Influence of a fine glass powder on cement hydration: Comparison to fly ash and modeling the degree of hydration

This paper reports the results of an investigation carried out to understand the influence of a fine glass powder on cement hydration. The pozzolanicity of the glass powder and a Class F fly ash for comparison was evaluated using strength activity index over a period of time, and a rapid electrical conductivity based method. Flame emission spectroscopy and electrical conductivity tests were used to quantify the alkali release from glass powder, and gain information on the rate of alkali release. It was found that the glass powder releases only a very small fraction of sodium ions into the solution. It was observed that the glass powder modified pastes show higher non-evaporable water contents than the plain paste and fly ash modified pastes, indicating that glass powder facilitates enhancement in cement hydration. An expression has been developed for the change in non-evaporable water content as a result of enhancement in cement hydration and the hydration of the cement replacement material. The efficiency of any cement replacement material with age in the paste system can be quantified using this parameter. Based on this parameter, a 5% cement replacement with glass powder was found to be effective at the chosen water-to-cementing materials ratio (w/cm), whereas at higher replacement levels, the dilution effect dominates. A model to predict the combined degree of hydration of cement pastes incorporating more than one cementing material is outlined. The measured and predicted combined degrees of hydration agree well.     

Material and environmental parameter effects on the leaching of cement pastes: Experiments and modelling

Results already published on the leaching of cement pastes have shown that the kinetics depends sensitively on the material and environment. However and because of the variability of the tested materials and leaching protocols, it is difficult to compare these data and quantify the effect of each parameter. In this paper, a large experimental database on the leaching kinetics of cement pastes is built. Four parameters are investigated: type of cement (portland cement, silica fume cement, slag cement, ternary cement with slag and fly ash); water-to-cement ratio (0.5; 0.4; 0.25), temperature (26 °C; 72 °C; 85 °C) and chemical composition of the leaching solution (pure water, mineralised water, ammonium nitrate solution). Firstly, the database is used to calculate the leaching kinetics of the cementitious materials. Secondly, a simplified model predicting the one-dimensional leaching kinetics for other water-to-cement ratios and temperature up to 85 °C is presented.     

Modeling the hydration of concrete incorporating fly ash or slag

Granulated slag from metal industries and fly ash from the combustion of coal are industrial by-products that have been widely used as mineral admixtures in normal and high strength concrete. Due to the reaction between calcium hydroxide and fly ash or slag, the hydration of concrete containing fly ash or slag is much more complex compared with that of Portland cement. In this paper, the production of calcium hydroxide in cement hydration and its consumption in the reaction of mineral admixtures is considered in order to develop a numerical model that simulates the hydration of concrete containing fly ash or slag. The heat evolution rates of fly ash- or slag-blended concrete is determined by the contribution of both cement hydration and the reaction of the mineral admixtures. The proposed model is verified through experimental data on concrete with different water-to-cement ratios and mineral admixture substitution ratios.     

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.     

Effect of TEA on fly ash solubility and early age strength of mortar

The investigations focused on the dissolution behaviour of fly ash in alkaline solution and the effect of triethanolamine (TEA) addition. TEA is known as a grinding aid in cement production and is an Al and Fe chelating agent. To determine the effect of TEA on the dissolution behaviour of fly ash constituents, fly ash was mixed with a KOH solution at pH 13 and different dosages of TEA. Samples were taken after different times and analysed by ICP-OES. The effect of TEA on the heat evolution rates of fly ash cement pastes was investigated using isothermal calorimetry. Strength tests were also conducted to investigate the effect of TEA on plain Portland cement and fly ash/cement mortars. TEA was found to increase the dissolution rate of Al, Ca and Fe from fly ash. A slight, but reproducible, effect on heat evolution rates and an increase in early age strength was observed for fly ash cements.     

Early hydration and setting of oil well cement

A broad experimental study has been performed to characterize the early hydration and setting of cement pastes prepared with Class H oil well cement at water-to-cement ratios (w/c) from 0.25 to 0.40, cured at temperatures from 10 to 60 °C, and mixed with chemical additives. Chemical shrinkage during hydration was measured by a newly developed system, degree of hydration was determined by thermogravimetric analysis, and setting time was tested by Vicat and ultrasonic velocity measurements. A Boundary Nucleation and Growth model provides a good fit to the chemical shrinkage data.Temperature increase and accelerator additions expedite the rate of cement hydration by causing more rapid nucleation of hydration products, leading to earlier setting; conversely, retarder and viscosity modifying agents delay cement nucleation, causing later setting times. Lower w/c paste needs less hydration product to form a percolating solid network (i.e., to reach the initial setting point). However, for the systems evaluated, at a given w/c, the degree of hydration at setting is a constant, regardless of the effects of ambient temperature or the presence of additives.     

Fire resistance of fired clay bricks-fly ash composite cement pastes

This work aims to study the effect of substitution of fly ash for homra on the hydration properties of composite cement pastes. The composite cements are composed of constant proportion of OPC (80%) with variable amounts of fly ash and homra. The addition of fly ash accelerates the initial and final sitting time, whereas the free lime and combined water contents decrease with fly ash content. The fly ash acts as nucleation sites which may accelerate the rate of formation of hydration products which fill some of the pores of the cement pastes. The fire resistance of composite cement pastes was evaluated after firing at 250, 450, 600, 800 °C with rate of firing 5 °C/min with soaking time for 2 h. The physico-mechanical properties such as bulk density and compressive strength were determined at each firing temperature. Moreover, the phase composition, free lime and microstructure for some selected samples were investigated. It can be concluded that the pozzolanic cement with 20 wt% fly ash can be used as fire resisting cement.     

Hydration of fly ash cement

It is necessary to establish the material design system for the utilization of large amounts of fly ash as blended cement instead of disposing of it as a waste. Cement blended with fly ash is also required as a countermeasure to reduce the amount of CO₂ generation. In this study, the influences of the glass content and the basicity of glass phase on the hydration of fly ash cement were clarified and hydration over a long curing time was characterized. Two kinds of fly ash with different glass content, one with 38.2% and another with 76.6%, were used. The hydration ratio of fly ash was increased by increasing the glass content in fly ash in the specimens cured for 270 days. When the glass content of fly ash is low, the basicity of glass phase tends to decrease. Reactivity of fly ash is controlled by the basicity of the glass phase in fly ash during a period from 28 to 270 days. However, at an age of 360 days, the reaction ratios of fly ash show almost identical values with different glass contents. Fly ash also affected the hydration of cement clinker minerals in fly ash cement. While the hydration of alite was accelerated, that of belite was retarded at a late stage.     

含C_4A_3矿物硅酸盐水泥中混合材反应程度的研究

采用非蒸发水量法测定含硫铝酸钙矿物水泥的水化程度 ,萃取法测定该水泥中混合材的反应程度 ,通过SEM/EDS观察分析水化产物形貌和种类。结果表明 :粉煤灰水泥的水化程度高于同龄期的矿渣水泥的水化程度 ,粉煤灰的反应程度高于矿渣的反应程度。同龄期粉煤灰水泥中的水化产物多于矿渣水泥的水化产物 ,且水化产物发育更良好     

矿物掺合料对蒸养水泥净浆性能的影响

研究了蒸养条件下粉煤灰、矿渣的掺量对水泥净浆化学结合水量和抗压强度的影响,揭示了矿物掺合料对蒸养水泥净浆水化性能和力学性能的影响。试验结果表明,与标准养护相比,蒸汽养护更有利于激发粉煤灰和矿渣的火山灰活性,促进水泥的早期水化,提高水泥浆体的早期强度;但无论是蒸汽养护还是标准养护,随着矿物掺合料掺量的增加,复合胶凝材料的水化性能和力学性能明显减弱,因此矿物掺合料掺量不宜太大。     

Effects of curing temperature and NaOH addition on hydration and strength development of clinker-free CKD-fly ash binders

Effects of curing temperature and NaOH addition on hydration and strength development of cement kiln dust (CKD)-fly ash (FA) binders were investigated. Pastes made with 50% CKD and 50% FA, having 0, 2, and 5% NaOH addition, and cured at temperatures of 24, 38, and 50 °C were evaluated. The hydration products of the binders were examined by thermogravimetric analysis (TGA) and X-ray diffraction (XRD) tests. The results indicate that the major crystalline hydration product of the CKD-FA binders is ettringite, and the ettringite is stable in the CKD-FA system at age over 100 days. Curing at elevated temperature is more effective for CKD-FA binder strength improvement than NaOH addition, the later often depressing ettringite formation in a CKD-FA system. At a proper curing temperature (38 °C), addition of a small amount of NaOH (2%) may increase CKD-FA binder strength; while at a high curing temperature (50 °C), addition of NaOH (2%) may reduce the binder strength.     

Influence of high-temperature and low-humidity curing on chloride penetration in blended cement concrete

The influence of high-temperature and low-humidity curing on chloride penetration in concrete containing cement replacement materials was investigated. Three different mixes were studied: a control mix in which no cement replacement materials were added and two mixes where cement was partially replaced by 20% fly ash and 9% silica fume (by weight), respectively, at a constant water-to-binder ratio of 0.45. High-temperature curing was employed to simulate concrete temperature in hot climate. The results show that at early periods of exposure, initial curing has a substantial influence on chloride penetration in concrete. The effect of initial curing is much reduced after a long period of exposure. The chloride penetration at early ages of exposure is directly related to the porosity of the binder phase and the absorption of concrete. Higher chloride penetration resistance was observed when cement is partially replaced with either fly ash or silica fume.     

MXene和纳米SiO2对粉煤灰水泥水化性能的影响

本文利用差热分析、X射线衍射、扫描电镜等手段研究了纳米Si O_2和MXene对粉煤灰水泥水化性能的影响。结果表明,单掺纳米Si O_2能够促进粉煤灰水泥早期水化,提高水化开始时的放热速率,并使粉煤灰水泥浆体更加密实;而单掺MXene、复掺纳米Si O_2和MXene对粉煤灰水泥后期水化的促进作用比较明显,能够促进水泥中期强度增长。     

Effect of treatment temperature on the early hydration characteristics of superplasticized silica fume blended cement pastes

In this investigation, two mixes were used: ordinary Portland cement (OPC) and a blended cement prepared with the partial substitution of OPC by 10 mass% silica fume (SF). The setting and hardening characteristics were monitored by the aid of electrical conductivity as a function of curing time. The shear stress and electrical conductivity were studied at different temperatures, namely, 20, 35, 45 and 55 °C. As the temperature increases, the shear stresses decrease with the increase of shear rate. The height of electrical conductivity peaks of superplasticized cement pastes increases due to the increase of the paste fluidity. In the presence of 1.0% polycarboxylate (PC), the electrical conductivity of cement pastes decreases from 1 to 28 days. PC retards the hydration of cement pastes. The presence of PC extended the setting times of cement pastes at 35 °C than at 20 °C due to the increase in the adsorption capacity at this temperature. PC extends the dormant stage of the hydration process and delays the onset of the accelerating stage, without affecting its rate.     

Nanoparticles and Apparent Activation Energy of Portland Cement

Although chemically inert nanosize mineral fillers have been shown to modify early cement hydration kinetics, with the effects dependent upon usage rate, particle size, and dispersibility, the effects of such fillers on the “apparent activation energy” (E_a) of cement has not been previously examined. Here, cement E_a was calculated from isothermal calorimetry performed at different temperatures with two different types of fillers (i.e., titanium dioxide and limestone) using a linear method as well as a modified ASTM C1074 method. The use of both types of nanoparticles increased the rate of cement hydration as well as accelerated the reaction rate, due to heterogeneous nucleation effect, as previously demonstrated. E_a increased in the presence of nanosized fillers, demonstrating an increased temperature sensitivity of the filler‐cement composites relative to ordinary cement. These results show that chemically inert nanoparticles behave fundamentally differently compared with supplementary cementitious materials such as fly ash and silica fume which instead decrease temperature sensitivity. The increased temperature sensitivity could thus be used to modify and optimize the reaction mechanism and kinetics of cement hydration, especially to increase the rate of cement hydration, to decrease setting time, and to achieve faster strength gain accounting for higher or lower temperatures during curing.     

Electrochemical studies on the corrosion performance of steel embedded in activated fly ash blended concrete

The use of fly ash to replace a portion of cement has resulted significant savings in the cost of cement production. Fly ash blended cement concretes require a longer curing time and their early strength is low when compared to ordinary Portland cement (OPC) concrete. By adopting various activation techniques such as physical, thermal and chemical methods, hydration of fly ash blended cement concrete was accelerated and thereby improved the corrosion-resistance of concrete. Concrete specimens prepared with 10-40% of activated fly ash replacement were evaluated for their open circuit potential measurements, weight loss measurements, impedance measurements, linear polarization measurements, water absorption test, rapid chloride ion penetration test and scanning electron microscopy (SEM) test and the results were compared with those for OPC concrete without fly ash. All the studies confirmed that up to a critical level of 20-30% replacement; activated fly ash cement improved the corrosion-resistance properties of concrete. It was also confirmed that the chemical activation of fly ash yielded better results than the other methods of activation investigated in this study.     

养护温度和粉煤灰对补偿收缩混凝土膨胀效能的影响

研究了养护温度和粉煤灰掺量对补偿收缩混凝土的膨胀效能和强度的影响。结果表明：20、40℃养护时混凝土的各龄期强度均匀增长;60℃养护能极大地促进早期强度增长,后期强度增长缓慢。粉煤灰在不同养护温度下对混凝土的早期强度发展都有抑制作用;长期高温养护后,粉煤灰活性逐渐显现,显著促进混凝土的强度增长,且粉煤灰掺量越大,混凝土强度增幅越大。硫铝酸钙–氧化钙类膨胀剂的膨胀效能发挥对温度非常敏感,养护温度越高,膨胀剂的水化速度越快,膨胀作用发挥越早;适量掺加粉煤灰有利于膨胀效能的发挥,掺量越大,膨胀随温度增长的增幅越大。大掺量粉煤灰补偿收缩混凝土的强度发展和限制膨胀率的温度敏感性均很高。     

Influence of nucleation seeding on the hydration kinetics and compressive strength of alkali activated slag paste

Addition of pure calcium silicate hydrate (C–S–H) to alkali-activated slag (AAS) paste resulted in an earlier and larger hydration rate peak measured with isothermal calorimetry and a much higher compressive strength after 1 d of curing. This is attributed to a nucleation seeding effect, as was previously established for Portland cement and tricalcium silicate pastes. The acceleration of AAS hydration by seeding indicates that the early hydration rate is controlled by nucleation and growth. For the experiments reported here, the effect of C–S–H seed on the strength development of AAS paste between 1 d and 14 d of curing depended strongly on the curing method. With sealed curing the strength continued to increase, but with underwater curing the strength decreased due to cracking. This cracking is attributed to differential stresses arising from chemical and autogenous shrinkage. Similar experiments were also performed on Portland cement paste.     

Influence of steam curing on the compressive strength of concrete containing supplementary cementing materials

Heat treatment is widely used to accelerate the strength-gaining rate of concrete. In general, the ultimate strengths of the heated-treated concrete are lower than those of the standard cured specimens. When ultrafine fly ash (UFA) is included in concrete, the pozzolanic reaction is accelerated through the heat treatment. Sometimes, various chemical activators were used to activate the reactivity of fly ash. In the current study, UFA and slag were used as a replacement for cement, steam curing and chemical activators were used to accelerate hydration of cement and fly ash, and then compared with moist curing. This paper presents the influence of steam curing on the compressive strength of concrete containing UFA with or without slag. The experimental results indicated that the concrete containing UFA has low early strength after 13-h steam curing and that the difference between the 28-day compressive strength of concrete through 13-h steam curing and that of moist-cured concrete is large, but the concrete with UFA and CaSO₄ or Ca(OH)₂ has a high early strength, thus, the reactivity of fly ash must be accelerated. Concrete containing UFA and ground slag was prepared, whose compressive strengths were improved.