Compressive strength of fly‐ash‐based geopolymer concrete at elevated temperatures

This paper presents the compressive strength of fly‐ash‐based geopolymer concretes at elevated temperatures of 200, 400, 600 and 800 °C. The source material used in the geopolymer concrete in this study is low‐calcium fly ash according to ASTM C618 class F classification and is activated by sodium silicate (Na₂SiO₃) and sodium hydroxide (NaOH) solutions. The effects of molarities of NaOH, coarse aggregate sizes, duration of steam curing and extra added water on the compressive strength of geopolymer concrete at elevated temperatures are also presented. The results show that the fly‐ash‐based geopolymer concretes exhibited steady loss of its original compressive strength at all elevated temperatures up to 400 °C regardless of molarities and coarse aggregate sizes. At 600 °C, all geopolymer concretes exhibited increase of compressive strength relative to 400 °C. However, it is lower than that measured at ambient temperature. Similar behaviour is also observed at 800 °C, where the compressive strength of all geopolymer concretes are lower than that at ambient temperature, with only exception of geopolymer concrete containing 10 m NaOH. The compressive strength in the latter increased at 600 and 800 °C. The geopolymer concretes containing higher molarity of NaOH solution (e.g. 13 and 16 m ) exhibit greater loss of compressive strength at 800 °C than that of 10 m NaOH. The geopolymer concrete containing smaller size coarse aggregate retains most of the original compressive strength of geopolymer concrete at elevated temperatures. The addition of extra water adversely affects the compressive strength of geopolymer concretes at all elevated temperatures. However, the extended steam curing improves the compressive strength at elevated temperatures. The Eurocode EN1994:2005 to predict the compressive strength of fly‐ash‐based geopolymer concretes at elevated temperatures agrees well with the measured values up to 400 °C. Copyright © 2014 John Wiley & Sons, Ltd.     

Bond strength between blended slag and Class F fly ash geopolymer concrete with steel reinforcement

In this paper, geopolymer concrete bond with both deformed and smooth reinforcing steel bars is investigated using the standard RILEM pull-out test. The geopolymer binder is composed of 85.2% of low calcium fly ash and 14.8% of ground granulated blast furnace slag (GGBFS). The tests were aimed to assess the development of the bond strength from 24 h to 28 days after casting, with different heat curing conditions. The results show that 48 h of heat curing at 80 °C is required in order to obtain similar or better performances to those of the reference 45 MPa OPC concrete. The 28-day bond strength and the overall bond stress–slip behaviour of the geopolymer concrete were similar to those previously reported for OPC-based concretes. Providing intensive heat curing, high early bond strength can be achieved showing that Class F fly ash geopolymer concrete is well suited for precast applications.     

Electrochemical studies on the corrosion performance of steel embedded in activated fly ash blended concrete

The use of fly ash to replace a portion of cement has resulted significant savings in the cost of cement production. Fly ash blended cement concretes require a longer curing time and their early strength is low when compared to ordinary Portland cement (OPC) concrete. By adopting various activation techniques such as physical, thermal and chemical methods, hydration of fly ash blended cement concrete was accelerated and thereby improved the corrosion-resistance of concrete. Concrete specimens prepared with 10-40% of activated fly ash replacement were evaluated for their open circuit potential measurements, weight loss measurements, impedance measurements, linear polarization measurements, water absorption test, rapid chloride ion penetration test and scanning electron microscopy (SEM) test and the results were compared with those for OPC concrete without fly ash. All the studies confirmed that up to a critical level of 20-30% replacement; activated fly ash cement improved the corrosion-resistance properties of concrete. It was also confirmed that the chemical activation of fly ash yielded better results than the other methods of activation investigated in this study.     

Effect of nano silica and fine silica sand on compressive strength of sodium and potassium activators synthesised fly ash geopolymer at elevated temperatures

Environment friendly geopolymer is a new binder which gained increased popularity due to its better mechanical properties, durability, chemical resistance, and fire resistance. This paper presents the effect of nano silica and fine silica sand on residual compressive strength of sodium and potassium based activators synthesised fly ash geopolymer at elevated temperatures. Six different series of both sodium and potassium activators synthesised geopolymer were cast using partial replacement of fly ash with 1%, 2%, and 4% nano silica and 5%, 10%, and 20% fine silica sand. The samples were heated at 200°C, 400°C, 600°C, and 800°C at a heating rate 5°C per minute, and the residual compressive strength, volumetric shrinkage, mass loss, and cracking behaviour of each series of samples are also measured in this paper. Results show that, among 3 different NS contents, the 2% nano silica by wt. exhibited the highest residual compressive strength at all temperatures in both sodium and potassium‐based activators synthetised geopolymer. The measured mass loss and volumetric shrinkage are also lowest in both geopolymers containing 2% nano silica among all nano silica contents. Results also show that although the unexposed compressive strength of potassium‐based geopolymer containing nano silica is lower than its sodium‐based counterpart, the rate of increase of residual compressive strength exposed to elevated temperatures up to 400°C of potassium‐based geopolymer containing nano silica is much higher. It is also observed that the measured residual compressive strengths of potassium based geopolymer containing nano silica exposed at all temperatures up to 800°C are higher than unexposed compressive strength, which was not the case in its sodium‐based counterpart. However, in the case of geopolymer containing fine silica sand, an opposite phenomenon is observed, and 10% fine silica sand is found to be the optimum content with some deviations. Quantitative X‐ray diffraction analysis also shows higher amorphous content in both geopolymers containing nano silica at elevated temperatures than those containing fine silica sand.     

Effect of cooling methods on residual compressive strength and cracking behavior of fly ash concretes exposed at elevated temperatures

This paper presents the effects of cooling methods on residual compressive strength and cracking behavior of concretes containing four different class F fly ash contents of 10%, 20%, 30% and 40% as partial replacement of cement at various elevated temperatures. The residual compressive strength of the aforementioned fly ash concretes is measured after being exposed to 200, 400, 600 and 800 °C temperatures and two different cooling methods, for example, slow cooling and rapid water cooling. Results show that the residual compressive strengths of all fly ash concretes decrease with increase in temperatures irrespective of cooling regimes, which is similar to that of ordinary concrete. Generally, control ordinary concrete and all fly ash concretes exhibited between 10% and 35% more reduction in residual compressive strength because of rapid cooling than slow cooling except few cases. Cracks are observed over concrete specimens after being exposed to temperatures ranging from 400 to 800 °C. Samples that are slowly cooled developed smaller cracks than those rapidly cooled. At 800 °C, all fly ash concretes that are exposed to rapid cooling showed the most severe cracking. X‐ray diffraction analysis shows reduction of Ca(OH)₂ peak and formation of new calcium silicate peak in concretes containing 20% and 40% fly ash when subjected to 800 °C in both cooling methods. Thermo gravimetric analysis and differential thermal analysis results show increase in thermal stability of concrete with increase in fly ash contents. The existing Eurocode also predicted the compressive strength of fly ash concretes with reasonable accuracy when subjected to the aforementioned elevated temperatures and cooling methods. Copyright © 2014 John Wiley & Sons, Ltd.     

Bond strength evaluation of palm oil fuel ash-based geopolymer normal weight and lightweight concretes with steel reinforcement

This article presents a comparison of the bond behaviour between palm oil fuel ash (POFA)-derived geopolymer and conventional cement-based normal weight and lightweight concretes. A total of 16 variables were tested, which includes concrete cover (50 and 100 mm), bar diameter (12 and 16 mm) and types of concrete (POFA-based geopolymer normal/ lightweight concrete and cement-based normal/lightweight concrete). Results showed that the bond strength of cement-based concretes had higher critical bond stress and ultimate bond strength as well as lower slip at the ultimate bond strength compared to the corresponding POFA-based geopolymer concretes. The cement-based and geopolymer lightweight concrete specimens also exhibited greater bond strength than the normal weight concrete specimens. All of the concrete specimens generally exhibited similar bond stress-slip curves. Besides that, bond strength models proposed in the past predicted satisfactory match (difference of up to 35%) to the experimental ultimate bond strength values in the case of cement-based normal weight concrete and geopolymer concrete whereas a difference in the range of 16–138% was found for the case of lightweight concrete.     

Durability study on coal fly ash-blast furnace slag geopolymer concretes with bio-additives

The objective of this research was to understand the positive impact of bio-additives such as Terminalia chebula and natural sugars (molasses/palm jaggery/honey) on the durability properties of coal fly ash-blast furnace slag (BFS) based geopolymer concrete under various chemical attacks. Various tests had been conducted by immersing specimens in 5% sulfuric acid, 5% sodium sulfate and 5% sodium chloride solution for different duration of 7, 14, 28, 56 and 90 days to determine the resistance of bio-additives added geopolymer concrete against chemical attacks. The durability was also related by mercury intrusion porosimetry to find out the porosity and pore size distribution. After 90 days of immersion, test results confirmed that bio-additives inclusion in coal fly ash-blast furnace slag based geopolymer concrete had undergone weight loss and compressive strength loss in the range of 2.82–3.91%, 9.67–12.05% under sulfuric acid attack, 0.38–0.68%, 2.15–2.95% under sodium sulfate attack and 0.28–0.51%, 0.83–1.33% under sodium chloride attack respectively. However in control ordinary geopolymer specimen, the weight loss and compressive strength loss was 13.97%, 33.57% under sulfuric acid attack, 1.64%, 6.45% under sodium sulfate attack and 0.86%, 2.05% under sodium chloride attack was observed. This led to the conclusion that bio-additives added geopolymer concretes had enhanced durability properties when compared to control ordinary geopolymer concrete examined in this study.     

Factors affecting the suitability of fly ash as source material for geopolymers

The suitability of fly ash stock piles for geopolymer manufacturing was studied. The results of chemical analyses, X-ray diffraction (XRD) and particle size distribution (PSD) of five sources of fly ash obtained from coal-fired power generating plants in the US are presented. Geopolymer paste and concrete specimens were prepared from each stock pile. The specimens were subjected to an array of chemical and mechanical tests including XRD, RAMAN spectroscopy, setting time and compressive strength. A correlation study was undertaken comparing the fly ash precursor chemical and crystallographic compositions as well as particle size distribution, with the mechanical and chemical characteristics of the resulting geopolymer. Factors inherent to the fly ash stockpile such as particle size distribution, degree of vitrification and location of the glass diffraction maximum were found to play an important role in the fresh and hardened properties of the resulting geopolymer.     

Assessment of recycling use of GFRP powder as replacement of fly ash in geopolymer paste and concrete at ambient and high temperatures

Environmental issues caused by glass fiber reinforced polymer (GFRP) waste have attracted much attention. The development of cost-effective recycling and reuse methods for GFRP composite wastes is therefore essential. In this study, the formulation of the GFRP waste powder replacement was set at 20–40 wt%. The geopolymer was formed by mixing GFRP powder, fly ash (FA), steel slag (SS) and ordinary Portland cement (OPC) with a sodium-based alkali activator. The effects of GFRP powder content, activator concentration, liquid to solid (L/S) ratio, and activator solution modulus on the physico-mechanical properties of geopolymer mixtures were identified. Based on the 28-day compressive strength, the optimal combination of the geopolymer mixture was determined to be 30 wt% GFRP powder content, an activator concentration of 85%, L/S of 0.65, and an activator solution modulus of 1.3. The ratios of compressive strength to flexural strength of the GFRP powder/FA-based geopolymers were considerably lower than those of the FA/steel slag-based geopolymers, which indicates that the incorporation of GFRP powder improved the geopolymer brittleness. The incorporation of 30% GFRP powder in geopolymer concrete to replace FA can enhance the compressive and flexural strengths of geopolymer concrete by 28%. After exposure to 600 °C, the flexural strength loss for geopolymer concretes containing 30 wt% GFRP powder was less than that of specimens without GFRP powder. After exposure to 900 °C, the compressive strength and flexural strength losses of geopolymer concretes containing 30 wt% GFRP powder were similar to those of specimens without GFRP powder. The developed GFRP powder/FA-based geopolymers exhibited comparable or superior physico-mechanical properties to those of the FA-based geopolymers, and thus offer a high application potential as building construction material.     

Properties of lightweight expanded polystyrene aggregate concretes containing fly ash

Lightweight concretes can be produced by replacing the normal aggregates in concrete or mortar either partially or fully, depending upon the requirements of density and strength levels. The present study covers the use of expanded polystyrene (EPS) beads as lightweight aggregate, both in concrete and mortar. The main aim of this programme is to study the mechanical properties of EPS concretes containing fly ash and compare the results with these in literature on concretes containing OPC alone as the binder. The effects of EPS aggregate on the green and hardened state characteristics of concretes containing fly ash were evaluated. The compressive strength of the EPS concretes containing fly ash show a continuous gain even up to 90 days, unlike that reported for OPC in literature. It was also found that the failure of these concretes both in compression and split tension was gradual as was observed earlier for the concretes containing plastic shredded aggregates. The stress-strain relations and the corresponding elastic modulus were also investigated.     

Effect of supplementary cementitious materials on the compressive strength and durability of short-term cured concrete

This research focuses on studying the effect different supplementary cementitious materials (silica fume, fly ash, slag, and their combinations) on strength and durability of concrete cured for a short period of time—14 days. This work primarily deals with the characteristics of these materials, including strength, durability, and resistance to wet and dry and freeze and thaw environments. Over 16 mixes were made and compared to the control mix. Each of these mixes was either differing in the percentages of the additives or was combinations of two or more additives. All specimens were moist cured for 14 days before testing or subjected to environmental exposure. The freeze-thaw and wet-dry specimens were also compared to the control mix.Results show that at 14 days of curing, the use of supplementary cementitious materials reduced both strength and freeze-thaw durability of concrete. The combination of 10% silica fume, 25% slag, and 15% fly ash produced high strength and high resistance to freeze-thaw and wet-dry exposures as compared to other mixes. This study showed that it is imperative to cure the concrete for an extended period of time, especially those with fly ash and slag, to obtain good strength and durability. Literature review on the use of different supplementary cementitious materials in concrete to enhance strength and durability was also reported.     

Suitability of Sarawak and Gladstone fly ash to produce geopolymers: A physical,chemical, mechanical,mineralogical and microstructural analysis

Two types of fly ash sourced from Sarawak, Malaysia and Gladstone, Australia reflect differences in chemical compositions, mineral phase and particle size distributions. In this paper, the Sarawak fly ash was used to produce geopolymer in comparison to the well-developed Gladstone fly ash-based geopolymer. Characteristics of fly ash and mixtures proportions affecting compressive strength of the geopolymers were investigated. It is found that the variations of both fly ash types on particle size distributions, chemical compositions, morphology properties and amorphous phase correspond to the compressive strength. The results obtained show that after 7 days, geopolymer using Sarawak fly ash has lower compressive strength of about 55 MPa than geopolymer using Gladstone fly ash with strength of about 62 MPa. In comparison with Gladstone fly ash-based geopolymer, it showed that Sarawak fly ash-based geopolymer can be a potential construction material. Moreover, the production of Sarawak fly ash-based geopolymer aids to widen the application of Sarawak fly ash from being treated as industrial waste consequently discharging into the ash pond.     

Combined effects of mineral admixtures and curing conditions on the sorptivity coefficient of concrete

The effect of mineral admixture and curing condition on the sorptivity of concrete are investigated. In the present work, the maximum particle size and the grading of coarse aggregate, the cement content and water/cement ratio of the concrete are kept constant. Then, in the ordinary Portland cement (OPC) 42.5 concrete, a portion of the sand is replaced by a mineral admixture such as fly ash (FA), limestone filler, sandstone filler or silica fume (SF). This paper presents the results of both the sorptivity coefficient and the compressive strength of OPC 42.5 concretes with these mineral admixtures, and concretes with OPC 32.5, blended cement (BC) or trass cement (TC). The results obtained indicate that the sorptivity coefficient of concrete decreases as the compressive strength of concrete increases. It is also shown that the sorptivity coefficient of concrete is very sensitive to the curing condition. The effect of curing condition on the sorptivity coefficient of concrete seems to be higher in low-strength concretes.     

Interaction between sulfate and chloride solution attack of concretes with and without fly ash

In this paper, the two sets of concretes under attack of erosion solution of sulfate and chloride salt were investigated. The one set is the plain concrete without fly ash addition. The other set is the concrete with 20% and 30% of fly ash addition, respectively. The corrosion solution includes three types: 3.5%NaCl, 5% Na₂SO₄, and a composite solution of 3.5%NaCl and 5% Na₂SO₄. In addition, two corrosion regimes were employed in this study: naturally immersion (stored in corrosion solution for long duration), drying-immersion cycles. The damage process of the two sets of concretes was systematically investigated under the above three types of corrosion solutions and two corrosion regimes. The interaction between sulfate and chloride salt was also quantitatively determined. The experimental results shown that a presence of sulfate in the composite solution increased the resistance to chloride ingress into concretes at early exposure period, but the opposition was observed at latter exposure period. For the damage of concretes, a presence of chloride in the composite solution reduces the damage of concrete caused by sulfate. Addition of fly ash may significantly improve the resistance to chloride ingress into concretes and the resistance to sulfate erosion when a suitable amount of fly ash addition and low water-to-binder (W/B) was employed. Studies of the different corrosion regimes indicate that concretes stored in corrosion solution for about 850 d, the changes in relatively dynamic modulus of elastically (RDME) could be described by three stages: linearly increasing period, steady period, and declining period. Whereas for drying-immersion cycles, an accelerated trend could be found. The changes in RDME included an accelerated decreased stage, linearly increased stage, and then a slowly decreased stage, finally accelerating failure stage. In order to elucidate the above experimental results in a microscopic scale, the mechanism was also investigated by the modern microanalysis techniques.     

Effect of transient creep on compressive strength of geopolymer concrete for elevated temperature exposure

The strength and transient creep of geopolymer and ordinary Portland cement (OPC)-based material (paste and concrete) were compared at elevated temperatures up to 550 °C. The strength properties were determined using an unstressed hot strength test and unstressed residual strength test for paste and concrete, respectively. At 550 °C, compared with the original strength, the strength of geopolymer was increased by 192% while the strength of OPC paste showed little change. However, after exposure to 550 °C, the residual strength percentage of both geopolymer and OPC concretes was similar. Transient creep data show that geopolymer had little change in transitional thermal creep (TTc) between 250 and 550 °C while OPC paste developed significant TTc in this temperature range. In comparison with OPC concrete, a higher strength loss of geopolymer concrete is thus believed to be due to the absence of TTc to accommodate nonuniform deformation during thermal exposure.     

Performance evaluation of basalt fiber-reinforced geopolymer composites with various contents of nano CaCO3

High carbon footprint of cement production is the major drawback of plain cement concrete resulting in environmental pollution. Geopolymer composites paste can be effectively used as an alternative to Portland cement in the construction industry for a sustainable environment. The demand for high-performance composites and sustainable construction is increasing day by day. Therefore, the present experimental program has endeavored to investigate the mechanical performance of basalt fiber-reinforced fly ash-based geopolymer pastes with various contents of nano CaCO₃. The content of basalt fibers was fixed at 2% by weight for all specimens while the studied contents of nano CaCO₃ were 0%, 1%, 2%, and 3%, respectively. The compressive strength, compressive stress-strain response, flexural strength, bending stress-strain response, elastic modulus, toughness modulus, toughness indices, fracture toughness, impact strength, hardness, and microstructural analysis of all four geopolymer composite pastes with varying contents of nano CaCO₃ using scanning electron microscopy (SEM) were evaluated. The results revealed that the use of 3% nano CaCO₃ in basalt fiber-reinforced geopolymer paste presented the highest values of compressive strength and hardness while the use of 2% nano CaCO₃ showed the highest values of flexural strength, impact strength, and fracture toughness of composite paste. The SEM results indicated that the addition of nano CaCO₃ improved the microstructure and provided a denser geopolymer paste by refining the interfacial zones and accelerating the geopolymerization reaction.     

Studies using Al MAS NMR of AFm and AFt phases and the formation of Friedel's salt

This paper describes the application of the magic angle spinning (MAS) NMR spectroscopy to study the chemical environment of ²⁷Al-bearing phases in Portland cement-based concrete. A specific methodology is described that allows reliable spectra to be determined for combinations of different types of cements and fillers (in this case, Portland cement, fly ash, slag, silica fume, metakaolin and limestone filler). As well as the study of ‘molecular structure’ of cement matrix, the paper reviews the mechanism of Friedel's salt formation in cement systems. Mechanisms based on ion exchange of chloride for hydroxide in hydroxy-AF_m and on chloride absorption on formation are discussed. Finally, the nature of the chloride/hydrate binding phenomena are described to provide a reasonable robust and fundamental picture of the role different cements can play in the provision of overall concrete durability to chloride ingress from a chemical perspective.     

Cement pastes alteration by liquid manure organic acids: chemical and mineralogical characterization

Liquid manure, stored in silos often made of concrete, contains volatile fatty acids (VFAs) that are chemically very aggressive for the cementitious matrix. Among common cements, blast-furnace slag cements are classically resistant to aggressive environments and particularly to acidic media. However, some standards impose the use of low C₃A content cements when constructing the liquid manure silos. Previous studies showed the poor performance of low-C₃A ordinary Portland cement (OPC). This article aims at clarifying this ambiguity by analyzing mechanisms of organic acid attack on cementitious materials and identifying the cement composition parameters influencing the durability of agricultural concrete. This study concentrated on three types of hardened cement pastes made with OPC, low-C₃A OPC and slag cement, which were immersed in a mixture of several organic acids simulating liquid manure. The chemical and mineralogical modifications were analyzed by electronic microprobe, XRD and BSE mode SEM observations. The attack by the organic acids on liquid manure may be compared with that of strong acids. The alteration translates into a lixiviation, and the organic acid anions have no specific effect since the calcium salts produced are soluble in water. The results show the better durability of slag cement paste and the necessity to limit the amount of CaO, to increase the amount of SiO₂ (i.e., reduction of the Ca/Si ratio of C-S-H is not sufficient) and to favor the presence of secondary elements in cement.     

Formation of ASR gel and the roles of C-S-H and portlandite

Experimental investigations of the reactions between silica, alkali hydroxide solution, and calcium hydroxide show that alkali-silicate-hydrate gel (A-S-H) comparable to that formed by the alkali-silica reaction (ASR) in concrete does not form when portlandite or the Ca-rich, Si-poor C-S-H of ordinary portland cement (OPC) paste is available to react with the silica. Under these conditions, we observe either the formation of additional C-S-H by reaction of Ca(OH)₂ with the dissolving silica or the progressive polymerization of C-S-H. The A-S-H dominated by Q³ polymerization forms only after portlandite has been consumed and the C-S-H polymerized. These conclusions are consistent with previously published results and indicate that the ASR gel of concrete forms only in chemical environments in which the pore solution is much lower in Ca and higher in Si than bulk pore solution of OPC paste. These results highlight the similarity between ASR and the pozzolanic reaction and are supported by data for mortar bar specimens.     

Corrosion resistance in activated fly ash mortars

The question of whether reinforcing steel can be protected with activated fly ash cement as effectively as with Portland cement is explored in this study. Corrosion potential (E_corr) and polarisation resistance (R_p) values for steel electrodes embedded in Portland cement mortar and two fly ash mortars, respectively activated with NaOH and waterglass+NaOH solutions, are monitored. Chloride-free activated fly ash mortars are found to passivate steel reinforcement as speedily and effectively as Portland cement mortars, giving no cause to fear that corrosion may limit the durability of reinforced concrete structures built with these new types of activated fly ash cement. The polarisation curves and the response to short-term anodic current pulses (galvanostatic pulse technique) obtained further corroborate the full and stable passivation of the steel by the concrete manufactured with these binders.