Utilization of fly ash with silica fume and properties of Portland cement-fly ash-silica fume concrete

This paper reports the normal consistency, setting time, workability and compressive strength results of Portland cement-fly ash-silica fume systems. The results show that water requirement for normal consistency was found to increase with increasing SF content while a decrease in initial setting time was found. Workability, measured in term of slump, was found to decrease with silica fume content (compared to blends without silica fume). However, it must be noted that despite the reduction in the slump values, the workability of Portland cement-fly ash-silica fume concrete in most cases remained higher than that of the Portland cement control concrete. Furthermore, the utilization of silica fume with fly ash was found to increase the compressive strength of concrete at early ages (pre 28 days) up to 145% with the highest strength obtained when silica fume was used at 10 wt%. Moreover, scanning electron micrographs show that utilization of fly ash with silica fume resulted in a much denser microstructure, thereby leading to an increase in compressive strength.     

High strength mortars containing condensed silica fume

A silica fume and a superplasticizer have been added to an ordinary easily flowing mortar composition. The optimal amounts of those constituents have been determined in order to obtain the maximum compressive strength for a constant workability. Large contents of silica fume and superplasticizer are needed (silica/cement=0.40 and superplasticizer/ce cement=2.4% in dry weights). The compressive strengths are approximately twice higher at 2,7 and 28 days as compared to the reference mortar. The drying shrinkage is slightly increased and the hydration kinetics is highly modified.     

An investigation on microstructure of cement paste containing fly ash and silica fume

In this study, high-calcium fly ash (HCFA) and silica fume (SF) were used as mineral admixtures. The effect of these admixtures on the microstructure of cement paste was investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The reaction of HCFA and SF with portlandite, which occurs in Portland cement (PC), forms a new calcium-silicate-hydrate (C-S-H) gel.     

粉煤灰/硅灰对低标号混凝土性能的影响

研究了单掺粉煤灰、硅灰和双掺粉煤灰/硅灰对低标号混凝土的工作性、强度和干缩性的影响。结果表明,粉煤灰和硅灰能改善新拌低标号混凝土的工作性,硅灰使得低标号混凝土在短期内的干缩变化加大,造成早期裂缝,短期内双掺粉煤灰/硅灰能有效提高低标号混凝土的抗压强度。     

Effect of silica fume and fly ash on heat of hydration of Portland cement

Results of calorimeter tests on Portland cement-silica fume-fly ash mixtures are presented. Data indicate that silica fume accelerates cement hydration at high water/cementitious ratios and retards hydration at low water/cementitious ratios. On the other hand, fly ash retards cement hydration more significantly at high water/cementitious ratios. When silica fume and fly ash are added together with cement, the reactivity of the silica fume is hampered and the hydration of the cementitious system is significantly retarded.     

Corrosion resistance in activated fly ash mortars

The question of whether reinforcing steel can be protected with activated fly ash cement as effectively as with Portland cement is explored in this study. Corrosion potential (E_corr) and polarisation resistance (R_p) values for steel electrodes embedded in Portland cement mortar and two fly ash mortars, respectively activated with NaOH and waterglass+NaOH solutions, are monitored. Chloride-free activated fly ash mortars are found to passivate steel reinforcement as speedily and effectively as Portland cement mortars, giving no cause to fear that corrosion may limit the durability of reinforced concrete structures built with these new types of activated fly ash cement. The polarisation curves and the response to short-term anodic current pulses (galvanostatic pulse technique) obtained further corroborate the full and stable passivation of the steel by the concrete manufactured with these binders.     

粉煤灰/硅灰复合掺合料对水泥净浆性能的影响

研究了水泥标准稠度用水量、粉煤灰掺量、硅灰掺量、粉煤灰与硅灰双掺对水泥净浆性能的影响.结果表明,硅灰使水泥净浆需水量明显增加,粉煤灰、硅灰双掺可克服单掺粉煤灰早期强度低的缺点,短期内能提高水泥净浆的抗压强度.     

Effect of silica fume, metakaolin, and low-calcium fly ash on chemical resistance of concrete

Effects of aggressive chemical environments were evaluated on the mortars prepared with ordinary portland cement (OPC) and silica fume (SF)/metakaolin (MK)/low-calcium fly ash at various replacement levels. The natural adverse chemical environmental conditions were simulated using sulfuric acid, hydrochloric acid, nitric acid, acetic acid, phosphoric acid, and a mixture of sodium and magnesium sulfates. Chemical resistance information was used in conjunction with compressive strength measurements to propose realistic OPC/mineral admixture proportions.     

Experimental study on properties of polymer-modified cement mortars with silica fume

This paper discussed the flexural and the compressive strengths of polyacrylic ester (PAE) emulsion and silica fume (SF)-modified mortar. The chloride ion permeability in cement mortar and the interfacial microhardness between aggregates and matrix were measured. The chemical reactions between polymer and cement-hydrated product were investigated by the infrared spectral technology. The results show that the decrease of porosity and increase of density of cement mortars can be achieved by the pozzolanic effect of SF, the water-reducing and -filling effect of polymer. Lower porosity and higher density can give cement mortars such properties as higher flexural and compressive strength, higher microhardness value in interfacial zone and lower effective diffusion coefficient of chloride ion in matrix.     

Use of ground coarse fly ash as a replacement of condensed silica fume in producing high-strength concrete

This paper presents a method of improving coarse fly ash in order to replace condensed silica fume in making high-strength concrete. The coarse fly ash, having the average median diameter about 90-100 μm, yields a very low pozzolanic reaction and should not be used in concrete. In order to improve its quality, the coarse fly ash was ground until the average particle size was reduced to 3.8 μm. Then, it was used to replace Portland cement type I by weights of 0%, 15%, 25%, 35%, and 50% to produce high-strength concrete. It was found that concrete containing the ground coarse fly ash (FAG) replacement between 15% and 50% can produce high-strength concrete and 25% cement replacement gave the highest compressive strength. In addition, the concrete containing FAG of 15-35% as cement replacement exhibited equal or higher compressive strengths after 60 days than those of condensed silica fume concretes. The results, therefore, suggest that the FAG with high fineness is suitable to use to replace condensed silica fume in producing high-strength concrete.     

Metakaolin and fly ash alkali-activated mortars compared with cementitious mortars at the same strength class

Alkali-activated and cementitious mortars belonging to R1 ≥ 10 MPa, R2 ≥ 15 MPa and R3 ≥ 25 MPa strength classes were tested and compared in terms of workability, dynamic modulus of elasticity, porosimetry, and water vapor permeability. Capillary water absorption, drying shrinkage, resistance to sulfate attack, and corrosion behavior of embedded bare and galvanized reinforcements were also investigated.In alkali-activated mortars, drying shrinkage is higher than that of cementitious mortars but restrained shrinkage is lower due to lower modulus of elasticity. Pore dimensions affect water vapor permeability, more pronounced in alkali-activated mortars, and capillary water absorption, much lower in fly ash ones. The high alkalinity of fly ash and metakaolin mortars delayed the achievement of the passive state in particular for the galvanized reinforcements but after 1 month of curing they reached the same corrosion rates of those embedded in cementitious mortars.     

Response of alkali activated fly ash mortars to microwave curing

Volumetric heating provided by microwave curing results in faster property development as compared to conventional heat curing that relies on heat conduction from the skin to the core. This paper discusses the compressive strength and microstructure development of microwave cured NaOH activated fly ash mortars, and relates them to the microwave energy absorption by the material which is a function of its dielectric properties. Microwave curing parameters are chosen so as to eliminate the effects of thermal runaway. Strengths that are comparable to or greater than those of mortars heat cured for 48 h at 75 °C are obtained in less than 120 min of microwave curing. The rate of energy absorption by the mortars is found to be relatively constant for a considerable fraction of the microwave curing duration, attributable to the compensation for the drop in dielectric loss factor as a result of moisture loss by the increase in internal electric field. Compressive strength is shown to be related to the microwave energy absorbed by the specimens, especially during the time when free water is present in the system.     

Alkali reactivity of mortars containing chert and incorporating moderate-calcium fly ash

This paper reports the results of an experimental program, which aimed to investigate the alkali reactivity of chert and the effect of a moderate-calcium fly ash on the alkali–silica reaction. To determine the expansions, mortar bars were cast and tested in accordance with ASTM C1260. Mortar aggregate was replaced by chert, in controlled amounts, to find out the pessimum limit, if any. To evaluate the degree of cracking, sonic pulse velocity measurements and petrographic analysis were carried out on the cracked bars and on the thin sections taken from these bars, respectively. In the next series of tests, limestone and chert were blended together as mortar aggregate and cement was replaced by different dosages of fly ash to examine the changes in the mortar bar expansion as well as in the chemistry of reaction products. Microstructural observations were done on polished sections using a scanning electron microscope, equipped with energy dispersive X-ray analysis. The results showed that the chert used in this investigation had a pessimum proportion in the range of 5–15%. Sufficient fly-ash additions suppressed the expansion caused by chert. The study also revealed out that as the CaO/Na₂O_eq of alkali–silica gel increased, the expansivity of the gel decreased.     

The effects of expanded perlite aggregate, silica fume and fly ash on the thermal conductivity of lightweight concrete

Thermal conductivity coefficients of concretes made up of mixtures of expanded perlite and pumice aggregates (PA) were measured. To determine the effect of silica fume (SF) and class C fly ash (FA) on the thermal conductivity of lightweight aggregate concrete (LWAC), SF and FA were added as replacement for cement by decreasing the cement weights in the ratios of 10%, 20% and 30% by weight.The highest thermal conductivity of 0.3178 W/mK was observed with the samples containing only PA and plain cement. It decreased with the increase of SF and FA as replacement for cement. The lowest value of thermal conductivity, which is 0.1472 W/mK, was obtained with the samples prepared with expanded perlite aggregate (EPA) replacement of PA and 70% cement+30% FA replacement of cement. Both SF and FA had a decreasing effect on thermal conductivity. EPA (used in place of PA) also induced a decrease of 43.5% in thermal conductivity of concrete.     

Relationship between resistivity, diffusivity and microstructural descriptors for mortars with silica fume

Mortars with a sand-to-cement ratio of 3 and water-to-cement ratio of 0.5 were made with 0% and 10% silica fume (SF). Resistivities were measured with alternating current impedance spectroscopy (ACIS). Diffusivities were determined with the propan-2-ol counterdiffusion method. Microstructure was investigated with mercury intrusion porosimetry. It was found that there is a relationship relating hydration time to the product of resistivity and diffusivity. Furthermore, the product of resistivity and diffusivity was related to porosity, mean and threshold pore diameters. The influence of silica fume in refining the pore structure was marked.     

Pozzolanic activity of clinoptilolite: A comparative study with silica fume, fly ash and a non-zeolitic natural pozzolan

Pozzolanic activity of clinoptilolite, the most common natural zeolite mineral, was studied in comparison to silica fume, fly ash and a non-zeolitic natural pozzolan. Chemical, mineralogical and physical characterizations of the materials were considered in comparative evaluations. Pozzolanic activity of the natural zeolite was evaluated with various test methods including electrical conductivity of lime-pozzolan suspensions; and free lime content, compressive strength and pore size distribution of hardened lime-pozzolan pastes. The results showed that the clinoptilolite possessed a high lime-pozzolan reactivity that was comparable to silica fume and was higher than fly ash and a non-zeolitic natural pozzolan. The high reactivity of the clinoptilolite is attributable to its specific surface area and reactive SiO₂ content. Relatively poor strength contribution of clinoptilolite in spite of high pozzolanic activity can be attributable to larger pore size distribution of the hardened zeolite-lime product compared to the lime-fly ash system.     

Use of ternary blends containing silica fume and fly ash to suppress expansion due to alkali-silica reaction in concrete

This paper investigates the effects of cementitious systems containing Portland cement (PC), silica fume (SF) and fly ash (FA) on the expansion due to alkali-silica reaction (ASR). Concrete prisms were prepared and tested in accordance with the Canadian Standards Association (CSA A23.2-14A). Paste samples were cast using the same or similar cementitious materials and proportions that were used in the concrete prism test. Pore solution chemistry and portlandite content of the paste samples are reported. It was found that practical levels of SF with low-, moderate- or high-calcium FA are effective in maintaining the expansion below 0.04% after 2 years. Pore solution chemistry shows that while pastes containing SF yield pore solutions of increasing alkalinity at ages beyond 28 days, pastes containing ternary blends maintain the low alkalinity of the pore solution throughout the testing period (3 years).     

Resistance of mortars containing silica fume to attack by a solution containing chlorides

Mortars prepared at water/cement + silica fume ratios, w/ (c+sf), of 0.45 and 0.60 were exposed to a solution containing a mixture of Mg-, Ca- and Na- chlorides to investigate their resistance to attack. Mixes contained 0, 10 and 30% silica fume, and sand-binder ratio was 2.25. Stiffness, pore-size distribution, Ca(OH)₂ content and non-evaporable water were measured before and after exposure to salt solution. Inclusion of silica fume resulted in increased durability of mortar, especially in specimens prepared at w/ (c+sf) of 0.45 and cured for 28 days. Specimens prepared at the same w/ (c+sf) with 30% silica fume were also durable even after seven days of curing. Calcium hydroxide in mortars was reduced to zero by exposure to the salt solution.     

Compressive strength studies on portland cement mortars containing fly ash and superplasticizer

The effects of two super water-reducing admixtures on the compressive strengths and elastic moduli of portland cement mortars containing fly ash were determined. The results show that the effectiveness of the chemicals for improving the strengths of mixtures in which portland cement is partially replaced by fly ash decreases with an increase in the percentage of ash. Increased strength can be nil or too low to justify the additional expense associated with the chemicals. The mixtures did exhibit normal stress-strain relationships: the elastic moduli were related to the ultimate compressive strengths in the usual manner. The results are based on mixes with equal workabilities.     

Effect of pumice and fly ash incorporation on high temperature resistance of cement based mortars

The effects of high temperature on the mechanical properties of cement based mortars containing pumice and fly ash were investigated in this research. Four different mortar mixtures with varying amounts of fly ash were exposed to high temperatures of 300, 600, and 900 °C for 3 h. The residual strength of these specimens was determined after cooling by water soaking or by air cooling. Also, microstructure formations were investigated by X-ray and SEM analyses.Test results showed that the pumice mortar incorporating 60% fly ash revealed the best performance particularly at 900 °C. This mixture did not show any loss in compressive strength at all test temperatures when cooled in air. The superior performance of 60% FA mortar may be attributed to the strong aggregate-cement paste interfacial transition zone (ITZ) and ceramic bond formation at 900 °C. However, all mortar specimens showed severe losses in terms of flexural strength. Furthermore, specimens cooled in water showed greater strength loss than the air cooled specimens. Nevertheless, the developed pumice, fly ash and cement based mortars seemed to be a promising material in preventing high temperature hazards.