Alkali-activated Fly Ash Mortars Research Articles

Bond strength of alkali-activated flyash based masonry system for sustainable construction

Bond strength is a crucial factor that impacts the performance, structural reliability and stability of masonry constructions. This paper aims to examine the efficacy of various masonry unit and mortar combinations and their bond strength thereby, evaluating their adhesion performance. It experimentally analyzes two masonry unit types paired with two mortar combinations. One is the traditional clay brick and the other is an alkali activated flyash based brick. Alkali activated flyash bricks and mortars use flyash as a sole binder, activated with popular alkalis, thereby reducing carbon footprints compared to cement manufacturing. Two types of mortar used are conventional cement mortar and alkali activated flyash mortar. Bonded prisms were tested to determine the compressive, tensile, shear, and flexural bond strengths. The results revealed significant variations in bond strength across different combinations of masonry units and mortar. Notably, it was observed that alkali-activated bricks bonded with alkali-activated mortar exhibited higher bond strength, compared to conventional cement mortar. These findings provide valuable insights in assessing the compatibility between masonry units and mortar, highlighting the potential of this technology for sustainable construction practices.

Impact of long-term natural weathering on the properties of alkali-activated fly ash mortars modified with silica fume and sodium tripolyphosphate

As known, exposure of any material to outdoor natural weathering, which has varied changes in temperature, relative humidity, percentage of sunny/cloudy, precipitation, CO2 and polluted air may adversely affect its properties compared to its unexposed counterpart cured at the lab. This paper aims to evaluate the effect of one year of outdoor natural weathering exposure on the properties of neat alkali-activated fly ash (AAFA) mortars and those modified with silica fume (SF) and sodium tripolyphosphate (STPP). To increase the resistivity of AAFA mortars to weathering exposure, the FA was partially substituted with 5–20 wt% SF. To further improvement, a fixed ratio of 4% STPP was added to the neat FA mortar specimens and those containing SF. After initial curing, half of the specimens were cured inside the lab for one year (unexposed specimens), whilst the other half were exposed to outdoor natural weathering conditions for the same period. The variations in the compressive strength, mineral composition, hydration products and microstructure were compared. The results showed that exposing specimens to natural weathering has a negative effect on their compressive strength. This negative effect can be mitigated by introducing SF up to 15%. To further reduce this negative effect, 4% STPP can be added.

Effect of superabsorbent polymer and polypropylene fiber on mechanical performances of alkali-activated high-calcium fly ash mortar under ambient and elevated temperatures

This study investigated the mechanical properties of alkali-activated fly ash (AAF) mortars containing superabsorbent polymers (SAPs) and polypropylene (PP) fibers before and after exposure to elevated temperatures. AAF mortars were synthesized using sodium silicate and high-calcium fly ash as the solid activator and aluminosilicate source, respectively. This procedure classified the binder as a one-part AAF mortar. The mechanical properties of the AAF mortar, including compressive and flexural strengths, were investigated considering eight mixes with varying SAP contents (0%, 0.5%, 1%, and 2% by weight of binder) and fiber contents (0 and 0.2% by volume). Moreover, physical properties including the permeability and sorptivity were examined, and physiochemical analysis was performed to evaluate the behavior of AAF mortar at room temperature, 300, 500, and 700 °C. The results show that the compressive strength of specimens at room temperature slightly decreased owing to the increased macroporosity caused by SAPs through pre-swelling, while the flexural strength was maintained via an internal curing effect. The PP fibers significantly enhanced the flexural capacity of the cement mortar, while the compressive strength initially decreased at 28 d and later exhibited a negligible effect at 90 d. The specimens produced using SAPs were effective in preserving the residual compressive and flexural strengths compared with those without SAPs after exposure to 300 °C. Meanwhile, the residual strength of PP fiber specimens diminished after elevated temperature exposure because the melted fiber led to the excessive formation of cavities.

Microstructure and macro properties of sustainable alkali-activated fly ash mortar with various construction waste fines as binder replacement up to 100%

Utilizing construction waste fines as eco-friendly binder for sustainable alkali-activated materials provides new approach to recycling construction waste. This work investigated the characterization of alkali-activated fly ash mortar (AAM) incorporating construction waste fines at varied types and replacement levels. The results showed that incorporating waste paste powder containing abundant hydrated products refined the pore size of AAM, but mixing waste mortar powder and waste concrete powder that contained massive inert quartz and calcite adversely affected the alkali-activated reaction and pore structure of AAM. The incorporation of construction waste fines was found to reduce the fluidity but increased the drying shrinkage of AAM. At identical replacement level of construction waste fines, the waste paste powder incorporated AMM has lower fluidity and greater drying shrinkage than the waste mortar powder or waste concrete powder incorporated AAM. Substituting waste paste powder for fly ash improved the mechanical strength of AAM, but the strength decreased as waste mortar powder or waste concrete powder was incorporated. Particularly, the AAM blended with 100% construction waste fines still owned good strength. Mixing construction waste powder reduced the chloride diffusion coefficient and chloride ingress depth of AAM, and the waste paste powder mixed AAM had better chloride resistance than the waste mortar powder or waste concrete powder mixed AAM. Therefore, the hydrated products in construction waste fines was revealed to benefit the micro-characteristics of alkali-activated materials, and optimizing the type and replacement level of construction waste fines can achieve sustainable AAM with good mechanical strength and chloride ingress resistance.

Effect of reactive magnesium oxide in alkali-activated fly ash mortars exposed to accelerated CO2 curing

The effect of reactive magnesium oxide (MgO) as partial fly ash replacement in alkali-activated fly ash mortars (0, 10 and 20 %) was studied under two different environments: conventional climatic chamber and climatic carbonation chamber with 0.04 and 5 % level of CO2 respectively. A complete characterisation of all raw materials was performed. The effect of replacing fly ash with MgO resulted in a decline of compressive (up to 52.3 % at 28 days of age and 20% of MgO replacement) and flexural strength, ultrasonic pulse velocity and dynamic Young’s modulus mainly due to the formation of weak and expansive brucite, detected by X-ray diffraction (XRD). Carbonation depth and shrinkage also were obtained. CO2 curing improved the physicomechanical properties, due to the formation of nesquehonite in samples with MgO. An improvement of CO2 absorption of 2 g CO2/kg mixes with the CO2 curing and the substitution of fly ash with MgO was obtained (determined by thermogravimetric and differential thermal analysis). The increase in mass due to CO2 capture was of 6.91%, 9.39% and 12.26% at the age of 7, 14 and 28 days, respectively, when compared to mixes cured with 0.04 (reference) and 5 % (CO2 curing) level of CO2.

Characterization and Comparison of Corrosion Layer Microstructure between Cement Mortar and Alkali-Activated Fly Ash/Slag Mortar Exposed to Sulfuric Acid and Acetic Acid.

In this study, we investigated the formation and evolution of the corrosion layers in alkali-activated mortar and ordinary Portland cement mortar exposed to sulfuric acid and acetic acid environments with different pH values, and explored the differences in the deterioration mechanisms. The experimental results indicated that ordinary Portland cement (OPC) mortars experienced more severe deterioration in terms of appearance, mass loss, and strength loss as compared with alkali-activated mortars exposed to an acetic acid environment, but their neutralization depths were smaller. Alkali-activated fly ash (AAF) mortar had a the relatively intact appearance but the greatest neutralization depth, which was due to its stable three-dimensional network but highly porous structure. To sum up, alkali-activated fly ash/slag (AFS) mortar had the best resistance to acid attack. In addition, the mortars exposed to acetic acid suffered greater deterioration than those exposed to sulfuric acid with the same pH values, which was mainly due to the highly porous corrosion layer formed in acetic acid, whereas crystallization of gypsum in sulfuric acid had a pore filling effect. However, for alkali-activated slag (AAS) and OPC mortars exposed to a sulfuric acid environment, extensive gypsum resulted in the formation of micro-cracks, and the corrosion layer of OPC mortar was more prone to fall off. OPC mortar also had the greatest resistance difference values of the continuously connected micro-pores before and after acid corrosion, followed by AAS, AAF, and AFS mortars, and these values for all the specimens were smaller in sulfuric acid. Furthermore, the gaps between acetic and sulfuric acid attacks increased with increased calcium content in binders, which were 7%, 13%, 21%, and 29% for AAF, AFS, AAS, and OPC mortars, respectively. Thus, it can be inferred that an appropriate amount of gypsum existed in the corrosion layer which could act as a barrier to some extent ina sulfuric acid environment.

Resistance of blended alkali-activated fly ash-OPC mortar to mild-concentration sulfuric and acetic acid attack.

The traditional cementitious product is prone to suffer from a high degree of deterioration in the case of exposure to acid solutions because of the decomposition of the binder network. However, the degradation of concrete structures in service by mild concentrations of acid under conditions involving sewage, industrial waters, and acid rain is more common and results in a significant environmental problem. The utilization of alkali-activated materials has been seen to potentially offer an attractive option with regard to acceptable durability and a low carbon footprint. With the aid of visual observation, mass loss, compressive strength tests, X-ray diffraction, Fourier transform infrared spectroscopy, and field-emission scanning electron microscopy with energy-dispersive X-ray spectroscopy, the acid resistance of alkali-activated fly ash mortars in which the precursor was partially replaced (0-30% by mass proportion) with ordinary Portland cement (OPC) was evaluated after 180 days of exposure to mild-concentration sulfuric and acetic solutions (pH = 3). A conventional cement mortar (100% OPC) was used as a reference group. The results demonstrate that the addition of OPC into the alkali-activated system causes a significant increase in compressive strength (around 16.08-36.61%) while showing an opposite influence on durability after acid attack. Based on a linear mean value and nonlinear artificial neural network model simulation, the mass losses of the specimens were evaluated, and the alkali-activated pure-fly ash mortar demonstrated the lowest value (i.e., a maximum of 5.61%) together with the best behavior in the aspect of discreteness at 180 days. The results from microstructure analysis show that the coexistence of the N-A-S-H and C-S-H networks in the blend system occurred by both OPC hydration and FA. However, the formation of the gypsum deposition within the fly ash-OPC blend systems at sulfuric acid was found to impose internal disintegrating stresses, causing a significant area of delamination and cracks. In addition, alkali metal ion leaching, dealumination, as well as the disappearance of some crystalline phases occurred in specimens immersed in both types of acids.

Alkali-Activated Mortars Modified by Epoxy-Carbon Fiber Composites Wastes

Short chopped fibers coated by epoxy resin of different length (5 to 10 mm length) were added at low volume content (about 4.6% on the composite) to alkali-activated fly ash or metakaolin mortars. These uncured scraps derive from the production of carbon fiber-reinforced polymer composites and they are not presently recycled, despite their outstanding mechanical properties. The workability, microstructure, porosity, and physical and mechanical properties (mainly flexural strength) of the derived materials were investigated. Superior flexural strength and increased toughness were obtained. An acid treatment of the scraps further improved the mechanical properties of the mortars by changing the chemical structure of the surface, thus increasing the interaction with the inorganic phase. These results foster the use of these wastes to improve the performance of low carbon footprint building materials such as alkali-activated composites in the building industry.

Research on the Micro-Pore Structures of AAFAM

The pore structure of alkali-activated fly ash mortar (AAFAM) under different mix ratios and curing system was measured by mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). Based on the fractal theory, the integral shape dimension of the pore surface of the AAFAM is obtained and discussed its relationship with total porosity, proportion of gel pores, transition pore, capillary pores and macropores, average pore diameter, median pore diameter, pore surface area, pore size distribution, and the influence of mass ratio of alkali activators, curing temperature, and curing age on pore structures. In addition, data regression analysis (DRA) is a statistical tool and applied to study the relationship between microscopic pore parameters and the fractal dimension of the AAFAM mixtures. The results show that the fractal dimension of the AAFAM is between 2.6 and 2.9, and the pore size distribution has a linear relationship with the fractal dimension, and the internal pore diameter of the AAFAM will gradually decrease and AAFAM’s matrix gradually becomes denser and the microstructure integrity is more stable as the curing age and temperature increases, and the mechanical strength will be higher. The AAFAM mixture composed of Na2SiO3/NAOH with a mass ratio of 1.0 and 100% fly ash is considered the most suitable mixture. AAFAM can make full use of industrial by-products (fly ash) and seems to be a suitable candidate for green environmental protection materials for engineering applications.

Mechanical, Pore Structure and Microstructural Properties of Alkali-Activated Fly Ash Mortars Containing Silica Aerogel

Silika aerojeller, çimento matrisine dâhil edildiklerinde yüksek poroziteleri aracılığıyla çimento esaslı malzemelerin akustik ve termal yalıtım performanslarına katkı sunabilmektedirler. Silika aerojel partikülleri, yüksek porozitelerine rağmen mekanik zorlamalar altında ihmal edilemeyecek düzeyde mekanik özellikler de gösterebilmektedirler. Bu çalışmada çimento ağırlığınca düşük katkı oranlarında (%0.25 ve %0.50) çimento matrisine dâhil edilen silika aerojellerin alkali – aktive edilmiş uçucu kül harçlarının mekanik, por yapısı ve mikro yapı özelliklerine olan etkisi araştırılmıştır. Deneysel çalışmadan elde edilen sonuçlar, ortalama 20 MPa basınç dayanımına sahip uçucu kül harçlarının basınç etkisi altındaki tokluk ve pik – sonrası tokluk kapasitelerinin sırasıyla yaklaşık %27 ve %70 düzeylerinde artabileceğini ortaya koymaktadır. Harç numunelerinin por yapısında mikro ve kapiler boyuttaki por çaplarının değişimine bağlı olarak elde edilen tokluk kazancı, gelecekte eşdeğer basınç dayanımları altında deformasyon davranışı geliştirilmiş silika aerojel katkılı blok elemanların üretimi için yenilikçi bir tasarım fikrini ortaya koymaktadır.

Influence of the CaO/SiO2 molar ratio on the technological properties of alkali-activated fly ash mortar with quicklime

This study aimed to investigate the influence of the CaO/SiO2 molar ratio on the physical, chemical and mechanical properties of alkali-activated fly ash with quicklime and assess the technical viability of the mortar with the highest compressive strength. Eight mortars were prepared, with a fixed sand to combined activator/precursor ratio of 3:1, varying the CaO/SiO2 ratio from 0.25 to 2.00 every 0.25. The doses that resulted in the highest and lowest compressive strength were CaO/SiO2 = 0.75 and 2.00, respectively, which reached 17.9 and 3.8 MPa at 28 days. Analytical techniques were performed to identify the products formed and the morphological differences in order to justify the variation in mechanical strength between mortars. EDS/SEM showed the formation of C-S-H, C-A-S-H and CH as well as other compounds in the mortars. XRD demonstrated greater CH formation in the mortars with the highest CaO/SiO2 molar ratio and CaCO3 in all the mortars, indicating that all the samples underwent carbonation. The TG/DTG results confirmed the presence of C-S-H, CH and CaCO3 phases in the mortars. Mercury intrusion porosimetry showed a significant difference in porosity between mortars with the highest (15.4%) and lowest (4.6%) compressive strength, confirming that this parameter also justifies mechanical strength differences.

Experimental Study on the Flexural Behavior of Alkali Activated Fly Ash Mortar Beams

This work aims to study the possibility of using alkaline activated fly ash in structural members. The work, of an experimental nature, focuses on the evaluation of the behavior of simply supported beams under two symmetrical loads (four-point tests). For such study, 10 beams were built, of which, five using fly ash and the remaining five using traditional Portland cement. The test results are compared. Conclusions on the practical application of fly ash in structures were explained and, as mention later in this document, there is room for improvement. This is one of very few works on fly ash alkali activated structures and further studies are necessary in the future. Some aspects, such as shrinkage and deformability are presented as some of the negative points concerning the potential use of fly ash. These are two aspects that need more attention in future investigations.

Curing Temperatures on the Compressive Strength and Drying Shrinkage of Alkali-Activated Fly Ash Mortars

This study investigated the effect of curing temperature on the compressive strength and drying shrinkage of alkali-activated fly ash (AAFA) mortars. Class F fly ash was used as the raw material, and sodium oxide and liquid sodium silicate were used as alkaline activators. AAFA mortars with an alkaline-solution-to-binder ratio of 0.5 were prepared under four different initial curing conditions. Test results showed that the curing temperature appreciably influenced the compressive strength development and drying shrinkage of AAFA mortars at early ages. A higher temperature led to more effective alkali activation of fly ash. Based on the results, AAFA mortars cured at 65 °C for 12 h had superior mechanical properties.

Crack initiation of selected geopolymer mortars with hemp fibers

The aim of this paper is to quantify particularly fracture properties of selected types of mortars prepared with an alkali activated binder and hemp fibers. The main attention is focused on evaluation of three-point bending fracture tests of prismatic specimens with an initial central edge notch made of alkali activated fly ash mortars. The load versus crack mouth opening displacement (F–CMOD) diagrams were recorded during the fracture tests and subsequently evaluated using the Double-K fracture model. This model allows the quantification of two different levels of crack propagation: initiation, which corresponds to the beginning of stable crack growth, and the level of unstable crack propagation. The course of fracture tests was also monitored by acoustic emission method.

Leaching, carbonation and chloride ingress in reinforced alkali-activated fly ash mortars

Alkali-activated fly ash mortars were studied with regard to durability-relevant transport coefficients and the electrochemical behaviour of embedded carbon steel bars on exposure of the mortars to leaching, carbonation and chloride penetration environments. The transport coefficients differed considerably between different formulations, being lowest for a mortar with BFS addition, but still acceptable for one of the purely fly ash-based mortars. Leaching over a period of ~300 days in de-ionized water did not lead to observable corrosion of the embedded steel, as shown by the electrochemical data and visual inspection of the steel. Exposure to 100 % CO2 atmosphere caused steel depassivation within approx. two weeks; in addition, indications of a deterioration of the mortar were observed. The results are discussed in the context of the different reaction products expected in highand low-Ca alkali-activated binders, and the alterations caused by leaching and carbonation.

Strength development and durability of alkali-activated fly ash mortar with calcium carbide residue as additive

The strength development and durability of alkali-activated fly ash (FA) mortar with calcium carbide residue (CCR) as additive cured at ambient temperature were investigated in this paper. CCR was used to partially replace FA as additional calcium in the alkali-activated binder system by a weight percentage of 0%, 10%, 20% and 30%. Sodium hydroxide and sodium silicate solutions were used as liquid alkaline activation in all mixtures. Test results show that the incorporation of CCR has an effect on the strength development of alkali-activated FA mortar with CCR. The setting time of alkali-activated FA mortar with CCR has decreased whereas its strength development has increased. This is further confirmed by XRD, SEM, and FTIR analyses, which show that the reaction products were increased when the alkali-activated FA incorporated with CCR. The highest 28-day compressive strength of alkali-activated FA mortar was found in the mix of 70% FA and 30% CCR, which is about 40.0 MPa. In addition, the resistances of alkali-activated FA mortar incorporated with CCR to tap water, 5% H2SO4 solution, and 5% MgSO4 solution are found to be superior to those of alkali-activated FA mortar without CCR as indicated by the relatively low strength loss. For the samples immersed in 5% H2SO4 solution and 5% MgSO4 solution for 120 days, the alkali-activated FA incorporated with 30% CCR showed a low strength reduction of around 71% and 53%, respectively.

Influence of duration of heat curing and extra rest period after heat curing on the strength and transport characteristic of alkali activated class F fly ash geopolymer mortar

In the study, the influence of heat curing duration and rest period after heat curing duration on the strength and transport characteristics of alkali activated fly ash (AAFA) mortar were investigated. A local class F type fly ash, CEN reference sand, sodium hydroxide and potable water were used in preparation of cement-less fresh geopolymer mortar mixture. Mixture ratios (in mass basis) were 3, 1, 0.29, and 0.1 for sand, fly ash, water and sodium hydroxide, respectively. Some samples were cured at 75°C temperature for 4h, 1, 2, 3 and 7days; then, they were tested after heat curing period. Additionally, some of the equivalent samples were kept in the laboratory at 23±2°C temperature with 50±5% relative humidity, until 28days after their initial heat curing period. Then, they were tested at 28days for combined curing conditions. The measured properties of AAFA mortar were unit weight, porosity, capillary water absorption, water absorption capacity, flexural-bending and split tension strength, compressive strengths and abrasion resistance. It was observed that the strengths of mortar were significantly increased with the increase in heat curing duration. A significant strength development was also observed after rest period. As a result of the study, high strength and abrasion resistant AAFA geopolymer mortar was produced. However, transport properties of AAFA geopolymer mortar found to be not as good as mechanical properties.

Fresh properties and compressive strength of high calcium alkali activated fly ash mortar

This paper reports the fresh properties and compressive strength of high calcium alkali-activated fly ash (AAFA) mortar. Two different sources of class C fly ash, with different chemical compositions were used to prepare alkali-activated mortar mixtures. Four different sodium silicate to sodium hydroxide (SS/SH) ratios of 0.5, 1.0, 1.5, and 2.5 were used as alkaline activators with a constant sodium hydroxide concentration of 10M. Two curing regimes were also applied, oven curing at 70°C for 24h, and ambient curing at 23±2°C. The rest time, i.e., the time between casting the mortar cubes and starting the oven curing was 2h. The results revealed that the setting time, and workability of mortar decreased with increasing the alkali to fly ash ratio, and decreasing the water to fly ash ratio. The optimum sodium silicate to sodium hydroxide ratio was 1.0, which showed the highest compressive strength and setting time. An increase of sodium silicate to sodium hydroxide ratio to 2.5 led to a significant reduction in the setting time, and workability of mortar. The 7-day compressive strength of the mortar approached 20.80MPa for ambient cured regime and 41.10 for oven cured regime.

The effects of the sequential addition of synthesis parameters on the performance of alkali activated fly ash mortar

Geopolymer is an energy efficient and sustainable material that is currently used in construction industry as an alternative for Portland cement. As a new material, specific mix design method is essential and efforts have been made to develop a mix design procedure with the main focus on achieving better compressive strength and economy. In this paper, a sequential addition of synthesis parameters such as fly ash-sand, alkaline liquids, plasticizer and additional water at well-defined time intervals was investigated. A total of 4 mix procedures were used to study the compressive performance on fly ash-based geopolymer mortar and the results of each method were analyzed and discussed. Experimental results show that the sequential addition of sodium hydroxide (NaOH), sodium silicate (Na2SiO3), plasticizer (PL), followed by adding water (WA) increases considerably the compressive strengths of the geopolymer-based mortar. These results clearly demonstrate the high significant influence of sequential addition of synthesis parameters on geopolymer materials compressive properties, and also provide a new mixing method for the preparation of geopolymer paste, mortar and concrete.

Corrosion behaviour of hybrid sol-gel coated steel embedded in carbonated Portland and fly ash mortars contaminated with chlorides

The protection of metals from their surrounding environment is usually achieved by deposition of protective coatings on the metal surface to establish a physical barrier against aggressive ions. The corrosion behaviour of hybrid organic–inorganic coatings for carbon steel embedded in carbonated ordinary Portland cement (OPC) and alkali activated fly ash (AAFA) mortars was studied, after different periods of immersion in a 3 wt.% sodium chloride solution using electrochemical techniques. Hybrid organic–inorganic films employed as protective coatings have been prepared via sol–gel using several silicon compounds: 3-methacryloxy-propyltrimethoxysilane or methyltriethoxysilane (MTES) and tetraethyl orthosilicate (TEOS) or tetramethyl orthosilicate (TMOS) precursors using a molar ratio of 1 and deposited on carbon steel substrates by dip-coating. The corrosion potential (Ecorr) values were higher and the current density (icorr) values were lower for AAFA mortars, indicating that the coatings were more efficient against rebar corrosion in this system than in OPC mortars. On the other hand, the hybrids synthesised with TEOS/MTES and TMOS/MTES for carbon steel embedded in carbonated AAFA mortars showed the lowest icorr values. This enhanced protection could be due to a structure denser and more compact and greater adhesion to the metallic surface.