Compressive strength and drying shrinkage of fly ash-bottom ash-silica fume multi-blended cement mortars

This paper studies the physical properties, compressive strength and drying shrinkage of multi-blended cement under different curing methods. Fly ash, ground bottom ash and undensified silica fume were used to replace part of cement up to 50% by weight. Specimens were cured in air at ambient temperature, water at 25, 40 and 60 °C, sealed with plastic sheeting for 28 days. The results show that absorption and volume of permeable pore space (voids) of blended cement mortars at 28 day under all curing methods tend to increase with increasing silica fume replacement. The compressive strength of blended cement with fly ash and bottom ash was lower than that of Portland cement control at all curing condition while blended cement with silica fume shows higher compressive strength. In addition, the compressive strength of specimens cured with water increased with increasing curing temperature. The drying shrinkage of all blended cement mortar cured in air was lower than that of Portland cement control while the drying shrinkage of blended cement mortar containing silica fume, cured with plastic sealed and water at 25 °C was higher than Portland cement control due to pore refinement and high autogenous shrinkage. However, the drying shrinkage of blended cement mortar containing SF cured with water at 60 °C was lower than that of Portland cement control due to lower autogenous shrinkage and the reduced microporosity of C–S–H.     

Mechanical and durability properties of ternary concretes containing silica fume and low reactivity blast furnace slag

In this study, the effect of incorporation of silica fume in enhancing strength development rate and durability characteristics of binary concretes containing a low reactivity slag has been investigated. Binary concretes studied included mixes containing slag at cement replacement levels of 15%, 30% and 50% and mixes containing silica fume at cement replacement levels of 2.5%, 5%, 7.5% and 10%. Ternary concretes included combinations of silica fume and slag at various cement replacement levels. The w/b ratio and total cementitious materials content were kept constant for all mixes at 0.38 and 420 kg/m³ respectively. Concrete mixes were evaluated for compressive strength, electrical resistance, chloride permeability (ASTM C1202 RCPT test) and chloride migration (AASHTO TP64 RCMT test), at various ages up to 180 days.The results show that simultaneous use of silica fume has only a moderate effect in improving the slow rate of strength gain of binary mixes containing low reactivity slag. However it improves their durability considerably. Using appropriate combination of low reactivity slag and silica fume, it is possible to obtain ternary mixes with 28 day strength comparable to the control mix and improve durability particularly in the long term. Ternary mixes also have the added advantage of reduced water demand.     

Effects of different curing regimes on the compressive strength properties of self compacting concrete incorporating fly ash and silica fume

This paper presents the effect of air curing, water curing and steam curing on the compressive strength of Self Compacting Concrete (SCC). For experimental study, SCC is produced with using silica fume (SF) instead of cement by weight, by the ratios of 5%, 10% and 15%, and fly ash (FA) with the ratios of 25%, 40% and 55%. It is observed that mineral admixtures have positive effects on the self settlement properties. The highest compressive strength was observed in the concrete specimens with using 15% SF and for 28 days water curing. Air curing caused compressive strength losses in all groups. Relative strengths of concretes with mineral admixtures were determined higher than concretes without admixtures at steam curing conditions.     

Transport and mechanical properties of self consolidating concrete with high volume fly ash

This paper presents the transport and mechanical properties of self consolidating concrete that contain high percentages of low-lime and high-lime fly ash (FA). Self consolidating concretes (SCC) containing five different contents of high-lime FA and low-lime FA as a replacement of cement (30, 40, 50, 60 and 70 by weight of total cementitious material) are examined. For comparison, a control SCC mixture without any FA was also produced. The fresh properties of the SCCs were observed through, slump flow time and diameter, V-funnel flow time, L-box height ratio, and segregation ratio. The hardened properties included the compressive strength, split tensile strength, drying shrinkage and transport properties (absorption, sorptivity and rapid chloride permeability tests) up to 365 days. Test results confirm that it is possible to produce SCC with a 70% of cement replacement by both types of FA. The use of high volumes of FA in SCC not only improved the workability and transport properties but also made it possible to produce concretes between 33 and 40 MPa compressive strength at 28 days, which exceeds the nominal compressive strength for normal concrete (30 MPa).     

A study on the effect of nano silica on compressive strength of high volume fly ash mortars and concretes

This paper presents the effect of nano silica (NS) on the compressive strength of mortars and concretes containing different high volume fly ash (HVFA) contents ranging from 40% to 70% (by weight) as partial replacement of cement. The compressive strength of mortars is measured at 7 and 28 days and that for concretes is measured at 3, 7, 28, 56 and 90 days. The effects of NS in microstructure development and pozzolanic reaction of pastes containing above HVFA contents are also studied through backscattered electron (BSE) image and X-ray diffraction (XRD) analysis. Results show that among different NS contents ranging from 1% to 6%, cement mortar containing 2% NS exhibited highest 7 and 28 days compressive strength. This NS content (2%) is then added to the HVFA mortars and concretes and the results show that the addition of 2% NS improved the early age (7 days) compressive strength of mortars containing 40% and 50% fly ash by 5% and 7%, respectively. However, this improvement is not observed at high fly ash contents beyond 50%. On the other hand, all HVFA mortars exhibited improvement in 28 days compressive strength due to addition of 2% NS and the most significant improvement is noticed in mortars containing more than 50% fly ash. In HVFA concretes, the improvement of early age (3 days) compressive strength is also noticed due to addition of 2% NS. The BSE and XRD analysis results also support the above findings.     

High-volume natural volcanic pozzolan and limestone powder as partial replacements for portland cement in self-compacting and sustainable concrete

A laboratory study demonstrates that high volume, 45% by mass replacement of portland cement (OPC) with 30% finely-ground basaltic ash from Saudi Arabia (NP) and 15% limestone powder (LS) produces concrete with good workability, high 28-day compressive strength (39 MPa), excellent one year strength (57 MPa), and very high resistance to chloride penetration. Conventional OPC is produced by intergrinding 95% portland clinker and 5% gypsum, and its clinker factor (CF) thus equals 0.95. With 30% NP and 15% LS portland clinker replacement, the CF of the blended ternary PC equals 0.52 so that 48% CO₂ emissions could be avoided, while enhancing strength development and durability in the resulting self-compacting concrete (SCC). Petrographic and scanning electron microscopy (SEM) investigations of the crushed NP and finely-ground NP in the concretes provide new insights into the heterogeneous fine-scale cementitious hydration products associated with basaltic ash-portland cement reactions.     

Optimization of concrete mixture with hybrid blends of metakaolin and fly ash using response surface method

This paper presents the experimental results of a research carried out on the strength and permeability related properties of high performance concretes made with binary and ternary cementitious blends of fly ash (FA) and metakaolin (MK). The replacement ratios for FA were 10% and 20% by weight of Portland cement and those for MK were 5% and 10%. Compressive strength, chloride permeability, water sorptivity, and water absorption properties of concretes were obtained in this study for different testing ages up to 90 days. The influences of fly ash, metakaolin, and testing age on the properties of concretes have been identified using the analysis of variance. The statistical based regression models and the response surface method with the backward stepwise techniques were employed in the multi-objective optimization analysis. That is carried out by maximizing compressive strength while minimizing chloride permeability, water sorptivity, and water absorption. It was observed that fly ash and especially metakaolin were very effective on the aforementioned properties of the concretes, depending mainly on replacement levels and duration of curing. The results indicated that the ternary use of fly ash and metakaolin with the approximate cement replacement values of 13.3% and 10% respectively has provided the best results for the testing age of 90 days, when the optimized strength and permeability based durability properties of the concretes are concerned.     

Effect of specimen size and shape on compressive strength of concrete containing fly ash: Application of genetic programming for design

The use of fly ash as a mineral admixture in the manufacture of concrete has received considerable attention in recent years. For this reason, several experimental studies are carried out by using fly ash at different proportions replacement of cement in concrete. In the present study, the models are developed in genetic programming for predicting the compressive strength values of cube (100 and 150 mm) and cylinder (100 × 200 and 150 × 300 mm) concrete containing fly ash at different proportions. The experimental data of different mixtures are obtained by searching 36 different literatures to predict these models. In the set of the models, the age of specimen, cement, water, sand, aggregate, superplasticizers, fly ash and CaO are entered as input parameters, while the compressive strength values of concrete containing fly ash are used as output parameter. The training, testing and validation set results of the explicit formulations obtained by the genetic programming models show that artificial intelligent methods have strong potential and can be applied for the prediction of the compressive strength of concrete containing fly ash with different specimen size and shape.     

Strength and resistance to sulfate and sulfuric acid of ground fluidized bed combustion fly ash–silica fume alkali-activated composite

Fluidized bed combustion (FBC) is an environmentally friendly process for burning of coal and is used in many small factories located in urban area. The FBC fly ash is an environmental problem and needs good disposal or utilization. This research studied the strength and resistance to sulfate and acid of alkali-activated FBC fly ash–silica fume composite. The FBC fly ash was interground with silica fume (at the dosage levels of 1.5%, 3.75% and 5.0%) to make the source material homogenous with increased reactivity. Addition of silica fume enabled the adjustment of SiO₂/Al₂O₃ ratios (6.55-7.54) of composite and improved the strength and resistance to sulfate and acid of composite. The composite with 3.75% silica fume showed the optimum strength with 28-day compressive strength of 17.0 MPa. The compressive strengths of composite with 3.75% silica fume immersed in 5% magnesium sulfate solution and 3% sulfuric acid solutions were substantially higher than the control. The strength loss was from the high calcium content of FBC fly ash and incorporation of silica fume thus increased the durability of the composite.     

Effect of fly ash fineness on compressive strength and pore size of blended cement paste

This paper presents an experimental investigation on the effect of fly ash fineness on compressive strength, porosity, and pore size distribution of hardened cement pastes. Class F fly ash with two fineness, an original fly ash and a classified fly ash, with median particle size of 19.1 and 6.4 μm respectively were used to partially replace portland cement at 0%, 20%, and 40% by weight. The water to binder ratio (w/b) of 0.35 was used for all the blended cement paste mixes.Test results indicated that the blended cement paste with classified fly ash produced paste with higher compressive strength than that with original fly ash. The porosity and pore size of blended cement paste was significantly affected by the replacement of fly ash and its fineness. The replacement of portland cement by original fly ash increased the porosity but decreased the average pore size of the paste. The measured gel porosity (5.7–10 nm) increased with an increase in the fly ash content. The incorporation of classified fly ash decreased the porosity and average pore size of the paste as compared to that with ordinary fly ash. The total porosity and capillary pores decreased while the gel pore increased as a result of the addition of finer fly ash at all replacement levels.     

Heat deformations of fly ash concrete

When dealing with concrete resistance to high temperatures it is important for design purposes to know the elastic parameters, such as the temperature–strain curves and the modulus of elasticity.Concretes containing a high volume of fly ash differ from conventional mixes in the cementitious phase. This results in a different behaviour under heating compared to plain Portland cement concretes. To find the elastic response of fly ash concrete four series of concrete mixtures were manufactured: one with cement only, another with 30% by mass partial replacement of cement by fly ash, and two with 30% and 40% by mass replacement of cement by ground fly ash. Tests were carried out on cylinders (150 × 300 mm). A high-calcium fly ash was used.The conditions were selected so that the applied level of stress corresponded to 25% or to 40% of the ultimate compressive strength of concrete, and a transient type of temperature regime was followed. Based on the experiments the critical temperature, the residual deformation and the modulus of elasticity were determined.The results indicate that concretes containing a high volume of fly ash are more sensitive to high temperatures, since they developed greater deformations. The fineness of the fly ash used also seems to influence the degree of deformation in an adverse way.     

Comparative study of the initial surface absorption and chloride diffusion of high performance zeolite,silica fume and PFA concretes

Zeolite, a type of natural pozzolanic material, has been used in producing blended cement and concrete in China. The purpose of this study is to evaluate the effectiveness of zeolite in enhancing the performance of concrete in comparison with silica fume and pulverized fuel ash (PFA). In the first series of experiments, zeolite, silica fume, and PFA were all used to replace 5%, 10%, 15% and 30% of cement by weight in concrete with water to total cementitious material ratio (W/(C + P)) kept at 0.28. The results showed that zeolite decreased bleeding and increased marginally the viscosity of concrete without significantly compromising the slump. And at 15% replacement level, it resulted in 14% increase in concrete strength at 28-day compared with the control concrete. The test results also showed that there existed an optimum replacement level for zeolite to effect a decrease in initial surface absorption and in chloride diffusion of concrete. The test results of the second series of experiments where zeolite, silica fume and PFA were in turn used to replace 10% of cement in concretes with W/(C + P) in the range of 0.27 to 0.45 appeared that zeolite performed better than PFA but was inferior to silica fume in terms of increasing strength, decreasing initial surface absorption and chloride diffusion. It was further found that when W/(C + P) was greater than 0.45, the strength of the concretes incorporating zeolite or PFA (by direct replacement) was lower than that of the control concrete. The micro-structural study on concrete with zeolite revealed that the soluble SiO₂ and Al₂O₃ could react with Ca(OH)₂ to produce C–S–H which densified the concrete matrix. Pozzolanic effect of zeolite improved the microstructure of hardened cement paste and reduced the content of the harmful large pores, hence made concrete more impervious.     

Seawater effect on durability of ternary cements. Synergy of chloride and sulphate ions

The durability of a new cementing composite (ternary cement pastes), containing thermally activated paper sludge, fly ash, and Portland cement was evaluated in present work. The study was carried out according to the Koch and Steinegger methodology by testing the flexural strength of pastes immersed in a mixed solution of sodium sulphate (4090 ppm) and sodium chloride (24,530 ppm) at 18 °C over a period of 180 days. The reaction mechanism of sulphate, chloride and sodium ions with the paste was evaluated by scanning electron microscopy, porosity and pore-size distribution, and X-ray diffraction. The new blended cements elaborated with activated paper sludge–fly ash pozzolan blends showed chemical resistant to the aggressive solutions selected in the present work. The chloride binding and formation of Friedel’s salt was partially inhibited by the presence of sulphate. Sulphate ion reacts preferentially with the calcium aluminate hydrates forming non-expansive ettringite which precipitated inside the pores; the microstructure was refined and the mechanical properties enhanced. The influence of the type of morphology of ettringite and Friedel’s salt compounds on the mechanical strength is evidenced.     

Role of microwave radiation in curing the fly ash geopolymer

Fly ash geopolymer requires rather long heat curing to obtain reasonable strength development at an early age. However, the long heat curing period limits the application of the fly ash geopolymer. High strength development and a reduction in heat curing duration have been considered for energy saving. Therefore, this research proposed a process using 90-W microwave radiation for 5 min followed by conventional heat curing for high-calcium fly ash geopolymer. Results showed that the compressive strengths of geopolymer with microwave radiation followed by conventional heat curing were comparable to those of the control cured at 65 °C for 24 h. Microwave radiation gave the enhanced densification. In addition, SEM images showed that the gels formed on the fly ash particles owing to the promoted dissolution of amorphous phases from fly ash. This method accelerated the geopolymerization and gave the high compressive strength comparable to the conventional curing.     

Effect of coarse aggregate blending on short-term mechanical properties of self compacting concrete

This investigation is mainly focused on finding the unit weight, compressive strength, modulus of elasticity (MOE) and splitting tensile strength (STS) of SCC mixes with different coarse aggregate blending (60:40 and 40:60) (20 mm and 10 mm) and coarse aggregate content (28% and 32%) and these properties were compared to a conventional concrete (CC). All SCC mixes had 35% replacement of cement with class F fly ash. The coarse aggregate blending did not affect the compressive strength of SCC mixes, but it affected the unit weight, MOE and STS of SCC mixes. A new parameter called coarse aggregate points (CAPs) has been introduced to study the effect of coarse aggregate blending in a particular coarse aggregate content on mechanical properties of SCC mixes. It is observed that for the given strength, SCC mixes with the same CAP value have shown similar mechanical properties. The measured MOE of all mixes were compared with ACI 363R and AASHTO LRFD/ACI 318 predicted equations. The measured STS of all mixes were compared with ACI 363R and CEB-FIP predicted equations.     

The hydration and microstructure of ultra high-strength concrete with cement–silica fume–slag binder

This study investigated the flowability, compressive strength, heat of hydration, porosity and calcium hydroxide content of ultra-high-strength concrete (UHSC) with cement–silica fume–slag binder at 20 °C. The composition of the binder was designed using seven-batch factorial design method. The relationships between the binder composition and the properties were expressed in contours. Results showed that proper silica fume content could improve the flowability and compressive strength of UHSC, reduce the porosity and calcium hydroxide content of UHSC. Slag reduced the flowability, compressive strength, porosity, and calcium hydroxide content of UHSC to certain extent. The silica fume and slag demonstrated positive synergistic effects on the flowability and 3 d compressive strength, but have negative synergistic effects on the total heat of hydration, hydration heat when the time is infinitely long(P₀), 56 d compressive strength, porosity and calcium hydroxide content of UHSC.     

Mechanical performance of low cement reactive powder concrete (LCRPC)

The possibility of producing a reactive powder concrete (RPC) with low cement content was aimed in the scope of this study. Cement was replaced with class-C fly ash (FA) up to 60% for this purpose. Three different curing conditions (standard water curing, autoclave curing and steam curing) were applied to specimens. Two series of RPC composites were prepared with bauxite and granite aggregates. Mechanical properties such as compressive strength, splitting tensile strength, flexural strength and fracture energy of composites were investigated. Test results showed that, compressive strength of 200 MPa can be reached with low cement by using high-volume fly ash. Thermally treated specimens showed compressive strength beyond 250 MPa and high volume fly ash RPC have superior performance. Furthermore, compressive strength values reached up to 400 MPa with external pressure application during setting and hardening stages.     

Mechanical performance of low cement reactive powder concrete (LCRPC)

The possibility of producing a reactive powder concrete (RPC) with low cement content was aimed in the scope of this study. Cement was replaced with class-C fly ash (FA) up to 60% for this purpose. Three different curing conditions (standard water curing, autoclave curing and steam curing) were applied to specimens. Two series of RPC composites were prepared with bauxite and granite aggregates. Mechanical properties such as compressive strength, splitting tensile strength, flexural strength and fracture energy of composites were investigated. Test results showed that, compressive strength of 200 MPa can be reached with low cement by using high-volume fly ash. Thermally treated specimens showed compressive strength beyond 250 MPa and high volume fly ash RPC have superior performance. Furthermore, compressive strength values reached up to 400 MPa with external pressure application during setting and hardening stages.     

环境湿度对玻璃纤维增强水泥力学性能的影响

通过开展在不同龄期、不同环境湿度下玻璃纤维增强水泥(GRC)试件的抗折强度、抗压强度试验和基体pH值测定,研究了环境湿度对掺加粉煤灰和硅灰等活性矿物掺合料的GRC试件力学性能的影响。结果表明:环境湿度对GRC试件的抗折强度有重要影响,相对湿度越大,随着龄期增加, GRC试件抗折强度降低越严重;在温度60℃、相对湿度95%条件下,经过56 d龄期后,掺有40%粉煤灰和10%硅灰的GRC试件抗折强度比未掺加粉煤灰和硅灰的GRC试件的抗折强度提高48.5%、抗压强度提高23.6%, GRC基体pH值降低6%。在相同的湿度条件下,掺有粉煤灰和硅灰试件的pH值在各个龄期都低于普通硅酸盐水泥试件,说明粉煤灰和硅灰的掺入能降低水泥水化液相的碱度,进而延缓了纤维受侵蚀的速度,显著改善了GRC试件的力学及耐久性能。通过对试验结果进行分析,利用MATLAB软件建立了GRC试件抗折强度和抗压强度与水泥砂浆基体pH值及时间的关系式。      

Portland cement-fly ash-silica fume systems in concrete

Laboratory flow, strength, and ultrasnic pulse velocity tests were performed on mortars made with 70% (by weight) of portland cement and 30% of pozzolanic materials where the pozzolanic materials consisted of various combinations of fly ash and silica fume. In addition to these ternary systems, binary blends, such as Portland cement and fly ash, and Portland cement and silica fume, along with 100% Portland cement mortars, were investigated for comparison. The purpose of the investigation, preliminary in nature, was to see under what circumstances, if any, would be a synergistic action when a ternary system of Portland cement-fly ash-silica fume is used in a mortar or concrete.Mortars were made with two cements of type I and two cements of type III along with class F and class C fly ashes. One silica fume was used. Standard flow tests were performed on the fresh mortars, and compressive strength as well as ultrasonic pulse velocity tests were performed with each hardened mortar at various ages up to 28 days. It is expected that the results and conclusions obtained here on mortars will be transferable to concretes.There are several novel, or at least lesser known, results of the investigation. For instance, a new explanation is offered for the plasticizing effect of fly ash which is based on the optimum particle-size distribution concept. Another such result is that ground fly ash produced greater flow increases with type I cement than with type III. A third finding is that the superplasticizer is more effective in increasing the flow as well as strength when the mortars contain fly ash and/or silica fume than in the case of mortars without mineral admixture. Also, it appears that when type I cement is used, the silica fume in the quantity of 5% of the weight of the cement produces relatively greater strength increase in the presence of fly ash than without fly ash.These promising results are preliminary in nature. Therefore, further research is justified with ternary systems in concrete. The presented work is a portion of a larger investigation.