Search

Alkali-activated binders (AAB) are low-carbon alternatives to cement used in traditional concrete manufacture. This paper presents a state-of-the-art survey of the historical background, synthesis development, reaction mechanism, and recent advances of the AAB. The parameters influencing the engineering performance of the AABs, including the physicochemical characteristics of the precursor materials, formulation and dosage of alkaline activators, and type and period of curing, were critically reviewed. In this regard, three different precursor material sources are identified: industrial by-products, natural aluminosilicates, and agricultural waste materials, which have various availability, reactivity, cost, and value worldwide. In addition, recent advances in the life cycle assessment of AAB mixtures are compiled. The challenges that AABs face towards commercial applications are also discussed. The selection of appropriate precursors, the production of low-cost alkaline activators, mix design parameters, fundamental mechanisms, engineering properties, construction processing and application, performance standards, and sustainability aspects constitute the challenges. Alkali-activated concrete can be a potential low-carbon substitute for conventional concrete if it ensures true sustainability and not just greenwashing. • Collation of recent technical updates on alkali-activated cementitious composites. • Informs readers of the nuances and challenges of alkali-activated materials. • Recent progress in understanding the reaction kinetics and engineering properties of alkali-activated binders summarized. • An overview of LCA studies conducted on alkali-activated binders. [ABSTRACT FROM AUTHOR]

This study monitors the early-age reaction kinetics and measures the heat of reaction of alkali-activated ground granulated blast-furnace slag (GGBFS) using an isothermal calorimeter, and the initial setting and possible product formations by measuring ultrasonic pulse velocity (UPV). The GGBFS was activated with alkaline activator solutions of different concentrations in terms of modulus ratio (Ms) and Na2O% content. Results reveal that GGBFS is highly sensitive to the compositions of the alkaline solutions. Increasing the alkalinity of the activator results in better dissolution of species and consequently an increased heat evolution with a relatively shorter setting time. The activator concentration also affects the rate of product formations as well as the morphological features. Activators with higher Ms lead to homogeneous slag morphology. The abundance of silicate species in the alkaline solution also plays an important role in its structural formation. scanning electron microscope with energy-dispersive X-ray spectroscopy (SEM-EDS) microstructural characterizations supports the advanced dissolution of particles in a more alkaline environment. The presence of C-(A)-S-H gels in alkali-activated slag pastes was detected. [ABSTRACT FROM AUTHOR]

• Studied the durability of seawater and sea sand-based ultra-high-performance concrete (SWSS-UHPC) with supplementary cementitious materials (SCM). • Non-destructive tests (i.e., ultrasonic pulse velocity, and electrical resistivity) were correlated to the mechanical and durability properties. • The chloride binding capacity was determined. • SCMs offer a significant enhancement in transport properties and durability. • Electrical resistivity can successfully predict chloride penetration resistance. While concrete production accounts for significant CO 2 emissions and natural resources (e.g., river sand and freshwater) depletion, the incorporation of sea sand, seawater, and industrial by-products in ultra-high-performance concrete (UHPC) manufacture could offset such impacts. However, the transport and durability performances of marine resource-based UHPC are not well documented. This paper presents an experimental investigation into the mechanical, transport, and chloride binding characteristics of seawater and sea sand-based ultra-high-performance concrete (SWSS-UHPC) with supplementary cementitious materials (SCM) (i.e., slag and silica fume). Mixes were developed with varying proportions of SCMs (up to 62.5%) and sources of aggregates. Early-age and long-term mechanical (i.e., modulus of elasticity, splitting tensile strength) and transport properties (i.e., water absorption, porosity, and ingress of chloride) were evaluated. Non-destructive tests (i.e., ultrasonic pulse velocity, and electrical resistivity) were correlated to the mechanical and durability properties. The chloride binding capacity was determined. Results reveal that although SCMs in SWSS-UHPC marginally improve its mechanical performance, it offers a significant enhancement in transport properties and durability, particularly at 50% cement replacement. The incorporation of marine resources can be beneficial against chloride diffusion and does not adversely affect the binding of external chloride. Electrical resistivity can successfully predict chloride penetration resistance. SCM-based SWSS-UHPC can be extremely durable for long-term exposure in spray and tidal/splash zones of maritime structures. [ABSTRACT FROM AUTHOR]

Effects of maximum aggregate size (12.5 mm, 19.0 mm, 25.0 mm, 37.5 mm, and 50.0 mm) on properties of concrete made with different sand to total aggregate volume ratio (0.40 and 0.45), W/C ratio (0.45, 0.50, and 0.55), and cement content (375 kg/m 3 and 400 kg/m 3 ) were investigated. Considering the variables, a total of 552 concrete cylinder specimens of diameter 100 mm diameter and height 200 mm were made for 52 numbers of independent cases. Brick aggregates were tested for specific gravity, absorption capacity, unit weight, and abrasion resistance. Concrete specimens were tested for compressive strength, stress-strain curve, splitting tensile strength, and Young's modulus. Results have revealed that for higher cement content (400 kg/m 3 ), concrete made with small aggregates give more compressive strength. However, for a cement content of 375 kg/m 3 , and W/C ratio of 0.45, the compressive strength is increased with an increase in maximum aggregate size up to 37.5 mm. The compressive strength of concrete increases with an increase in s/a ratio from 0.40 to 0.45. Relationships between mechanical properties of concrete, and stress-strain relationships are proposed for different maximum aggregate sizes. [ABSTRACT FROM AUTHOR]

• Influence of supplementary admixtures on fresh properties, hydration and strength are compared. • Silica fume substantially increases yield stress, while glass powder reduces yield stress. • Final setting time is reduced in biochar-cement, while fly ash and glass powder delays the setting. • Biochar has the highest accelerating effect on hydration kinetics due to higher surface area for nucleation. • Silica fume and biochar offer 14% higher strength than control at 28-day mark. The selection of mineral admixtures to develop low-carbon cement composites must be diversified to balance the increasing demand and low supply of certain widely used admixtures. The research aims to advance scientific knowledge into the comparative influence of several admixtures recovered from different waste streams including silica fume (SF), fly ash (FA), pulverized glass powder (PGP), biochar (BC), and high-performance ash (HPA) on static yield stress, dynamic yield stress, plastic viscosity, setting, micro-structural build-up, degree of hydration and compressive strength of cementitious paste. Experimental findings show that adding 10 wt% silica fume results in a 300% increase in yield stress (under static and dynamic shear rate) and plastic viscosity than control, attributed to its fine particle size and higher gelling rate during early hydration. HPA leads to an 83% increase in static yield stress compared to control at 70 mins, but the application of a high shear rate leads to a marginal increase (18%) in yield stress than control. PGP leads to improvement in workability by reducing the dynamic and static yield stress by 50 – 55% than the control without significant change in plastic viscosity. Comparing the influence of BC, FA, and PGP, which have comparable particle size distribution in this research, the addition of biochar accelerates micro-structural build-up and final setting, evident from higher ultrasonic pulse velocity and faster hydration kinetics by 1.20 h. Cement pastes with FA, PGP and HPA demonstrate significantly lower compressive strength at 3-day and 7-day age than control but offer similar 28-day strength. The addition of SF and BC do not compromise 3-day and 7-day strength while offering 16% enhancement in 28-day strength than control attributed to a higher degree of hydration due to pozzolanic and filler effects respectively. The findings suggest that PGP, BC, and HPA could be potential alternatives to more widely used FA and SF as supplementary mineral admixtures in cement-based construction materials. [ABSTRACT FROM AUTHOR]

Investigation was carried out to study the effects of maximum aggregate size (MAS) (12.5 mm, 19.0 mm, 25.0 mm, 37.5 mm, and 50.0 mm) on ultrasonic pulse velocity (UPV) of concrete. For investigation, first class bricks were collected and broken to make coarse aggregate. The aggregates were tested for specific gravity, absorption capacity, unit weight, and abrasion resistance. Cylindrical concrete specimens were made with different sand to aggregate volume ratio (s/a) (0.40 and 0.45), W/C ratio (0.45, 0.50, and 0.55), and cement content (375 kg/m 3 and 400 kg/m 3 ). The specimens were tested for compressive strength and Young’s modulus. UPV through wet specimen was measured using Portable Ultrasonic Non-destructive Digital Indicating Tester (PUNDIT). Results indicate that the pulse velocity through concrete increases with an increase in MAS. Relationships between UPV and compressive strength; and UPV and Young’s modulus of concrete are proposed for different maximum sizes of brick aggregate. [ABSTRACT FROM AUTHOR]

Research has shown that reaction kinetics and physical transformation during alkali-activation of aluminosilicate materials are sensitive to both the compositions of raw precursors and the alkaline solution. Methods that allow for early observations relating to reaction kinetics, and that capture early-age physicochemical transformations with sufficient accuracy, can assist design engineers in bringing this new generation of binders to market. In this study, reaction heat and ultrasonic pulse velocity (UPV) evolutions of low calcium fly ash activated with alkaline solutions of different concentrations, and substituted with various dosages of ground granulated blast furnace slag (GGBFS), were systematically monitored using isothermal calorimetric and portable ultrasonic testing facilities. The incorporation of slag in a fly ash-based system influences the reaction kinetics, rate of physical transformation and morphological features. The study shows that UPV can capture the effect of slag addition on the microstructure densification, which is confirmed by measurements of heat evolution and SEM-EDS microstructural observations. A linear correlation of UPV was identified with the heat evolved. It was further shown that UPV tests can successfully capture workable time of alkali-activated fly ash paste blends that contain at least 10% of slag, noting that mixtures without any slag did not set within the first 24 h of activation. Further, improved dissolution of species, leading to an increased heat evolution, was observed with increasing activator alkalinity. SEM-EDS microstructural characterisation reveals an advanced dissolution of particles in a more alkaline environment, while higher silicate content leads to a more homogeneous morphology. The findings of this research will assist in the utilisation of alkali-activated materials in construction practice. [ABSTRACT FROM AUTHOR]

• Geopolymer batching should integrate at least 10 min mixing. • Short mixing duration can lead to faster concrete setting and hardening. • A 10 min mixing confirms superior product quality. • VCMs are not to be employed in geopolymer concrete batching. Despite being investigated for over 50 years, the adaptation and application of geopolymers in construction are still very limited. One major reason behind this slow and often underappreciated acceptance of geopolymer concrete in industries is the lack of reports outlining observations and knowledge gained in field applications. Although geopolymers are well-accepted in the research community owing to their comparable or even better performance as a substitution of cement as a concrete binder, major initiatives still need to be undertaken in studying and reporting field observations of this product. The flash set phenomenon of a proven geopolymer concrete mix on a field trial motivated this investigation to identify the effects of mixing duration on geopolymer concrete performance. As such, this paper describes laboratory investigations undertaken to study the fresh, hardened, and durability properties of geopolymer concrete incorporating 65% fly ash and 35% slag, mixed for durations of 1 min and 10 min. The longer mixing duration significantly improved the workable time, at least five times longer workable time was achieved. It also resulted in relatively better mechanical strength and durability. The importance of proper mixing of geopolymer concrete constituents has been emphasized based on knowledge acquired from a combination of field trials and subsequent laboratory experiments. [ABSTRACT FROM AUTHOR]

The incorporation of supplementary cementitious materials (SCMs) into cementitious materials can be used to offset the overall carbon footprint of cement in addition to improving performance and promoting circular economy. Synthesized silicomanganese fume (SiMnF), silica fume (SF), and ordinary portland cement (OPC) based binary and ternary cementitious mortar specimens were designed and optimized using the Taguchi method. Four factors with three levels each were investigated—SiMnF content of 0%–40% and SF content of 0%–10% (by mass) of the total cementitious, sand-to-binder of 1.5–2.5, and water-to-binder ratio of 0.35–0.45. Based on the orthogonal array proposed by the Taguchi method, nine mortar mixes were batched and their flow after mixing and compressive strength at 3, 7, and 28 days of casting were measured. The strength data were statistically analyzed using ANOVA to investigate the effects of the chosen experimental variables. It was observed that the strength is considerably reduced from the addition of SiMnF, but the reduction is marginal from increasing the sand-to-binder ratio. The addition of 5% SF increased the strength. A restricted analysis indicated that specimens prepared with 20% SiMnF, or 20% SiMnF and 5% SF can yield mortar strengths of up to 30.5 MPa and 48.8 MPa, respectively. Microstructural investigations revealed that the mixes with SiMnF have detectable pores at 1,000× magnification, however, the addition of 5% SF densifies the matrix with no visible pore at the same magnification. This corroborates the strength data. The life-cycle assessment (LCA) indicates that the utilization of SiMnF in the mortar mixtures can reduce CO2 emissions by up to 25% at a reasonably acceptable compressive strength. [ABSTRACT FROM AUTHOR]

• High-density geopolymer concrete Hanbar armour units were made with SFS aggregates. • SFS aggregate expansion can be eliminated in low-calcium content geopolymer binders. • Observations of the field batching of geopolymer concrete are presented. Marine environments can pose continuous threats to coastal infrastructure, especially during episodic storm events. Thus, monitoring and maintenance of rock or concrete units-armoured coastal breakwaters are required, especially under present climate change scenarios, where changes in wave behaviour are anticipated and coastal structures are likely to be exposed to even greater wave energy resulting in higher rates of damage. A possible pathway towards the upgrading of existing breakwaters is to introduce high-density armour units, suggested by theoretical estimates and physical model test findings. In this project, a unique, sustainable high-density geopolymer concrete (GPC) mix was developed and trialled in fabricating armour units for upgrading existing coastal breakwaters. The system developed in laboratories was upscaled for field applications and is being tested at the Northern breakwater of NSW Ports' Port Kembla Harbour. The concrete uses steel furnace slag (SFS) aggregate in an alkali-activated blended fly ash-blast furnace slag binder proportioned to facilitate the elimination of delayed expansion of the aggregate. The concrete properties were measured and microstructural analyses undertaken. The results show that SFS aggregate offers higher bulk density to the concrete alongside satisfactory strength in on-site curing and can reduce armour mass requirements at the same level of structural stability. Microstructural analyses confirm the elimination of SFS aggregate free-lime expansion. This important result provides a novel approach to both repair of existing structures and construction of new structures with reductions to both cost and carbon footprint. [ABSTRACT FROM AUTHOR]

• SFS aggregate can be accommodated in low-calcium content geopolymer binders. • The high-density geopolymer concrete results in better breakwater stability and reduced material consumption. • The geopolymer concrete developed in this research can be utilised in breakwater armour unit fabrication. Anticipated changes in coastal wave conditions due to various climate change impact scenarios along coastlines may expose coastal protection structures to greater wave energies and higher damage rates than designed for, especially during episodic storm events. Some existing coastal breakwaters need upgrading to withstand the projected conditions. Breakwater armour unit design equations and physical model tests predict a large gain in stability with a modest increase in the armour material density and indicate reduced armour unit size requirements when utilising high-density concrete. In this study, a high-density geopolymer concrete mix with steel furnace slag (SFS) aggregate was developed based on several trials; the material properties were evaluated for on-site applications under ambient curing conditions. The use of SFS aggregate offers higher bulk density to concrete and mixes were proportioned to achieve good workability and setting time. Most importantly, the fly ash-blast furnace slag blended binder used in this study leads to adequate strength gain in ambient curing and allows the diffusion of the free lime associated with the SFS aggregate into the geopolymer matrix to eliminate the delayed hydration and expansion of the aggregate. This research provides a pathway to both upgradings of existing breakwaters and construction of new structures with a reduction to the carbon footprint in breakwater construction. [ABSTRACT FROM AUTHOR]

As a pozzolan, waste glass powder (WGP) can improve the mechanical properties and durability of concrete, especially at a later age. However, the modification mechanisms should be fully understood and discussed. One of the effective approaches is to investigate the improvement in interfacial transition zones (ITZs) between aggregate and mortar matrix. In this paper, soda-lime WGP was used as a supplementary cementitious material and replaced 20% by weight cement in the sample with modeled aggregate. The phases and chemical composition within the ITZs and the adjacent mortar matrix were investigated by backscattered electron (BSE) imaging combined with image analysis. Advanced nanomechanical techniques were simultaneously applied, and the results obtained from nanoindentation and nanoscratch were compared. The results show that WGP can significantly reduce the width of ITZs and increase the content of calcium-silicate-hydrate (C-S-H) and Si/Ca ratio in both ITZs and matrix. The high heterogeneity of ITZ with WGP made it susceptible to errors caused by unreacted WGP during the measurement of C-S-H nano- and micromechanical properties. For the control group without WGP, the test results of nanoindentation and nanoscratch showed a high degree of consistency. However, for the samples with 20% by weight WGP, there are some differences in the test results between nanoindentation and nanoscratch. Nanoscratch was able to acquire abundant data in a short testing time and proved more suitable for the tests of homogeneous materials. On the other hand, for the sample with 20% by weight WGP, nanoindentation was found to more likely to reflect the real micromechanical properties of ITZs based on abundant indentation tests. [ABSTRACT FROM AUTHOR]

An investigation on the flexural performance of reinforced concrete beams made with recycled brick aggregate was conducted. For this, 24 reinforced concrete beam specimens of width 200 mm, height 250 mm and length 2100 mm were made with recycled brick aggregate, picket brick aggregate, and brick aggregate. Other variables include steel ratio, doubly reinforced, and singly reinforced concrete beams. Flexural tests of the beams were performed by four-point loading test. During loading, deflections of the beams at the middle and middle-third locations were recorded. Crack widths and crack spacing were also recorded. The beams were loaded gradually till failure. The results of beams made with recycled brick aggregate were compared with the same results of beams made with virgin brick aggregates, i.e. picket brick aggregate and brick aggregate. It is revealed that the design equations proposed in ACI 318-14 for design of reinforced concrete beams can be used for design of beams made with recycled brick aggregate. [ABSTRACT FROM AUTHOR]

• Combination of biochar and silica fume enhance carbon sequestration. • Enhanced hydration in biochar-foamed mixtures improve strength and sorptivity. • Adding biochar reduces yield stress and plastic viscosity of foamed paste mixtures. • Biochar-foamed mortar retain higher strength after prolonged carbonation. • Adding biochar reduces shrinkage associated with only carbonation. Enhancing carbon sequestration in Portland cement-based foamed mortar is a potential means to reduce carbon emissions associated with lightweight building components. This research aims to investigate the influence of biochar, prepared from wood waste, in enhancing carbon sequestration in foamed mortar of three target densities – 1150 kg/m3 and 1300 kg/m3 and 1450 kg/m3 respectively through accelerated carbonation (CO 2 concentration of 2%) technique. Effect of biochar on rheology, and influence of carbonation on micro-structure, compressive strength, water absorption and shrinkage are investigated. Based on thermogravimetry analysis, carbon dioxide uptake of 26.80% and 30% by mass of cement are achieved in foamed mortar containing biochar, silica fume and fly ash after 7 days and 28 days respectively, which is higher by 3% compared to control (without biochar). Adding biochar to replace 3% by mass of fly ash (density of 1150 kg/m3) and 20% by mass of silica fume (density 1450 kg/m3) improves the workability of foamed mortar mixtures, evident from the 32% and 44% reduction in yield stress up to 35 min from the time of wet mixing. For density groups of 1300 kg/m3 and 1450 kg/m3, addition of biochar reduces water-accessible porosity by 2 – 3%, leading to 17% and 23% higher compressive strength compared to the controls after 28 days of water curing. After carbonation biochar-foamed mortars retain 20–38% higher strength than the respective controls, although reduction in strength by 40 – 60% compared to water curing is observed, ascribed to degradation of hydration products during prolonged carbonation. Addition of biochar results in 10–12% higher shrinkage under an air-drying regime, while it reduces chemical shrinkage associated with carbonation by 15–20% than control. Overall, this study demonstrates a promising potential for developing biochar-enhanced low-carbon lightweight building products. [ABSTRACT FROM AUTHOR]

[Display omitted] • Biochar enhances carbon sequestration through mineralization in cementitious mixes. • Higher strength than control is maintained in biochar-added mortar after carbonation. • Chemical contaminants in biochar can increase drying shrinkage. • Biochar addition to fly ash-mix reduces water absorption and chemical shrinkage due to carbonation. A possible reduction in high CO 2 emission associated with cement production can be achieved by sequestering CO 2 as carbonate mineral in hydrated cement matrix. This study aims to investigate application of biochar and its combination with fly ash to improve CO 2 uptake and carbonate mineralization in cement mortar. Effect of fly ash and biochar, from waste peanut shell, on carbonate mineralization, strength, permeability and shrinkage of mortar are investigated. Experimental findings show that combination of biochar and fly ash, used to replace 23% of cement by mass, enhances carbon sequestration through mineralization by 7–13 % (by weight of cement) than control. Biochar addition at 3% contributes to higher carbon sequestration than plain mortar, while improving strength and reducing capillary water absorption. Although relatively high content of sodium (42%) and potassium (8%) in the produced biochar lead to 39% higher drying shrinkage than control, addition of biochar reduces the chemical shrinkage associated with only carbonation reaction by 30–60%. 23–25% of carbonation shrinkage in biochar-mortar can be recovered through 4–7 days of rewetting. Overall, the research suggests a strong possibility of developing low carbon building materials using biochar as supplementary admixture, while offering improved mechanical and durability performance. [ABSTRACT FROM AUTHOR]

• The absolute slope in the post-peak stage is flattener with increased regeneration cycles. • The energy evolution law of Multi-RAC is similar to the counterpart of NAC. • Damage initiation threshold increases as increased confining pressure, but rate tends to decline. • Regeneration cycle shows significant grey correlation effect on mechanical performance. • Multi-RA has significant impact on triaxial confining performance of Multi-RAC. In this paper, the stress–strain behaviour, mechanical properties, characteristic stress, and damage evolution of multi-recycled aggregate concrete (Multi-RAC up to 3 recycling cycles) under triaxial compression were investigated, considering the effects of confining pressure and number of recycling cycles. Furthermore, a parametric sensitivity evaluation of key parameters using grey correlation theory to analyse the peak stress, peak strain, and elastic modulus of Multi-RAC. The results indicate that both peak stress and peak strain increase with rising lateral confining pressure. While the peak stress of Multi-RAC is lower than that of natural aggregate concrete (NAC), the deformation capacity of Multi-RAC is greater. The deformation and failure of Multi-RAC under triaxial loading can be categorized into five stages. Notably, crack initiation stress (σ ci ) and crack expansion stress (σ cd ) demonstrate nonlinear growth as lateral confining pressure increases. In particular, the second generation of Multi-RAC (RACII) exhibits superior performance in resisting crack initiation and expansion, while the third generation of Multi-RAC (RACIII) performs relatively less favourably. The energy evolution characteristics and division of failure stages of Multi-RAC are effectively characterized by the evolution of elastic strain energy (U e) and dissipation energy (U d). The degree of progressive damage aligns with the increase in dissipation energy. Among the five key parameters, the number of recycling cycles shows the most significant grey correlation effect on the peak stress, peak strain, and elastic modulus of Multi-RAC. These findings contribute to a deeper understanding of the mechanical properties and failure behaviours of Multi-RAC under the complex triaxial loading conditions, thus promoting the application of Multi-RAC as a structural concrete. [ABSTRACT FROM AUTHOR]