Addition Of Supplementary Cementitious Materials Research Articles

Numerical Study of Potential Delayed Ettringite Formation in Cemented Nuclear Wasteforms

This paper presents a numerical study to investigate delayed ettringite formation (DEF) that may pose a long-term durability risk by altering the microstructure with consequent swelling leading to cracking. A chemo–thermal model is used to predict the evolution and distribution of temperature and hydration phases in a wide range of blended cements. In particular, the influence of nuclear waste loading, waste package size, and the addition of supplementary cementitious materials (SCMs) on DEF is systematically and numerically investigated. The analyses show that higher amounts of ordinary Portland cement (OPC) and waste loadings result in higher hydration temperatures and consequently increased DEF potential by enhancing sulfoaluminate dissolution and hydrogarnet precipitation. Partial replacement of OPC with SCMs reduced hydration heat and mitigated DEF risks. The analysis indicated that the DEF evolution may be different for waste packages of different sizes due to a shift from sulfate-controlling to aluminate-controlling reactions at high temperatures. Interestingly, higher temperatures did not necessarily induce higher DEF potential due to the excessive precipitation of aluminates in the form of hydrogarnet. This research enriches our understanding of DEF’s complex behavior, providing valuable insights for engineering applications beyond civil engineering, such as nuclear waste conditioning.

MgO‐based binders

AbstractTo limit global temperature rise to below 2°C, we need to radically and rapidly change the way we build and use materials, since construction is responsible for 20‐40% of industrial CO2 emissions. Magnesium carbonate‐based cements have the potential to become a major carbon sink in construction industry, as CO2 will not be emitted during their production, but CO2 will rather be bound during hardening. Two different reaction mechanisms lead to setting and hardening of such HMC cements: i) hydration of MgO‐Mg‐carbonate, also in blends with silica and other mineral additions, in the presence of water or salt solutions (such as sodium bicarbonate solution) at ambient conditions or ii) carbonation hardening of MgO‐based systems at increased CO2 partial pressure and/or at increased temperatures.However, the utilization of hydrated magnesium carbonate and magnesium silicate cements is currently hampered by the lack of systematic experiments and of fundamental understanding of the factors affecting the hardening process, mechanical properties, long‐term behavior and durability. In particular, the role of temperature, relative humidity and addition of supplementary cementitious materials and/or industrial by‐products has not been systematically investigated. We need knowledge both on the practical side through systematic experiments with pastes, mortars and concretes as well as on a fundamental level through solubility and sorption experiments and thermodynamic modelling. In addition, reaction kinetics need to be optimized as well as the early formation of the preferred phases (stability enhancement) to develop efficient HMC cements for construction. Due to the socio‐economic relevance of cement and concrete, the impact of such “carbon”‐negative cements may go beyond a purely scientific one and lead to technological breakthroughs to the benefit of environment and society.

Effect of silicate modulus and alkali content on the microstructure and macroscopic properties of alkali-activated recycled powder mortar

The use of recycled powder (RP) as a green raw material to prepare alkali-activated materials provides a promising approach for the sustainable utilization of construction waste. However, limited attention has been paid in the literature to the impact of silicate modulus and alkali content of alkaline activators on the performance of alkali-activated recycled powder mortar (AAM). Therefore, this study investigates the influence of different silicate modulus and alkali content on the microstructure, mechanical properties, water absorption and porosity of AAM. The results revealed that the RP exhibits irregular shapes, comprising hydrated products and inert materials like quartz and calcite, and can participate in polymerization reactions under the influence of alkaline activators. Suitable silicate modulus and alkali content can promote further dissolution of RP to form C-A-S-H/N-A-S-H gels, which benefits the development of the alkali-activated recycled powder paste microstructure. When the silicate modulus is constant, the mechanical performance of AAM initially improves with increasing alkali content, but eventually decreases beyond a certain threshold, especially at an alkali content of 8%. The 28-d compressive strength of M1.8N8 decreased by 4.92% compared to M1.8N6. Meanwhile, water absorption and porosity decline with increasing alkali content. When alkali content is constant, increasing the silicate modulus is beneficial for the development of the microstructure of the alkali-activated recycled powder paste, and improves the AAM mechanical performance. The 28-d flexural strength of N6M1.8 increased by 17.63% compared to N6M1.2. The addition of supplementary cementitious materials (SCMs) can reduce the porosity and water absorption of AAM and significantly improve its mechanical properties, with the best performance enhancement achieved by metakaolin (MK). Therefore, RP can be used as a raw material to prepare environmentally friendly alkali-activated materials with excellent macroscopic performance.

Experimental Behavior of High-Strength Concrete Reinforced with Aramid Fiber and Polyurethane Resin

Over the past few decades, research has been conducted to develop high-strength concrete (HSC) for high-rise structures and bridge decks. The research discussed in the study focuses on using polyurethane resin (PR) and aramid fibers (AF) to develop HSC, which enhances its strength, durability, and structural properties without increasing the cementitious content. This approach can lead to more sustainable and cost-effective construction practices by reducing the cementitious materials required. In the present investigation, M50-grade concrete mixes were designed in accordance with the guidelines mentioned in Indian Standard (IS) 10262 along with the addition of supplementary cementitious materials, such as fly ash and silica fume. Initially, varying percentages of AF (0% to 3%) and PR (0% to 6%) were added into the concrete mixes and detailed experimental investigations were completed on workability, strength, durability, and structural properties. It is concluded that the addition of AF and PR shows significant improvements in strength, durability, and structural properties compared to traditional HSC created with zero AF and PR content. As reinforced concrete (RC) elements serve as the final product for human construction projects, it is crucial to ensure that their structural properties are reliable. In order to validate the findings from experimental investigations, numerical simulations were conducted using the ANSYS commercial package software. Specifically, the structural properties of RC beams were analyzed using this software, allowing for further validation and verification of the experimental results. From the detailed investigation, it is concluded that 2.5% addition of AF and 4% addition of PR demonstrates better results and is considered the optimum ingredient dosage, which can be used as a reference for future studies and practical applications. These findings can result in the development of new and improved building materials and techniques that can potentially lead to safer, more durable, and sustainable structures.

Numerical study of the autogenous shrinkage of cement pastes with supplementary cementitious materials based on solidification theory

The addition of supplementary cementitious materials (SCMs), which is an effective method to reduce the CO2 emission during the cement production, will change the autogenous shrinkage of early-age hydrating cementitious systems. In this paper the effect of different SCMs, i.e., silica fume and fly ash, on the physical properties and autogenous shrinkage of cement paste is studied, both experimentally and numerically. The experiment results show that autogenous shrinkages of cement pastes reduce significantly with the increase of water-binder ratio. The addition of fly ash will lead to smaller autogenous shrinkage while the influence of the dry densified silica fume on the autogenous shrinkage is not pronounced. A numerical simulation model, in which autogenous shrinkage is split up into an elastic part and a time-dependent part is proposed in this paper. The two parts are calculated separately. The simulation of the visco-elastic response of the cement paste to the internal driving force was performed by using a solidification theory model that takes the aging into consideration. The physical properties of different kinds of cement paste are experimentally investigated and the measured results are taken as input parameters of the simulation model. The simulated autogenous shrinkage of different kinds of cement pastes are compared with the measured results to verify the prediction of the proposed model.

Influence of Calcined Clay Pozzolan and Aggregate Size on the Mechanical and Durability Properties of Pervious Concrete

Pervious concrete has been reported as a viable solution to reduce stormwater run-off, the heat-island effect, road noise, and pavement flooding. Previous researchers have focused on analysing the structural properties and functionality of pervious concrete. However, relatively few studies have been conducted into the addition of supplementary cementitious materials (SCMs), such as calcined clay, in pervious concrete and its effect on long-term durability. This paper has studied the effect of calcined clay pozzolan as a partial substitute for Portland cement in pervious concrete, together with the influence of coarse aggregate size. A water–binder ratio of 0.4 and aggregate–binder ratio of 4.0, as well as a superplasticiser content of 0.95%, were maintained for all mixes. Two sizes of coarse aggregates were used for this study: 9.5 mm and 20 mm. CEM-I cement was partly substituted with calcined clay in dosages of 0 to 30% in replacement intervals of 5%. The mechanical tests conducted included the split tensile test, compressive strength test, and flexural strength test. Durability measurements such as the rapid chloride permeability test (RCPT), thermal conductivity and sulphate resistance tests were also carried out. The mechanical properties of the pervious concrete followed a similar trend. The results showed that at 20% replacement with calcined clay, the compressive strength increased by 12.7% and 16% for 9.5 mm and 20 mm aggregates, respectively. The flexural strength improved by 13.5% and 11.5%, whereas the splitting tensile strength increased by 35.4% and 35.7%, respectively, as compared to the reference concrete. Beyond 20% replacement, the tested strengths declined. The optimum calcined clay replacement was found to be 20% by weight. Generally, pervious concrete prepared with 9.5 mm obtained improved mechanical and durability properties, as compared to those of 20 mm aggregates.

Study on the effect of supplementary cementitious materials on the volume stability of mortar mixed with laboratory-made high-content fluorine-type accelerator

The high-content fluorine-type accelerator (AF) can be prepared from fluorine-containing wastewater due to its low cost and is widely used in many projects. In this study, the fluorine-type alkali-free liquid accelerator was synthesized by simulating fluorine-containing waste liquid in the experiment, and its performance was tested and shown to meet the standard requirements. Subsequently, three supplementary cementitious materials (granulated blast furnace slag-GBFS, fly ash-FA, and silica fume-FS) were used to study the effect of supplementary cementitious materials on the setting time, mechanical properties and volume stability of mortar mixed with accelerators. The results show that the addition of supplementary cementitious materials shortens the setting time of the cement paste, while maintaining the requirements on compressive strength. Among these materials, the incorporation of silica fume makes the best effect of shortening the setting time, and yields the highest increase in compressive strength. In the 180-day test, both FA and GBFS can reduce the autogenous shrinkage and drying shrinkage of mortar, and the shrinkage reduction effect of FA is more prominent. FS significantly increases the autogenous and drying shrinkage of the mortar. By observing the changes to mass and humidity under the drying conditions of mortar, it is found that FS can effectively reduce mass loss and humidity change. This is because the early hydration reaction of FS occurs earlier than that of FA and GBFS, and it can play a role in refining the pore structure. However, the addition of FS aggravated the shrinkage of the mortar by increasing the pore size of 5–50 nm, resulting in higher capillary tensile stress and greater shrinkage driving force. FA increased the mass loss and internal humidity reduction of the mortar, but was less driven by capillary tensile stress due to coarsening of the pore structure. This study provides theoretical guidance for practical construction application.

A Review of the Durability-Related Features of Waste Tyre Rubber as a Partial Substitute for Natural Aggregate in Concrete

As the number of discarded tyres continues to increase, causing serious environmental problems, the need of recycling the waste tyre rubber become extremely urgent in worldwide. Today, there is an increasing focus on recyclable materials. The reuse of waste tyre rubber in concrete contributes to sustainable development. In the past 10 years, numerous experiments on the recovery of rubber from waste tyres to produce concrete products have been conducted. In this review, we conclude the major achievement of rubberized concrete (RC) durability, discuss and analyse the influence of rubber replacement rates, replacement patterns, particle size and treatment methods. Results show that an increase in rubber content can improve the chloride penetration resistance, acid and sulphate attack resistance, freeze–thaw resistance, and alkali–silica reaction damage resistance of concrete, and the content of 5–20% has a significant improvement effect. Rubber replacing fine aggregate is the best scheme for durability, followed by cement and coarse aggregate. In addition, the recommended rubber particle size is 0–3 mm. However, the rubber particle has adverse effects on abrasion resistance, impermeability, water absorption resistance and carbonation resistance. The pre-treatment of rubber or the addition of supplementary cementitious materials are effective and viable ways of improving the durability of RC. Further research is needed on the long-term durability of RC, as well as on ductility, energy absorption, and thermal and corrosion resistance.

Variation of Electrical Resistivity and Charge Passed in High-Performance Concrete

This study investigated the variation of bulk resistivity (BR) and charge passed for various high-performance concrete (HPC) mixtures based on significant factors (i.e., geometric size, operation frequency, and mixture constituents and proportions) using three testing instruments. These instruments were a surface resistivity (SR) meter and two bulk conductivity meters: one for using the BR data at a constant frequency, and the other at a wide range of frequencies. These HPC mixtures were categorized into several groups based on various supplementary cementitious materials (SCMs). The variation and distribution of BR and the charge passed were investigated and statistical analysis results showed that the addition of SCMs and their varying replacement level remarkably influenced the reduction of charge passed in each group over an extended period. The results revealed that, for fly ash-based ternary mixtures, the addition of 3% metakaolin or 12% silica fume resulted in the highest reduction of charge passed over time (82% and 90%, respectively). For cost purposes, 5% silica fume replacement in ternary mixtures was chosen as an optimal solution. Finally, this study offered promising options for charge passed computation to assess corrosion in light of simple, rapid, and reliable SR/BR measurement.

Designs of Abrasion Resistant and Durable Concrete Pavements Made with SCMs for Cold Climates

Rutting from studded tire wear is a typical pavement distress in cold climates such as that of Alaska and other northern states. Current state-of-the-art advancements in material technology and concrete pavement design have allowed for implementation of improved materials and concrete pavement sections that are more resistant to rutting. The addition of supplementary cementitious materials (SCMs) has been identified as one effective way to produce concrete pavements with better abrasion resistance. The objective of this study was to identify and develop concrete pavement mix designs containing SCMs that can provide excellent abrasion resistance and durability to address rutting from studded tire wear and accommodate extreme climate conditions in cold regions. This study involved two phases of work. During Phase I, a series of ternary mixes containing silica fume with either slag or class F fly ash were produced and tested. The results were statistically analyzed using Minitab version 19.2.0 to identify mix designs with good performance in terms of workability, compressive strength, and flexural strength requirements for pavement applications. In Phase II, the mechanical properties and durability of concrete specimens with selected mix designs from Phase I were further evaluated to identify the optimum mix design with SCMs. This included tests for compressive strength, drying shrinkage, abrasion resistance, and other dualities such as scaling resistance to deicer salts, freeze-thaw resistance, and chloride ion penetration resistance. In terms of the properties evaluated within this study along with a cost analysis, five mixes, including four optimal mixes and the control, all provided good performance, but a quaternary mix design containing primarily silica fume and slag (SL12 SF4 FA1 mix) appeared to provide the overall best performance considering strength, durability, abrasion resistance, and cost.

Assessment of Reactive powder concrete subjected to three different sodium sulfate Concentrations: Compressive Strength, Absorption, Porosity, Microstructure, and durability

Reactive Powder Concrete (RPC) is a relatively new type of concrete material characterized by very low water-to-binder ratios (w/b), the addition of supplementary cementitious materials like silica fume (SF), and the presence of very fine aggregates instead of conventional coarse aggregates. The durability of RPC with various w/b (0.18, 0.22, and 0.26) and SF content (15%, 20%, and 25%) against external sulfate attack (ESA) at various sodium sulfate (Na2SO4) concentrations (0.35 M, 0.7 M and 1.4 M) was investigated in this study through examinations of compressive strength, expansion and mass change, microstructural characteristics, mineralogy, porosity, and sorptivity. The results indicated that: 1) owing to its improved microstructure, RPC could accommodate expansive products of ESA reactions, which can provide additional strength; 2) RPC showed very high resistance to Na2SO4 attack under the ASTM C1012 criteria; 3) considerable sulfate ingress did not result in significant expansion and strength reduction for RPC mixtures at early stages of exposure, especially for the mixtures with high initial sorptivity and high interparticle connectivity; 4) considering the coupled effect of w/b and SF content on the number of RPC characteristics, mixture with w/b = 0.22 and 20% SF content showed the best performance upon exposure to ESA.

Modeling of Chloride Binding Capacity in Cementitious Matrices Including Supplementary Cementitious Materials

The improvement in the chloride binding capacity of concrete has been shown to increase corrosion resistance. The addition of supplementary cementitious materials (SCMs) to Portland cement has been proven to increase the binding capacity, except for silica fume, whereas the impact of chemical additives is not extensively addressed in the literature. This work studies the influence of SCMs and chemical additives, i.e., calcium nitrite inhibitor (CNI), migrating corrosion inhibitor (MCI), and Caltite as a hydrophobic material, on binding capacity. The addition of both corrosion inhibitors (MCI and CNI) has minimal effect on the binding capacity, while the addition of Caltite reduces the binding capacity by limiting the contact of the samples with the salt in water due to its hydrophobic nature. In addition, the study compares the performance of the available fitting–binding models against the available experimental work in the literature, and shows that the Freundlich isotherm is the best fitting model for describing the relationship between the binding capacity and the free chloride. The study further relates the binding capacity to different compositions in cement and SCMs, and shows, by conducting quantitative analysis, that the Al2O3 content is the dominant factor affecting the binding capacity. Finally, this work proposes a new model, which uses Al2O3 content and free salt concentration to predict the bound chloride. The model shows adequate correlations to the experimental work and, further, can be used in service-life modeling of concrete.

Appraisal on low cost and sustainable ECC using microfibers in hybridization at later age

The addition of supplementary cementitious materials and fiber plays an important role in the mechanical and durability performance of ECC matrix. In the present research work, the assessment of the performance of ECC matrix with the utilization of iron industry waste and microfibers has been done. Three types of microfibers, i.e., polyvinyl alcohol (PVA) fiber, polyester (PET) fiber, and microsteel (MSE) fiber, were used at various percentages in hybridization to prepare total seven mixes. First, PVA was switched by PET fiber at dosages 5%, 10%, 15%, 20%,and 25% and afterwards another 25% by MSE fiber. The performance of various matrix proportions was judged based on the flexural response, electrical resistivity, air permeability, and sorptivity characteristics to introduce sustainable and cost effective ECC matrix. Test results revealed that hybridization of fibers enhanced the flexural and durability performance of ECC and also produced a cost effective and sustainable ECC matrix.

Reducing detrimental sulfate-based phase formation in concrete exposed to sodium chloride using supplementary cementitious materials

The sodium chloride (NaCl) can chemically interact with the monosulfate (AFm) within the concrete matrix, leading to a detrimental sulfate-based phase formation during thermal cycling. This paper studies the potential of reducing the sulfate-based phase in the cementitious pastes made using different types of supplementary cementitious materials (SCMs) used as a partial replacement of ordinary portland cement (OPC). A low-temperature differential scanning calorimetry was used to quantify the amount of the sulfate-based phase. The results indicated that the addition of SCMs as a partial replacement of OPC revealed different effects on forming the sulfate-based phase through pozzolanic activities and/or dilution effects. The addition of slag or class F fly ash can lower the formation of the sulfate-based phase through the dilution effect, while the addition of silica fume can substantially lower the formation of the sulfate-based phase by both dilution and pozzolanic activities. The use of class C fly ash, however, showed negative effects through increasing the formation of the sulfate-based phase. Thermodynamic modeling was used to predict the phase equilibrium of hydrated cementitious systems. Results from thermodynamic predictions were in agreement with the experimental results, indicating that the formation of the sulfate-based phase was associated with the amount of AFm present in the cementitious systems.

Mix design of high-volume fly ash ultra high performance concrete

The addition of supplementary cementitious materials (SCMs) to replace cement, especially with a high volume (> 50%), is an effective way to reduce the environmental impact due to the CO2 emissions generated in the production of ultra-high performance concrete (UHPC). Unfortunately, no official guidelines of UHPC using a high volume of SCMs have been published up to now. This paper proposes a new method of mix design for UHPC using high volume fly ash (HVFA), that is referred to the particle packing optimization of the Compressive Packing Model proposed by F. de Larrard. This proposed method also considers the heat treatment curing duration to maximize the compressive strength of HVFA UHPC. The experimental results using this proposed mix design method show that the optimum fly ash content of 50 wt.% of binder can be used to produce HVFA UHPC with a compressive strength of over 120 MPa and 150 MPa under standard curing and heat treatment, respectively. Moreover, the embodied CO2 emissions of UHPC reduces 56.4% with addition of 50% FA.

Review on Accelerated Carbonation on the Properties of Concrete

Concrete is an essential material in all constructions throughout the world. It has lot of uses in our daily routine life. Every material has to deteriorate and damage due to many factors in the same way the concrete also deteriorates. The carbonation process is identified as a main reason for the corrosion in reinforcement concrete structure. The mechanism of carbonation which includes the entrance of carbon dioxide (CO2) into the solid permeable framework of concrete to shape a situation by decreasing the pH around the fortification and inception of the corrosion procedure. This paper investigates the impact of the carbonation on the characteristics of the concrete like strength in compression, split tensile strength, flexural strength, shear strength and durability. The addition of supplementary cementitious materials like fly ash, GGBFS, rice husk ash, metakaolin is known to enhance the strength and durability of concrete in construction. In this paper an accelerated carbonation test has been done to assess concrete carbonation on specimens made with cement and with the partial replacement of cement by fly ash GGBFS, rice husk ash, metakaolin. An accelerated carbonation chamber has been constructed for creating an environment of carbonation process to occur and also the passage of carbon dioxide gas is kept constant for all the cubes, cylinders and beams. Concrete cubes, cylinders and beams are prepared for M30 grade subjected to different percentages of carbonation.

Green Concrete for a Circular Economy: A Review on Sustainability, Durability, and Structural Properties.

A primary concern of conventional Portland cement concrete (PCC) is associated with the massive amount of global cement and natural coarse aggregates (NCA) consumption, which causes depletion of natural resources on the one hand and ecological problems on the other. As a result, the concept of green concrete (GC), by replacing cement with supplementary cementitious materials (SCMs) such as ground granulated blast furnace slag (GGBFS), fly ash (FA), silica fume (SF), and metakaolin (MK), or replacing NCA with recycled coarse aggregates, can play an essential role in addressing the environmental threat of PCC. Currently, there is a growing body of literature that emphasizes the importance of implementing GC in concrete applications. Therefore, this paper has conducted a systematic literature review through the peer-reviewed literature database Scopus. A total of 114 papers were reviewed that cover the following areas: (1) sustainability benefits of GC, (2) mechanical behavior of GC in terms of compressive strength, (3) durability properties of GC under several environmental exposures, (4) structural performance of GC in large-scale reinforced beams under shear and flexure, and (5) analytical investigation that compares the GC shear capacities of previously tested beams with major design codes and proposed models. Based on this review, the reader will be able to select the optimum replacement level of cement with one of the SCMs to achieve a certain concrete strength range that would suit a certain concrete application. Also, the analysis of durability performance revealed that the addition of SCMs is not recommended in concrete exposed to a higher temperature than 400 °C. Moreover, combining GGBFS with FA in a concrete mix was noticed to be superior to PCC in terms of long-term resistance to sulfate attack. The single most striking observation to emerge from the data comparison of the experimentally tested beams with the available concrete shear design equations is that the beams having up to 70% of FA as a replacement to OPC or up to 100% of RCA as a replacement to NCA were conservatively predicted by the equations of Japan Society of Civil Engineers (JSCE-1997), the American Concrete Institute (ACI 318-19), and the Canadian Standards Association (CSA-A23.3-14).

A Comprehensive Review on Effects of Seawater on Engineering Properties of Concrete

The concrete and natural seawater have a complex relationship between them, which requires special attention. Therefore, various studies have been conducted to consider the effects of natural seawater on the properties of concrete. However, this study aimed to summarize and analyze the previous findings and recommendations. It was noticed that the natural seawater has both positive and negative implications on concrete. Thus, resistance of concrete against the seawater can be improved by adding supplementary cementitious materials (SCM) like copper slag, coal bottom ash, fly ash and others with appropriate proportions. Moreover, the problem of corrosion of reinforcement due to seawater influence, can be avoided using corrosion inhibitor and/or corrosion resistant reinforcement. It was also noticed that the addition of SCM could increase the performances of concrete like strength and durability of concrete exposed to the marine environment. This paper provides a deep insight about experimental information and future direction for the concrete exposed to the marine environment.

Effects of packing density and water film thickness on fresh and hardened properties of ternary cement pastes

Optimisation of the packing density of cementitious materials by the addition of supplementary cementitious materials (SCMs) is a way of obtaining the same service properties with less embodied energy and reduced carbon dioxide emissions. In this paper, ternary cement pastes containing metakaolin (MK) and limestone filler (LF) finer than ordinary Portland cement (OPC) were studied to evaluate the effects of these SCMs on packing density and water film thickness (WFT). The flowability of the pastes at different water to cementitious material ratios by volume ((w/cm)v = 0·5 to 1·0) was evaluated by mini-slump and Marsh cone tests to evaluate the opposite effects caused by the incorporation of finer particles (increase of free water to lubricate the particles due to higher packing and increase of specific surface area). The incorporation of LF and MK increased the flowability when (w/cm)v was increased over the optimum water demand corresponding to the maximum packing density. The rheological properties of the pastes were primarily dominated by the relative solid packing fraction and the flowability was found to be mainly governed by the WFT. The incorporation of finer particles accelerated the setting time. The reactive particles (OPC and MK) can be replaced by LF up to 20% to obtain a similar compressive strength, as validated by Powers’ model.

Development of normal and very high strength geopolymer binders based on concrete waste at ambient environment

The objective of this study was to optimize the composition of concrete waste (CW)-based geopolymer binders at ambient environment. The goal was to evaluate the reliability of a new algorithmic mix design method that relies on pre-targeting ratios of SiO2/Al2O3, Na2O/SiO2 and liquid/solid of CW precursor powder and sodium silicate and sodium hydroxide alkaline reagents. The effects of elevated temperature curing and the addition of supplementary cementitious materials (SCMs) were also considered. The optimized CW-mixes at ambient curing were subjected to an initial 24 h curing at 50, 75, and 100 °C, and Class C fly ash (FA-C), Class F fly ash (FA-F), metakaolin (MK) and ground granulated blast furnace slag (GGBS) replaced CW at wt.% of 15%, 30% and 45%. The relationship between SiO2/Al2O3 and Na2O/SiO2 molar ratios and the fresh properties, including flowability, initial and final setting times, and mechanical strengths were assessed. Furthermore, an extensive microstructural study using X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with energy-dispersive X-ray (EDS) spectroscopy and Fourier transformed infrared (FTIR) spectroscopy was performed for various mix compositions and curing conditions. The results proved that fresh and mechanical properties are highly dependent on the targeted chemical ratios. Enhanced mechanical strengths of CW-compositions were achieved at adjusted ratios of SiO2/Al2O3 = 12.9 and Na2O/SiO2 = 0.23, which were significantly increased by incorporating SCMs with high calcium content. Microstructural assessments indicate that the degree of geopolymerization of CW-based geopolymers and related C(N)-A-S-H formations can be optimized at ambient environment.