Alkali-activated Concrete Research Articles

Analysis of the environmental impacts of alkali-activated concrete produced with waste glass-derived silicate activator – A LCA study

Concrete is responsible for a significant share of global GHG emissions, which can be mainly ascribed to the production of clinker. Alkali-activated concretes have been investigated in literature as a possible alternative, but the sustainability still appears reduced by the high embodied energy of chemicals typically used for the activation step. This paper investigates concrete belonging to strength classes 35, 50 and 70 MPa and produced with a silicate activator derived from waste glass (AABR). Through a Life Cycle Assessment (LCA), the investigation aims to compare the AABR to Ordinary Portland Cement (OPC) concrete and alkali-activated concrete produced with commercially available chemicals (AABC). The effects produced by the variations of some key parameters (impact allocation of precursors, energy mix, amount of activator in the concrete, distance of procurement of raw materials) over the total impact of the AABR are also investigated. Results show that the adoption of alkali-activated concretes instead of OPC concrete allows a significant reduction in environmental categories of global warming (averagely 64% reduction for AABC and 70% for AABR), acidification potential (averagely 23% for AABC and 35% for AABR), and terrestrial eutrophication (averagely 53% for AABC and 60% for AABR). In addition, the study evidenced that the use of waste glass-based activator allows a significant reduction in every environmental category when compared to the use of commercially available chemicals.

Cradle-to-Gate Life Cycle and Economic Assessment of Sustainable Concrete Mixes—Alkali-Activated Concrete (AAC) and Bacterial Concrete (BC)

The negative environmental impacts associated with the usage of Portland cement (PC) in concrete induced intensive research into finding sustainable alternative concrete mixes to obtain “green concrete”. Since the principal aim of developing such mixes is to reduce the environmental impact, it is imperative to conduct a comprehensive life cycle assessment (LCA). This paper examines three different types of sustainable concrete mixes, viz., alkali-activated concrete (AAC) with natural coarse aggregates, AAC with recycled coarse aggregates (RCA), and bacterial concrete (BC). A detailed environmental impact assessment of AAC with natural coarse aggregates, AAC with RCA, and BC is performed through a cradle-to-gate LCA using openLCA v.1.10.3 and compared versus PC concrete (PCC) of equivalent strength. The results show that transportation and sodium silicate in AAC mixes and PC in BC mixes contribute the most to the environmental impact. The global warming potential (GWP) of PCC is 1.4–2 times higher than other mixes. Bacterial concrete without nutrients had the lowest environmental impact of all the evaluated mixes on all damage categories, both at the midpoint (except GWP) and endpoint assessment levels. AAC and BC mixes are more expensive than PCC by 98.8–159.1% and 21.8–54.3%, respectively.

Life cycle assessment and cost analysis of fly ash–rice husk ash blended alkali-activated concrete

The utilization of industrial and agricultural by-products for the production of alkali activated concrete (AAC) has the potential to yield significant benefits towards sustainability goals. To be a viable material, the construction industry requires a construction material that achieves the requisite strength and the other property requirements as specified in codes and standards while demonstrating improved sustainability criteria. Fly ash and Rice Husk Ash (RHA) are abundantly available waste products, principally located in Asian countries. Currently, a significant proportion of these materials are disposed of in landfills, lagoons and rivers but offer potential to utilize in AAC. Hence, the identification of variables associated with fly ash and fly ah-RHA blended AAC by utilizing fly ash and RHA is vital. This study quantifies the environmental and economic factors by assessing the Greenhouse gas (GHG) emission, environmental impacts and benefits, and cost analysis of utilizing fly ash and RHA in AAC compared to Portland Cement (PC) concrete. Alkaline activator is a key component responsible for the highest GHG emission, cost and environmental impact amounts obtained for fly ash geopolymer and blended alkali-activated concrete compared with PC concrete. Alkali activators contribute to 74% of the total GHG emission, while heat curing contributed only 9% to the total GHG emission. The addition of 10% RHA to alkali-activated concrete showed a slight benefit for the analysis. Utilization of waste fly ash and RHA is responsible for providing significant benefits in terms of fresh and marine water ecotoxicity by avoiding waste disposal at the dumpsites, rivers, and storage lagoons.

Investigation on Electrical Resistance of Chloride Penetration of Alkali Activated Slag Concrete

Alternating-current method for measuring chloride penetration resistance of concrete, test method for coulomb electric flux and rapid chloride migration coefficient (RCM) were applied to evaluate the resistance of chloride penetration in alkali-activated slag concrete in this paper. At the same time, the applicability of the above three electrical parameters test methods to the alkali slag concrete was discussed. The results show that NaOH activated slag concrete behaves higher resistance to chloride penetration than water glass activated slag concrete. Blend of fly ash increases the porosity of alkali-activated slag concrete and weakens the resistance of chloride penetration. Correlation coefficient between chloride migration coefficient and AC electrical resistivity is 0.99. There are good correlations among the evaluation results of three electrical parameters test methods, and all of them behave sound applicability to alkali-activated slag concrete.

Optimization of alkali-activated concrete based on the characteristics of binder systems

The characteristics of binder systems played vital roles on the properties of alkali-activated concretes (AACs), from strength development to long-term durability. Here, the sum of 102 binder systems with three levels of alkalinity of 5%, 6%, and 7% and three levels of blast furnace slag to fly ash mass ratio of 50:50, 60:40, and 70:30 activated by diverse compositions of activators (single sodium hydroxide, sodium silicate, and sodium carbonates or combination) were investigated. Based on that, a series of compositions-properties contours/maps considering the hardening strength, shrinkage behavior, and mechanical characters of these binder systems have been established. After that, a modified overall-calculation method was applied for the mix design of concrete. The AACs performance including basic mechanical behaviors in compression, flexure, and tensile load and volume stability (including drying shrinkage and alkali-aggregates reaction) were checked. It’s suggested that the performances of AACs were in a broad range depended on the binder systems. Thus, the selection of binder systems was in the most important position in designing concrete even other aspects, for example, aggregates type would also affect performances like mechanical and shrinkage of AACs. However, the complexity of alkali-activated binder systems make it difficult to establish the quantitative relationships between compositions and performances of AACs. Therefore, the compositions-properties contours/maps established here would deep the understanding for roles of constituents on the performances of AACs and guide the optimization of AACs in practice.

Mechanical properties, shrinkage, and heat evolution of alkali activated fly ash concrete

Use of high calcium fly ash as a binder to replace the ordinary Portland cement possibly cause a false set for alkali-activated concrete. Therefore, the appropriate mixed proportion of alkali-activated concrete must be considered for further applications. This research investigated the mechanical properties, shrinkage, and heat evolution of concrete using high-calcium fly ash as a binder with two alkali activators: sodium silicate (Na2SiO3) and sodium hydroxide (NaOH). Ratios of sodium silicate to sodium hydroxide (Na2SiO3/NaOH) of 0.3, 0.4, and 0.5 by weight, and NaOH concentrations of 2, 4, 6, and 8 Molar were used. The ratio of liquid to binder was 0.7, and the fly ash content was 400 kg/m3. The results indicated that increasing the Na2SiO3/NaOH ratio and NaOH concentration improved the compressive strength of the concrete. The maximum value for the compressive strength, 27.5 MPa, was observed for a Na2SiO3/NaOH ratio of 0.5 and NaOH concentrations of 2 Molar at 90 days. The modulus of elasticity of alkali-activated fly ash concrete was found to be similar to that of conventional concrete. Additionally, the shrinkage of concrete tended to decrease with increasing NaOH concentration, and increasing the Na2SiO3/NaOH ratio led to increased concrete shrinkage. Finally, the heat evolution of alkali-activated fly ash concrete was found to be two times lower than that of concrete made from Ordinary Portland cement at the same compressive strength. The increase of NaOH concentration and Na2SiO3/NaOH ratio resulted in higher temperature rise of alkali-activated fly ash concrete.

Effects of nanosilica and steel fibers on the impact resistance of slag based self-compacting alkali-activated concrete

In this research, the effects of nanosilica and steel fibers on the impact resistance of ground granulated blast furnace slag based self-compacting alkali-activated concrete were investigated. Nanosilica volume fraction was kept constant at 2%. Two types of hooked-end steel fibers (Kemerix 30/40 and Dramix 60/80) and steel fiber volume contents (0.5% and 1%) were utilized to highlight the combined effects of nanosilica and steel fiber on the impact behavior. The fresh state and mechanical properties such as slump flow, L-box, V-funnel, compressive strength, modulus of elasticity, splitting tensile strength, and flexural strength were evaluated. The microstructure of the samples was examined using a scanning electron microscope. The impact resistance of the specimens was measured by a drop-weight test. Acceleration-time and force-time graphs were plotted and evaluated together with the crack photos of the specimens for the first and failure impactor drops. The incorporations of nanosilica and steel fiber improved splitting tensile strength, flexural strength, impact resistance, and energy absorption capacity, while they decreased compressive strength and modulus of elasticity. For the specimens without nanosilica and with 2% nanosilica, the impact energy improvements were five times and 12.5 times higher for 0.5% short fibrous, 20.5 times and 44.5 times higher for 1% short fibrous, 23.5 times and 31 times higher for 0.5% long fibrous, and 64 times and 144.5 times higher for 1% long fibrous specimens than the specimens without nanosilica and steel fiber, respectively. The long fibers were found more effective in mechanical strength and impact energy than short fibers, and the reinforcing efficiency of fibers enhanced with higher steel fiber volumes. The combined utilization of nanosilica and steel fibers have the potential to delay the crack formation and dissipate energy to the surrounding zones, and this potential increased with higher steel fiber lengths and volume ratios.

The performance of fiber GGBS based alkali-activated concrete

The fast construction and renovation of roads and bridges and the infrastructures in Egypt and developing countries cause concrete and cement production on a large scale. This huge market demand for precast units and concrete production harms the environment and global warming. Thus, the Ordinary Portland Cement (OPC) should reduce by developing new materials such as geopolymer concrete or alkali-activated concrete. This study investigated alkali-activated slag concrete (AAC) performance reinforced with structural polypropylene and steel fibers cured at ambient room temperature. The structural polypropylene and steel fibers were incorporated into the alkali-activated mix by 1.5% and 5% of the total binder weight, respectively. A wide range of engineering properties was assessed, including compressive strength, split tensile strength and stress-strain behavior, density, water absorption, and porosity. Microstructural analysis of the AAC samples was evaluated through scanning electron microscopic (SEM). The results showed that compressive strength at 1-day curing developed about 85% of the compressive strength for ACC at 28-days, which was considered as early high strength. A 10% enhancement of the AAC specimens' average compressive strength was reported after 7 and 28 days while fiber addition. The split tensile strength significantly increased by 19.28 and 26.80% by incorporating structural polypropylene and steel fibers, respectively. Comparison between the ACI 318 and ECP 203 provisions was handled along with those experimental results obtained elastic and rupture modulus. The ACI 318 provisions provided a more conservative prediction to the experimental results than the ECP 203 provision, which overestimated the experimental values by an average value of 1.1. The electron microscopy images confirmed the improvement in the engineering properties by showing a proper and coherent bond between the mortar matrix and the added fibers.

Coupling machine learning with thermodynamic modelling to develop a composition-property model for alkali-activated materials

Alkali-activation is one of the most promising routes for utilisation of versatile aluminosilicate resources. However, the variations of chemical compositions in these resources have increased the challenge of designing alkali-activated materials (AAMs) with multiple sources, posing the demand for establishing composition-property correlations that can represent a wide range of AAMs. This study proposes a data-driven approach to develop such composition-property correlations combining machine learning with global sensitivity analysis and thermodynamic modelling. The strength performance of alkali-activated concretes was investigated for a benchmark study (196 data inputs). The impact of the five key chemical compositions, CaO–SiO2–Al2O3–MgO–Na2O, has been assessed. The results show that despite the use of different aluminosilicate precursors, there appear to be coherent connections between bulk binder chemical compositions, phase assemblages, and the performance of AAMs. The composition-property correlations established via machine learning can be used to facilitate the on-demand design of AAMs utilising varying aluminosilicate resources.

Durability and microstructure studies on Slag-Fly Ash-Glass powder based alkali activated pavement quality concrete mixes

The present study attempts a detailed assessment of the performance of air-cured Alkali Activated Concrete (AAC) mixes produced using powdered waste glass as one of the ingredient, designed for highway applications. In this current investigations, the alkaline solution is prepared using sodium hydroxide flakes and liquid sodium silicate solution. Initially, AAC mixes are designed to obtain a pavement quality concrete (PQC) of grade M−45 using Ground Granulated Blast Furnace Slag (GGBS) and fly ash (FA) as binding ingredients in ratios of 75:25, incorporating graded crusher dust as fine aggregates. Based on the preliminary investigations, the design Na2O dosage of 4% and the modulus of activator (Ms) = 1.25 was selected and this dosage was maintained for the extended investigations. The further investigations were carried out for various trial dosages of waste glass powder (GP) in the concrete mix. The selected AAC mixes were tested with compressive and tensile strengths, flexural strength, unit weight, modulus of elasticity, durability properties (i.e, prolonged strength, acid attack, sulphate attack, and saline water attack tests); the microstructure studies using SEM with EDAX and the obtained results were discussed. The study reveal that, the AAC mixes display satisfactory performance for normal working environments for their use in roadway appliances. Thus, the use of AAC mixes produced using GGBS-FA-GP in association with the graded crusher dust fine aggregates is considered to be an economical and eco-friendly alternative to use of conventional concretes for highway applications.

Experimental Study of the Dynamic Compressive and Tensile Strengths of Fly Ash and Slag Based Alkali-Activated Concrete Reinforced With Basalt Fibers

The use of industrial by-products, e.g., fly ash, slag, as complete replacement of Portland cement to make alkali-activated concrete (AAC) has become a hot topic due to the contribution to sustainability in construction industry. AAC has comparable compressive strength compared to the ordinary Portland cement concrete (OPC) and has many advantages, such as excellent durability and corrosion resistance. However, similar to OPC, AACmaterial still has certain shortcomings such as brittleness, low tensile strength, and poor impact resistance, which can be improved by incorporating fibers in the matrix. This paper considers the basalt fiber-reinforced alkali-activated concrete (BFRAAC), and explores the dynamic compressive and tensile strengths through a series of impact tests. The test results show that the dynamic strength of BFRAAC exhibits significant strain rate effect, that is, the material strength increases with the strain rate. Compared to the compressive strength of the material, the strain rate sensitivity of its tensile strength is more marked. Based on the test results, empirical formulas describing the relation between dynamic strength and strain rate of BFRAAC are proposed.

Sustainable utilization of ultrafine rice husk ash in alkali activated concrete: Characterization and performance evaluation

This paper investigates the novel effect of ultrafine rice husk ash (URHA) on the properties of alkali-activated concrete. Rice husk ash was obtained as waste from biomass energy production and further processed to obtain URHA. The URHA was first characterized using various techniques to understand its morphology, structure and chemical composition. The URHA was then used to replace fly ash in alkali-activated fly ash/slag concrete up to 10%, and the effects on the performance were evaluated. The optimum dosage of URHA was assessed based on the compressive strength and the resistance to chemical attacks was carried out on the control mixture and that incorporating the optimum URHA. Microstructural properties of some of the control mixture and that incorporating the optimum URHA content was also investigated. Results from this study showed that the use of URHA as a 5% replacement of the FA is optimum that ensured high performance.

Sustainable alkali activated concrete with fly ash and waste marble aggregates: Strength and Durability studies

This study examines the use of waste marble aggregates (WMA) as a substitute to natural aggregates (NA) to produce sustainable alkali activated concrete (AAC). Sodium hydroxide (NaOH) of 8M concentration and sodium silicate (Na2SiO3) were used as an alkaline activator for high calcium fly ash based AAC. NA were replaced with WMA at different weight ratios, namely 0%, 25%, 50%, 75% and 100%. Different properties like density, workability, air content, compressive, flexural and tensile strengths, along with modulus of elasticity of fresh and hardened concrete were investigated. Also, rapid chloride penetration test, oxygen permeability index test, depth of water penetration, electrical resistivity and acid resistance were also conducted to study the durability characteristics of the sustainable AAC. Results of this study showed that WMA affected the pore structure and new calcium-based products were formed along with polymeric products. New products in the matrix accelerate the polymerisation process and significantly enhanced the strength properties and durability of AAC. The outcomes of this study revealed that WMA can be employed to replace up to 50% of NA in the production of sustainable AAC. Its high potential to replace NA could lead to significant saving in energy, cost and, can further reduce the hazardous environmental impacts caused by marble industries during mining, processing and polishing phases.

Assessing the sustainability potential of alkali-activated concrete from electric arc furnace slag using the ECO2 framework

Alkali-activated materials are regarded as a potential sustainable building material with industrial by-products fully replacing ordinary Portland cement. Five million tonnes of electric arc furnace slag are produced annually mostly to be recycled as low value aggregates in several construction applications. This study examined the possibility of valorising the understudied slag as a precursor in alkali-activated concrete. The material, supplied free and available in abundance as a waste, presents a significant potential to produce sustainable concrete. Hence, the mechanical and durability properties of electric arc furnace slag-based alkali-activated concrete were examined. After that, using a sustainability assessment framework called ECO2, the combined whole-life cycle assessment of the environmental and economic impact was calculated for several mixes that combined electric arc furnace slag and fly ash as precursors. The increasing amount of slag content led to a decline in mechanical performance, though there was an equivalent durability-related performance; mixes with electric arc furnace slag showed equivalent slump and resistance to carbonation, and enhanced resistance to chloride ion penetration. Furthermore, slag-based concrete exhibited significant improvement in the overall ECO2 sustainability score due to its minimal environmental and economic impact.

Compressive Strength Performance of Alkali Activated Concretes under Different Curing Conditions

This study investigated the influence of different curing conditions on the compressive strength (CS) of the different alkali activated concrete (AAC) specimens at the ages of 2, 28, and 90 days for the structural utilization and standardization process of AAC instead of OPC concrete. For this aim, 100% slag (S100), 75% slag and 25% fly ash (S75FA25), and 50% slag and 50% fly ash based (S50FA50) AAC specimens were produced. Based on the oven-curing (O), water-curing (W), and ambient-curing (A) methods, the influence of 2O for 2 days, 26A2O, 2O26A, 28A, 28W, 26W2O, and 2O26W for 28 days, and 88A2O, 2O88A, 90A, 88W2O, 2O88W, 90W for 90 days on the CS of the AAC were examined in details. In addition, the influence of delayed oven-curing conditions on CS development was also investigated. The results indicated that curing conditions significantly affected on the CS and the water-curing condition could provide a better CS for those of AAC at 90 days. Although, the oven-curing enhanced CS of the S100 specimens at initial ages (first oven-curing applied), delayed oven-curing (oven-curing applied later) was found significant for S75FA25 and S50FA50 specimens. The delayed oven-curing affected more on the CS of the AAC when fly ash content increased. The most of AAC specimens with oven-curing had significantly enhanced the CS at 28 days, but S50FA50 at the age of 90 days decreased. Different curing regimes were proposed for the superior compressive strength values for each AAC specimens at the ages of 28 and 90 days.

A review on alkali-activated slag concrete

Alkali-activated concrete (AAC) has attracted considerable attention since its first use as an alternative material to the well-known traditional Portland cement concrete (PCC) due to their superior properties and environmental impact. In this paper, a comprehensive review on the present knowledge about the AAC in terms of historical background, environmental impact, constituent materials and characteristics of alkali-activated slag concrete (AASC) is presented. The following topics are reviewed in details for AASC: historical background, environmental impact, constituent materials, reactions mechanism, hydration products, compressive strength, stress–strain behavior, elasticity modulus, Poisson’s ratio, tensile strength, bond characteristics with reinforcing steel bars and behavior under elevated temperature. Most studies have demonstrated the superior mechanical properties of AASC and their applicability in the construction engineering field. Moreover, AASC exhibits bond performance and elevated temperature resistance better than PCC. However, the review reveals that more studies and investigations related to mix design including mix proportions, mixing procedures and curing regime, with which the AASC could demonstrate the best engineering properties, are required.

Properties of a Lightweight Fly Ash–Slag Alkali-Activated Concrete with Three Strength Grades

Lightweight alkali-activated concrete (LAAC) is a type of highly environmentally friendly concrete, which can provide the benefits of both alkali-activated material and lightweight concrete. The study aimed to investigate the influence of different water/solid (W/S) ratios on the properties of normal-weight/lightweight fly ash–slag alkali-activated concrete manufactured at ambient temperature. The relative performance of the alkali-activated concrete (AAC) mixes with limestone and sintered fly ash lightweight aggregates as the coarse aggregates was also compared to the conventional ordinary Portland cement (OPC) concrete mix in terms of their compressive stress–strain relationship, splitting tensile strength and fracture parameters. The morphologies and microstructure of the four types of interfacial transition zones (ITZs) were characterized by scanning electron microscopy (SEM). Results indicated that the AAC had a higher tensile strength, stress intensity factor, brittleness and lower elastic modulus than its cement counterpart. With the decrease in the W/S ratio, the density, compressive and tensile strength, ultrasonic pulse velocity, fracture energy, brittleness and elastic modulus of the AAC increase. However, the influence of the W/S ratio on the mechanical properties of the LAAC with lightweight porous aggregates was less than that of the normal-weight AAC. Predictive models of the splitting tensile strength, fracture energy and elastic modulus of the AAC were also suggested, which were similar to those of the OPC concrete. Furthermore, the microstructure investigation showed that no wall effect occurred in the ITZ of the AAC. The ITZ structure of the hardened AAC was also more compact and uniform than that of the OPC concrete.

Durability Characteristics of Slag / Fly Ash Based Alumina Silicate Concrete at Ambient Temperature

To diminish the environmental pollution in the making of conventional concrete using cement, an urgent demand to use other materials to manufacture concrete is required. In this current situation, the advanced material such as alkali-activated concrete is to be replaced for concrete, which are the combinations of ingredients containing Ground Granulated Blast Furnace Slag (GG BFS), Fly ash.(FA) These mineral admixtures contains rich amount of aluminium and silicon. This article presents the results on Strength, Water absorption and Water Penetrability, Sorptivity and Resistance against Chloride Ion Penetration Test (RCPT) of Alumina Silica Concrete based on GGBFS blended with fly ash. GGBFS is partially substituted with various combinations of Zero to fifty percentages in a standard concentration of 12 M under ambient curing. From the experimental investigation, the robustness of alkali activated concrete (GGBFS alone) performing better in comparison to alkali activated concrete with GGBFS blended with FA. Same results are also observed in water absorption and sorptivity related properties. In the replacement levels, 40% replacement (GPC-E) performed better compared to other replacement levels. Thus, alumina silicate concrete with partial substitution of 40 percentage of fly ash to GGBFS is the precise level of substitution which satisfies the above mentioned durable parameters as it is cheaper than GGBFS.

Properties of alkali-activated concrete (AAC) incorporating demolished building waste (DBW) as aggregates

This study was undertaken to evaluate the potential of demolished building waste (DBW) as aggregates in alkali-activated concrete (AAC). A recent road-widening activity led to the demolition of commercial buildings along National Highway 275, Bangalore-Bantwal, India. DBW was collected from these sites and processed manually at the laboratory facility of CHRIST (Deemed to be University). Processing of DBW was done to obtain both waste coarse and fine aggregates from demolished concrete and brick waste units, respectively. AAC was synthesized by fly ash, ground granulated blast furnace slag, sodium hydroxide, sodium silicate, along with waste aggregate replacement rates of 0, 25, 50, and 75% by weight of natural aggregates. Fresh and hardened properties of developed concrete mixtures were experimentally determined. Results of the study indicate that 28-day compressive strength of 30.4 and 21 MPa was obtained for AAC with 25 and 50% DBW aggregates, which was 8.6 and 36.9% lower than control mix, respectively. Further, there was an increase in the water absorption and a reduction to acid resistance for all the AAC mixes with DBW aggregates. Based on the results obtained, it was observed that AAC with 25 and 50% DBW aggregates find great potential in civil engineering applications.

Effect of Bauxite Residue Content as A Replacement of Slag on the Strength, Shrinkage, and Abrasion of Alkali-Activated Slag Concrete

Effect of Bauxite Residue Content as A Replacement of Slag on the Strength, Shrinkage, and Abrasion of Alkali-Activated Slag Concrete