Replacement Level Of Metakaolin Research Articles

Early age and long‐term properties of mortars containing metakaolin and limestone powder as SCMs

AbstractIn this study we focus on the development of plastic shrinkage and capillary pore pressure on mortars containing metakaolin and limestone powder as cement replacement. Isothermal calorimetry was also used to compliment the early age properties of the investigated mortars. Strength development and phase assemblage using thermal analysis were also investigated. Five different replacement levels of metakaolin and LSP and two different amounts of SP were used. Our data show that the shrinkage strain values increased as the amount of SCMs increased, while capillary pressure showed a mixed trend. The hydration kinetics did not appear to be influenced much from the SCMs used. Strength development improved in general when metakaolin was used while decreasing when LSP was added, as already expected. The TGA data show an increase in the consumption of portlandite as the amount of SCMs was increased.

Hydration kinetics, microstructure and physicochemical performance of metakaolin-blended cementitious composites

This paper presents a comprehensive investigation on influences of metakaolin on the early and later performances of cementitious composites. Systematic measurements from hydration to physicochemical performance and thermodynamic modeling based on ternary compatible phase diagram were carried out. Different fineness of metakaolin with d50 of 25, 15.3 and 3.37 μm was adopted to replace ordinary Portland cement. The replacement level of metakaolin varied at 5 %, 10 %, 15 % and 20 %. Effects of metakaolin in view of pozzolanic activity and filling capacity on the microstructural formation of cementitious composites were quantified based on the Frattini test and the strength activity index. It is found that a higher addition or specific surface area of metakaolin decreases the fluidity of cementitious composites and meanwhile the setting time was delayed. The filling effect of metakaolin on cement hydration works primarily in the early 3 d while pozzolanic effect plays a major role afterwards. The relationship between particle size of MK and its pozzolanic activity becomes pronounced for metakaolin content greater than 10 %. The presence of metakaolin contributes to strength development mainly at late age. Finer particle size of metakaolin does not ensure a pronounced filling effect, which depends on the content of metakaolin. The metakaolin with high fineness d50 = 3.37 μm enables to promote cement hydration. The decrease of metakaolin particle size results in a higher content of C-A-S-H and high alumina-rich hydrates, as demonstrated by both experiments and simulation.

Modelling the compressive strength of high-performance concrete containing metakaolin using distinctive statistical techniques

Cement consumption is rapidly increasing around the world causing environmental concerns over higher CO2 emissions. Metakaolin obtained through the calcination of kaolinite clay is an eco-friendly substitution for clinker in portland cement production. The raw materials required to produce metakaolin are easily accessible and cheap. Addition of metakaolin in concrete alters mechanical properties, notably compressive strength. Prior knowledge of the strength characteristics of concrete play a vital role in determining the design aspects of the buildings. Robust prediction models that can effectively predict the compressive strength of pozzolan-based concrete are highly essential in the field of concrete technology in order to save time and reduce laboratory testing samples. In this regard, A total of 152 experimental datasets from the literature were collected and evaluated. Influencing parameters such as binder content (385–600 kg/m3), water to cementitious material ratio (0.25–0.45), metakaolin replacement levels (0%–50%) and SiO 2/Al2O3 ratio in the cement paste (1.05–12) were chosen as independent variables. The present study was aimed at developing prediction models using different statistical techniques such as Linear regression (LR), Multi-logistic regression (MLR), Nonlinear regression (NLR) and Response surface methodology (RSM) to predict the 28-day compressive strength of High-performance concrete containing metakaolin. Also, the performance of the proposed models was examined using different statistical metrics such as coefficient of correlation (R), root mean squared error (RMSE), mean absolute error (MAE) and scatter index (SI). The quadratic RSM model outperforms all the proposed models in the present study with R, RMSE, MAE and SI of 0.92, 6.5 MPa, 5.2 MPa and 0.08. The models were evaluated in strength ranges other than HPC, and the QRSM model showed a maximum residual error of ±20%. Furthermore, the results of sensitivity analysis indicate that binder content, w/b ratio and SiO2/Al2O3 are the most influential parameters controlling the compressive strength of concrete.

Strength properties and hydration of ultra-high performance concrete incorporating calcined clay and limestone with steam curing regimes

UHPC blended with low-energy mineral admixture and cement supplementary material is beneficial to the environmental protection and energy conservation. This paper investigated the hydration, micro-structure and strength development of LC3 based UHPC matrix with steam curing at 55 and 90 ℃ for 1 d and 2 d. The compressive and flexure strengths of UHPC mixtures incorporating limestone and various replacement levels of metakaolin were measured. Hydration, pore structure and micro-morphology of LC3 based UHPC mixture were investigated by various modern analysis and testing technologies. The test results indicate that the 2 d or 3 d compressive strengths for 90 ℃ steam cured UHPC mixture containing 15 % MK are about 60 % greater than that of standard cured mixture. While its flexure strengths for steam cured UHPC matrix blended with LC3 are generally smaller than that of standard cured mixture. It is noted that the degrading effect of flexure strength can be relieved when the UHPC mixtures are steam cured at 55 ℃ for 1 d. For the hydration of UHPC matrix, steam curing temperature is a key factor for stimulating the MK pozzolanic reaction, which contributes to a 30 % and 50 % higher MK reaction degree than that of standard mixture at 28 d when the steam curing temperature are 55 and 90 ℃, respectively. In addition, the significant MK pozzolanic reaction does not obviously restrict the cement hydration of steam cured LC3 based UHPC matrix. The pore size distributions of LC3 based UHPC matrix with steam curing regime are similar with that of standard cured mixture at 28 d, among which the ratios of pores with sizes smaller than 20 nm are over 70 %. Therefore, satisfactory mechanical strengths and micro-structures of LC3 based UHPC matrix can be obtained with steam curing regimes.

Mechanical and Durability Evaluation of Metakaolin as Cement Replacement Material in Concrete.

Due to the increasing prices of cement and its harmful effect on the environment, the use of cement has become highly unsustainable in concrete. There is a considerable need for promoting the use of cement replacement materials. This study investigates the effect of variable percentages of metakaolin (MK) on the mechanical and durability performance of concrete. Kaolin clay (KC) was used in the current research to prepare the MK by the calcination process; it was ground in a ball mill to its maximum achievable fineness value of 2550 m2/Kg. Four replacement levels of MK, i.e., 5%, 10%, 15%, and 20% by weight of cement, in addition to control samples, at a constant water-to-cement (w/c) ratio of 0.55 were used. For evaluating the mechanical and durability performance, 27 cubes (6 in. × 6 in. × 6 in.) and 6 cylinders (3.875 in. diameter, 2 in. height) were cast for each mix. These samples were tested for compressive strength under standard conditions and in an acidic environment, in addition to being subjected to water permeability, sorptivity, and water absorption tests. Chemical analysis revealed that MK could be used as pozzolana as per the American Society for Testing and Materials (ASTM C 618:2003). The results demonstrated an increased compressive strength of concrete owing to an increased percentage of MK in the mix with aging. In particular, the concrete having 20% MK after curing under standard conditions exhibited 33.43% higher compressive strength at 90 days as compared to similarly aged control concrete. However, with increasing MK, the workability of concrete decreased drastically. After being subjected to an acid attack (immersing concrete cubes in 2% sulfuric acid solution), the samples exhibited a significant decrease in compressive strength at 90 days in comparison to those without acid attack at the same age. The density of acid attack increased with increasing MK with a maximum corresponding to 5% MK concrete. The current findings suggest that the local MK has the potential to produce good-quality concrete in a normal environment.

Evaluation of Strength Parameters of Metakaolin and Nanosilica Incorporated High-Performance Concrete

This article presents the evalution of strength properties of High-Performance Concrete (HPC) due to the addition of nanosilica and metakaolin. Metakaolin partially replaces the cement at 10%, 20%, and 30% by weight. The addition proportions of nanosilica are 0%, 1. 5%, 3.0%, and 4.5% by weight of binder for each of the metakaolin replacement levels. Water-to-binder ratios of 0.3, 0.325, 0.35, 0.375, 0.4, and 0.425. The hardened HPC mixes were tested for their compressive, split tensile, and flexural strength at 7 and 28 days. Additionally, the compressive strength test was carried out after 90 days of curing to study the long-term strength development. The obtained results were compared with conventional concrete as a reference mix. HPC with 10% metakaolin, 3% nanosilica and 0.3 water-to-binder ratio yielded the best performance with 36.11%, 31.75% and 34.59% improvement in 7, 28 and 90 days compressive strength respectively. The improvement in split tensile strength were 25.24% and 40.26% at 7 and 28 days respectively. The flexural strength was enhanced by 41.10% and 30.30% at 7 and 28 days respectively. Furthermore, mathematical relationships between cube compressive strength and split tensile strength, flexural strength were derived from the experimental results of this investigation.

Insights to the sulfate resistance and microstructures of alkali-activated metakaolin/slag pastes

In this work, the effects of slag addition on the geopolymerization process and sulfate resistance of alkali-activated cement were studied. The substitution of metakaolin by slag increased the reactivity of precursors, contributing to a higher reaction rate and accelerating geopolymerization process. The mechanical properties and sulfate resistance were significantly improved with the incorporation of slag. The results show that the transformation of gel phases and development of pore structures were the vital reasons for improvement of sulfate resistance. The quantity of C-A-S-H generation increased with the slag content and consequently to a more compact microstructure. The refinement of pore structures came from the higher space-filling capacity of C-A-S-H phase and the filling effect of slag particles. The optimum replacement level of metakaolin was 30 wt%, in terms of sulfate resistance development and the potential damage of ettringite. This work provides a reference for the utilization of granulated blast furnace slag in metakaolin based geopolymer, which is of great value in urging geopolymer towards practical application in harsh environment.

Effect of high temperature on the performance of self-compacting mortars produced with calcined kaolin and metakaolin

The effects of high temperature up to 900 °C on the mechanical characteristics of self-compacting mortars (SCMs) containing different proportions of calcined kaolin (CK) and metakaolin (MK) were investigated in this study. The percentages of CK and MK that replace cement in this work were 0%, 5%, 10%, 15% and 20% by weight. The mechanical performance was assessed from compressive strength and flexural strength on the specimens of 40x40x160 mm size exposed 3, 28 and 90 day- water curing at ambient temperature and on the 28 day-specimens at high temperature while the durability of 28 day-SCMs samples was assessed from porosity, water absorption, sorptivity and density tests on water cured specimens of 50x50x50 mm size. The fresh properties of mortars were researched by slump and V-funnel tests. The rheological properties of fresh mortars were investigated by conducting the viscosity test. The outcomes of the study reveal that workability of mixtures was decreased with increasing proportion of MK replacement. The results also revealed that at 28 and 90 days of curing ages, the specimens of SCMs with MK15 gained the highest strength of 65.40 MPa and 79.01 MPa for about 10.13% and 15.46% more than the control specimens at the respective ages. Results illustrated that while the temperature rises, the compressive strength reduction of CK blended SCMs samples decreases with increasing replacement ratio of CK. A remarkable reduction in the total water absorption and porosity values of MK blended SCMs samples was observed. The highest density values at all SCMs mixes were observed for MK5 blended samples about 8.44% increase. 15% replacement level of MK gives the lowest sorptivity values.

Use of metakaolin as supplementary cementitious material in concrete, with focus on durability properties

Numerous research efforts on metakaolin as a supplementary cementitious material (SCM) have been undertaken in the past 20 years. This material, while relatively expensive mainly due to low production volumes worldwide, nevertheless has a significantly lower production cost than Portland cement. However, industry remains tentative in considering metakaolin in concrete. This paper takes the view that industry should consider investing in the production and application of metakaolin in appropriate concrete projects, particularly in aggressive environments where plain Portland cement may be inadequate, and where other SCMs may not readily be available. The main contribution of the paper is a global review of recent studies on the use of metakaolin in different types of concrete. This international experience is then compared with results from a study on the durability performance of metakaolin concrete using local materials in the Western Cape province of South Africa, as a means of concrete performance improvement. The study investigates concrete durability properties: penetrability (sorptivity, permeability, conductivity and diffusion), mitigation of Alkali-Silica Reaction (ASR), and carbonation resistance. The concretes were prepared with three water-binder ratios (0.4, 0.5 and 0.6), and with metakaolin replacement levels of 0% (control), 10%, 15% and 20%. Performance results show that, with increasing metakaolin content, the transport properties of concrete are considerably improved, ASR expansion due to a highly reactive local aggregate decreases to non-deleterious levels, while no detrimental effect on carbonation is observed. Thus, metakaolin could serve as a valuable SCM to enhance the durability performance of concrete in local aggressive environments.

Microstructure and mechanical, physical and structural properties of sustainable lightweight metakaolin-based geopolymer cements and mortars employing rice husk

ABSTRACTThis work focuses on an in-depth investigation of the formation of pores in the structure of lightweight geopolymer cements and mortars using rice husk as a foaming agent. The hardener used in this study was sodium waterglass. Metakaolin was replaced by 0, 10, 20, 30 and 40 % by mass of husk and the obtained powders were used to produce lightweight geopolymer cements and mortars. The formation of pores in the lightweight geopolymer cements was monitored using X-ray diffractometry and infrared spectroscopy while those in the mortars were assessed using apparent density and compressive strength measurements, mercury intrusion porosimetry and optical and scanning electron microscopy. The values for the compressive strength and apparent density were in the ranges of 28.92–0.75 MPa and 1.88–1.70 g/cm3, respectively. The results indicated that the values for the compressive strength and apparent density of geopolymer mortars decreased while those of the cumulative pore volume increased with increases in the metakaolin replacement level. Stereomicroscopic and scanning electron microscopic images showed the presence of rice husk and fibres of rice husk, respectively, in the networks. It was found that rice husk can be used as a foaming agent for producing sustainable lightweight geopolymer mortars.

Microstructural and mechanical properties of (Ca, Na)-poly(sialate-siloxo) from metakaolin as aluminosilicate and calcium silicate from precipitated silica and calcined chicken eggshell

The aim of this work is to investigate the influence of the amorphous calcium silicate on the microstructural, physical and mechanical properties of (Ca, Na)-poly(sialate-siloxo) networks. The calcium silicate with molar ratio CaO/SiO2 equal to 1.0 was prepared from the precipitated silica and calcined chicken eggshell. The X-ray patterns and infrared spectra of the synthesized calcium silicate indicate that it mainly constitutes of the amorphous calcium silicate hydrate. The X-ray patterns of geopolymer cements indicate that the prepared amorphous calcium silicate hydrate does not include in the network. The compressive strengths of the geopolymer mortars decrease from 28.92 to 12.03 MPa with increasing the replacement level of metakaolin (from 0 to 25% by mass). Whereas, the values of the apparent densities increase with increasing the replacement level of metakaolin. It seems that the amorphous calcium silicate hydrate in the structure of calcium silicate affects negatively the mechanical properties of the (Ca, Na)-poly(sialate-siloxo) network. The heterogeneous structure of geopolymer mortars containing 25% of calcium silicate could be related to the higher calcium content and the amorphous structure of calcium silicate hydrate. It was typically found that in the absence of significant levels of dissolved calcium, the precursors such as silicate and aluminate condense to form (Ca, Na)-poly(sialate-siloxo) networks. This implies the homogenous structure and the higher values of the compressive strengths of geopolymer mortars containing 0, 5, 10 and 15% of calcium silicate. Whereas those from the replacement level of metakaolin by 20 or 25% of calcium silicate contain significant levels of dissolved calcium and amorphous calcium silicate hydrate in their structure which hinder the formation of the precursors and therefore impede the formation of the (Ca, Na)-poly(sialate-siloxo) networks.

Design of low cost semi-crystalline calcium silicate from biomass for the improvement of the mechanical and microstructural properties of metakaolin-based geopolymer cements

Abstract The main objective of this work is to use low-value SiO2 and CaO-rich wastes like rice husk ash and chicken eggshell powders, respectively for producing semi-crystalline calcium silicate with molar ratio CaO/SiO2 equal to 0.4. The prepared valuable semi-crystalline calcium silicate was used as an additive for a replacement level of metakaolin (0, 1, 3, 5, 7 and 10% by mass). The resulting powders were used to synthesize geopolymer cements using hardener obtained by mixing commercial sodium silicate, sodium hydroxide and distilled water. The final products were characterized by compressive strengths, apparent density, X-ray diffractometry, infrared spectroscopy, scanning electron and optical microscope observations. The results show that the compressive strengths of geopolymer cements were between 33.25 and 51.38 MPa. They increase when metakaolin was replaced by 5, 7 and 10% of semi-crystalline calcium silicate, but no significant change was observed on the strengths of geopolymer cements content 0, 1, 3 and 5% of semi-crystalline calcium silicate. The micrograph images of the selected geopolymer cements were homogeneous and compact and the one from 10% of semi-crystalline calcium silicate shows a coarse microstructure. It was found that the low-value silica and calcium-rich wastes such as rice husk ash and eggshell can be used to prepare semi-crystalline calcium silicate. The obtained product could be used as a low-cost additive to enhance the microstructural and mechanical properties of poly(sialate-siloxo) network.

The influence of aluminum from metakaolin on chemical binding of chloride ions in hydrated cement pastes

Abstract In this study, chloride ions binding capacity of Portland cement blended with 10–30% of metakaolin was investigated. Chemical binding of chloride ions was found depending more on phase assemblage of pastes rather than merely on available aluminum content in the blends. More monosulfoaluminate formed at low replacement levels, while at high replacement levels of metakaolin, stratlingite formation in the blends was favored. When exposed to 0.53 M NaCl solution at ambient temperature, monosulfoaluminate converted to Friedel's salt completely, and played a principal role in chemical binding of chloride ions while ettringite and stratlingite did not participate in the chemical binding of chloride ions. Considering the exposure condition, the contribution of monocarboaluminate in chemical binding was less significant than that of monosulfoaluminate. Results demonstrated that well formulated Portland cement-metakaolin blends with appreciate metakaolin replacement level and sulfate supply are expected to have good chloride binding capacity.

Comparison of The Compressive Strength and The Microstructure of Metakaolin Metastar and Metakaolin Bangka as Additive in Ordinary Portland Cement

Various This paper presents the results of the investigation on the use of Metakaolin (Al2Si2O2) as a supplementary cementing materials to improve the strength of cement. The most effective way to increase the strength of cement is the substitution of a proportion of cement with supplementary cementing materials. One of them was Metakaolin. Metakaolin was produced by thermal treatment calcination from Kaolin at 600-800 Celcius and has highest alumina and silicate purity. By added Metakaolin to Portland Cement type I (OPC), the amount of Calcium Silicate Hydrate (CSH) will increase through binding with Calcium Hydroxide (CaOH). There were two kinds of Metakaolin used in this investigation, commercial metakaolin named Metakaolin Metastar compared with Metakaolin Bangka which derived from Indonesia local resources, Bangka Island. Four Metakaolin replacement levels were employed in this investigation: 5%, 0%, 15%, and 20% with water per cement ratio 0.35, 0.40, and 0.50 both of Metakaolin Metastar and Metakaolin Bangka. The cement pastes cured at room temperature for 7, 14, and 28 days. The mechanical strength examined by compressive strength test, the microstructure were examined by SEM-EDS. The results of the study revealed both Metakaolin Metastar and Metakaolin Bangka enhanced the compressive strength of OPC. The most appropriate strength was obtained for a substitution of 20% metakaolin metastar which had 46,15% higher than OPC and 5% metakaolin Bangka which had 39,06% higher than OPC. The hydration rate was examined by Thermal Analysis Monitor. The results indicated that metakaolin metastar released higher heat than metakaolin Bangka. It can be concluded that Metakaolin metastar was more effective than metakaolin Bangka as additive in OPC.

Mechanical and microstructural properties of HFRHSCs containing metakaolin subjected to elevated temperatures and freezing-thawing cycles

Mechanical and microstructural properties of hybrid fibers reinforced high strength concretes (HFRHSCs) containing metakaolin (MK) and metakaolin + silica fume (MK + SF) subjected to elevated temperatures and freezing-thawing (F-T) cycles were investigated in this paper. In the concrete mixtures without and with hybrid fibers, MK and MK + SF were replaced with eight percentage of cement by weight. A total of sixteen concrete mixtures were produced at a water-to-binder ratio of 0.25. Ultrasound pulse velocity (Upv), compressive strength (fc), splitting tensile strength (fsts) and flexural strength (ffs) tests were performed to evaluate the properties of HFRHSCs subjected to ambient and elevated temperatures (300, 400 and 500 °C). The Upv and fc tests of HFRHSCs subjected to F-T cycles were also conducted. Moreover, the alterations in the matrix, interface zone and aggregate of concretes without hybrid fibers subjected to ambient temperature, elevated temperatures and F-T cycles were analyzed by scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) spot and polarized light microscope (PLM). The results of tests have shown that the replacement levels of MK and MK + SF have important influences on the mechanical and microstructural properties of high strength concretes. Moreover, the mechanical results have shown that the concretes with hybrid fibers have higher fc, fsts and ffs than other concretes. The results have also shown that the Upv, fc, fsts and ffs values of concretes without and with hybrid fibers decrease, as the temperature increases.

Effect of alkali-activated metakaolin cement on compressive strength of mortars

This paper reports the compressive strength and microstructure characteristic of alkali-activated metakaolin (MK) cement under two curing methods. MK was used to replace part of Portland cement (PC) at 70, 80, 85, 90, 95 and 100% by mass of binder. Sodium hydroxide (NaOH) and sodium silicate (Na2SiO3) were used as activating agents (Na2SiO3 to NaOH ratio of 0.67). NaOH with 10 molar concentrations, liquid alkali/binder ratio of 0.8 and water base was used. Mortar and paste specimens were cured at 23±2°C (55% RH) and 60°C (95% RH). The results indicated that calcium silicate hydrate (C–S–H) and calcium carbonate (CaCO3) phases were detected by thermogravimetry (TGA). Setting time and drying shrinkage decreased when metakaolin replacement level increased, the compressive strength of alkali-activated metakaolin cement increased with the increase of PC content and increased with temperature. The curing of specimens at 60°C (95% RH) enhanced the compressive strength, matrices appeared denser than when cured at 23±2°C (55% RH) and phases of (C, N)-A-S-H gel and or C–S–H gel can be seen clearly.

Effect of metakaolin on the properties of conventional and self compacting concrete

Supplementary cementitious materials (SCM) have turned out to be a vital portion of extraordinary strength and performance concrete. Metakaolin (MK) is one of SCM material is acquired by calcinations of kaolinite. Universally utilised as pozzolanic material in concrete to enhance mechanical and durability properties. This study investigates the fresh and hardened properties of conventional concrete (CC) and self compacting concrete (SCC) by partially replacing cement with MK in diverse percentages. In CC and SCC, partial replacement of cement with MK varies from 5-20%. Fresh concrete properties of CC are conducted by slump test and compaction factor tests and for SCC, slump flow, T500, J-Ring, L-Box, V-Funnel and U-Box tests. Hardened concrete characteristics are investigated by compressive, split tensile and flexural strengths at age of 7, 28 and 90 days of curing under water. Carbonation depth, water absorption and density of MK based CC and SCC was also computed. Fresh concrete test results indicated that increase in MK replacement increases workability of concrete in a constant w/b ratio. Also outcomes reveal that concrete integrating MK had greater compressive, flexural and split tensile strengths. Optimum replacement level of MK for cement was 10%, which increased mechanical properties and robustness properties of concrete.

Properties of metakaolin-blended oil palm shell lightweight concrete

In this paper, the effect of partial replacement of cement with metakaolin (MK) on the strength properties of oil palm shell (OPS) lightweight concrete was investigated. Generally, 5–20% of partial cement replacement with MK enhanced the compressive, splitting tensile and flexural strengths as well as modulus of elasticity of OPS concrete (OPSC). Though the 28 day compressive strength of the OPSC was found in the range of 39–47 MPa, the highest compressive strength was obtained for the mix with 10% replacement of MK. Similar replacement level of MK was found optimum for both the splitting tensile and flexural strengths. In addition, the strength efficiency of OPSC with MK as partial cement replacement was also investigated and it was found that the increase in the replacement level of MK up to 20% resulted in 49% increase in the strength efficiency for the OPSC.

Effect of metakaolin on the chloride ingress properties of concrete

Research and practical experience have shown that partial replacement of cement by metakaolin improves concrete durability as a result of the refinement of the pore structure. While much research has been presented on concrete performance with metakaolin, it is scarce concerning the transport properties of chlorides in concrete with metakaolin, in natural conditions (i.e., un-accelerated). This study determines the chloride diffusion coefficients for several concretes with vary levels of cement replacement with metakaolin, and compares these results with chloride migration coefficients obtained from accelerated laboratory testing. In this study, two cement types (CEM I 42,5R and CEM IV/A 42,5) and two cement contents levels where used with metakaolin replacement levels varying from 10-20%. Concretes where tested for fresh properties, compressive strength, electrical resistivity, chloride ingress characteristics (natural diffusion and migration), and mercury intrusion porosimetry. The results show improved strength, durability properties and chloride penetration resistance of concretes with metakaolin. Furthermore, the use of metakaolin in fly ash concrete improves the early age performance of the concrete (<90 days), counteracting the delay in strength and durability gain typically associated with fly ash concrete. The results obtained from this study fulfil the lack of critical input for service life design models of reinforced concrete structures in chloride environments, with emphasis on concretes with metakaolin replacement.

Influence of Metakaolin on Strength and Microstructure of Steam-Cured High-Strength Concrete

The influence of metakaolin (MK) on strength and microstructure of steam-cured high-strength concrete has been investigated using compressive strength test, XRD, MIP and SEM. Three MK replacement levels were considered in the study: 5%, 10% and 15% by weight of cement. A mix double blended with 10% MK and 10% slag was prepared too. Test results have indicated that MK can increase the compressive strength of steam-cured concrete, especially at early age. Compressive strength up to 90MPa at 1 and 28 days is obtained with the incorporation of 10% MK and 10% slag. When the replacement level of MK is higher than 10%, the enhancement of strength becomes less significant. Content of Ca (OH)2 crystals is decreased while content of hydrates with Al is increased due to the pozzolanic reactivity of MK. Concrete pore structure is significantly refined and a denser hydrates structure is obtained due to the pozzolanic reaction and filler effect of MK. Meanwhile, combination of aggregate and paste is enhanced too. The improvements of strength and microstructure become more obvious when MK and slag are double incorporated.