High Volume Fly Ash Cement Research Articles

Modification of high-volume fly ash cement with metakaolin for its utilization in cemented paste backfill: The effects of metakaolin content and particle size

High-volume fly ash (FA) cement presented a slow setting and lower early compressive strength, restricting the utilization in cemented paste backfill (CPB). The present work investigated the feasibility of using metakaolin with various particle sizes and content to modify the properties of FA cement. Reaction kinetics, rheological properties, setting time, compressive strength, hydration products, and the CPB samples' microstructure were thoroughly studied. The results showed that 5 wt% of super-fine metakaolin (SMK) could strongly promote the binder hydration, leading to the formation of a more compact matrix with higher early compressive strength. The 3 d-compressive strength of the CPB samples was increased by around 216% after replaced by 5 wt% of SMK. Besides, the binder doped with 5 wt% SMK showed better environmental and cost performance.

Influence of fly ash and GGBFS on the pH value of cement mortar in different curing conditions

The pH of cement-based materials (CBMs) is an important factor for their durability, sustainability, and long service life. Currently, the use of supplementary cementitious materials (SCMs) is becoming mandatory due to economic, environmental, and sustainable issues. There is a decreasing trend in pH of CBMs due to incorporation of SCMs. The determination of numerical values of pH is very important for various low and high volume SCMs blended cement mortars for the better understanding of different defects and durability issues during their service life. In addition, the effect of cement hydration and pozzolanic reaction of SCMs on the pH should be determined at initial and later ages. In this study, the effect of low and high-volume fly ash (FA) and ground granulated ballast furnace slag (GGBFS) cement mortars in different curing conditions on their pH values has been determined. Thermal gravimetric analysis (TGA) was carried out to support the findings from pH measurements. In addition, thermal conductivity (k-value) and strength activity indices of these cement mortars were discussed. The results showed that pH values of all blended cement mortars were less than ordinary Portland cement (OPC) mortar in all curing conditions used. There was a decreasing tendency in pH of all mortars with passage of time. In addition, the pH of cement mortars was not only dependent on the quantity of Ca(OH)2. The effect of adding SCMs on the pH value of cement mortar should be monitored and measured for both short and long terms.

Improving durability of heat-cured high volume fly ash cement mortar by wet-grinding activation

This work investigated the durability performances of heat-cured high volume (30 wt%–70 wt%) fly ash (FA) cement mortars by using different particle sizes (19.7 μm, 4.29 μm and 2.51 μm) and amounts of wet-grinded fly ash slurries. Wet-grinding technology was used to prepare ultrafine fly ash slurry and improve the durability of heat-cured high volume fly ash cement mortar. Results indicated that the smaller the particle size of wet-grinded fly ash, the better the durability improvement effect of heat-cured high volume fly ash blended mortar. Autogenous shrinkage was increased with the reduced particle size of wet-grinded fly ash. Sulfate attack resistance coefficient, capillary water absorption and electric flux of heat-cured high volume FA mortar were efficiently decreased by wet-grinded FA, within 50 wt% content. As for carbonation resistance, lower carbonation depth than the reference mortar was observed with wet-grinded F3 content of 30 wt%. Results highlight that wet-grinding treatment of fly ash could provide an efficient way to improve the durability performance of heat-cured high volume fly ash composites.

Temperature-Dependent Hydration and Mechanical Properties of High-Volume Fly Ash Cement with Chemical Additives

AbstractThis research investigates the effects of Na2SO4 and Na2CO3 on the mechanical and hydration properties of high-volume fly ash (FA) cement (HFC) at three curing temperatures: 25°C, 40°C, and...

Hydration and pore-structure characteristics of high-volume fly ash cement pastes

The hydration characteristics and microstructure of HVFA cement were investigated according to the FA replacement ratio and the correlation between the degree of hydration and pore structure was analyzed. As the FA replacement ratio increased, the degree of hydration of cement increased, but the degree of reaction of FA decreased. The gel/space ratio of the HVFA cement paste showed strong correlation with the compressive strength. As the FA replacement ratio increased, the surface fractal dimension decreased in the micro region. The surface fractal dimension in the micro region showed strong correlations with the compressive strength and gel/space ratio.

Effects of calcium formate on early-age strength and microstructure of high-volume fly ash cement systems

Replacement of Portland cement (PC) by fly ash (FA) is currently limited to 15–30% by mass, mainly due to low early age strength development of concrete. This research uses calcium formate (Ca(HCO2)2; CF) as an admixture to high-volume FA (HVFA) composites to improve its strength properties. HVFA represents 60–70% of cement replaced by FA and dosage of CF varies from 0.5% up to high dosage of 9% of cement content. Compressive strength, isothermal calorimetry, thermogravimetric analysis, X-ray diffraction and scanning electron microscopy were conducted to investigate the effects of CF on hydration and microstructural aspects. The results show that both HVFA pastes with 60% and 70% FA achieved the highest strength at the CF dosage of 3%. At the age of 28 days, adding 3% CF to HVFA mixes led to higher consumption of FA as well as higher formation of calcium hydroxide (Ca(OH)2), calcium silicate hydrates, calcium carbonate (CaCO3) and ettringite, which contribute to the increase of strength. The addition of very high dosages of CF at 9% increased the hydration of tricalcium aluminate but could hinder the hydration of tricalcium silicate in both PC and HVFA pastes with 60% and 70% FA.

Performance based energy, ecological and financial costs of a sustainable alternative cement

This paper presents a comparison of the energy and ecological impacts and financial costs of Alkali Pozzolan Cement (APC), High Volume Fly Ash (HVFA) cement and Ordinary Portland Cement (OPC) mixtures. The embodied energy, carbon dioxide emission and financial costs were calculated per unit weight of dry binder powder; per unit volume per unit strength of hydrated hardened binder; and finally per unit volume per unit strength per year of service. The performance based indicators therefore consider both strength and durability; with the latter based both on carbonation and chloride environments. The statistical means and variability of the costs were also computed. The results indicate that the energy, ecological and financial costs of APC are very attractive compared to those of OPC or HVFA, when considered either as the cost per unit weight of binder or as the cost per unit performance, whether related to unit strength or both unit strength and year of service. The variability of the APC cost indicators was also the lowest in general among the cements considered. However, OPC is still arguably the best when only carbonation exposure is involved during service.

Effect of steam curing on compressive strength and microstructure of high volume ultrafine fly ash cement mortar

Utilization of high volume fly ash is generally limited in steam-cured concrete products due to the delayed strength development. In present work, ultrafine fly ash (UFA) with a median particle size of 2.3 μm was prepared with wet-grinding technology. The influence of different FA fineness, UFA content (30–70%) and curing regimes on the compressive strength and microstructure of steam-cured high volume fly ash cement composites were studied. Both the early and late compressive strength of steam-cured FA blended composites were enhanced by wet-milled UFA with replacement level as high as 50%, and the strength was even higher than that of pure cement group. Pore structure of steam-cured high volume FA blended composites was effectively refined by UFA. The amount of CH after steam curing was generally lower than that with standard curing. Furthermore, the main chain length and Al/Si of C-S-H gels under steam curing were improved by UFA. Compared with prolonging the curing duration, increasing the curing temperature seems more efficient for the incorporation of aluminum from fly ash into the calcium silicate hydrates.

Improving properties of high-volume fly ash cement paste blended with β-hemihydrate from flue gas desulfurization gypsum

The present study investigated the effects of utilizing the treated flue gas desulfurization (FGD) gypsum on the improvement of properties of high volume fly ash (FA) cement pastes. The FGD gypsum was subjected to heat treatment at 150 °C in a kiln to change its compositions and convert into β-hemihydrate gypsum. The ordinary Portland cement (OPC) was partially used in the FA-cement paste at two different levels: 50 wt% for Set I mixture and 30 wt% for Set II mixture. The β-hemihydrate was added to partially replace the FA with 3, 5, and 10 wt%, respectively, for both Sets. Experimental works, including workability, setting time, compressive strength, dynamic moduli, and microstructural analysis by scanning electron microscopy (SEM) and X-ray diffraction (XRD) tests were conducted. The workability of fresh blended pastes was found to be decreased with the increase of β-hemihydrate amount, while the compressive strength, dynamic moduli and thermal conductivity were improved. Lower amount of β-hemihydrate was found to be more effective in improving the early-age strength, while higher amount of β-hemihydrate had more effect on enhancing the later-age strength. Based on the microstructural analysis, the addition of β-hemihydrate increased the sulfate compound in the binder and in turn increased the formation of ettringite.

Modification of water absorption and pore structure of high-volume fly ash cement pastes by incorporating nanosilica

High-volume (>50%) fly ash (HVFA) is used to partially replace cement in concrete to reduce the greenhouse gas emission, which is beneficial to both cement industry and environment. However, one drawback of using high-volume fly ash cement (HVFAC) is the increase of the water absorption and the reduction of the durability of cement-based materials. In this study, the effect of nanosilica (NS) on the compressive strength and water absorption of HVFAC pastes were investigated. In addition, the phase composition, microstructure and pore structure of the pastes were analyzed through X-ray diffraction (XRD), scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP). Results showed that the addition of NS increased the compressive strength and reduced the water absorption of HVFAC pastes. NS could react with Portlandite to produce additional C–S–H gel, and simultaneously a higher content of fly ash was hydrated in the presence of NS. Therefore, the incorporation of NS refined the pores and reduced the pore volume of HVFAC pastes. The pores with diameter larger than 10 nm were more pronounced for the water absorption ability of HVFAC composites. This study provided a possible way to reduce the negative effect brought by the increased water absorption when using HVFA in cementitious materials.

Reduction of heavy metals leaching and pore volume in high-volume fly ash cement pastes by adding nano-SiO2.

This study investigated the compressive strengths and leaching of heavy metals in high-volume fly ash cement (HVFAC) pastes with and without nano-SiO2 (NS). Further, scanning electron microscopy, X-ray diffraction, and mercury intrusion porosimetry were used to characterize the microstructure of the specimens. The results showed that the addition of NS increased the compressive strength of HVFAC specimens. NS accelerated the hydration of both cement and fly ash to form more hydration products, and it reacted with portlandite to simultaneously generate more C-S-H gel. Thus, the total pore volume, especially the capillary pores, in the NS-mixed HVFAC specimens was reduced, and their microstructure became denser because of the filling of more hydration products in the pores. This resulted in the reduction of the leaching of heavy metals (Cr, Pb, Cd, As, Zn, and Cu) in the NS-mixed HVFAC specimens compared with the HVFAC specimen containing no NS. According to the Chinese national standard (GB 3838-2002), the water quality of leachates from the HVFAC specimens mixed with both 3% and 5% NS reached grade I, thus causing nearly no harm to the water and soil environment.

Using nano-silica to improve mechanical and fracture properties of fiber-reinforced high-volume fly ash cement mortar

Using nano-silica (NS) in cement-based materials has attracted extensive attention. However, whilst most studies focused on the hydration, fresh, mechanical and durability properties, there is limited information on the effect of NS on the fracture properties of fiber-reinforced high-volume fly ash mortars (HVFAM). In this paper, a series of experiments were carried out to evaluate the effect of NS on the fracture and mechanical properties of polyvinyl alcohol (PVA) fiber-reinforced HVFAM, with the fly ash/binder ratio fixed at 50% by weight. Four NS/binder weight ratios of 0% 0.5%, 1.0% and 1.5%, and four PVA fiber volume dosages of 0%, 0.2%, 0.5% and 1.0% were adopted. Compared to 0.2–1.0 vol% PVA fiber-reinforced HVFAM without NS, the addition of 0.5 wt% NS could further increase the flexural strength, fracture energy, fracture toughness, critical crack tip opening displacement and brittleness index by 3–5%, 8–29%, 17–28%, 18–38% and 34–95%, respectively; the addition of 1.0 wt% NS could increase the flexural strength, fracture energy, fracture toughness, critical crack tip opening displacement and brittleness index by 4–9%, 15–41%, 26–46%, 18–48%, and 62–79%, respectively. The mineralogy and microstructure characterizations showed that the NS condensed the fiber/matrix interface. These findings provide insights into designs and applications of fiber-reinforced high-volume pozzolan cement-based materials with nano-particles.

Combination of precipitated-calcium carbonate substitution and dilute-alkali fly ash treatment in a very high-volume fly ash cement paste

Poorer reactivity of fly ash (FA) than Portland cement brings to the problems of low strength and setting time delays of very high-volume fly ash cement paste. This study combines precipitated-calcium carbonate (PCC) substitution and dilute-alkali fly ash treatment for overcoming those drawbacks. The strength was examined at 3-, 7-, 28- and 56-day. Scanning electron microscopy and Fourier-transform infrared spectroscopy were applied for characterisations. PCC substitution improved the strength and shortened the setting time; then additional dilute-alkali fly ash treatment results in better properties. Combination of PCC substitution and dilute-alkali treatment enhances cement hydration and FA pozzolanic reaction simultaneously.

Mechanical and Transport Properties of High Volume UFA Cement Composites

In this study, mechanical activation is used to generate ultra-fine fly ash (UFA) for high volume fly ash（FA）cement composites. The effects of different content and medium particle size of FA on mortar`s electrical resistivity, chloride penetration and mechanical properties are investigated. The results show that the compressive strength and resistance to chloride permeability of specimens with UFA have been enhanced, owing to higher pozzolanic reaction and higher dissolution rate of Si and Al units of UFA to accelerate the generate of reaction products. However, At the early ages, electrical resistivity of specimens increases with the increase of UFA; at later ages, specimens have a higher electrical resistivity with the increase of UFA.

Effect of Nano Silica on Hydration and Microstructure Characteristics of Cement High Volume Fly Ash System Under Steam Curing

The influences of nano silica (NS) on the hydration and microstructure development of steam cured cement high volume fly ash (40 wt%, CHVFA) system were investigated. The compressive strength of mortars was tested with different NS dosage from 0 to 4%. Results show that the compressive strength is dramatically improved with the increase of NS content up to 3%, and decreases with further increase of NS content (e g, at 4%). Then X-ray diffraction (XRD), differential scanning calorimetry-thermogravimetry (DSCTG), scanning electron microscope (SEM), energy disperse spectroscopy (EDS), mercury intrusion porosimeter (MIP) and nuclear magnetic resonance (NMR) were used to analyze the mechanism. The results reveal that the addition of NS accelerates the hydration of cement and fly ash, decreases the porosity and the content of calcium hydroxide (CH) and increases the polymerization degree of C-S-H thus enhancing the compressive strength of mortars. The interfacial transition zone (ITZ) of CHVFA mortars is also significantly improved by the addition of NS, embodying in the decrease of Ca/Si ratio and CH enrichment of ITZ.

Effects of hybrid fibers on the development of high volume fly ash cement composite

Abstract High volume fly ash (HVFA) is used as cement substitute in many cementitious materials. In recent years, hybrid fibers such as steel and polypropylene have been added to improve the mechanical characteristics. The usage of three fibers combinations is relatively a new concept in HVFA concrete. In this paper, silane coated basalt fiber has been combined with steel and polypropylene due its availability and low costs. Varied hybrid fiber combinations (both two types and three types) were tested on cement mortar specimens to identify the optimal fiber percentages. Based on the experimental results, equations to predict the compressive and flexural strength were developed. Optimal percentages identified from the experimental data were then used to test concrete specimens. It was found that the compressive strengths of three types hybrid fiber concrete showed appreciable increase by 5.44% and the splitting tensile strength increase was 6.77% in comparison with two type hybrid. Furthermore, the long-term durability issue of the selected basalt fiber was examined using the Scanning Electron Microscopy (SEM) and Computed Tomography (CT) X-ray scan. These two techniques confirmed the reduction in porosity and hence the durability of concrete, suggesting the benefit of the added silane coated basalt fiber.

Durability properties of alkali pozzolan cement (APC)

This paper compares the durability properties of alkali pozzolan cement (APC) with high volume fly ash (HVFA) cement and ordinary Portland cement (OPC). The susceptibility to durability issues was investigated through the water absorption test using 28-day-old samples. Durability against chemical attacks was investigated through sulphuric acid attack, chloride ion penetration and carbonation tests on 1-year-old samples. The results indicated that APC occupies a median position in relation to OPC and HVFA, having greater resistance than HVFA for water absorption and carbonation, and greater resistance than OPC for sulphuric acid attack and chloride ion penetration. Chemical and microstructural reasons for the above behaviours are presented. A key index of durability performance is the free calcium hydroxide available and/or generated during cement hydration and durability exposure.

Mechanical Properties and Sulfate Resistance of High Volume Fly Ash Cement Mortars with Air-Cooled Slag as Fine Aggregate and Polypropylene Fibers.

The depletion of natural sand and production of the huge amount of cement in the construction industry are serious threats to the environment, which can be reduced by the utilization of by-products as cement replacement material. In this study, cement was replaced with fly ash up to 45% (by weight). In addition, the natural fine aggregate was replaced with air-cooled blast furnace slag aggregate (here referred to as “slag aggregate”) at a level of 50% and 100% (by weight). Polypropylene fiber was also added, at a dosage of 0.25% of binder weight. Mortar specimens were prepared and analyzed using tests for compressive, flexure, and splitting tensile strength, as well as for microhardness, and ultrasonic pulse velocity. In addition, the specimens were exposed to sulfate solution and investigated for changes in length, mass, and compressive strength. Electron microscopy and X-ray diffraction analysis were performed to examine the microstructure and phase changes of mortar specimens exposed to sulfate solution. The results indicate that mortar specimens made with 50% slag aggregate and 0.25 % fiber showed enhanced mechanical properties. The performance of slag aggregate mortars under sulfate attack was improved significantly.

Properties and performance of a high volume fly ash grout

This study investigated the penetrability of high volume fly ash cement suspensions prepared with and without superplasticizer into sandy soil having different relative densities with 30%, 60%, 73%, and 83% through permeation grouting. Class C fly ash was used due to its pozzolanic activity and fineness. Due to engineering characteristics and cost, cementitious grouts are the most commonly used grout in both waterproofing and ground strengthening. Fly ash-cement grouts have relatively constant and low viscosity values for a reasonable period after preparation, exhibit limited or negligible bleed capacity and set and develop satisfactory strength within a relatively short period. Modeling of grouting of soil was done in laboratory and improvements in physical and mechanical properties of grouted soil were analyzed. Unconfined compressive strength, shear strength and permeability characteristics of grouted soil were studied as a result. Unconfined compressive strength values of grouted sand with high volume fly ash ranged between 410 and 1107 kPa. Morover, cohesion values were comparable to microfine cement grouting ranging from 373 to 511 kPa. Furthermore, permeability values were also approximately equal to the permeability of impervious liners, which is around 1 × 10−7 cm/s. The findings support the applicability of grouting in different applications.

Early strength of various fly ash based concrete in peat environment

Fly ash is a by-product of coal combustion in a power station and usually used as additive or cement replacement material to improve properties of concrete in aggressive environments such as acid, chloride, and sulphate. Peatland is one of acidic environment that is common in Riau province with high organic content and low pH that is damaging for concrete, especially when exposed to concrete at an early age. This paper aims to study the early compressive and tensile strength of the various type of fly ash based concrete subjected to peat water. Seven fly ash-based concrete mixtures investigate were, i.e., geopolymer hybrid using 15% of Ordinary Portland Cement (OPC) as an additive, high volume fly ash using 25%, 50% and 75% of fly ash as cement replacement material, and blended OPC with fly ash with different grade of 15, 21 and 29 MPa. The OPC concrete with a target strength of 20 MPa was a control mix. The OPC based-specimens were cast and cured in water for 28 days before placed in peat water for another 28 days before the testing date. Compressive strength and tensile strength values of the concrete at 7 and 28 days were taken. Results show the type of concrete, fly ash content, and concrete grade significantly influence the early strength properties and resistance of the concrete to the acid attack. Four concrete mixtures with decreasing vulnerability to the attack were distinguished: OPC, high volume fly ash, geopolymer hybrid and blended cement concrete.