Alkali-activated Mortars Research Articles

Effect of incorporating municipal solid waste incinerated bottom ash in alkali-activated fly ash concrete subjected to accelerated CO2 curing

The effect of incorporating bottom ash from municipal solid waste incineration (MIBA) as partial replacement for fly ash (FA) (0%, 15% and 30%) was studied on alkali-activated mortars under two curing environments: conventional climatic chamber (∼0.04% CO2) and carbonation chamber (5% CO2). The microstructural characterization showed that the use of MIBA resulted in a decrease of calcium silicate hydrate gel (C–S–H) and sodium aluminosilicate hydrate (N-A-S-H) observed as “rounded growths” worsening the macrostructural behaviour (compressive/flexural strength, ultrasonic pulse velocity, dynamic Young’ modulus, dry bulk density, shrinkage, depth of carbonation and porosity accessible to water). The accelerated CO2 curing stage produced high amounts of aragonite, observed as “needle-like” formations improving the macrostructural behaviour. One tonne of mixture with MIBA and CO2 curing could decarbonise approximately 3695, 7391 and 10163 m3 of air volume at the age of 7, 14 and 28 days. The combined substitution of FA with MIBA and CO2 curing can be a highly viable technique in plain precast products.

Recycling air-cooled blast furnace slag in fiber reinforced alkali-activated mortar

Fiber reinforced alkali-activated materials (FR-AAM) present as one type of sustainable and resilient materials. However, the thermal degradation mechanism of FR-AAM remains unclear. In this study, FR-AAM incorporating air-cooled blast furnace slag (ACBF), ground granulated blast furnace slag (GGBS) and different types of fibers (steel, glass, and polypropylene) are produced and exposed to elevated temperatures. Test results show that ACBF (replacing 30% of river sand) improved the thermal resistance of FR-AAM due to the ameliorated interfacial transition zone (ITZ) and channels for the release of vapor pressure. Relatively, steel fibers better retain mechanical performance, whilst polypropylene fibers better provide channels for the release of vapor pressure after melting. Gel decomposition and micro crack development are the main causes for the thermal deterioration of FR-AAM. Based on non-destructive tests, damage degree is proposed to quantitatively evaluate the usability and deterioration coefficient (K) is adopted to controll the strength retention of FR-AAM at high temperatures. Economically and environmentally, the development of FR-AAM is promising in shaping a sustainable and resilient future.

Alkali-activated materials produced using high-calcium, high-carbon biomass ash

Eucalyptus ash (EA) was used in this study as a high calcium ash (HCA) precursor for alkali-activated binders. The EA used also has high carbon unburned (High loss on ignition). This type of ash is one of the waste products from biomass-fuelled thermoelectric plants, and annually thousands of tons are discarded as a by-product of the energy generation process in Brasil, but it is rich in unburnt carbon which means that it is challenging to use in cementitious systems. Eucalyptus is a biomass that removes CO2 from the atmosphere by photosynthesis and part of this carbon content remains in the ashes, generating CO2 capture when EA is incorporated in the production of alkali-activated binders. The objective of the present study was to evaluate the properties of the material obtained by the alkali-activation of the high-calcium high-carbon biomass ash to generate a cementitious binder, with different pastes proportions of EA and silica fume (SF), activated by sodium hydroxide. With the different pastes, mortars were produced using standardized sand. The results obtained from the pastes and mortars were satisfactory in several aspects. The mechanical results of the alkali-activated mortars were comparable those of Portland cement mortars. Mortars degraded methylene blue more efficiently in illuminated conditions, even after high adsorption for 24 h in the dark. The content of leached ions in the remaining solutions met potability standards.

Alkali-activated mortars: porosity and capillary absorption

Studies conducted on alkali-activated materials in late years attested that they could be a replacement to ordinary Portland cement (OPC). Nevertheless, their performances in aggressive conditions require profound investigations to evaluate the ability to the fluid transfer. This article assesses the pore structure of alkali-activated mortars produced using one-part metakaolin combined with ground granulated blast furnace slag and other-part using only alkali-activated ground granulated blast furnace slag. The pore characteristics of alkali-activated mortars were compared to those of OPC as a reference. Tests consist in the measurement of the porosity accessible to water, water absorption and sorptivity. The results revealed that the alkali-activated based metakaolin combined with ground granulated blast furnace slag have greater porosity, water absorption, sorptivity and rate of water saturated porosity than alkali-activated based ground granulated blast furnace slag and Portland cement mortars.

---Mechanical behavior of textile reinforced alkali-activated mortar based on fly ash, metakaolin and ladle furnace slag.

The need for repair and maintenance has become dominant in the European construction sector. This, combined with the urge to decrease CO 2 emissions, has resulted in the development of lower carbon footprint repair solutions such as textile reinforced mortars (TRM) based on alkali-activated materials (AAM). Life cycle studies indicate that AAM CO 2 savings, when compared to Portland cement, range from 80% to 30%. Furthermore, in this study, recycled aggregates were considered with the aim to promote a circular economy mindset. AAM mortars formulation based on fly ash, ladle furnace slag and metakaolin were tested for compressive and flexural strength. Three out of all formulations were chosen for an analysis on the potential of these mortars to be used for TRM applications. Tensile and shear bond tests, combined with a concrete substrate, were executed as indicators of the TRM effectiveness. Scanning electron microscopy and chemical analysis based on energy dispersive X-ray spectroscopy were used to interpret the results and reveal the reasons behind the different level of performance of these composites. Results indicated that TRM based on high calcium fly ash are unsuitable for structural strengthening applications due to low bond between matrix and/or substrate and fibers. Metakaolin-based TRM showed good performance both in terms of tensile strength and bond capacity, which suggests potential as a repair mortar.

Structure, morphology and compressive strength of Alkali-activated mortars containing waste bottle glass nanoparticles

Alkali-activated materials (AAMs) have emerged as a sustainable, clinker free binder alternative to portland cement, with reduced consumption of raw materials and very low CO2 emissions. They are produced via the activation of aluminosilicates with alkaline solutions. Millions of tons of bottle glass waste are generated annually worldwide and only a small amount are re-utilized and recycled. Various recycling processes need to be innovated to reduce the volume of waste glasses, which is often landfilled, causing environmental concerns. Waste glass has been utilized as fine aggregate in concrete or as partial replacement for cement. In this study, waste bottle glass nanoparticles (WBGNPs) were incorporated as a precursor combined with ground blast furnace slag (GBFS) and fly ash (FA) to achieve high-strength alkali-activated mortars (AAMs). The effect of WBGNPs, sodium hydroxide molarity, solution modulus (SiO2:Na2O), alkaline solution content, and binder-to-aggregate ratio on the compressive strength development of AAMs were investigated. The strength characteristics of the studied AAMs were assessed by developing an informational model united with a metaheuristic shuffled frog-leaping algorithm (SFLA). The experimental results indicated that after 28 days of curing, AAM prepared with 5% of WBGNPs (precursor) as GBFS/FA replacement attained an improved compressive strength in the range of 16 to 18.4%. Additionally, the optimum mixture containing 5% of WBGNPs as GBFS replacement achieved a strength of 68.6 MPa after 28 days of curing. It was shown that the novel informational SFLA model attained accurate predictions and could appreciably simplify generative design in future computational intelligence of a construction materials platform in civil engineering.

Self healing of alkali active mortars with expanded perlite aggregate

The use of alkali-activated mortars instead of cemented mortars provides an advantage in terms of various properties such as high strength, fast curing, and high durability by terminating cement production. However, concrete cracks reduce durability and strength, thus, it is crucial that they be healed. This study aimed to investigate the healing of early microcracks in geopolymer mortars by using Sporasarcina pasteurii, which makes carbonate precipitation, and expanded perlite aggregate as the bacteria carrier. This is the first study to investigate the bacterial healing of an alkali-activated geopolymer mortar using expanded perlite aggregate without applying a complex encapsulation process to the bacterial agents. For this purpose, optical, physical, and mechanical changes caused by self-healing were observed ahead of and after the cure period with tests like water absorption, compressive strength, UPV, SEM, FTIR, and optical microscope images. The results showed that bacterial cells could sporulate directly on the expanded perlite aggregate, visually improving the crack closure rate of the samples by a majority to 100 %, and the unit weights of all samples healed in DM and containing bacteria increased by up to 5 %.

Influence of Ca and Al source on elevated temperature behavior of waste ceramic sanitaryware-based alkali-activated mortars

Influence of Ca and Al source on elevated temperature behavior of waste ceramic sanitaryware-based alkali-activated mortars

Strength properties and microstructural characteristics of clay treated with alkali activated mortar and fiber

Chemical stabilization, along with strengthening and reinforcing the soil with the use of fibers are among the methods employed for soil improvement. In recent years, the application of alkali-activated mortars has received considerable attention due to less CO2 emissions along with no need to precure the pozzolans used to make the mortar. Application of fibers in addition to chemical stabilizers can be effective to increase the ductility and tensile strength of improved soils and delay the rupture process. Basalt fibers, which are of natural origin and have very high resistance in acidic, alkaline, and saline environments, are considered as suitable options for reinforcement of alkali-activated mortars. In the present study, the improvement of a type of clay has been examined using basalt fibers and the alkali-activated mortar. To investigate the strength parameters of the improved clay soil, a combination of basalt fibers and alkali-activated mortar (Clay improved with 3% wollastonite and 16% potassium hydroxide alkaline solution) was prepared and was then subjected to uniaxial compressive strength and indirect tensile strength tests after curing times of 7, 14, 28 and 90 days. Also, electron microscope images (SEM) and EDX analyses were performed on the samples to interpret the results. The results showed that in samples improved with alkali-activated mortars, the formation of calcium silicate hydrate(C-S-H) and calcium aluminosilicate hydrate (C-A-S-H) gels and their polymerization after the curing time increased the compressive and tensile strengths. In the samples improved with alkali-activated mortars reinforced with basalt fibers, the ductility of the samples also increased in addition to the increase in the compressive and tensile strengths as the gels formed were interlocked among the fibers.

Advanced Composites with Alkali-Activated Matrices for Strengthening of Concrete Structures: Review Study

Old and seismically prone buildings are in need of strengthening in order to comply with the latest building codes and to prolong their service life. For over two decades fiber-reinforced polymers (FRP) have been successfully used for this purpose. However, the poor performance in high temperatures of organic matrices has led researchers to investigate the use of inorganic matrices. Consequently, textile-reinforced mortars (TRM) have been opted for strengthening, since they incorporate textiles impregnated in inorganic cementitious matrices. Lately, in order to promote sustainability and lower the high carbon emissions of cement, alkali-activated mortars, also called geopolymers, have been investigated as an alternative. Their high performance and fireproof properties have made them excellent candidates as matrices in advanced composites for strengthening. This study aims to provide an overview of research in the field of advanced composites with alkali-activated matrices used for strengthening of concrete members. Systems implementing either fiber sheets or meshes have been used so far to strengthen reinforced concrete members, indicating promising results of the new advanced composite.

Reusing Construction and Demolition Waste to Prepare Alkali-Activated Cement.

Large amounts of waste are derived not only from construction processes, but also the demolition of existing buildings. Such waste occupies large volumes in landfills, which makes its final disposal difficult and expensive. Reusing this waste type is generally limited to being employed as filler material or recycled aggregate in concrete, which limits its valorisation. The present work proposes reusing construction and demolition waste to manufacture alkali-activated cement to improve its sustainability and recovery. Construction and demolition waste (C&DW) from a demolition waste collection plant in Valencia (Spain) was physically and chemically characterised. This residue contained large fractions of concrete, mortar, bricks, and other ceramic materials. X-ray fluorescence (XRF) analysis showed that its chemical composition was mainly CaO, SiO2 and Al2O3. X-ray diffraction (XRD) analysis revealed that it presented some crystalline products, and quartz (SiO2) and calcite (CaCO3) were the main components. Blends of C&DW and blast furnace slag (BFS) were alkali-activated with mixtures of sodium hydroxide and sodium silicate. The corresponding pastes were characterised by techniques such as thermogravimetry and scanning electron microscopy (SEM). The alkali-activated mortars were prepared, and the resulting mortars’ compressive strength was determined, which was as high as 58 MPa with the 50% C&DW-50% BFS mixture. This work concluded that it is possible to make new sustainable binders by the alkali activation of C&DW-BFS without using Portland cement.

Optimization of Curing Parameters of Class C Fly-Ash-Based Alkali-Activated Mortar

Optimization of Curing Parameters of Class C Fly-Ash-Based Alkali-Activated Mortar

Shrinkage behavior and mechanical properties of alkali activated mortar incorporating nanomaterials and polypropylene fiber

Blended ground granulated blast slag (GGBS), low-calcium fly ash (FA), nano silica (NS), and nano alumina (NA) with/without polypropylene fiber (PPF) had a significant effect on the development length of alkali activated mortar (AAM) cured at various humidity levels and curing ages. This paper presents the behavior of alkali activated mortar with 50% FA and 50% GGBFS binder materials and the alkali ratio of sodium silicate to sodium hydroxide (SS/SH) is 2.5. The molar concentration of sodium hydroxide (NaOH) used was 12 M. Feasibility Comparisons between the different humidity 60%–98% of at 23 ± 3 C° were examined. The strength behavior of alkali-activated mortar with different curing ages was also evaluated. Scanning electron microscopy (SEM) analysis and X-ray diffractions (XRD) were also conducted to clarify the effect of nanomaterials and PPF on the microstructures of AAM. It was found that the shrinkage values of AAM were decreased with the addition of polypropylene fiber and nanomaterials. The combined use of both nanomaterials had better performance than the use of nano SiO2 and/or nano Al2O3 alone. The combined use of PPF with nanomaterials had a superior reduction in shrinkage and expansion and increment on strength values; the minimum shrinkage, expansion values, and maximum strength were found at AAM mix incorporating 2%NS-1%NA-0.5%PPF. The SEM analysis and XRD evaluation indicates the significant effect of nanomaterials on the microstructures and bond strength of AAM. The microstructure of the mixes incorporating both nanomaterials and PPF was denser than other mixes without nanomaterials and/or PPF and showed lower micro cracks.

The Influence of Accelerated Carbonation on Physical and Mechanical Properties of Hemp-Fibre-Reinforced Alkali-Activated Fly Ash and Fly Ash/Slag Mortars.

The physical and mechanical properties of hemp-fibre-reinforced alkali-activated (AA) mortars under accelerated carbonation were evaluated. Two matrices of different physical and chemical properties, i.e., a low Ca-containing and less dense one with fly ash (FA) and a high Ca-containing and denser one with FA and granulated blast furnace slag (GBFS), were reinforced with fibres (10 mm, 0.5 vol% and 1.0 vol%). Under accelerated carbonation, due to the pore refinement resulting from alkali and alkaline earth salt precipitation, AA hemp fibre mortars markedly (20%) decreased their water absorption. FA-based hemp mortars increased significantly their compressive and flexural strength (40% and 34%, respectively), whereas in the denser FA/GBFS matrix (due to the hindered CO2 penetration, i.e., lower chemical reaction between CO2 and pore solution and gel products), only a slight variation (±10%) occurred. Under accelerated carbonation, embrittlement of the fibre/matrix interface and of the whole composite occurred, accompanied by increased stiffness, decreased deformation capacity and loss of the energy absorption capacity under flexure. FA-based matrices exhibited more pronounced embrittlement than the denser FA/GBFS matrices. A combination of FA/GBFS-based mortar reinforced with 0.5 vol% fibre dosage ensured an optimal fibre/matrix interface and stress transfer, mitigating the embrittlement of the material under accelerated carbonation.

Study on the Influence of Processed Rice Straw Ash as a Binder on Compressive Strength and Water Absorption of Alkali Activated Mortar

Major concern today with cement production is its environmental issues. For the sustainable development the usage of cement is reduced day by day in the construction sector. The alkali activated binders (AAB) are eco-friendly and they do not cause any environmental effects which can be used as one of the alternatives to cement. AAB is a hardened cementitious paste made of industrial pozzolanas such as Fly Ash and GGBS, and alkaline solutions like sodium hydroxide and sodium silicate. Currently Fly Ash is the most commonly used AAB; but fly ash may become scarce because coal- fired power plants are declining due to the increasing popularity of renewable energy.
 Rice straw ash (RSA) is an agricultural waste which has the pozzolanic characteristics and contains about 70%-80% of amorphous silicon dioxide (SiO2) when burnt at 650c. Rice straw can be used as replacement of fly ash in Alkali Activated Mortar (AAM) by burning it at 650c and milling it for 10 minutes, 20 minutes and 30 minutes. The specimens are casted for Rice Straw based AAM and compressive strength and water absorption are studied for 7.5, 10, 16 molarity of NaOH and also RSA milled for 10,20 and 30 minutes and SEM EDX is studied for the hardened specimen at 28 days. 
 The RSA has significant positive effects on performance of AAM. The replacement of RSA by 20% has shown greater results and as the milling time of RSA is raised the surface area of RSA is increased which in turn resulted in improved strength and as the molarity of NaOH is increased the AAM at 3, 7, 28 and 56 days has the higher Compressive Strength and lower Water absorption.

Influence of SiO2 /Na2O molar ratio on mechanical properties and durability of metakaolin-fly ash blend alkali-activated sustainable mortar incorporating manufactured sand

Manufactured sand (MS) is an optimal alternative for river sand used in sustainable construction materials to decrease the detrimental effects caused by excessive sand mining. In the current study, the mechanical performance and durability of alkali-activated sustainable mortar incorporating 100% MS and using various SiO2/Na2O molar ratios was investigated. The metakaolin-fly ash (MK-FA) blend activated by sodium-based alkaline liquid was used as the alkali-activated binder, and MS from crushed limestone was used as fine aggregate for the preparation of alkali-activated mortar (AAM) to ensure green and sustainable development. The properties of MK-FA based AAM using MS (MKF3A2MS) were measured including cube compressive, splitting tensile, flexural strengths, and modulus of elasticity, water permeability, and cracking resistance. The influence of molar ratio on mechanical performance and durability of MKF3A2MS were analyzed. An exponential fitting model for the compressive and tensile strengths of MKF3A2MS was proposed. In addition, analysis of variance (ANOVA) was adopted to discuss the statistic effect for the factors. The results demonstrate that the molar ratio of SiO2/Na2O has remarkable influence on mechanical performance and durability of MKF3A2MS. The compressive, splitting tensile and flexural strengths, and elastic modulus of MKF3A2MS first increased and then decreased, while the depth of water penetration and cracking index first decreased and then increased as the molar ratio was within the scope of 1.1–1.6. The mechanical properties and durability of MKF3A2MS using a molar ratio of 1.2 were optimal.

The role of dissolved rice husk ash in the development of binary blast furnace slag-sewage sludge ash alkali-activated mortars

This study evaluated the compressive strength performance and microstructure of binary blast furnace slag-sewage sludge ash (BFS-SSA) alkali-activated mortars, in which rice husk ash (RHA) was dissolved in NaOH solution to replace commercial sodium silicate. The dissolution of RHA, performed in a thermal bottle with NaOH solution, enhanced the compressive strength of the alkali-activated mortars based on BFS up to 4.5-fold. BFS-SSA based mortars (10–40% SSA), activated with NaOH/RHA-based sodium silicate suspensions, reached a compressive strength up to 30 MPa after 28 curing days at 20°C. The RHA dissolution enhanced the strength and sustainability of the BFS-SSA alkali-activated mortars.

The Volume Stability of Alkali-Activated Electric Arc Furnace Ladle Slag Mortar and Its Performance at High Temperatures

In this study, the engineering properties of Ordinary Portland Cement (OPC) and alkali-activated slag (AAS) mortar with electric arc furnace ladle slag (EAFLS) were investigated to reveal the effects of EAFLS on the expansion of cementitious mortars. Additionally, the effects of these two types of mortar were explored based on their compressive strength, especially at high temperatures. EAFLS in OPC mortars significantly reduced the compressive strength and caused serious soundness problems in the mortars after autoclaving due to the presence of free-CaO and free-MgO in the EAFLS slag. On the other hand, the AAS mortars produced with EAFLS had compressive strength comparable to ordinary OPC mortars and maintained soundness after autoclaving. During a 550 °C heat treatment, the OPC mortar cracked and lost residual strength, but the AAS mortar retained more than 90% of its residual strength. Even after an 800 °C heat treatment, the AAS mortar maintained 14% of its residual strength (about 4 MPa), sufficient to prevent the collapse of the specimen structure. The main reason is that alkali-activated technology can accelerate the hydration process and solve the delayed hydration problem. The results of this study indicated that EAFLS is suitable to partially replace the binder used in the production of AAS mortars, and the resulting AAS mortars have high volume stability, high compression strength, and good high temperature resistance.

Effect of elevated temperatures on mechanical and microstructural properties of alkali-activated mortar made up of POFA and GGBS

Alkali-activated cement systems (Geopolymers) are essential alternatives to OPC in the sustainable concrete production industry. In general, the alkali-activated mortar and concrete have good fire resistance due to their ceramic-like properties. Due to the limited resources investigating POFA-GGBS based geopolymer exposed to elevated temperatures, this study will help researchers for the first time understand the geopolymeric binder characteristics after exposure to elevated temperature on strength, thermo-physical stability, and microstructure properties. The main objective is to investigate the role of calcium coupled with aluminum and their effect on geopolymeric gel stability. The alkali-activated mortar was synthesized with liquid sodium silicate and sodium hydroxide. Cubic specimens were heated to a range of temperatures between 100 °C and 800 °C to evaluate strength loss and gel thermal microstructure damage. Scanning electron Microscopy test confirmed the finding of the ability of POFA with GGBS to be used as a novel environmentally-friendly material to produce sustainable mortar and concrete with advanced performance under high temperatures. Thermal-physical stability of POFA-GGBS mortar due to the spherical pore system allows the vaporized water to be released outside the samples. FTIR spectra results revealed two mechanisms explaining the effect of GGBS by lowering the wavelength through shifting Si-O peaks with the aluminium substitution producing C-A-S-H and changing the FTIR spectra by generating new peaks. TGA/DTG and DSC tests confirmed the formation of a robust gel with a higher degree of crystallinity and very high thermal stability due to the Ca and crosslink effect of Al in the geopolymeric system. The excellent thermal performance of POFA-GGBS mortar under elevated temperature would increase the applications for this novel environmentally-friendly material.

High performance mortars from vitrified bauxite residue; the quest for the optimal chemistry and processing conditions

This study investigates the transformation of bauxite residue into a reactive precursor after heat treatment at 1200–1300 °C and the synthesis of high performance inorganic polymer mortars thereof. Minor amounts of C, CaO and SiO2 were added to bauxite residue, and the melt was water-quenched resulting in amorphous phase (25 up to 62 wt%), the rest being mainly iron-rich phases. After milling the vitrified bauxite residue, alkali-activated mortars were produced with a maximum compressive strength of 131 MPa after 28 days at ambient curing. Calcium was identified as key element in increasing the compressive strength, reduction in shrinkage and permeability. The release of heavy metals and radionuclide concentration were below legislative limits. This work identified an ideal chemistry for producing high-performance binders from precursors containing more than 81 wt% of bauxite residue, opening the possibility of upscaling and, eventually, the real-life transformation of bauxite residue into a product.