Free CaO Research Articles

Properties of CaO-MgO-SiO2 insulating refractories prepared from phosphorus tailings

CaO-MgO-SiO2 insulating refractories were prepared using phosphorus tailings through in-situ pore-forming technology in this work, aiming to achieve the efficient utilization of solid waste phosphorus tailings. microsilica was used to react with free CaO generated by the decomposition of dolomite from phosphorus tailings during the calcination in order to prevent the hydration of CaO in the materials. The influence of microsilica addition and heating temperature on the properties of insulating refractories prepared from phosphorus tailings was investigated. The results showed that CaO-MgO-SiO2 insulating refractories with 8.8 wt% microsilica addition exhibited excellent comprehensive performance after heating at 1300 °C for 3 h. The insulation refractories prepared from phosphorus tailings were anti-hydration, had lower thermal conductivity, and could be used as insulation refractories for high-temperature kilns.

Properties of cement raw meals used as sorbents in a calcium looping process

Calcium looping (CaL) is a promising carbon capture and storage (CCS) technology that has the potential to significantly reduce CO2 emissions in cement production. Integrating CaL with cement production provides a viable solution to the high CO2 emissions generated during the calcination process. This study examines the behavior of two industrial cement raw meals from different sites (A-RM and B-RM) under CaL conditions, focusing on phase composition, particle size distribution, and clinker phase evolution up to 1450 °C. Calcination and CaL experiments were conducted in a CO2-rich atmosphere, with materials characterized using quantitative X-ray diffraction (Q-XRD) and high-temperature X-ray diffraction (HT-XRD). The results showed that both raw meals absorbed similar amounts of CO2 during the cyclic CaL experiments. A-RM formed C2S and other silicates, while B-RM retained more free CaO due to a less effective reaction with coarser quartz (SiO2) particles. HT-XRD revealed delayed clinker-phase evolution in the 1000–1400 °C range in CaL-treated raw meals. However, CaL-treated raw meals achieved low free CaO at 1450 °C, suggesting that optimal kiln conditions can produce the desired phase composition. These findings indicate that integrating CaL-treated raw meals into cement production requires careful optimization of operational parameters to maintain clinker quality and minimize energy consumption. Further research should focus on improving the efficiency and reactivity of CaL-treated raw meals to enhance their suitability for industrial cement production.

Influence of Top Slag Containing TiO2 and VOx on Hot Metal Pre-Desulfurization

The desulfurization capacity of top slag in the process of pre-desulfurization of hot metal containing vanadium and titanium was researched. The top slag system of CaO-SiO2-MgO-Al2O3-TiO2-VOx that was formed by blast furnace slag and a CaO desulfurization agent reduced the sulfur in hot metal from 0.08 wt.% to 0.02 wt.%. It was found that the resulfurization of the slag happened in the later periods of the desulfurization process. The vanadium–titanium oxides were both acidic in the desulfurization slag. TiO2 and VOx reacted with the basic oxides to form CaTiO3 and MgV2O4 at 1623 K, which reduced free CaO and was not conducive to top slag desulfurization. The results of calculation showed that the top slag desulfurization accounted for 15% of the total desulfurization. Using the ionic and molecule coexistence theory of slag structure, it is shown that the desulfurization efficiency could be enhanced by adjusting both the amount of desulfurization agent and the composition of the blast furnace slag before pre-desulfurization.

Characterization of Limestone Surface Impurities and Resulting Quicklime Quality

Quicklime, rich in CaO(s), is generated by calcining limestone at high temperatures. Parallel-flow regenerative lime kilns are the most energy-effective industrial method available today. To prevent major disruptions in such kilns, a high raw material quality is necessary. Under some conditions, impurity-enriched material may adhere to limestone pebbles and enter the kiln. In this study, limestone and corresponding quicklime were analyzed to evaluate the extent and composition of surface impurities and assess the effect on quicklime product quality, here defined as free CaO. This was performed by sampling and analyzing limestone, quarry clay, laboratory-produced quicklime, and industrially produced quicklime with XRF, SEM/EDX, and XRD; interpretations were supported by thermodynamic equilibrium calculations. In the laboratory-produced quicklime, the surface impurities reacted with calcium forming Larnite, Gehlenite, Åkermanite and Merwinite, reducing the quicklime quality. The results showed that the limestone surface layer comprised 1.2 wt.-% of the total mass but possessed 4 wt.-% of the total impurities. The effect on industrially produced quicklime quality was lower; this indicated that the limestone surface impurities were removed while the material moved through the kiln. Multicomponent chemical equilibrium calculations showed that the quarry clay was expected to be fully melted at 1170 °C, possibly leading to operational problems.

Enhanced Carbonation of Free CaO in Basic Oxygen Furnace Slag Under High Temperature and Moderate Pressure and Its Kinetics

Enhanced Carbonation of Free CaO in Basic Oxygen Furnace Slag Under High Temperature and Moderate Pressure and Its Kinetics

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.

Potassium citrate-activated pure BOF slag-based mortars utilizing carbonated and autoclaved BOF slag aggregates

This study proposes novel basic oxygen furnace (BOF) slag mortars formulated with tri-potassium citrate (TPC) activated BOF slag as binder and treated BOF slag (carbonated (CB) and autoclaved (AB)) as aggregates, aiming to maximize the utilization of BOF slag in the construction industry. Untreated BOF slag (UB) and natural sand (NS) were employed as reference aggregates. The effects of carbonation and autoclaving were investigated, focusing on the interfacial transition zone (ITZ) properties through SEM-BSE greyscale thresholding and constituent segmentation analysis. Results indicate that both treatments reduce the free CaO, promote CaCO3 formation, and modify the aggregate surface. The 28 d compressive strength of pure BOF slag-based mortars with UB, CB, and AB aggregates are 25.8, 29.4, and 34.8 MPa (average at varying TPC dosages), significantly higher than that with NS (19.6 MPa). Pure BOF slag-based mortars also show narrower ITZ thicknesses and the absence of bond cracks. Mortars with CB and AB aggregates exhibit fewer unreacted particles and more hydration products within the integral binder region compared to those with UB. Overall, pure BOF slag-based mortars present a more robust aggregate/binder interface that is further enhanced by the aggregate treatments. Leaching values of all mortars are well below the Dutch legal limits. The findings suggest promising application prospects for pure BOF slag-based mortars in substituting cement mortars and valorizing BOF slag.

Improving skid-resistance durability of ultra-thin friction course in asphalt pavements through recycled steel slags as basalt replacements

The ultra-thin friction course (UTFC) has emerged as an effective solution for restoring skid-resistance on worn pavements. However, the scarcity and high cost of high-quality basalt aggregates pose challenges for UTFC implementation. This study explores the potential of utilizing recycled steel slags (blast furnace slag (BFS), basic oxygen furnace slag (BOFS), and electric arc furnace slag (EAFS) as alternatives to basalt aggregates in UTFC applications. The durability of skid-resistance and shear strength of these steel slag-UTFCs are investigated, and the chemical and mineral compositions of the steel slags are analyzed to assess their suitability. The findings reveal that EAFS-UTFC exhibits superior surface roughness, enhanced wear resistance, and comparable shear strength to Basalt-UTFC. The dynamic friction coefficient (μ) between EAF-UTFC and rubber at 120 km/h is 1.12 times higher than that of basalt at 0 wear cycles, and this advantage amplifies to 3.11 times higher after 400k wear cycles. However, BFS and BOFS are not suitable for direct use in UTFC, which have lower wear resistances and shear strengths compared to basalt, attributed to their poor bulk stability caused by high levels of free CaO and MgO. Consequently, a significant portion of their surface aggregates are dislodged under repeated wheel loading before sufficient polishing occurs. This study offers insights into sustainable solutions for improving skid-resistance on asphalt pavements.

Semi-dry and aqueous carbonation of steel slag: Characteristics and properties of steel slag as supplementary cementitious materials

Accelerated carbonation of steel slag presents a viable method for CO2 sequestration and the valorized utilization of steel slag. This study aimed to investigate the impact of semi-dry and aqueous carbonation, as well as the carbonation degree, on the microstructure and properties of steel slag. Additionally, it evaluated the feasibility of utilizing carbonated steel slag as supplementary cementitious materials (SCMs). Steel slag was subjected to CO2 (20% volume percentage) at 2 bar pressure for both semi-dry and aqueous carbonation process at room temperature with water/solid ratio of 0.08 and 5, respectively. The carbonation duration was varied (10 min, 30 min, 60 min, and 120 min) to adjust the degree of carbonation. Mortars and pastes with 30 wt% carbonated steel slag were formulated and their compressive strength and volume expansion values were quantified. The findings revealed that the CO2 uptake values for steel slag after 2 hours of semi-dry and aqueous carbonation were only 3.75% and 1.50%, respectively. Compounds such as Ca(OH)2, Mg(OH)2, and Ca2SiO5 demonstrated high carbonation reactivity during the semi-dry carbonation process, whereas Ca(OH)2 and Mg(OH)2 exhibited only a marginal increase in aqueous carbonated steel slag. The specific surface area of steel slag increased with an increase in the carbonation degree, attributed to the formation of silica gel or amorphous C-S-H during carbonation. However, the hydration activity of carbonated steel slag diminished with an increased degree of carbonation due to the consumption of alkaline minerals. Conversely, pozzolanic activity was enhanced with a higher degree of carbonation, owing to the formation of silica gel. The cumulative heat release of cement containing SDC30 (semi-dry carbonation for 30 min) within 24 h and 72 hours showed increases of 16.0% and 3.5%, respectively, aligning with the compressive strength results. Semi-dry carbonation treatment significantly enhanced the early-stage cementitious properties of steel slag/cement composites. Specifically, the 3-day compressive strength of mortar containing 30 wt% SDC30 increased by 22.7% relative to the reference mortar, composed of 30 wt% uncarbonated steel slag and 70 wt% Portland cement. Carbonation markedly improved the volumetric stability of steel slag with the extent of improvement was positively correlated with the carbonation degree for both semi-dry and aqueous carbonated steel slag. Aqueous carbonation was more effective in promoting the hydration process of free CaO and MgO more effectively than semi-dry carbonation.

Effect of Carbonation Treatment on the Properties of Steel Slag Aggregate.

Steel slag is the waste slag generated after steel smelting, which has cementitious activity. However, untreated steel slag can damage the integrity of steel slag concrete due to its harmful expansion. This study prepared porous aggregates by mixing powdered steel slag, fly ash, and cement and carbonated them with CO2 under high pressure conditions (0.2 MPa). The effect of carbonation on the performance of steel slag aggregate was studied using volume stability and crushing value. The effect of different carbonation conditions on the products was studied using X-ray diffraction (XRD) and thermogravimetric (TG) analyses, and the carbon sequestration efficiency of steel slag under different treatment methods was quantitatively evaluated. The research results indicate that untreated steel slag was almost completely destroyed and lost its strength after autoclave curing. With the increase in temperature and carbonation time, the performance of steel slag aggregate gradually improved and the pulverization rate, expansion rate, and crushing value gradually decreased. According to the experimental results of XRD and TG, it was found that the reaction between f-CaO (free CaO) and CO2 in steel slag generated CaCO3, filling the pores inside the aggregate, which was the internal reason for the improvement of aggregate performance. After comparison, the best carbonation method was maintained at 55 °C for 72 h. After carbonation, the steel slag aggregate had a pulverization rate of 2.4%, an expansion rate of 0.23%, a crushing value of 23%, and a carbon sequestration efficiency of 11.27% per unit weight of aggregate.

Performance enhancement of free CaO containing magnesia by in situ intergranular CaAl2O4 and MgAl2O4

Magnesia refractory raw material with free CaO has inherent weaknesses of low resistance to hydration, thermal shock and slag penetration. In this work, micro-sized Al2O3 powder was added into such magnesia with the purpose of improving its comprehensive performances. The results showed that with increasing Al2O3 content, CaAl2O4 and MgAl2O4 phases were sequentially formed at the MgO grain boundaries, which enlarged the MgO grains and reduced the apparent porosity. Owing to the transformation of free CaO to CaAl2O4, reduction of apparent porosity and covering of intergranular MgAl2O4 on MgO grains, hydration of the specimens was inhibited effectively by introducing the Al2O3 addition. The formed CaAl2O4 and MgAl2O4 phases lowered the thermal expansion coefficient and induced crack deflection, and therefore residual flexural strength ratio after thermal shock tests increased remarkably from 15.1% to 33.2% with increasing Al2O3 addition from 0 wt% to 7 wt%. Moreover, slag penetration resistance of the specimens was improved effectively with raising the Al2O3 content to 3 wt%, attributing to the increase in MgO grain size, decrease in apparent porosity and formation of high stable intergranular CaAl2O4 phase. However, further increasing Al2O3 content would dramatically accelerate the slag penetration owing to the decrease in grain size and formation of more unstable intergranular MgAl2O4 phase.

The Characteristics of Self-Hydration and Carbonation Reaction of Coal Ash from Circulating Fluidized-Bed Boiler by Absorption of CO2.

The by-products of the circulating fluidized-bed boiler combustion (CFBC) of coal exhibit self-hardening properties due to the calcium silicates generated by the reaction between SiO2 and CaO, and the ettringite generated by the reaction of gypsum and quicklime with activated alumina. These reactions exhibit tendencies similar to that of the hydration of ordinary Portland cement (OPC). In this study, the self-hydration and carbonation reaction mechanisms of CFBC by-products were analyzed. These CFBC by-products comprise a number of compounds, including Fe2O3, free CaO, and CaSO4, in large quantities. The hydration product calcium aluminate (and/or ferrite) of calcium aluminate ferrite and sulfate was confirmed through instrumental analysis. The CFBC by-products attain hardening properties because of the carbonation reaction between calcium aluminate ferrite and CO2. This can be identified as a self-hardening process because it does not require a supply of special ions from the outside. Through this study, it was confirmed that CFBC by-products generate CaCO3 through carbonation, thereby densifying the pores of the hardened body and contributing to the development of compressive strength.

Low-temperature calcination composite binder from dolomite and its application to facing board materials

The article presents the research to obtain low-temperature calcination binder from dolomitic limestone with a MgO content of less than 20%. The initial dolomite material was calcinated at the following temperature range from 700 to 800 ℃ and firing time 25–45 min. The result of the selection of the optimal firing conditions is semi-calcination product caustic dolomite with a minimum content of residual Mg(CO3)2 and free CaO with bending strength 10–11 MPa. Physical-mechanical characteristics and mineral composition of samples were determined by the use of differential thermal analysis and infrared spectroscopy studies. Preliminary experimental samples of facing composite board materials were prepared and the main physico-chemical parameters were determined: bending strength 4–7.6 MPa, density 800–1400 kg/m3 and water absorption 10–25%. A three-dimensional model for the production of glass dolomite slabs has been developed.

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.

Phase evolution and burnability of cement raw meal

The use of high-temperature X-ray diffraction (HT-XRD) to study the mass transfer of raw meal constituents towards forming clinker phases and the occurrence of free lime (calcium oxide), also known as burnability, was assessed. A measuring strategy with temperature ranging from 1000°C to 1450°C was developed and compared with a conventional burnability method. The free lime determined by the methods showed that HT-XRD produced good results for the evaluation of burnability. In addition, HT-XRD revealed the formation of intermediate phases, providing insight into early reactions in a cement kiln. The particle size of quartz was found to affect crystal expansion of the phase at a high temperature, subsequently affecting the formation of silica polymorphs. The different raw meals used in this study also indicate that the formation of different silica polymorphs affects the formation of C2S. The lack of knowledge regarding the influence of β-quartz on the reduction of free lime is highlighted.

Utilisation of raw oil shale as fine aggregate to replace natural sand in concrete: Microstructure, surface chemistry and macro properties

This study set out to examine the unconventional use of raw oil shale (OS) as fine aggregate in concrete instead of its traditional utilisation as a pollutant energy source aiming to hinder the depletion of natural resources. Oil shale particles have been used to replace sand in concrete with 30 wt.-% and 50 wt.-% ratios. Part of the used OS particles was treated with silane prior to their addition in concrete. The fresh, mechanical, and durability properties, along with the micro-properties and surface chemistry of mixtures, were analysed by running the slump, compressive strength, permeability, scanning electron microscopy (SEM) and fourier transform infrared spectroscopy (FTIR) tests, respectively. Five concrete mixtures consisting of 30 samples with the size of 150 mm x 150 mm x 150 mm were prepared to serve the testing programme of the study. The results revealed the formation of a well-bonded interfacial transition zone between untreated OS particles and the cementitious matrix due to the presence of pozzolanic materials in OS. However, micro cracks and enlarged capillary pores were witnessed in treated OS mixtures along with a weak interfacial transition zone between the treated OS and the cementitious matrix. Moreover, the use of 30 wt.-% untreated OS reduced the water absorption of concrete by 9% (absorption rate of 9.5%), while other mixtures experienced an increase in their water absorption with a maximum increase of 42% when using 50 wt.-% treated OS (absorption rate of 10.7%). All mixtures attained a reduction in their compressive strength compared to control sample when employing treated and untreated OS in concrete, with a minimum reduction of 13% when using 30 wt.-% untreated OS (compressive strength of 26 MPa). The strength reduction in untreated OS mixtures is due to the formation of free CaO and free SiO2, which promote concrete expansion. The interference between silane and kerogen in OS was the main reason behind the strength reduction in treated OS mixtures. However, this reduction in mixtures’ strength is considered minimal, allowing for their potential use in different construction applications like rigid pavement.

Effects of the mineralogical composition and particle size distribution of ladle furnace slag as a cement/fine aggregate replacement in concrete

Ladle furnace slag (LFS) shows excellent potential for valorization. Despite this, landfills are typically its final destination, mainly because of technological barriers in its valorization process. This work examines the potential use of LFS as a partial cement/fine aggregate replacement, focusing on the effects of LFS composition and particle size distribution on concrete physico-mechanical properties. Chemical/mineralogical characterization of raw/hydrated samples, fresh-/hardened-state concrete properties, and volumetric instability tests were evaluated. Our results show reduced mechanical performance with LFS replacement, reaching compressive strength values of 32-42 MPa after 28 days. LFS mineralogical characterization reveals the absence of free CaO and the presence of periclase with its hydration/carbonation products. Therefore, the weathering/maturity process mainly affects free CaO. Furthermore, the observed volumetric instability issues were within the Code on Structural Concrete (Spanish abbreviation: EHE) established limits (0.04%), suggesting that the remaining periclase could be responsible for this expansive behavior.

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.

Influences of solid content of slurry on the microstructure of CaO crucible prepared by slip casting

AbstractCaO crucible was prepared from Ca(OH)2 slurry via a slip‐casting method in order to avoid hydration problem caused by free CaO during preparation process. The effect of solid content of Ca(OH)2 slurry on microstructure of CaO crucible was investigated. It was found that the rheological property of the slurry was significantly improved when the solid content of the slurry within a certain range. The change of solid content of slurry affects the microstructure of the final products. When the solid content of the slurry was 71 wt%, the manufactured CaO crucible was of smallest mean pore size (2.58 μm) after 1600°C heating treatment. The heated CaO sample was of lower apparent porosity (4.1%) and higher bulk density (2.93 g/cm3). And the size distributions of most CaO grains are between 10 and 40 μm.

Performance improvement of free CaO containing magnesia with ZrSiO4 addition

Free CaO containing magnesia has limited application due to poor resistance to hydration, slag penetration and thermal shock. This work aimed to improve the performances of such magnesia by introducing microscale ZrSiO4 powder. The results showed that the relative density of the specimens reduced from 92.1 % to 90.9 % when adding 1 wt% ZrSiO4 due to formation of Ca2SiO4 and CaZrO3, but then gradually increased to 96.2 % with further increasing ZrSiO4 to 7 wt% owing to the enhanced mass transfer and MgO grains growth caused by generation of c-ZrO2 and CaMgSiO4. The addition of ZrSiO4 obviously improved hydration resistance of the specimens, whose weight gain rate after the hydration test reduced significantly from 0.38 % to 0.01 % as the ZrSiO4 increasing from 0 wt% to 7 wt%. Thermal shock resistance (TSR) of the specimens reduced slightly when adding 1 wt% ZrSiO4 owing to the presence of more irregular pores, but then improved dramatically with further increasing ZrSiO4 due to the reduction in thermal expansion coefficient and occurrence of crack deflection. Besides, slag penetration was accelerated in the specimen with 1 wt% ZrSiO4 due to its lowered densification, while further increasing ZrSiO4 effectively improved the slag penetration resistance, attributing to the increase in MgO grain size and density and formation of the CaZrO3 containing dense layer preventing further slag infiltration.