Alkali Activated Concrete Mixes Research Articles

An investigation of the mechanical strength, environmental impact and economic analysis of nano alkali-activated composite

The current study explores both the mechanical strength and life cycle assessments of fly ash (FA)-based alkali-activated concrete (AAC) with/without colloidal nano-silica (NS) addition at high and low molar concentrations of sodium hydroxide in the activator fluid (12 M and 3 M). The cradle-to-gate approach is used to establish the system boundaries for life cycle assessments (LCA). The environmental impact of AAC (with/without NS) and conventional cement concrete (CC) mixtures was determined using the ReCiPe midpoint and endpoint approaches across various impact categories. The AAC mixes with and without NS addition reduce greenhouse gas (GHG) emissions by 30 % and 25 % with respect to CC mix, respectively. Within the majority of impact categories, sodium silicate and sodium hydroxide production are the primary contributors, accounting for up to 91 %. However, in water depletion, global warming potential, and fuel depletion, the overall contribution is as high as 65 %. Also, the AAC mix with and without NS addition consumes 34 % and 14 % less electricity during its production with respect to the CC mixture. The CC mixes have almost 70 % more ecological damage and approximately 30 % more health risk than AAC with and without NS. However, the NS-modified AAC shows 20 % lower ecological and health risks than AAC without NS. The effect of resource scarcity on NS modified AAC is 50 % less than that of other mixes, including both AAC mix without NS and CC mix. In comparison to AAC without NS, the NS modified AAC demonstrates an 18 % reduced impact on resource scarcity. Finally, cost analysis demonstrates that AAC with NS is 16 % less expensive than CC, while AAC without NS is 26 % cheaper than CC. AAC containing 6 % NS inclusions exhibits environmental and economic benefits in addition to its enhanced mechanical performance, indicating its potential for widespread practical usage.

Assessing the effect of sodium nitrite as corrosion inhibitor against the corrosion of steel rebar in alkali-activated concrete

The aim of this study is to investigate the effectiveness of corrosion inhibitor against chloride-induced corrosion of steel rebar in fly-ash (FA) and GGBS (ground granulated blast furnace slag) and Nanofine-based alkali-activated concrete. The Alkali-Activated Concrete (AAC) was prepared from FA, ground granulated blast furnace slag (GGBS), and Nanofine with Sodium hydroxide (NaOH) and Sodium Silicate (Na2SiO3) solutions. The concentration of NaOH varies to 10 M, 12 M, and 14 M, and all the specimens were subjected to accelerated corrosion, to perform the non-destructive test after the corrosion of the steel rebars. Fourier Transform Infrared (FTIR), scanning electron microscope (SEM), Energy dispersive X-ray spectroscopy (EDS), and Electron Backscatter Diffraction (EBSD) were used to examine the microstructure of AAC in the presence of Nano-Fine GGBS (NF), Sodium Chloride (NaCl), and NaNO2 and to compare the strength and corrosion properties with AAC and OPC. The ratio of sodium silicate to sodium hydroxide was taken as 2.5 and the solution-to-binder ratio was taken as 0.5. The compressive strength and ultrasonic pulse velocity (UPV) test of the Nano-fine based alkali-activated concrete increases as compared to the control concrete. The increase in the dosages of corrosion inhibitor and nano-fine increases the compressive strength of the control concrete and alkali-activated concrete. The half-cell potential of the control concrete and nano fine admixed with alkali-activated concrete mixes with different molarities is uncertain. The increases in the dosages of corrosion inhibitors reduce the occurrence of corrosion in the steel reinforcement rebars.

Investigating the rheological characteristics of alkali-activated concrete using contemporary artificial intelligence approaches

Abstract Using artificial intelligence-based tools, this research aims to establish a direct correlation between the alkali-activated concrete (AAC) mix design factors and their performances. More specifically, the machine learning system was fed new property data obtained from AAC mixes used in laboratory experiments. The rheological parameters (yield stress [static/dynamic] and plastic viscosity) of AAC were predicted using the multilayer perceptron neural network (MLPNN) and bagging ensemble (BE) models. In addition, the R 2 values, k-fold analyses, statistical checks, and the dissimilarity between the experimental and predicted compressive strength were employed to assess the performance of the created models. Also, the SHapley additive exPlanation (SHAP) approach was used for examining the relevance of influencing parameters. The BE approach was found to be significantly accurate in all prediction models, with R 2 greater than 0.90, and MLPNN models were found to be moderately precise, with R 2 slightly below 0.90. However, the error assessment through statistical checks and k-fold analysis also validated the higher precision of BE models over the MLPNN models. Building models that can calculate rheological properties of AAC for different values of input parameters could save a lot of time and money compared to doing the tests in a laboratory. In order to ascertain the required amounts of raw materials of AAC, investigators, as well as businesses, may find the SHAP study helpful.

Mechanical and microstructural study of slag based alkali activated concrete incorporating RHA

The objective of the present research is to evaluate the effect of incorporation of rice husk ash (RHA) into ground granulated blast furnace slag (GGBS) based alkali activated concrete (AAC) at ambient temperature to develop a cement free binder that composed of waste products. The hardened properties of AAC have been evaluated in accordance with Indian Standard code in terms of destructive tests such as compressive, splitting tensile and flexural strength, as well as non-destructive tests such as rebound number, ultrasonic pulse velocity (UPV) and dynamic modulus. Using scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR), the microstructural characteristics of AAC mixes were evaluated. The outcomes of the test indicate that the compressive strength of AAC achieves maximum strength when RHA component of up to 10% is incorporated into the AAC mixture. The compressive strength declined beyond 10% RHA, however the AAC has lower strength of 39.5 N/mm2 (M7 mix at 7 days) among all mixes, which was adequate for the structural applications. Corresponding to compressive strength, the split tensile and flexural strengths exhibit similar performance. Statistical analysis also carried out to determine the correlation between different parameters. The SEM test shows the formation of dense microstructure at a lower percentage of RHA content in the mix, whereas cracks and voids were detected in case of a higher percentage of RHA. Therefore, using GGBS with lower RHA content as binder cured at ambient temperature enhances the performance of AAC and also facilitates the use of AAC instead of ordinary concrete.

Numerical study on fracture behavior of alkali activated concrete mixes for different beam sizes

The present study considers the use of AAC mixes, since the raw materials that are being used to produce concrete are non-renewable natural resources, particularly Ordinary Portland Cement (OPC). Also, production of OPC contributes a greater amount of CO2 to the environment. Hence, an attempt has been made to study alternative construction materials keeping in mind sustainable development. The present numerical study compares the fracture behaviour and size effect of Alkali Activated Concrete (AAC) mixes with Ordinary Portland Cement Concrete (OPCC) mixes by numerically modelling the beams of AAC and OPCC using FEA software ABAQUS. Three different mixes of AAC, characterized by the variation of percentage volume of water-quenched slag (WQS) as a substitute for Natural sand (RS) and one OPCC mix, were studied. A three-point bending test was performed in ABAQUS for the beams of three different sizes, which are geometrically similar with constant thickness. Numerical simulation is performed under displacement control. Load versus CMOD graphs are plotted from the output data obtained from the numerical modelling. Fracture parameters such as Fracture energy (GF), Brittleness number (B), Characteristic length (lch), and Stress intensity factor (KIC) are then calculated from the obtained data and compared to understand the influence of fracture properties and size effect on the behaviour of structural elements since limited research works are available from the literature. From the results, it is observed that OPCC mixes show superior strength, high resistance to crack propagation, more toughness, and high fracture energy when compared to AAC mixes. AAC-50 concrete mix has shown better performance compared to the other two AAC mixes, i.e., AAC-0 and AAC-100. The results of this work can certainly be used to predict the non-linear behaviour of concrete and can be adopted in the design of structural elements.

Alkali Activated Concrete using Industrial and Agro Waste-Mix proportioning and Experimental Investigation

The demand for concrete is increasing day by day due to vast development in urbanisation. One of the important ingredients to manufacture concrete is cement whose production is leading to large amount of carbon dioxide (CO2) releases attributing to global warming. To reduce the usage of cement in concrete, Supplementary Cementitious Materials (SCM) are given importance and plenty of research is done on them. SCM’s can replace cement upto permissible limits and enhance the concrete properties. The concretes that can replace the cement content completely are Geo Polymer Concrete (GPC) and Alkali Activated Concrete (AAC). They are eco-friendly concretes in which the wastes coming from various types of industries, treatment plants and agro wastes can be used as binders. Alkaline activator solution is used to activate the binder materials which is a combination of silicates and hydroxides. GPC and AAC has wide range of applications like making of concrete benches in park, balusters, paver blocks and non-load bearing components etc. In this paper, a novel mix design for AAC for a constant alkaline solution to binder ratio of 0.4 is presented. The molarity of sodium hydroxide used in this study is 10M. Various mix combinations of binders using Fly ash, Ground Granulated Blast Furnace Slag (GGBS) and Rice husk ash (RHA) were considered. Workability, Physical and mechanical properties of AAC mixes were determined including Non-Destructive tests (NDT). Mixes with RHA resulted in poor compressive strength as the water demand was high and curing under ambient conditions proved to be of low impact especially for AAC with RHA.

Influence of alkali content and silica modulus on the carbonation kinetics of alkali-activated slag concrete

Carbonation is inevitable process during the service life of concrete structures, where CO2 causes decalcification of the calcium-bearing phases. These changes affect the durability of concrete and accelerate the corrosion of reinforcement. Alkali-activated materials (AAMs) are alternative, cement-free binders based on aluminosilicate rich precursor and alkaline activator. The interest in AAMs increased during the last century, due to the production process with low CO2 footprint comparing to Portland cement (PC) concrete, the possibility to use wide range of industrial by-products as precursors and comparable performance to PC concrete. Despite the extensive research in this field, the carbonation resistance of AAMs needs to be better understood, due to the differences and complexity of binder chemistry compared to PC concrete. The propagation of carbonation process will depend on chemical composition of the precursors and the type and dosage of activators. This paper presents the results of microstructural changes of three alkali-activated concrete mixes after exposure to accelerated carbonation. Ground granulated blast furnace slag was used as a precursor and sodium hydroxide and sodium silicate as activators. Three mixes have constant water to binder ratio and slag content, while alkali content and silica modulus were varied. The carbonation resistance was evaluated by testing carbonation depth after 7 and 28 days of exposure in carbonation chamber. Microstructural changes during carbonation were investigated by thermogravimetric analysis and mercury intrusion porosimetry.

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.

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.

Chloride diffusion and chloride-induced corrosion of steel embedded in natural pozzolan-based alkali activated concrete

Durability of emerging alkali-activated binder (AAB) widely viewed as alternative to ordinary Portland cement (OPC) is yet to be fully understood particularly when exposed to chloride-contaminated service environment. In this study, the resistance of natural pozzolan (NP) based alkali activated concrete (AAC) incorporating nanosilica (nSiO2) to chloride penetration and chloride-induced corrosion was evaluated. The performance of AAC upon exposure to chloride environment was assessed by conducting rapid chloride permeability, chloride migration and chloride-induced corrosion by measuring half-cell corrosion potentials, corrosion current density (Icorr) and gravimetric weight loss of embedded rebar. Additionally, accelerated corrosion by impressed anodic potential was conducted to validate the results. The chloride permeability was classified between moderate and high, while the chloride migration coefficient was in the range of 6.22 to 12.82 (×10−12) m2/s, lowest in the 7.5%-nSiO2 modified AAC followed by 5%-nSiO2. Though the corrosion potentials were more negative, the Icorr was in the range of 0.385 to 0.636 μA/cm2 in various AAC mixes as compared to 0.425 μA/cm2 recorded in the OPC concrete. The gravimetric weight loss of steel embedded in the AAC prepared with 0, 1, 2.5, 5 and 7.5% nSiO2 was 1.61, 0.99, 0.79, 0.67 and 0.68%, respectively. Therefore, it is postulated that the durability of AAC was comparable to that of OPC concrete exposed to chloride environment and the blend of 5–7.5% of nSiO2 in to NP outperformed other AAC mixes. However, it appears from the results that the electrochemical measurements do not reflect the accurate condition of the rebar embedded in AAC that could be attributed to lack of oxygen at the rebar level and lower pH in these binders. Hence, the specifications originally designed for OPC to assess the condition of the rebar may not be appropriate to be applicable for AAC.

Experimental investigation on physical and mechanical properties of alkali activated concrete using industrial and agro waste

Increasing infrastructure demands the need for using huge quantities of concrete. Concrete manufacture requires large amounts of cement whose production of one-ton results nearly one ton of CO2 emission into the atmosphere attributing to global warming. As a step towards sustainability, the use of supplementary cementitious materials (SCMs) for cement has been adopted globally. Alkali Activated Concrete (AAC) and Geo polymer Concrete (GPC) are great alternatives for regular conventional concrete because they make use of industrial and agro wastes as binders without cement. AAC is composed of two components-paste and aggregates. The paste is essentially a mixture of rich Calcium silicate or Alumino silicate materials as solid precursor and alkaline activator. Activators like alkali silicates, alkali hydroxides, alkali sulphates, alkali carbonates can be used to increase the value of pH of the mixture and quicken the splitting of the solid precursor. The present study proposed the novel mix proportioning of alkali activated concrete with possible industrial and agro wastes as precursors and homogeneous mixture of sodium silicate (Na2SiO3) and sodium hydroxide (NaOH) as alkaline solution activator followed by experimental investigations. Series of experimental investigations are conducted to evaluate the fresh and hardened properties of AAC with constant NaOH molarity of 10 M and alkaline solution to binder ratio of 0.5. Several combinations involving fly ash, fly ash and slag, slag, fly ash and rice husk ash, slag and rice husk ash, etc will be adopted for the experimental investigations. Fresh concrete properties like workability and mechanical properties like compressive strength will be evaluated based on the work carried out using different combinations. Curing of AAC specimens is carried out in ambient temperature and the expected outcome will be a benchmark in making small scale concrete products like balusters, concrete benches at parks, paver blocks, and any other non-load bearing components using AAC mixes.

RILEM TC 247-DTA round robin test: mix design and reproducibility of compressive strength of alkali-activated concretes

The aim of RILEM TC 247-DTA ‘Durability Testing of Alkali-Activated Materials’ is to identify and validate methodologies for testing the durability of alkali-activated concretes. To underpin the durability testing work of this committee, five alkali-activated concrete mixes were developed based on blast furnace slag, fly ash, and flash-calcined metakaolin. The concretes were designed with different intended performance levels, aiming to assess the capability of test methods to discriminate between concretes on this basis. A total of fifteen laboratories worldwide participated in this round robin test programme, where all concretes were produced with the same mix designs, from single-source aluminosilicate precursors and locally available aggregates. This paper reports the mix designs tested, and the compressive strength results obtained, including critical insight into reasons for the observed variability in strength within and between laboratories.

Enhancing the engineering properties and microstructure of room temperature cured alkali activated natural pozzolan based concrete utilizing nanosilica

The strength gain of alkali-activated binders (AAB) when cured at room temperature is delayed due to the slow polymerization process. This is the major impediment in the utilization of this green concrete in cast in-situ concrete structures. In order to expand the applications of these binders to various fields of construction, attempts have to be intensified to overcome this obstacle. Incorporation of finer materials with high specific surface area to enhance the reactivity of alkali activated concrete (AAC) is one of the potential approaches to accelerate their strength gain. In this regard, nanomaterials, such as nanosilica (NS), can play a key role. Therefore, to enable curing at room temperature, incorporation of NS as partial replacement of natural pozzolan (NP) in developing AAC is the focus of this research work. AAC mixes were prepared by partially replacing NP with NS up to 7.5% by weight. The compressive strength development was monitored until 180 days of room temperature curing (23 ± 2 °C). Flexural strength and modulus of elasticity of concrete were also measured. Constitutive models relating the engineering properties of the developed AAC were formulated using regression analysis. In addition, SEM and XRD techniques were used to assess the nature of the binder formed during alkali activation. The data developed in the reported study indicated that NP, without incorporating NS, can be used as precursor material for producing AAC that could be potentially used as a construction material. However, AAC mixtures incorporating NS exhibited remarkable enhancement in the mechanical properties and microstructural characteristics. The addition of, 5% and 7.5% NS resulted in superior mechanical properties and a denser microstructure due to the greater transformation of precursor material in to polymeric compounds. The tangible outcomes of this research highlights the importance of nanomaterials in improving the properties of green building materials.

Effect of alkaline activators and binder content on the properties of natural pozzolan-based alkali activated concrete

The stringent conditions are being imposed by the global community to reduce greenhouse gas emissions, will push the search for low carbon footprint alternative binders to ordinary Portland cement. In this pursuit numerous studies have been carried out to synthesize alkali activated concrete (AAC) utilizing industrial by-products, such as, fly ash that is rich in silica and alumina. Natural Pozzolan (NP) is one of such alternative cementitious materials having desirable chemical compositions that could be potentially used to develop AAC. The present work focuses on the effect of curing period, composition of alkaline activators and binder content on the properties of natural pozzolan-based AAC. A total of 15 AAC mixes were prepared by varying the sodium silicate to sodium hydroxide (SS/SH) by weight ratio from 2.0 to 2.75 and binder content between 350kg/m3 and 450kg/m3. Fresh properties, such as, setting time of alkali activated pastes (AAP) and workability of concrete were determined. Compressive strength was measured on the specimens cured for 0.5, 1, 3, 7, 14 and 28days in an oven maintained at 60°C. For selected mixtures, scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques were utilized to study the morphology and mineral composition of the developed alkali activated paste (AAP), respectively. Additionally, FTIR was used to identify functional groups formed in the AAP. The results showed that the composition of alkaline activators and binder content have significant effect on the properties of AAC. Considerable strength gain was recorded after 3days of curing and further curing up to 7days improved the strength. However, continued curing up to 28days marginally decreased the strength. Further, SS/SH ratio of 2.5 and binder content of 400kg/m3, resulting in alkaline activators to binder ratio of 0.525, was found to be suitable for achieving superior strength and microstructural characteristics.