Alkali-activated Concrete Research Articles

Comparison of axial behavior of RC columns using alkali-activated slag-based concrete and OPC concrete during and after fire

As no study on fire behavior of RC columns using alkali-activated concrete has been reported so far, this paper presents an experimental program comparatively investigating the thermal response and structural behavior of reinforced columns using alkali-activated slag-based concrete (AAC) and Portland cement-based concrete (PCC) during and after subjecting to the standard fire curve, with concrete strength and fire duration as test parameters. The experimental and analytical studies showed that compared with PCC columns, AAC columns had slightly lower temperature field but faster increase of axial deformation during fire. AAC and PCC columns using normal-strength concrete had equivalent residual axial capacity after exposure to fire. However, AAC columns using high-strength concrete retained greater load-bearing capacity than their PCC counterparts due to the spalling of high-strength PCC columns. AAC columns had lower axial stiffness than PCC columns at ambient temperature due to lower elastic modulus of AAC, but they had similar axial stiffness after exposure to fire of 2 h since PCC underwent more degradation of elastic modulus under high temperature. Furthermore, as concrete strength and fire duration increased, the axial contraction under heating and axial capacity loss after heating of AAC columns increased. A numerical method based on finite difference was used to predict the temperature field and corresponding residual axial capacity of RC columns after the fire.

Data driven design of alkali-activated concrete using sequential learning

This paper presents a novel approach for developing sustainable building materials through Sequential Learning. Data sets with a total of 1367 formulations of different types of alkali-activated building materials, including fly ash and blast furnace slag-based concrete and their respective compressive strength and CO2-footprint, were compiled from the literature to develop and evaluate this approach. Utilizing this data, a comprehensive computational study was undertaken to evaluate the efficacy of the proposed material design methodologies, simulating laboratory conditions reflective of real-world scenarios. The results indicate a significant reduction in development time and lower research costs enabled through predictions with machine learning. This work challenges common practices in data-driven materials development for building materials. Our results show, training data required for data-driven design may be much less than commonly suggested. Further, it is more important to establish a practical design framework than to choose more accurate models. This approach can be immediately implemented into practical applications and can be translated into significant advances in sustainable building materials development.

Enhanced microstructure and mechanical properties of cementless ultra-high-performance fiber-reinforced alkali-activated concrete with silicon dioxide nanoparticles

This study investigates the effect of silicon dioxide nanoparticles (nano-SiO2) on the microstructure and mechanical properties of eco-friendly, cementless ultra-high-performance fiber-reinforced alkali-activated concrete (UHP-FRAAC). Various amounts of nano-SiO2 were adopted in a range of 0%-5% of the mass of silica fume (SF). The experimental results showed that the addition of nano-SiO2 improves the packing density of ultra-high-performance alkali-activated concrete (UHP-AAC) and generates abundant calcium (alumino)silicate hydrate (C-(A-)S-H) gels to increase its density. Therefore, the interfacial bond strength and pull-out energy of steel fibers from UHP-AAC could be enhanced. At 2% nano-SiO2 replacement, the compressive strength of UHP-FRAAC was the highest (184.2 MPa), and its total porosity was the lowest, decreasing from 9.72% to 7.53%. The highest equivalent bond strength and strain energy density of UHP-FRAAC occurred at 2% nano-SiO2 replacement, i.e., 10.9 MPa and 72.5 kJ/m3, respectively. The maximum tensile strength of 14.5 MPa was observed at a nano-SiO2 content of 3%. Higher dosages of nano-SiO2, 3% or more, negatively affected the interfacial bond and tensile properties of UHP-FRAAC due to its agglomeration, but still exhibited higher performance than that of the plain UHP-FRAAC up to the dosage of 5%. Therefore, the optimal dosage of nano-SiO2 in UHP-FRAAC was suggested to be 2% by mass of silica fume.

Effects of rice husk ash on strength and durability performance of slag‐based alkali‐activated concrete

AbstractDespite their low impact on the environment and excellent mechanical strength, alkali‐activated concretes (AAC) can potentially replace ordinary Portland cement based concrete (OPC). However, AAC can be eco‐friendly and more sustainable by incorporating agricultural waste such as rice husk ash (RHA). Therefore, this study investigated the impact of RHA on the strength and durability performance of ground granulated blast furnace slag (GGBS) based AAC. For this purpose, seven mixes were made, in which RHA partially substituted GGBS with an increment of 5% up to 30%. The results of the experiments show that the workability and unit weight of AAC mixes decreased as the amount of RHA in the mix increased. The compressive strength of AAC mixes lies between 39.78 and 64.80 MPa, which is adequate for structural application. The AAC showed similar trends for all the mixes in terms of water absorption, permeable voids, apparent porosity and sorptivity. Compared to other mixes, the 5% GGBS substituted with RHA yields the highest resistance against carbonation. Compared to the sulfuric acid (H2SO4) cured sample, the specimen treated with hydrochloric acid (HCl) performed better in loss in mass, strength and ultrasonic pulse velocity. From scanning electron microscope test, the dense microstructure with pore refinement was observed in GGBS based AAC mix with 10% RHA content. According to the findings, RHA content up to 10% substitution can be used as a substitute for the binder to produce sustainable AAC with greater durability and could eventually replace conventional concrete in structural applications.

Investigation of Mechanical and Durability Properties of Alkali-Activated Concrete Prepared with Ternary Blended Ground Granulated Blast Furnace Slag, Fly Ash, and Jajrood Trass

Investigation of Mechanical and Durability Properties of Alkali-Activated Concrete Prepared with Ternary Blended Ground Granulated Blast Furnace Slag, Fly Ash, and Jajrood Trass

Hybrid reinforcement of steel–polyethylene fibers in cementless ultra-high performance alkali-activated concrete with various silica sand dosages

The effects of steel and polyethylene (PE) fiber hybrid reinforcement on the mechanical performance of cementless ultra-high performance alkali-activated concrete (UHP-AAC) containing an industrial by-product and various amounts of silica sand were investigated. Replacing steel fibers with PE fibers reduced the compressive and tensile strengths, with the best performance was observed in the hybrid specimens with a medium-grade fine aggregate-to-binder (FA/B) ratio. Increasing the PE fiber replacement ratio and decreasing the FA/B ratio increased the occurrence of multiple microcracks. The FA/B ratio had a greater effect on the variation in the pseudo-strain hardening indices than the fiber replacement ratio. The highest complementary energy, 1,762.5 J/m2, was obtained for PE fiber-reinforced UHP-AAC at an FA/B ratio of 0.16, while the lowest value, 309.6 J/m2, was obtained for steel-fiber-reinforced UHP-AAC at the same FA/B ratio. In conclusion, the hybrid reinforcement of UHP-AAC can be optimized by considering the FA/B ratio and fiber replacement ratio.

Realistic Evaluation of Reinforcement Bond Strength in Alkali-Activated Slag Concrete Exposed to Elevated Temperature

Alkali-activated concrete (AAC) has attained great popularity since finding it as an alternative to Portland cement concrete due to its superior characteristics in terms of mechanical properties and durability, and its low negative environmental impact. This research investigated both experimentally and analytically the bond behavior between alkali-activated slag concrete (AASC) and steel rebars considering some important parameters (rebar diameter and development length-to-diameter ratio) before and after exposure to elevated temperature using beam-end bond testing technique. The obtained experimental results were compared with those obtained from applying the CEB-FIP model and the well-known available equations in the literature. A modified model was proposed for predicting the bond behavior of AASC. Results have showed that the CEB-FIP model provides more conservative values for bond strength compared to the experimentally obtained results which increases the safety level when estimating the bond strength for design purposes. The proposed modified model achieved a higher correlation with the experimental results than the CEB-FIP model at ambient temperature.

Influence of mix proportioning parameters and curing regimes on the properties of ultra-high strength alkali-activated concrete

This study investigates the influence of mix proportioning parameters and curing regimes on the material properties of alkali-activated binder-based ultra-high strength concrete (AAB-UHSC). The effects of five groups of mix proportioning parameters (i.e., fly ash replacement ratio, silica fume replacement ratio, alkali to binder ratio, water glass modulus, and steel fiber content) and three curing regimes(i.e., ambient curing, standard curing, and hybrid curing) on the workability, compressive strength, flexural strength and microstructure development of AAB-UHSC were systematically evaluated. The results indicated that increasing the steel fiber and silica fume dosage negatively affected the flowability of freshly mixed AAB-UHSC. There existed an optimal alkali-to-binder ratio and water glass modulus in terms of flowability, compressive strength, and flexural strength, but the fly ash replacement ratio had a complex effect on the material performance of the AAB-UHSC. The microstructures of the specimens cured in hybrid curing regimes were denser, and fewer cracks were observed, which resulted in higher strength compared to specimens cured under ambient curing and standard curing conditions. The three types of curing regimes adopted barely alter the phases and molecular bonds of hydration products based on the XRD and FTIR results. In addition, the correlations between the compressive strength of the AAB-UHSC obtained under ambient curing conditions, standard curing conditions, and obtained under hybrid curing and standard curing conditions were analyzed, and the fitted results had higher correlation coefficients (R2).

The Effect of Preconditioning Temperature on Gas Permeability of Alkali-Activated Concretes.

Alkali-activated materials (AAM) are binders that are considered an eco-friendly alternative to conventional binders based on Portland cement. The utilization of industrial wastes such as fly ash (FA) and ground granulated blast furnace slag (GGBFS) instead of cement enables a reduction of the CO2 emissions caused by clinker production. Although researchers are highly interested in the use of alkali-activated concrete (AAC) in construction, its application remains very restricted. As many standards for hydraulic concrete's gas permeability evaluation require a specific drying temperature, we would like to emphasize the sensitivity of AAM to such preconditioning. Therefore, this paper presents the impact of different drying temperatures on gas permeability and pore structure for AAC5, AAC20, and AAC35, which contain alkali-activated (AA) binders made from blends of FA and GGBFS in slag proportions of 5%, 20%, and 35% by the mass of FA, respectively. The preconditioning of samples was performed at 20, 40, 80, and 105 °C, up to the obtainment of constant mass, and then gas permeability was evaluated, as well as porosity and pore size distribution (mercury intrusion porosity (MIP) for 20 and 105 °C). The experimental results demonstrate up to a three-percentage-point rise in the total porosity of low-slag concrete after 105 °C in comparison to 20 °C, as well as a significant increase in gas permeability, reaching up to 30-fold amplification, contingent upon the matrix composition. Notably, the alteration in pore size distribution, influenced by the preconditioning temperature, exhibits a substantial impact. The results highlight an important sensitivity of permeability to thermal preconditioning.

Tension Stiffening and Cracking Behavior of Axially Loaded Alkali-Activated Concrete.

Alkali-activated concrete is an eco-friendly construction material that is used to preserve natural resources and promote sustainability in the construction industry. This emerging concrete consists of fine and coarse aggregates and fly ash that constitute the binder when mixed with alkaline activators, such as sodium hydroxide (NaOH) and sodium silicate (Na2SiO3). However, understanding its tension stiffening and crack spacing and width is of critical importance in fulfilling serviceability requirements. Therefore, this research aims to evaluate the tension stiffening and cracking performance of alkali-activated (AA) concrete. The variables considered in this study were compressive strength (fc) and concrete cover-to-bar diameter (Cc/db) ratios. After casting the specimen, they were cured before testing at ambient curing conditions for 180 days to reduce the effects of concrete shrinkage and obtain more realistic cracking results. The results showed that both AA and OPC concrete prisms develop slightly similar axial cracking force and corresponding cracking strain, but OPC concrete prisms exhibited a brittle behavior, resulting in a sudden drop in the load-strain curves at the crack location. In contrast, AA concrete prisms developed more than one crack simultaneously, suggesting a more uniform tensile strength compared to OPC specimens. The tension-stiffening factor (β) of AA concrete exhibited better ductile behavior than OPC concrete due to the strain compatibility between concrete and steel even after crack ignition. It was also observed that increasing the confinement (Cc/db ratio) around the steel bar delays internal crack formation and enhances tension stiffening in AAC. Comparing the experimental crack spacing and width with the values predicted using OPC codes of practice, such as EC2 and ACI 224R, revealed that EC2 tends to underestimate the maximum crack width, while ACI 224R provided better predictions. Thus, models to predict crack spacing and width have been proposed accordingly.

Study on Shrinkage in Alkali-Activated Slag-Fly Ash Cementitious Materials.

Traditional silicate cement materials produce a large amount of CO2 during production, making it urgent to seek alternatives. Alkali-activated slag cement is a good substitute, as its production process has low carbon emissions and energy consumption, and it can comprehensively utilize various types of industrial waste residue while possessing superior physical and chemical properties. However, the shrinkage of alkali-activated concrete can be larger than that of traditional silicate concrete. To address this issue, the present study utilized slag powder as the raw material, sodium silicate (water glass) as the alkaline activator, and incorporated fly ash and fine sand to study the dry shrinkage and autogenous shrinkage values of alkali cementitious material under different content. Furthermore, combined with the change trend of pore structure, the impact of their content on the drying shrinkage and autogenous shrinkage of alkali-activated slag cement was discussed. Based on the author's previous research, it was found that by sacrificing a certain mechanical strength, adding fly ash and fine sand can effectively reduce the drying shrinkage and autogenous shrinkage values of alkali-activated slag cement. The higher the content, the greater the strength loss of the material and the lower the shrinkage value. When the fly ash content was 60%, the drying shrinkage and autogenous shrinkage of the alkali-activated slag cement mortar specimens decreased by about 30% and 24%, respectively. When the fine sand content was 40%, the drying shrinkage and autogenous shrinkage of the alkali-activated slag cement mortar specimens decreased by about 14% and 4%, respectively.

Performance of Alkali-Activated Concrete Mixes Incorporating Precious Slag as Fine Aggregate

Performance of Alkali-Activated Concrete Mixes Incorporating Precious Slag as Fine Aggregate

Development of Alkali-Activated 3D Printable Concrete: A Review

The construction world has changed day by day and is becoming more digitalized by introducing new technologies. Three-dimensional concrete printing (3DCP) is one such technology that has automated building process along with several benefits such as reduced material waste, reduced human hazard, and time savings. Traditionally, this technique utilizes cement to construct numerous structures, resulting in a significant carbon footprint and negative environmental impact. There is a need to find alternate solutions to reduce cement consumption. Alkali activation technology has replaced cement completely. The scope of development of alkali-activated 3D printable concrete utilizing agro-industrial byproducts is presented in this study. A review of the fresh and hardened properties of alkali-activated 3D printable concrete was the primary objective. The change in properties of 3D concrete mixes with the variation of additives that influence the ultimate strength parameters is presented. This study explores the curing conditions and in-depth behavior of uses of 3DCP in the construction industry. The environmental benefits over conventional concreting technology are presented. As per previous studies, the optimum mix composition per cubic meter concrete is 600–700 kg/m3 of binder content, 450 kg/m3 of alkali activator solution, and 600–800 kg/m3 of fine aggregate content. This study contributes to the making of 3D printable alkali-activated concrete.

Examining the use of Lime and Gypsum in GGBS based alkali activated concrete

Alkali Activated Concrete (AAC) using Ground Granulated Blast Furnace Slag (GGBS) and Fly ash as precursors attracted several researchers in developing no cement concretes. Use of Fly ash as precursor needs high temperature for curing. GGBS as precursor avoids temperature curing, but setting is too fast to accept or compare with conventional or ordinary Portland Cement. Many researchers have used the combination of GGBS and Fly ash as precursor in developing AAC. Lime resources are plentily available, and use of lime be encouraged for developing sustainable concretes. Thus, an attempt has been made to examine the use of lime to some extent in the AAC. The present paper examines the use of lime and gypsum along with GGBS and metakaolin as precursor in developing AAC. 4 M Sodium hydroxide and sodium silicate solution is used as alkaline activator. Consistency, setting times and compressive strength of these mixes are reported. It is found that lime and gypsum addition has increased the consistency as well as setting times. It is suggested that precursor with 75% GGBS, 20% Metakaolin and 5% Lime showed good compressive strength. Addition of gypsum lowered the compressive strength of the mix.

Mechanical properties of alkali-activated concrete exposed to cryogenic temperature

As a promising and sustainable construction material, alkali-activated concrete (AAC) may be exposed to harsh cryogenic environments. However, the mechanical properties of AAC at cryogenic temperatures remain unexplored. This is probably the first study that focuses on the mechanical performances, i.e., compressive strength, elastic modulus, splitting tensile strength, fracture energy, characteristic length and fracture toughness of AAC exposed to cryogenic temperatures as low as −160 °C. According to the test results, the mechanical properties of the AAC enhanced dramatically as temperature decreased. Due to the high moisture content and water-accessible porosity, above hardened properties of AAC with lower strength grades were more susceptible to low temperatures. Compared to ordinary Portland cement-based concrete (OPCC) with similar strength grades, AAC exhibited superior mechanical characteristics at comparable temperatures. Based on the test data, empirical relations were proposed to correlate the mechanical performances of AAC and temperatures.

Prediction & optimization of alkali-activated concrete based on the random forest machine learning algorithm

Alkali-activated concrete (AAC) is regarded as a promising alternative construction material to reduce the CO2 emission induced by Portland cement (PC) concrete. Due to the diversity in raw materials and complexity of reaction mechanisms, a commonly applied design code is still absent to date. This study attempts to directly correlate the AAC mix design parameters to their performances through an artificial intelligence approach. To be specific, 145 fresh property data and 193 mechanical strength data were collected from laboratory tests on 52 AAC mixtures, which were used as inputs for the machine learning algorithm. Five independent random forest (RF) models were established, which are able to predict fresh and hardened properties (in terms of compressive strength, slump values, static/dynamic yield stress, and plastic viscosity) of AAC with equivalent accuracy reported in the literature. Moreover, an inverse optimization was performed on the RF model obtained to reduce the sodium silicate dosages, which may further mitigate the environmental impact of producing AAC. The present RF model gives practical information on AAC mix design cases.

A mix design methodology of blast furnace slag and fly ash-based alkali-activated concrete

Although the application of blast furnace slag and fly ash-based alkali-activated concrete (BFS/FA-AAC) has both economic and environmental benefits, it is limited by the lack of a straightforward mix design method. In this paper, an experiment was conducted to investigate the effect of control factors, including the Na2O/binder ratio, the SiO2/Na2O ratio, the BFS/binder ratio, the water/binder ratio, and the water content on the workability (slump and rheology) of BFS/FA-AAC, and the effect of control factors include the Na2O/binder ratio, the SiO2/Na2O ratio, the BFS/binder ratio, the water/binder ratio, and the curing time on the compressive strength of BFS/FA-AAC. As a result, the influence degree and mechanism of each control factor on the performance of BFS/FA-AAC were quantitively explored and the accuracy of an empirical compressive strength formula was validated. Based on that, a practical mix design method of BFS/FA-AAC was eventually established. It is found that the mixture composition and content of paste can significantly influence the workability of BFS/FA-AAC. The compressive strength of BFS/FA-AAC is determined by control factors when the water content is within 160–195 kg/m3. The mechanical predictive method of BFS/FA-AAC is proven of high accuracy. The mixture designed by this methodology exhibits satisfied fresh and hardened performance as well as high environmental benefits.

Compressive and splitting tensile impact properties of rubberised one-part alkali-activated concrete

This paper presents an experimental assessment of the compressive and splitting tensile properties of rubberised one-part alkali-activated concrete under quasi-static and low-velocity impact loading. An optimised mix design, employing blast furnace slag and fly ash as precursors and anhydrous sodium metasilicate as a solid activator, is used as a reference. Rubber contents of up to 60% volumetric replacement of total natural aggregates are considered. Quasi-static tests are performed using servo-hydraulic machines, whilst the impact tests are performed in an instrumented drop-weight loading rig. Digital image correlation is used to get displacement measurements under both quasi-static and impact loading conditions. Three impact velocities of 5, 10, and 15 m/s are considered, giving rise to strain-rates in the range of 3–270 s−1. The quasi-static results show shape- and size-dependency and characteristically lower compressive and splitting tensile strengths with higher rubber content. The dynamic properties are notably influenced by the rubber content, with a higher ratio resulting in greater impact duration under compressive loading, reduced peak compressive strength, and reduced peak splitting tensile strength. The shape of the stress-strain response under compressive loading changes with rubber addition, showing two major peaks as opposed to a single peak for the non-rubberised specimens. The dynamic mechanical properties are also strain-rate dependent, exhibiting an increase with higher strain-rates. The rubberised specimens exhibit higher strain-rate sensitivity in splitting tension than compression, signified by higher dynamic increase factors for a given strain-rate and lower critical transition strain-rates. A higher rubber content in the mix also result in reduced critical transition strain-rates for the compressive strength, axial crushing strain, and splitting tensile strength. Based on the results of this study, analytical expressions are provided for predicting the dynamic increase factors for the compressive strength, axial crushing strain, elastic modulus, and splitting tensile strength.

Mechanical Properties Investigation and Optimization of Ambiently Cured Alkali-Activated Concrete

The mechanical, microstructural and statical interpretation of the Alkali Activated Concrete (AAC), prepared using fly ash with crushed granulated corex slag as the binder ingredient, are examined in this work. Here, cast in-situ alkali-activated concrete strengths and substitute slag concentrations of 0-50 % by fly-ash weight were undertaken to determine a workable manufacturing process and were all considered for up to 28 days. Moreover, in microstructural research, C-A-S-H gel is produced by adding Ground Granulated Corex Slag (GGCS) to the binder-generated structural changes in the in-situ alkali-activated concrete. As a result, this research obtained perfect mixture fractions by synthesizing 25 % GGCS with 0.4 % of the liquid-to-binder ratio, in addition, a unified desirability of 80 % was attained.

Improvement of Durability Aspects of CAC-Based Alkali-Activated Concrete

Improvement of Durability Aspects of CAC-Based Alkali-Activated Concrete