Free CaO Content Research Articles

Evaluating the potential of high free-CaO fluidized bed boiler ash as a cement admixture

This study investigates the feasibility of utilizing ash from circulating fluidized bed combustion (CFBC) boilers as a cement admixture. Two types of CFBC ash, distinguished by their free-CaO contents, are examined to evaluate their impact on the hydration reactions and mechanical properties of cement. The incorporation of CFBC ash with elevated free-CaO content into cement facilitates a hydration reaction, which results in accelerated setting and reduced compressive strength. To effectively utilize CFBC ash as a cement admixture, it is necessary to reduce the free-CaO content to an appropriate level, and carbonation curing is found to be an efficient method. It is confirmed that even CFBC ash with a high free-CaO content can effectively be utilized as a cement admixture through pretreatment with carbonation curing. This study may have valuable implications for the ability of CFBC ash to reduce carbon dioxide via mineral carbonation and its potential use as a cement admixture.

Sustainable Application of Pyrolytic Oxygen Furnace Slag in Cement-Stabilized Macadam: Volume Stability, Mechanical Properties, and Environmental Impact

As an industrial waste, basic oxygen furnace (BOF) slag is an ideal substitute for natural rubble and sand. However, its potential instability of volume restricts the application of the BOF slag in engineering. This study aims at investigating the volume stability and mechanical properties of BOF slag and its application as an aggregate in cement-stabilized macadam. As part of this research, the physicochemical properties, especially the volume stability, of two types of BOF slags and andesite were first studied. Then, mechanical properties, volume stability, and an environment analysis are used to evaluate the application of pyrolytic BOF slag in cement-stabilized macadam. The experimental results show that different types of BOF slags have similar thermal expansion coefficients, which are higher than andesite. The free CaO content of pyrolytic BOF slag is much lower than that of ordinary BOF slag and the volume expansion of pyrolytic BOF slag is less than 0.5%. The unconfined compressive strength (UCS) of cement-stabilized macadam using pyrolytic BOF slag is about 30% higher than that of andesite. Although the water loss rate is higher than a natural aggregate, dry shrinkage of pyrolytic BOF slag cement-stabilized macadam is about 30–50% less than that of a natural aggregate. Meanwhile, its shrinking speed is also slower than that of a natural aggregate. The micro-expansion properties of pyrolytic BOF slag could effectively partially offset the shrinkage characteristics of cement-stabilized macadam. Finally, the Toxicity Characteristic Leaching Procedure (TCLP) test results indicated that the metal leaching concentration meets the Chinese environmental standards. This study provides a direction for the large-scale and effective sustainable application of pyrolytic BOF slag.

High-temperature modification of steel slag using composite modifier containing silicon calcium slag, fly ash, and reservoir sediment

Steel slag (SS) is a kind of industrial solid waste, and its accumulation brings certain harm to the ecological environment. In order to promote the building material utilization of SS, high-temperature modification (HTM) of SS is performed using a composite modifier (CMSFR) containing silicon calcium slag (SCS), fly ash (FA), and reservoir sediment (RS). Then, the authors investigated the effect of CMSFR on the cementitious properties and volume soundness of SS mixture after HTM (SMHTM). After that, the mineral composition and microstructure of SMHTM were investigated through X-ray fluorescence analysis (XRF), X-ray diffraction (XRD), scanning electronic microscopy (SEM), energy dispersive spectrometry (EDS), and particle size analysis. It was found that the free CaO (f-CaO) content obviously decreased, and the cementitious properties improved in SMHTM. When the CMSFR content was 20% (SCS: FA: RS = 9:7:4), and the modification temperature (MT) was 1,250°C, the mass fraction of f-CaO in SMHTM dropped from 4.81% to 1.90%, down by 60.5%; the 28-day activity index of SMHTM increased to 85.4%, 14.3% higher than that of raw SS, which meets the technical requirement of Steel slag powder used for cement and concrete (GB/T 20491-2017): the activity index of grade I SS powder must be greater than or equal to 80%. As the mass fraction of CMSFR grew from 10% to 30%, new mineral phases formed in SMHTM, including diopside (CMS2), ceylonite (MgFe2O4), gehlenite (C2AS), tricalcium aluminate (C3A), and magnetite (Fe3O4). The HTM with CMSFR promotes the decomposition of RO phase (a continuous solid solution composed of divalent metal oxides like FeO, MgO, MnO, and CaO) in raw SS, turning the FeO in that phase into Fe3O4. The above results indicate that the SMHTM mixed with CMSFR can be applied harmless in cement and concrete, making low-energy fine grinding of SS a possibility.

Research Progress on CO2 Capture, Utilization, and Storage of Fly Ash

The research progress of different technologies of fly ash for CO2 capture, utilization, and storage at home and abroad was summarized, and the research opportunities were discussed. Fly ash could mineralize, capture, and store CO2 through direct dry, semi dry, wet, and indirect methods, reducing the leaching of heavy metals in fly ash while mineralizing CO2. The mineralized fly ash was more suitable for making concrete additives because it could effectively reduce the content of free CaO and MgO. Fly ash could also be made into activated carbon, zeolite, porous silica, and other products for physical adsorption and capture of CO2. The type of products depended mainly on the composition and physical/chemical properties of fly ash. In terms of CO2 utilization, fly ash could not only expand the utilization of building materials but also be made as catalysts or catalyst carriers required for various chemical processes of CO2 and new materials such as pseudo boehmite. The proposal of "double carbon" in China and the physical/chemical characteristics of fly ash from coal-fired power plants provide a new method for comprehensive utilization of fly ash.

A Ten-Year Study on Alkali Content of Coal Fly Ash

After years of decline, coal consumption has risen significantly in the last year (2021), driven mainly by the ever-increasing demand in fast-growing Asian countries and fostered by rising gas prices in Europe and the United States. Coal is both the largest electricity production source and the largest source of carbon dioxide emission. Coal-fired plants produce electricity by generating steam by burning coal in a boiler, but also large amounts of coal fly ash. Coal fly ash contains essential constituents for cement production, such as Ca, Si, Al, and Fe. Application of coal-fired ash to produce clinker at high doses may reduce the limestone content in the raw mix. Furthermore, coal fly ash is one of the industrial source materials utilized in the development of low-carbon cements and concretes on account of its chemical characteristics. The monitoring methodology is based fundamentally on the analysis of a set of variables (Na2Oe, Na2O, K2O, free CaO, and reactive silica content and fineness) over time. Weak relations between Na2O and K2O, and Na2Oe, and reactive silica content were found. This applied research has been done to verify previously done research. The scope of this paper is to assess the alkaline content of coal fly ash over a period of 10 years. The Na2O-equivalent of coal fly ash ranged from 0.35% to 2.53%, with an average value of 0.79%. These values should be taken into account producing concretes made with potentially reactive aggregates in order to mitigate the alkali–silica reaction (ASR).

Effect of carbonation curing on durability of cement mortar incorporating carbonated fly ash subjected to Freeze-Thaw and sulfate attack

High-calcium fly ash (HFA) is available in large quantity produced by coal-based electrical power plants. Up to now, the utilization of HFA in construction materials is limited. In this research, carbonation treatment of HFA in association with carbonation curing was applied for stabilization of hazardous residues and CO2 sequestration. The improvement on durability of carbonation-cured cement mortar incorporating carbonated HFA against coupled freeze–thaw and sulfate attack was investigated. The results show that the synergy of carbonation treatment and carbonation curing mitigated the deterioration of relative dynamic elastic modulus and compressive strength caused by coupling action of freeze–thaw and sulfate attack. The collaborative carbonation treatment and carbonation curing can also reduce the content of expansile free-CaO, as well as the components that would expand under sulfate attack conditions, such as CH and aluminates. On the other side, the results of MIP show that carbonation curing improved the pore structure and decreased the permeability of cement mortars specimens.

Safe disposal and recyclability of MSWI fly ash via mold-pressing and alkali-activation technology: Promotion of metakaolin and mechanism

The long-term, efficient and low-cost immobilization of the heavy metals contained in municipal solid waste incineration (MSWI) fly ash, is an important path to realize its harmless and resource treatment. The disposal of MSWI fly ash into geopolymers through alkali activation technology (AAT) can effectively simultaneously immobilize heavy metals and reuse MSWI fly ash. However, how to reduce the dosage of alkaline activator (AA) and improve the compressive strength of treated materials has always been a difficulty in the field. In this study, metakaolin (MK) was added (0 −20 wt%) as an aluminosilicate source to explore its effects on the compressive strength and heavy metal immobilization efficiency of the geopolymers; additionally, mold-pressing method was used to investigate its feasibility of reducing the consumption of AA. The results revealed that, in the case of the 20 wt% MK samples, the compressive strength of the geopolymers can significantly increase to 30.31 MPa, meanwhile, the AA requirement is only 25 wt% of the MSWI fly ash and MK, and the leaching toxicities of various heavy metals were below the limit established by the Chinese standard (GB 16889–2008). The content of free CaO in the optimal samples was observed to decrease to < 1 wt%. In addition, the results revealed that an increase in mold pressure tended to coincide with an increase in the proportion of gel pores (up to 6.98%), which increased geopolymer compressive strength. The main products yielded by the mold-pressed MSWI fly ash geopolymer are sodium-aluminosilicate-hydrate (N-A-S-H) gels.

Experimental Investigation and Multi-Gene Genetic Programming Simulation of Portland Clinker Burnability

In this study, the effect of chemical composition of the raw material on the clinker burnability was studied by determination of free CaO (wt %) content of clinker. The burnability of two types of Portland clinker was investigated for silica modules of 2.3, 2.5 and 2.7 and lime saturation factor of 0.88–0.98. In addition, using the Multi-gene genetic programming (MGGP) model, the burnability of clinker was predicted. The results of MGGP model indicated that the performance of the model for predicting the amount of free CaO (wt %) was acceptable. Moreover, using MGGP, a promising correlation was introduced for accurately calculating the amount of free CaO (wt %). The performance of this correlation was compared with FL-Smidth, and it was established that the average errors of MGGP correlation and FL-Smidth equation were 2.95 and 7.45 %, respectively.

Enhancing Burnability Characteristics of Low-Temperature Burnt-Cement Clinker by Recycling Phosphogypsum Wastes

In the present study, the effect of phosphogypsum, a byproduct from the phosphoric acid production industry, on the clinkerization process was researched in order to improve the burnability of clinkers at lower burning temperatures. Raw meal samples containing phosphogypsum at 0%, 1%, 3%, and 5% by weight were burned at 1,350°C, 1,400°C, 1,450°C, and 1,500°C. The introduction of phosphogypsum reduced the presence of unbounded (free) CaO relatively up to 43%. Thus, the mineralization effect of phosphogypsum promotes lower burning temperatures. The free CaO content of the 5% phosphogypsum containing clinker decreased to 0.94% at 1,400°C after 120 min of burning time. The free CaO content was found to be almost identical (0.75%) to that of industrial clinker burned in a rotary kiln at 1,500°C. Due to the low free CaO content (<1%) and the burnability index (73), the recycling of phosphogypsum wastes was found to be suitable for the production of eco-efficient clinkers. The reduced energy consumption and energy-related CO2 emissions in such modified clinkers can contribute to the economic and ecological goals of the cement industry.

Effect of binary basicity and ferric oxide content on the soundness, grindability, and cementitious activity of converter steelmaking slag

In this study, the effects of binary basicity on the stability, grindability, and cementitious activity of steelmaking slag were investigated. The free CaO content, relative grindability index, and cementitious activity of the samples considered were determined using the glycol method, specific surface method, and activity testing technology, respectively. The results obtained reveal that an increase in slag basicity from 1.8 to 4 and a decrease in its Fe2O3 content from 30% to 10% increase the free CaO content and lead to improvements in the slag grindability and cementitious activity. Accordingly, a basicity value of approximately 2 is determined ideal for building materials.

Carbonation curing of cement mortars incorporating carbonated fly ash for performance improvement and CO2 sequestration

Carbonation treatment of fly ash (FA) is proposed as a promising method for stabilization of hazardous residues and CO2 sequestration. This paper aims to investigate the performance of cement mortar incorporating carbonated FA and the synergistic effect with carbonation curing. The results show that the content of free CaO and the hydration heat were reduced apparently by carbonation treatment. The synergy of carbonation treatment and carbonation curing compensated for the strength loss caused by FA incorporation, but can also delayed strength development brought by pozzolanic effect. CO32- on the surface of FA introduced by carbonation treatment competed with Cl− for binding AFm, consequently lead to decreased resistance of chloride ion penetration. Carbonation curing mitigated this degradation by improving the pore structure of cement mortar significantly. In summary, carbonation curing can be an appropriate complement to carbonation treatment of FA for performance improvement of cement-based materials and more efficient CO2 sequestration.

Corrosion evaluation of CA-50 steel in pore waters extracted from cement pastes with steel slags using electrochemical techniques

Abstract There is an interest in the cement industry in the use of steel slags in cement, but chemical and/or pyrometallurgical modifications are necessary to reduce the free CaO, MgO, and iron contents. However, the potential effects of its application in the reinforcement corrosion, whether in solid (concrete) or liquid medium (pore water), have not yet been addressed. In this context, the present study shows a corrosion analysis of the CA-50 steel in pore water medium extracted from cement pastes with 25% by weight of steel slag, in natural state or pyrometallurgical modified, by means of polarization curves, electrochemical impedance spectroscopy (EIS), and microstructural analysis. For comparison, tests were performed in pore waters presented in literature and representative from Ordinary Portland Cement (OPC) or activated Blast Furnace Slag (BFS) and, to simulate aggressive conditions, 1.0% NaCl was also added to the solutions. The steel remained passive in all media without 1.0% NaCl, but the EIS results indicated more protective characteristics in the medium simulating the modified steel slag. The main corrosion product identified by SEM images after the tests on aggressive media was lepidocrocite (γ-FeOOH), and the steel did not corrode in media with steel slags and 1.0% NaCl. This was attributed mainly to the higher alkalinity of these media in comparison to other usual pore waters, promoting longer protection of the steel.

Investigation of Permanent Deformation Behavior of Steel Slag Asphalt Mixture under Indoor Simulation

Previous studies have indicated that steel slag can be used as a substitute for natural aggregates in asphalt mixture, while little attention has been paid to systematic investigation of the influences of various external environmental factors on deformation resistance of steel slag asphalt mixture. In order to understand the service behavior of steel slag asphalt mixture, its permanent deformation under different condition was investigated based on an indoor simulation test. The chemical composition, microscopic structure, surface texture, and volume stability of steel slag were firstly characterized. The uniaxial repeated loading test and standard wheel‐tracking test were applied to evaluate the effect of temperature, stress levels, and water damage on the permanent deformation of AC‐16 and AC‐20 steel slag asphalt mixtures. The results indicate that a higher content of alkaline oxide and high‐grade texture index existing in steel slag contribute to its strong absorptivity and adhesion of asphalt. The steel slag demonstrates fine volume stability due to its lower free‐CaO (f‐CaO) content, autoclave chalked ratio, and immersion expansion ratio. The permanent deformation of steel slag asphalt mixtures increases rapidly under higher stress and temperatures in contrast to lower increment at lower stress and temperatures. Asphalt mixtures at higher stress and higher temperatures and water condition exhibit larger rutting deformation and inferior rutting resistance. AC‐16 steel slag asphalt mixture has superior resistance to permanent deformation than AC‐20 asphalt mixture. Rutting factors show different degrees of impact in a decreasing order of temperature, water damage, and stress levels. The findings have significant implications for providing a theoretical basis for reusing steel slag in pavement construction and facilitating engineering application of steel slag asphalt mixture.

A Review of the Influence of Steel Furnace Slag Type on the Properties of Cementitious Composites

The type of steel furnace slag (SFS), including electric arc furnace (EAF) slag, basic oxygen furnace (BOF) slag, ladle metallurgy furnace (LMF) slag, and argon oxygen decarburization (AOD) slag, can significantly affect the composite properties when used as an aggregate or as a supplementary cementitious material in bound applications, such as concretes, mortars, alkali-activated materials, and stabilized soils. This review seeks to collate the findings from the literature to express the variability in material properties and to attempt to explain the source(s) of the variability. It was found that SFS composition and properties can be highly variable, including different compositions on the exterior and interior of a given SFS particle, which can affect bonding conditions and be one source of variability on composite properties. A suite of tests is proposed to better assess a given SFS stock for potential use in bound applications; at a minimum, the SFS should be evaluated for free CaO content, expansion potential, mineralogical composition, cementitious composite mechanical properties, and chemical composition with secondary tests, including cementitious composite durability properties, microstructural characterization, and free MgO content.

Stabilization of a Clayey Soil with Ladle Metallurgy Furnace Slag Fines.

The research study described in this paper investigated the potential to use steel furnace slag (SFS) as a stabilizing additive for clayey soils. Even though SFS has limited applications in civil engineering infrastructure due to the formation of deleterious expansion in the presence of water, the free CaO and free MgO contents allow for the SFS to be a potentially suitable candidate for clayey soil stabilization and improvement. In this investigation, a kaolinite clay was stabilized with 10% and 15% ladle metallurgy furnace (LMF) slag fines by weight. This experimental study also included testing of the SFS mixtures with the activator calcium chloride (CaCl2), which was hypothesized to accelerate the hydration of the dicalcium silicate phase in the SFS, but the results show that the addition of CaCl2 was not found to be effective. Relative to the unmodified clay, the unconfined compressive strength increased by 67% and 91% when 10% and 15% LMF slag were utilized, respectively. Likewise, the dynamic modulus increased by 212% and 221% by adding 10% and 15% LMF slag, respectively. Specifically, the LMF slag fines are posited to primarily contribute to a mechanical rather than chemical stabilization mechanism. Overall, these findings suggest the effective utilization of SFS as a soil stabilization admixture to overcome problems associated with dispersive soils, but further research is required.

Obtaining low-energy cement from industrial raw material

The results of obtaining Portland cement by lowenergy technology using technogenic raw materials are presented. Coal wastes and tephritobasalt in a ratio of 1: 1 were used as the clay component, instead of scarce pyrite cinder, granular lead slags were used. The regularities of the influence of the compositions of the blends, saturation coefficient, silicate and alumina modules, firing modes on the chemical and mineral composition and quality of clinker and cement are established. The composition of the raw mixes was calculated according to the ROCS program, the content of free CaO in the clinker was determined. The strength of experimental cement was tested after 7 and 28 days of hardening. Its physical and mechanical characteristics and structure were studied.

Preparation Low-Energy Content Cement from Technogenic Raw Materials

Results of preparing Portland cement by low-energy technology using technogenic raw materials are presented. Coal mining waste and tephritobasalt in a ratio of 1:1 are used as the clay component, and granulated lead slags are used instead of scarce pyrite cinder. Correlation of the influence of charge compositions, saturation coefficient, silicate and alumina moduli, firing regimes with chemical and mineral composition and clinker and cement quality are established. Composition of the raw material mixtures is calculated according to a ROCS program, and the free CaO content in clinker composition is determined. Experimental cement strength is tested after hardening for 7 and 28 days. Its physicomechanical characteristics and structure are studied.

The potential of FBC fly ashes to reduce CO2 emissions

The production of electricity and heat in Poland is the reason why the commercial power industry is the largest emitter of CO2. At the same time, significant amounts of solid by-products of combustion, which can be used to bind CO2 by mineral carbonation, are generated during the production processes. The article presents the results of research on mineral sequestration of CO2 (suspension-CO2) using fluidized bed combustion (FBC) fly ashes from hard coal combustion. The analyzed fluidized bed combustion (FBC) fly ashes were characterized by a significant free CaO content (1.7–6.8%) and a high CO2 binding potential ranging from 9.7 to 15.7%. In the case of fluidized bed combustion (FBC) fly ashes suspensions, the basic product of the carbonation process is calcium carbonate, which is clearly indicated by the results of the phase composition determination of solidified suspensions of fluidized bed combustion (FBC) fly ashes. The degree of carbonation, i.e. the degree of CO2 binding, calculated on the basis of the calcium carbonate content, in the analyzed suspensions was up to 1.1%. Mineral carbonation also reduces the leachability of pollutants such as: Zn, Cu, Pb, Ni, As, Hg, Cd, Cr, Cl, and SO42-. The pH is also reduced from about 12 to about 9. Aqueous suspensions of fluidized bed combustion (FBC) fly ashes with introduced CO2 can potentially be used in underground mining. These activities are in line with the concepts of Carbon Capture and Utilization and the idea of circular economy.

Alkali Elution Behavior of Steelmaking Slag Packed in an Open Channel Vessel in Seawater

The alkali elution behavior of steelmaking slag in seawater was kinetically investigated and simulated under continuous flow in an open channel vessel with packed bed of steelmaking slag. Two types of steelmaking slags, viz. decarburization slag and dephosphorization slag, were used in this study. The alkali elution rate of decarburization slag was larger than that of dephosphorization slag due to larger free CaO content. The pH value for dephosphorization slag was almost the same as the seawater pH value in 3–4 days, whereas that for decarburization slag was stabilized in 3 days although the pH value was slightly larger than that of seawater. The capacity coefficients of alkali elution for dephosphorization and decarburization slags decreased together in an exponential manner with time. Based on a regression equation on the mass transfer capacity coefficient change with time, the alkali elution behavior was simulated and the calculated results agreed well with the experimental ones. The temporal pH change was predicted by changing slag surface area and seawater flow rate as a parameter. According to the simulation results for dephosphorization slag, the seawater pH value did not reach a high level in the ocean area.

Hydration of clinkers doped with baryte with and without addition of gypsum

The use of alternative fuels and raw materials in cement industry leads to the incorporation of some undesirable compounds into the clinker phases. One of them is barium oxide which influences the formation of clinker phases and related properties of the produced cement. Barium in clinker was already studied; the phase composition is changed depending on the amount of barium. However, detailed research focused on hydration properties of these clinkers and cements has not been carried out, yet. A clinker with barium oxide addition to raw meal was prepared. As a source of barium oxide, barium sulphate (baryte) was used in amount of 4% calculated to BaO in the clinker. Barium incorporated in clinker inhibits formation of alite; therefore, lime saturation factor was slightly reduced. Compared to barium carbonate—another source of barium oxide—barium sulphate influences the ratio of clinker phases less significantly. This clinker and a reference sample without barium were milled with and without addition of gypsum. Hydration of barium clinker and cement was examined by XRD in situ hydration and isothermal calorimetry. Hydration products were evaluated by means of thermogravimetric and differential thermal analysis. An increase in total heat of hydration can be seen in samples with barium. The heat is influenced by different hydration of clinker phases with incorporated barium, sulphates and by higher content of free CaO and C3A. According to XRD in situ hydration, hydration of samples with barium is suppressed, the amount of portlandite during the whole measurement is much higher in samples without barium.