showcased notable reductions in initial and final setting times. Djobo et al. [38] documented that a milling time of 60 minutes led to increased fineness in VA particles while causing a reduction in the setting time by nearly 95%.

Mineral additives further impact setting times within AAMs. Lemougna et al. [92] identified that replacing 10% of VA materials with GBFS diminished the setting time from 7 days down to 6.7 hours when assessed at 25 °C. Additionally, Robayo-Salazar et al. [27] reported faster setting times for alkaline-activated pastes when incorporating NH and NS in conjunction with GBFS instead of pure VA during production. Although pastes composed entirely of VA necessitated in excess of 48 hours to set, the addition of 30% GBFS within the VA mix decreased initial and final setting durations to 22 and 30 minutes, respectively along with a total heat of reaction for 48 hours registered at 44.48 J/g—a dramatic reduction of up to 76% relative to the 190 J/g observed in the 100% OPC paste.

Tchakouté et al. [34] investigated the mortar blends developed with alkaline-activated (sodium silicate solution) mixtures of VA to MK at ratios 70/30, 60/40, 50/50, and 40/60. Their findings indicated a decrease in setting time as MK percentages approached 60% in the mixture formulation (see Figure 4). The high proportions of MK increased the availability of Al and Si species, accelerating the formation of a polymeric binder compared to blends with lower MK concentrations.

Figure 4.

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Setting time of fused alkali-activated mortars, distinguished by MK proportions of 30%, 40%, 50%, and 60%, respectively labeled as Z1, Z2, Z3, and Z4 [34].

5.2. Density, Apparent Porosity, and Water Absorption

Djobo et al. [22] revealed that mixtures cured at 27 and 80 °C exhibited analogous density values around kg/cm3, details captured in Table 5. Contrastingly, specimens cured at 80 °C demonstrated minimized water absorption coupled with reduced apparent porosity values in comparison to 27 °C-cured samples. This distinction led researchers to conclude that higher degrees of geopolymerization were attained in specimens subjected to elevated curing temperatures. Maximum water absorption results ranged from 7.03% for the 27 °C-cured specimens, declining to 5.91% for those cured at 80 °C over a 28-day period.

Lemougna et al. [36] crafted mortar samples characterized by a Na2O/SiO2 molar ratio of 0.3. Despite the VA-based mortars blending with 10 wt.% sand exhibiting the lowest bulk density of kg/m3 and the highest water absorption rate (14.61%), mixtures containing 25 and 40 wt.% sand possessed relatively higher densities and reduced absorption capacities. Inclusion of sand serves to diminish manufacturing costs, provided that proportions remain within acceptable limits to preserve intended properties for respective applications.

Ghafoori et al. [46] indicated that mixtures featuring high hydroxide concentrations, enhanced alkaline activation, coupled with reductions in solution-to-binder (S/B) ratios fostered denser matrices while minimizing water absorption. Research indicated absorption levels dropped from 13.2% to 2.4% when solution molarity increased from 2.5 to 12.5 M. The porosity correspondingly reduced by 37% from 2.5 to 12.5 M concentrations.

5.3. Compressive Strength of Mortars

According to findings reported by Ghafoori et al. [46], mortar samples with an S/B ratio of 0.58, compounded utilizing a 12.5 M NaOH solution and an aggregates/binder ratio of 2 (see Table 5). The mortars experienced compressive strengths of 31.8, 26.6, and 37.7 MPa after exposure to dry, wet, and sealed curing protocols at 80 °C, respectively. While mortar samples under sealed curing conditions achieved peak compressive strengths, those under wet conditions resulted in the lowest strengths. Similarly, Kantarci et al. [55] demonstrated that maximum compressive strengths of 37 MPa at 90 days emerged from mortar samples enduring sealed curing for 72 hours at 120 °C. The investigation determined that mixes generated through alkaline activation of VA using a 16 M NaOH solution, with an S/B ratio of 0.45 alongside an aggregates/binder ratio of 2 yielded promising results; however, it was evident that high-temperature curing conditions might compromise the geopolymer gel's structure due to excessive shrinkage.

Djobo et al. [22] also affirmed that mortar samples cured at 80 °C exhibited superior compressive strengths (38 MPa at the 90-day mark) when juxtaposed with those cured at 27 °C. High-temperature curing initiated highly cross-linked AAM materials and a favorable transitional interface between paste and aggregates, ultimately enhancing compressive strength in the geopolymer mortar [93]. Notably, the bonding strength of geopolymer gels correlates directly with the extent of geopolymerization, evidenced by higher performance in specimens cured at elevated temperatures compared to those cured under ambient temperatures.

Findings from Tchakouté et al. [34] indicated that MK incorporation as a mineral additive enhanced hardened properties of alkali-activated VA-based mortars. These mortars were produced by activating fused VA, MK, and river sand blends at a 40/60 ratio, utilizing sodium silicate solutions (refer to Table 4). The aggregate-to-binder ratio was drawn to 2 by weight. MK’s fine particle size of 9.95 µm alongside its high SSA of 20.5 m2/g facilitated significant polycondensation phenomena, cultivating a polymeric binder. Consequently, the compressive strength of these binders reached 68.8 MPa (see Table 4).

The research conducted by Lemougna et al. [36] concentrated on producing mortar for structural and refractory applications. The researchers demonstrated that sand addition minimized porosity and enhanced the bulk density of alkali-activated mortars. Nevertheless, compressive strength exhibited a decrement from 55 to 30 MPa as sand proportions increased from 10% to 40% relative to the weight of VA (see Figure 5). The reported adherence of 30 MPa with 40% sand fulfilled ASTM C216 requirements for severe weathering pertaining to building materials. Furthermore, the incremental use of sand from 10% to 40% reduced the standard deviation of compressive strength from 2.5 MPa to 0.5 MPa, respectively (see Figure 5) and concurrently diminished manufacturing costs.

Figure 5.

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Influence of the addition of mortar on the compressive strength of a mortar with a molar composition of Na2O/SiO2 = 0.30 [36].

6. Engineering Properties of Alkali-Activated Volcanic Ash-Based Concretes

Numerous researchers stress the potential of utilizing raw VA material to fabricate alkali-activated concretes [5,57,58,94]. Table 6 outlines the mixture proportions for these alkali-activated concretes. The mixture proportions and curing conditions for the concrete types are illustrated in Table 7.

Table 6.

Mixture proportions (kg/m3) for alkali-activated concretes developed in distinct investigations as captured in Table 7.

Research Author VA 1 (kg/m3) CA 2
(kg/m3) FA 3
(kg/m3) Silica Sand
(kg/m3) NS 4
(kg/m3) AH 5
(kg/m3) W 6
(kg/m3) SP 7
(%) w/b 8 Ibrahim et al. [57] 400 650 ' 150 60 100 ' 0.25 Robayo-Salazar et al. [5] 400 933 763 ' 146 103 ' 0.35 Haddad et al. [58] 410 740 666 444 132 53 83 3 0.40 Bondar et al. [94] 391 578 ' 34 * 66 180 ' 0.45 344 702 ' 37 * 72 195 ' 0.55 417 499 ' 32 * 67 180 ' 0.42 417 499 ' 32 * 67 180 ' 0.42 Open in a new tab

Table 7.

Summary of studies that produced alkali-activated VA-based concretes.

Precursors
VA 1 (SSA 2) Alkaline Activators
NS 3, NH 4, KH 5 Aggregates
(FA 6, CA 7) Curing Conditions CS 8 Optimal Conditions Ref. VA (SSA, 442 m2/kg) NS + NH(14 M)
NS/NH = 2.0, 2.5, 2.75 FA: Dune Sand
CA: Limestone Oven at 60 °C for 1, 3, 7, 14 and 28 days 37.52 MPa at 7 days NS/NH = 2.5
7 days of curing Ibrahim et al. [57] 70% VA (SiO2/Al2O3, 6.79) +
30% GBFS (SiO2/Al2O3, 4.93) NS + NH
(SiO2/Na2O, 1.09) FA: Sand
CA: Gravel Room temperature at 25 °C and RH > 80% until reaching their test age. 21 MPa at 28 days and 33 MPa at 360 days NS/NH = 2.5
Room temperature at 25 °C and RH > 80% Robayo-Salazar et al. [5] VA (fineness 700 m2/kg,
SiO2/Al2O3) NS + NH(14 M)
NS/NH = 0.5'3.0 FA: Mineral (SG, 2.53)
CA: Limestone (SG, 2.44)
SP 9: Polycarboxylate up to 6% by wt. of VA Dry curing at room temperature for 7 and 28 days 24.1 MPa at 28 days NS/NH = 2.5
dry curing period for 24 h at 80 °C Haddad & Alshbuol [58] Dry curing at 40, 80, and 120 °C for 24 h and 48 h, maintained in air for the remainder of the 28 days until contesting 31.9 MPa at 28 days VAs
<75 µm: Type 1
SSA, cm2/g NS+ KH (7.5 M)
KH/NS = 7.1 FA: sand
CA: Gravel (SG, 2.6) Sealed curing at 20, 40 and 60 ± 2 °C
Fog curing at 20, 40 and 60 ± 2 °C 40.97 MPa at 180 days Cured at 40 °C sealed Bondar et al. [94] Type 2
SSA, cm2/g KH/NS = 7.7 33.15 MPa at 90 days 40 °C cured sealed Type 3 (Calcined)
SSA, cm2/g KH/NS= 7.7 40.56 MPa at 180 days Cured at 20 °C sealed Open in a new tab

Haddad and Alshbuol [58] created alkali-activated concretes utilizing 100% VA as a binder. The mixture consisted of coarse limestone, silica sand fine aggregates, a water-to-binder (w/b) ratio of 0.40, and a superplasticizer (reference Table 6). Various sodium silicate to sodium hydroxide (NS/NH) ratios were integrated as alkali activators in tandem with curing approaches at normal and elevated temperatures. The NS was comprised of 29 wt.% SiO2, 11 wt.% Na2O, and 60 wt.% H2O; employments of 14 M sodium hydroxide (NH) were enforced as well. The concrete gained through the 2.5 NS/NH activator gave favorable fresh properties. Impressively, the utmost compressive strength of 31.9 MPa was attained under a 24-hour dry-cured interval at 80 °C, with strength test values remaining consistent as reflected in a standard deviation of roughly 4%. Alarmingly, excessive curing temperatures above 80 °C appeared to jeopardize the compressive strength of alkali-activated VA concretes. SEM images validated that the dense aluminosilicate gel formed, while limited in microcrack occurrence, supported this compressive strength behavior.

Bondar et al. [94] noted that alkali-activated VA constructions exhibited lower compressive and tensile strength alongside elasticity modulus when compared to OPC mixtures, particularly in the early stages. However, activated VA materials with a w/b ratio of 0.45, particularly when calcined VA underwent sealed curing protocols at 40 °C and 20 °C, achieved equivalent or superior mechanical properties compared to OPC mixtures after extending curing durations (post-365 days): reflecting increases of up to 15% in compressive strength and 20% in elastic modulus. The attained values were 43.5 MPa and 33.6 GPa respectively. Conversely, the activated VA mix with a w/b ratio of 0.42, implemented under sealed curing at 60 °C demonstrated significantly lower values for static modulus of elasticity and strength relative to OPC mixtures. The VA mixtures yielded values of 10.7 GPa, and 35 MPa in compressive strength, while OPC secured figures of 29 GPa and 38 MPa. Excessive curing temperatures of 60 °C led to water evaporation alongside incomplete alkaline activation, culminating in depreciated compressive strength and static modulus attributes.

Ibrahim et al. [57] also substantiated the use of alkali-activated VA-based concretes. They amalgamated 70% VA with 30% GBFS to create binders in the concrete fabrication process. In comparison to OPC concrete, the mixed concrete exhibited equivalent or heightened compressive strength values of approximately 33 MPa. Additionally, alkali-activated VA-GBFS concrete mixtures operated with a 44.7% lower carbon footprint: 210.90 kg CO2 eq/m3 for alkali-activated concrete as opposed to 381.17 kg CO2 eq/m3 for OPC concrete. Consequently, alkali-activated VA compositions present formidable alternatives to conventional OPC concrete.

7. Durability Properties of Alkali-Activated Volcanic Ash-Based Materials

Few studies have concentrated on evaluating the durability of alkali-activated VA-based cements or concretes [22,46,94,95,96,97]. Yet, alkali-activated blended materials have manifested satisfactory durability characteristics regarding oxygen and chloride permeability, resistance to sulfate and sulfuric acid corrosion, drying shrinkage/crack susceptibility, and efflorescence.

7.1. Permeability

Bondar et al. [96] examined the permeability of two Iranian VAs, accounting for various synthesis conditions. Their findings asserted that under high curing temperatures of 40 °C and 60 °C with seclusion methods, oxygen permeability diminished for mixtures encapsulating lower w/b ratios; mixtures produced with a w/b ratio of 0.42 showcased the lowest permeability compared against those generated at ratios of 0.45 or 0.55. Furthermore, alkali-activated concretes surfaced with oxygen permeability reduced by as much as 35% compared to that of OPC concrete at the 90-day mark. Similarly, the findings indicated reduced chloride penetration within mixtures assessed after the sealed and heated curing processes at 40–60 °C.

According to Ghafoori et al. [46], the chloride penetration rate in VA-based mortars dropped from 45.62 to 4.49 mm with increasing NaOH solution concentrations, adjusting from 2.5 to 12.5 M. Chloride penetration depths were also diminished by a range of 14.1% and 11.4% when sodium hydroxide S/B ratios decreased from 0.58 to 0.54 and accordingly from 0.54 to 0.50.

7.2. Sulphate and Acid Resistance

The water-to-binder (w/b) ratio emerges as a pivotal factor influencing the sulphate resistance of alkali-activated VA-based concrete. Lower w/b ratios yield less weight loss in concrete samples post-sulphate resistance tests. For example, reports by Bondar et al. [95] elucidated the compressive strength evaluations for alkali-activated natural pozzolan concrete samples subjected to sulphate solution over 2 years. The immersed samples registered compressive strength reductions spanning from 8% to 19.5% compared to untreated samples cured out of the solution. The maximum expansion percentage of the AANP mixtures remained below 0.1%, thereby reaffirming compliance with ASTM C standards for OPC concrete exposed to mild sulphate conditions at six-month intervals.

Post-analyses conducted by Djobo et al. [22] aimed at assessing acid resistance amidst alkali-activated VA-based mortars in contact with 5% H2SO4. The synthesis parameters stated (refer to Table 5) demonstrated that samples sealed at 27 °C exhibited better acid resistance compared to heat-cured samples at 80 °C. The presence of a sodium-rich gel for samples cured at 27 °C minimized pH levels via acid-base reactions, curtailing gypsum formation rates. Compression strength for samples cured at 27 °C experienced a 24% decrease, compared to significant deterioration at 60% in samples due to acid exposure after 180 days. Observational findings confirmed that samples cured at 27 °C possessed densely packed pores or exhibited lower internal connectivity compared to counterparts cured at 80 °C. Figure 6 visually presents the differences in alkali-activated VA-based mortars cured at varying temperatures following their acid exposure.

Figure 6.

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Visual representation of alkali-activated VA-based mortar cores cured at 27 and 80 °C subsequent to (a) 90 days and (b) 180 days of exposure to 5% H2SO4 [22].

7.3. Drying Shrinkage/Crack

As reported by Bondar et al. [94], the employed curing regimes and durations during alkali-activated VA-based concrete synthesis play an integral role in determining drying shrinkage amounts. Specimens undergoing 60 °C sealed curing for 3 days, alongside high w/b ratios of 0.55 sealed under similar conditions for 7 days, reported minimal drying shrinkage levels. The utmost final recorded drying shrinkage (at 180 days) asserted values at a w/b ratio of 0.55, measuring 514 × 10–6—marking a 43% reduction compared to 0.45 samples (~10–6). Findings implied that samples with lowered w/b ratios did not support effective cross-linking, leading to volume loss as moisture left fresh alkali-activated VA concrete, possibly inhibiting fullness of cross-linking. Moreover, the reduced drying shrinkage observed at high temperatures correlated with both moisture loss and enhanced hydration product cross-linking. Meanwhile, conditioning via fogging at 40 °C correlated with higher drying shrinkages at time points of 3 and 7 days’ retention, likely due to the retention of moisture by the alkali-activated binder mix, ultimately yielding a more porous concrete microstructure [98].

7.4. Efflorescence

Efflorescence manifests through free alkalis within the pore solution reacting with humid air containing CO2, leading to the formation of characteristic white deposits on concrete surfaces akin to carbonate compounds (e.g., Na2CO3 or K2CO3). Allahverdi et al. [97] elucidated that by optimizing alkaline activator compositions along with w/b ratios, efflorescence severity could be controlled, demonstrating no or only scant efflorescence emergence in samples activated with Na2O at 4 wt.%, within a silica modulus between 0.52 and 0.68, and a w/b ratio ranging from 0.36 to 0.44. However, this sample set failed to reach acceptable 28-day compressive strength comparative to samples comprising 10 wt.% Na2O which encountered intensified efflorescence.

Additional studies illuminated that the introduction of suitable amounts of active aluminum into the alkali-activated VA-based binders using minerals like MK, GBFS, and CA can substantially alleviate efflorescence prevalence [35,53,84]. Similarly, improvements in efflorescence reduction could stem from modifying curing conditions to enhance aluminum discharge from less reactive precursors. Hydrothermal conditions at temperatures exceeding 65 °C typically mitigate efflorescence extent alongside resulting slight increases in compressive strength amongst samples [52,97].