Sodium silicate activated slag-fly ash binders: Part III-Composition of soft gel and calorimetry (original) (raw)
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Alkali silicate activated slag and class F fly ash-based binders are ambient curing, structural materials that are feasible replacements for ordinary Portland cement (OPC). They exhibit advantageous mechanical properties and less environmental impact than OPC. In this work, five sodium silicate activated slag-fly ash binder mixtures were developed and their compressive and flexural strengths were studied as a function of curing temperature and time. It was found that the strongest mixture sets at ambient temperature and had a Weibull average flexural strength of 5.7 AE 1.5 MPa and Weibull average compressive strength of 60 AE 8 MPa at 28 days. While increasing the slag/fly ash ratio accelerated the strength development, the cure time was decreased due to the formation of calcium silicate hydrate (C-S-H), calcium aluminum silicate hydrate (C-A-S-H), and (Ca,Na) based geopolymer. The density, microstructure, and phase evolution of ambient-cured, heat-cured, and heat-treated binders were studied using pycnometry, scanning electron microscopy, energy dispersive X-ray spectroscopy (SEM-EDS), and X-ray diffraction (XRD). Heat-cured binders were more dense than ambient-cured binder. No new crystalline phases evolved through 28 days in ambient-or heat-cured binders.
Materials
The growing use of blends of low- and high-calcium solid precursors in combination with different alkaline activators requires simple, efficient, and accurate experimental means to characterize their behavior, particularly during the liquid-to-solid transition (setting) at early material ages. This research investigates slag-fly ash systems mixed at different solution-to-binder (s/b) ratios with sodium silicate/sodium hydroxide-based activator solutions of varying concentrations. Therefore, continuous non-destructive tests—namely ultrasonic pulse velocity (UPV) measurements and isothermal calorimetry tests—are combined with classical slump flow, Vicat, and uniaxial compressive strength tests. The experimental results highlight that high alkali and silica contents and a low s/b ratio benefit the early-age hydration, lead to a faster setting, and improve the early-age strength. The loss of workability, determined from the time when the slump flow becomes negligible, correlates well wi...
Shrinkage and Related Properties of Alkali-Activated Binders Based on High MgO Blast Furnace Slag
Doctoral thesis, 2019
Concrete is the second most used material in the world just after water. A drawback is that it is mostly based on Portland cement, which has an extremely high carbon footprint reaching a staggering 900 kg/tonne. The carbon dioxide emissions related to the production of the Portland cement accounts for nearly 8 % of the global total. Consequently, the construction sector is engaged in an active search for sustainable alternatives. Over the past few decades, alkali-activated materials (AAMs) emerged as one alternative and attracted strong scientific and commercial interests. Many industrial by-products produced in large volumes can be used as precursors for the AAMs system. The most common include blast furnace slag, fly ash, mine tailings, metallurgical slags, and bauxite residues. So far, products based on ground granulated blast furnace slag (GGBFS) showed the best price/performance ratio. Still, there are a number of unresolved issues, which must be addressed to ensure the economical and safe full-scale utilisation of that material. The research work presented in this thesis focuses on alkali-activated concretes based on Swedish water-cooled high-MgO ground granulated blast furnace slag. The objective of this work was to identify experimentally factors that are controlling the shrinkage and the creep of concretes made with this type of GGBFS and to understand their influence on various physical and chemical properties of fresh and solidified systems. Liquid sodium silicate, powder sodium carbonate and a combination of both were used to activate the binder chemically. Two curing procedures were followed; laboratory curing and heat curing at 65°C applied for 24 hours. Various properties were determined including workability, setting time, hydration heat development, shrinkage, creep, efflorescence, carbonation, compressive strength, microstructure and phase composition. Additionally, the effects of the activator type, dose, binder fines, binder composition and curing regime were investigated. The results revealed that the particle size distribution of the binder as well as the activator type and its dosage have strong effects on the produced materials. Increasing the activator amount or decreasing the alkali modulus of the used sodium silicate activator improved the early-age compressive strength and accelerated the hydration reaction. Alkali-activated high-MgO slag concrete showed higher autogenous and drying shrinkage, as well as higher creep in comparison to the Portland cement-based reference concrete. The sodium silicate increased the slump, shortened the setting time, increased the compressive strength and shrinkage but lowered the creep in comparison with the sodium carbonate-activated mixes. Replacing 20% of the slag with fly ash and decreasing the alkali modulus of the sodium silicate activator increased the autogenous shrinkage but decreased the ultimate drying shrinkage. Application of a heat treatment produced in general a higher early age compressive strength, a lower VI later strength development, a more porous microstructure and a decreased ultimate measured shrinkage. Sealed curing decreased the ultimate shrinkage by up to 50%. Some of the produced mixes showed strong efflorescence. Two years of curing in laboratory conditions resulted in an extensive carbonation of some of the mixes. This weakened the silicate binding of the gel and produced a coarser porosity due to the decalcification of C-(A)-S-H. The heat-cured samples activated with sodium silicate were the most affected. Many mixes showed an extensive microcracking of the binder matrix. However, the within this study newly developed mixes were substantially less effected. These optimised mixes were based on a combination of sodium silicate and sodium carbonate activators, combined with a heat treatment and partial replacement of the slag with fly ash. The main hydration phase that formed was C-(A)-S-H, with gaylussite, calcite, nahcolite and hydrotalcite as secondary phases. The partial replacement of slag with fly ash resulted in a dominant formation of N-(A)-S-H and C-(A)-S-H
Sodium silicate-activated slag-fly ash binders (SFB) and slag-metakaolin binders (SMKB) are room-temperature hardening binders that have excellent mechanical properties and a significantly lower carbon footprint than ordinary Portland cement (OPC). The aim of this study was to use nuclear magnetic resonance (NMR) spectroscopy to study the nanostructure of poorly ordered phases in SFB by varying slag/fly ash ratio, curing time, and curing temperature. Fly ash was completely substituted with metakaolin and the effect of this substitution on the poorly ordered phases was studied. It was observed that the proportion of geopolymer was generally higher in SMKB when compared to SFB. Although C-N-A-S-H and geopolymer coexisted in SFB and SMKB, C-N-A-S-H was the major product phase formed. The mean chain length (MCL) and the structure of C-N-A-S-H gel were estimated as a function of time, temperature, and slag/fly ash ratio. The MCL was found to have a negative correlation with slag/fly ash ratio and Ca/(Si+Al) ratio, but positive correlation with curing temperature. The average Si/Al atom ratios for geopolymers were also estimated. Lastly, the increased proportion of five-coordinated aluminum (Al(V)) in metakaolin resulted in the decreased unreacted metakaolin in the hardened binder but did not increase the geopolymer content.
Shrinkage and strength development of alkali-activated fly ash-slag binary cements
Construction and Building Materials, 2017
Shrinkage characteristics of different alkali-activated fly ash-slag binders were evaluated. Utilization of higher-pH sodium silicate activator mitigated autogenous and drying shrinkage. Strength development and setting time properties of different binary binders were measured. Addition of larger amount of slag to binary binder led to a quicker set, higher strength and stiffness. Activating by higher-pH sodium silicate solution resulted in a slower setting, higher strength and stiffer matrix. a b s t r a c t This paper evaluates the effect of fly ash and slag proportions and the type of activating solution on shrinkage and strength development of alkali-activated binary fly ash-slag mixtures (mortar and paste), cured at room temperature. Three different volumetric ratios of slag/fly ash were considered: 10%, 15%, and 20%. Two activators with different pH and modulus, n = (SiO 2 /Na 2 O) mol were utilized. The liquid to solid volume ratio of all binders was maintained at 0.75. The results showed that while the addition of slag significantly shortens the time of setting (up to 178 min), and increases the compressive strength (up to 93%) and bulk modulus (up to 43%), it also results in higher autogenous shrinkage, but smaller mass loss during drying. Measured drying shrinkage of mixtures with various slag contents was similar, likely due to counteracting effects of binder stiffness and degree of saturation. Fly ash-slag binders activated at higher pH exhibited larger chemical shrinkage, but lower autogenous (up to 21%) and drying shrinkage (up to 47%) magnitude.
Effect of Adding Silica Fume, Calcium Carbonate or Silicon Carbide on the Properties of an Alternative Binder Based on Anhydrite and Blast Furnace Slag, 2022
The effect of the addition of silica fume, calcium carbonate or silicon carbide in an alternative cementitious matrix based on anhydrite and blast furnace slag is reported in this paper. The addition of any of these materials improved the compressive strength of the systems, with the one containing 5% silica fume exceeding 22 MPa at 28 days and 26 MPa at 360 days of curing, showing the best compressive strength over the test period. Increasing the addition of silica fume resulted in a decrease in compressive strength because silica fume interferes with the hydration reaction of anhydrite. The systems with silica fume showed lower expansions, of 0.06%, compared to 0.11% for the other systems, because the pozzolanic reaction of the silica fume to form calcium silicate hydrate densifies the matrix preventing the ettringite from finding a place to nucleate. The hydration products were identified as gypsum and ettringite, in addition to calcium silicate hydrate, which covered the gypsum crystals forming an intermixed structure between the two phases. Despite the appearance of ettringite, no cracking or spalling was observed, even after 360 days of curing.
materials, 2023
Concrete is the most used construction material, needing large quantities of Portland cement. Unfortunately, Ordinary Portland Cement production is one of the main generators of CO2, which pollutes the atmosphere. Today, geopolymers are an emerging building material generated by the chemical activity of inorganic molecules without the Portland Cement addition. The most common alternative cementitious agents used in the cement industry are blast-furnace slag and fly ash. In the present work, the effect of 5 wt.% -limestone in mixtures of granulated blast-furnace slag and fly ash activated with sodium hydroxide (NaOH) at different concentrations was studied to evaluate the physical properties in the fresh and hardened states. The effect of -limestone was explored through XRD, SEM-EDS, atomic absorption, etc. The addition of -limestone increased the compressive strength reported values from 20 to 45 MPa at 28 days. It was found by atomic absorption that the CaCO3 of the -limestone dissolved in NaOH, precipitating Ca(OH)2 as the reaction product. SEM-EDS analysis showed a chemical interaction between C-A-S-H- and N-A-S-H-type gels with Ca(OH)2, forming (N, C)A-S-H- and C-(N)-A-S-H-type gels, improving mechanical performance and microstructural properties. The addition of -limestone appeared like a promising and cheap alternative for enhancing the properties of low-molarity alkaline cement since it helped exceed the 20 MPa strength recommended by current regulations for conventional cement.
Journal of Cleaner Production, 2015
Alkali-activated Portland fly ash cement (FA/OPC) and alkali activated blast furnace slag-fly ash cement (FA/GBFS) were prepared using 70% of a low quality fly ash (FA). The low quality is associated with a high content of unburned material (loss of ignition of 14.6%). The hybrid cements were activated by the alkaline solution in order to obtain an overall SiO 2 /Al 2 O 3 molar ratio of 5.0 and 6.0 and unique overall Na 2 O/SiO 2 molar ratio of 0.21. The microstructural characterization of the blended pastes generated in the systems showed the coexistence of amorphous gels C-A-S-H and N-A-S-H gels in the hybrid systems. The addition of OPC or GBFS increases the compressive strength (at 28 days of curing) up to 127% compared with the geopolymer systems based only on FA used in this study. The content of silicates soluble also plays an important role in the reaction products and higher SiO 2 /Al 2 O 3 lead to obtain higher mechanical performance and denser structure. The results obtained show that these hybrid cements are an effective way for valorization the waste used in this study for the production of high strength and lowcarbon footprint cement-type material.
Use of Biomass Ash for Development of Engineered Cementitious Binders
ACS Sustainable Chemistry & Engineering, 2018
Using waste agriculture and power plant byproducts to replace materials that are energy-intensive to produce can make these materials more "green". Improved compressive strength can be obtained when rice husk ash (RHA) partially replaces ordinary Portland cement, a substance potentially hazardous and energy-intensive to make. When RHA is combined with other nanoparticles to replace ordinary Portland cement (OPC), strength can be further enhanced. The microstructure of cement binders with replacement of OPC by combinations of coal fly ash, silica fume, RHA, nanosilica, and metakaolin were investigated using X-ray diffraction, backscattered electron imaging with energy-dispersive spectrum analysis, X-ray photoelectron spectroscopy, and nitrogen sorptiometry. A combination of sustainable, renewable RHA and the waste product coal fly ash was found to synergistically improve cement binder strength. Analyses suggested the enhanced strength was due to RHA increasing amorphous reactive silica and coal fly ash contributing alumina to form calcium−silicate−hydrate (C−S−H) gel along with calcium−aluminum−silicate−hydrate (C−A−S−H). Thus, this work shows the potential benefits of merging residual wastes from the agricultural sector with wastes from coal in combustion-based power plants.
Composition and microstructure of alkali activated fly ash binder: Effect of the activator
Cement and Concrete Research, 2005
The alkali activation of fly ashes is a chemical process by which the glassy component of these powdered materials is transformed into very well-compacted cement. In the present work the relationship between the mineralogical and microstructural characteristics of alkaline activated fly ash mortars (activated with NaOH, Na 2 CO 3 and waterglass solutions) and its mechanical properties has been established. The results of the investigation show that in all cases (whatever the activator used) the main reaction product formed is an alkaline aluminosilicate gel, with low-ordered crystalline structure. This product is responsible for the excellent mechanical-cementitious properties of the activated fly ash. However the microstructure as well as the Si/Al and Na/Al ratios of the aluminosilicate gel change as a function of the activator type used in the system. As a secondary reaction product some zeolites are formed. The nature and composition of these zeolites also depend on the type of activator used.