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Engineering, Civil
For several decades, class F fly ash has been an attractive supplementary cementitious material, at least in part, due to its ability to reduce Portland cement consumption and mitigate alkali-silica reactions in concrete. However, fly ash availability is becoming uncertain as the energy industry decommissions coal burning power plants as it transitions to renewable energy production. This situation creates a need to identify viable and sustainable alternative supplementary cementitious materials. There are several types of supplementary cementitious materials, such as natural pozzolans, metakaolin, or ground granulated blast-furnace slag, which appear to be potential alternatives to fly ash in concrete. In this research, a locally available natural pozzolan (pumicite) was selected to replace fly ash in concrete. After conducting alkali-silica reaction tests on mortar mixtures, rheological and strength properties, shrinkage, resistance to freezing and thawing, and chloride ion permeability of concrete mixtures containing different amounts of fly ash and natural pozzolan were evaluated. The results showed that pumicite was more effective than fly ash at mitigating the alkali-silica reaction, and a pumicite content of 20% was necessary to mitigate the alkali-silica reaction. Ternary mixtures containing both pumicite and fly ash were the most effective cementitious materials combinations for mitigating the alkali-silica reaction expansion. Additionally, pumicite provided acceptable compressive strength and modulus of rupture values (greater than 4.0 MPa) that exceeded the flexural strengths provided by established mixtures containing only fly ash. Shrinkage and durability factor values for all mixtures were less than 710 μstrain and greater than 75, which are generally considered acceptable. Additionally, all mixtures with acceptable alkali-silica reaction expansions had very low chloride permeability. These results indicate that pumicite can be a reliable alternative for fly ash.
- alkali-silica reaction
- durability
- fly ash
- natural pozzolan
- pumicite
- shrinkage
- freeze–thaw
- permeability
1. Introduction
The concrete industry has traditionally used ordinary Portland cement as the binder in concrete mixtures. However, Portland cement production is a large contributor to global carbon dioxide (CO2) emissions [1,2,3]. One way to reduce CO2 emissions is to decrease cement usage by partially replacing cement with supplementary cementitious materials (SCMs). Class F fly ash, a byproduct of burning coal to produce electricity, has been a reliable SCM used to replace a substantial portion of cement and mitigate the alkali-silica reaction (ASR) in concrete. Fly ash also improves mechanical and other durability properties of concrete (such as corrosion resistance), reduces materials costs, and improves sustainability [4,5,6]. However, environmental considerations and changes in the energy industry have reduced availability of fly ash [7,8,9].
To compensate for anticipated fly ash shortages, cost-effective and eco-friendly alternative SCMs must be identified and investigated. There are several types of SCMs, such as natural pozzolans, metakaolin, or ground granulated blast-furnace slag (GGBFS) that appear to be potential alternatives to fly ash in concrete. As the effects of metakaolin and GGBFS in ultra-high performance concrete mixtures have been evaluated in previous studies conducted by the authors [10,11], a natural pozzolan (pumicite) was selected in this study since it is locally available in New Mexico, USA, where the research was conducted. In the United States, natural pozzolans have not been cost-competitive with fly ash until recently so they have not been widely used as SCMs. Consequently, comprehensive durability studies on concrete mixtures containing pumicite as a SCM are needed.
ASR is a prominent durability concern for concrete, especially in locations with highly reactive aggregates. ASR is an expansive reaction that can occur between siliceous aggregates and alkalis that may be present in Portland cement. Concrete expansion due to the ASR can cause tensile stresses that can lead to internal cracking which can significantly decrease the service life of a concrete structure. As ASR expansion cannot be controlled just by using low-alkali cement in locations with highly reactive aggregates [12], such as New Mexico, USA, class F fly ash has historically been used in combination with low-alkali cement to mitigate ASR. Another common durability problem in concrete is corrosion of reinforcing steel that is often initiated by chloride ions migrating through concrete and reaching the reinforcing steel. Other durability-related issues that can accelerate degradation in concrete include shrinkage cracking and frost damage. Consequently, any SCM used to replace fly ash in concrete should also have the ability to mitigate ASR and provide the concrete with resistance to chloride ion penetration and frost resistance while also limiting concrete shrinkage.
This research examined the effects of using a locally sourced natural pozzolan (pumicite) mined from a geological deposit located near Española, New Mexico, USA, in concrete mixtures and its ability to provide protection against durability issues such as ASR and corrosion. The first goal of this study was to identify combinations of cementitious materials (including Portland cement, fly ash, and pumicite) capable of mitigating ASR. Once cementitious materials combinations with acceptable ASR resistance were identified, the next goal was to assess the possibility of producing concrete mixtures containing a natural pozzolan (pumicite) or containing a blend of fly ash and pumicite that had rheological, mechanical, and durability properties comparable to concrete mixtures containing only fly ash. In this research, concrete mixtures were characterized by assessing slump, air content, compressive strength, flexural strength, shrinkage, resistance to freezing and thawing, and chloride permeability (measured by rapid chloride permeability and surface resistivity tests).
2. Background
2.1. Alkali-Silica Reaction
ASR expansion has been shown to increase with greater alkali concentration. Alkalis can be provided by aggregates, admixtures, or SCMs, so it is important to consider the total alkali content of a concrete mixture. Depending on aggregate reactivity, maximum permissible alkali contents of 2.5 to 4.5 kg/m3 equivalent sodium oxide have been reported [13].
Some studies have focused on identifying the aggregate mineral components, assessing aggregate reactivity, and identifying mechanisms for ASR reactions [14,15]. Research has shown that not all siliceous aggregates are susceptible to ASR. In another study, Multon et al. [16] reported that ASR expansion can be seven times greater with coarser aggregate particles than with the finer particles, even for aggregates containing approximately the same amount of reactive silica. Additionally, ASR cannot be activated without moisture [17]. Research has also shown that concrete with an internal relative humidity less than 80% will not experience expansive ASR [18,19].
Methods to mitigate ASR include replacing a portion of the cement with SCMs, using non-reactive aggregates, limiting the alkali content in the cement by using low-alkali cement, and using lithium compounds to inhibit the reaction [17,18,20,21]. However, using non-reactive aggregates or using low-alkali cement is not always practical [17]. For example, New Mexico, USA, contains some of the most reactive aggregates in the world, and the ASR expansion cannot be controlled just by using low-alkali cement [12]. Replacing a portion of Portland cement with SCMs is a cost-effective and environmentally friendly method for mitigating ASR since many SCMs are waste products or naturally occurring materials [20,22].
2.2. Other Durability Issues
SCMs in concrete generally improve mechanical and durability properties through pozzolanic reactions that lead to secondary calcium silicate hydrate (CSH) production that fills pore space, improves density, and reduces permeability [10,11,23,24,25,26]. Decreasing concrete permeability increases resistance to chloride ion penetration, which reduces the probability of reinforcing steel corrosion and extends the life expectancy of a concrete structure. While using SCMs is an effective way to reduce concrete permeability and to protect a structure against corrosion, moisture and harmful chemical compounds may still penetrate dense concrete through cracks caused by freezing and thawing cycles or shrinkage. Studies have shown that damage caused by freezing and thawing cycles can be avoided by using air-entraining admixtures (AEAs) or foaming agents to develop an appropriate air void system and by selecting frost-resistant aggregates [27,28,29,30,31]. Cracking caused by shrinkage can also be limited by using SCMs such as fly ash and GGBFS that reduce concrete shrinkage [32].
2.3. Supplamantry Cementiouos Materials
For decades, SCMs have been used to reduce Portland cement content and mitigate durability issues such as ASR expansion, degradation caused by freezing and thawing, chloride ion penetration, and attack by sulfates in different types of concrete [1,10,11,26,33,34]. In many cases, it has been demonstrated that SCMs can also enhance mechanical properties of concrete. Fly ash, GGBFS, silica fume, and natural pozzolans are some of the most commonly used SCMs in the concrete industry [10,35,36,37]. Other SCMs, such as rice husk ash and metakaolin, have also been used in the concrete industry to partially replace Portland cement and mitigate ASR [11,38,39,40,41]. Additionally, Thomas et al. [21] highlighted studies where highly reactive recycled glass (crushed) and ground reactive aggregates were used to mitigate ASR.
2.4. Natural Pozzolan
Natural pozzolans include a wide range of naturally occurring siliceous or siliceous and aluminous materials that individually lack significant cementitious ability but can form compounds possessing cementitious properties in the presence of water [42]. Pumicite is an amorphous natural pozzolan that can be produced by gases released when volcanic lava solidifies. Pumicite has a porous structure caused by gas bubbles being trapped in rapidly cooling molten lava [43].
Studies have shown that concrete slump decreases with the addition of pumicite powder [44]. In a study conducted by Kabay et al. [45], it was observed that using pumicite powder, fly ash, and blends containing both SCMs to replace Portland cement in concrete mixtures resulted in reduced sorptivity, lower water absorption, decreased void content, as well as reduced compressive and splitting tensile strengths at early ages. Additionally, using more, or coarser, pumicite in concrete mixtures may also lead to reduced heat of hydration and compressive strength along with increased setting time and water demand as well as improved ASR resistance and sulfate resistance [46,47]. Another study [48] found that the use of pumicite alone (without catalysts such as sodium silicate and potassium fluoride) showed limited pozzolanic activity.
Sarıdemir [49] also used pumicite in high strength concrete and found that using 25% pumicite decreased 28-day compression strength and elastic modulus. Another study [50] also reported that replacing 10% of Portland cement with pumicite in high strength concrete significantly improved microstructural density, increased resistance to chloride ion penetration, enhanced 180-day compressive and indirect tensile strengths, and decreased 90-day water absorption. Tangtermsirikul and Kitticharoenkiat [51] showed that using fly ash, pumicite, or a blend of both reduced 91-day compressive strength although strength development after 28 days was more rapid than for ordinary concrete. Using pumicite in self-compacting concrete also resulted in acceptable workability and greater 120-day compressive strength than mixtures without pumicite [52]. Öz et al. [53] also reported that replacing up to 20% of the Portland cement in concrete mixtures with pumicite resulted in acceptable frost resistance. However, concrete frost resistance decreased when 30% pumicite was used [53]. Ghafari et al. [44] reported that mixtures with pumicite had similar strength, elastic modulus, and bulk resistivity to mixtures containing class C fly ash and mixtures with only Portland cement at 56 days.
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