Pozzolanic activity of various siliceous materials

The accelerated pozzolanic activity of various siliceous materials, like silica fume, fly ash (as received and fine ground), quartz, precipitated silica, metakaolin and rice husk ash (RHA; various fineness and carbon content), has been determined. The compressive strength of accelerated tests has been compared with cubes cured in water at 7 and 28 days. Maximum activity has been observed in case of RHA (<45 μ), followed by quartz and silica fume. The 10% replacement of cement by sand has shown accelerated pozzolanic index of 92% compared with 85% required in ASTM for silica fume as mineral admixture.     

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.     

Magnesium sulfate attack on hardened blended cement pastes under different circumstances

This paper describes the sulfate resistance of some hardened blended Portland cement pastes. The blending materials used were silica fume (SF), slag, and calcium carbonate (CaCO₃, CC?). The blended cement pastes were prepared by using W/S ratio of 0.3. The effects of immersion in 10% MgSO₄ solution under different conditions (room temperature, 60 °C, and drying-immersion cycles at 60 °C) on the compressive strength of the various hardened blended cement pastes were studied. Slag and CC? improve the sulfate resistance of ordinary Portland cement (OPC) paste. Mass change of the different mixes immersed in sulfate solution at 60 °C with drying-immersion cycles was determined. The drying-immersion cyclic process at 60 °C accelerates sulfate attacks. This process can be considered an accelerated method to evaluate sulfate resistance of hardened cement pastes, mortars, and concretes.     

Zero-eccentricity direct-tension testing of thermally damaged cement-based materials

This study presents some results on direct-tension strength of two cementitious mortars using a test set-up specifically designed to virtually eliminate any load eccentricity. The tests were conducted on cement mortars with and without condensed silica fume after being exposed to high temperature (200, 300, 400 and 500 °C). Direct-tension tests were also carried out at room temperature (20 °C) for reference. The specimens were exposed to high temperature and were then allowed to cool to room temperature before testing up to failure. The strength values measured in this study exhibit a trend that is similar to that exhibited by the compressive strength cited in the literature. The results show that mortar specimens exhibited a small increase in strength at temperatures up to 200 °C for regular mortar and up to 230 °C for silica fume mortar. At temperatures above 200/230 °C, the residual tensile strength of the mortar decreases significantly and rapidly. Adding silica fume to the cement mortar increases the resistance to high temperature.     

海水环境下矿物掺合料对硫铝酸盐水泥的水化影响

研究了海水环境下掺入硅灰、粉煤灰、矿渣对硫铝酸盐水泥抗压强度、化学收缩和水化产物的影响规律.结果表明:当硅灰的掺量为2.5%时,水泥浆体的抗压强度比空白组高.矿渣掺量为10%的水泥浆体28 d抗压强度明显超过掺入硅灰和粉煤灰时的强度,60 d强度高于空白组.掺入2.5%硅灰后,水泥浆体的化学收缩增大;在水化早期,粉煤灰和矿渣的火山灰活性很低,导致水泥浆体的化学收缩降低.掺入10%硅灰加快了硫铝酸盐水泥3 d水化反应,钙矾石生成量增多,水泥浆体早期强度比掺其它掺合料有所提高,但体积过快膨胀会破坏其内部结构,对水泥浆体的强度发展不利.     

Effects of densified silica fume on microstructure and compressive strength of blended cement pastes

Some experimental investigations on the microstructure and compressive strength development of silica fume blended cement pastes are presented in this paper. The silica fume replacement varies from 0% to 20% by weight and the water/binder ratio (w/b) is 0.4. The pore structure by mercury intrusion porosimetry (MIP), the micromorphology by scanning electron microscopy (SEM) and the compressive strength at 3, 7, 14, 28, 56 and 90 days have been studied. The test results indicate that the improvements on both microstructure and mechanical properties of hardened cement pastes by silica fume replacement are not effective due to the agglomeration of silica fume particles. The unreacted silica fume remained in cement pastes, the threshold diameter was not reduced and the increase in compressive strength was insignificant up to 28 days. It is suggested that the proper measures should be taken to disperse silica fume agglomeration to make it more effective on improving the properties of materials.     

Properties and hydration of blended cements with calcareous diatomite

In this paper the effect of diatomite addition on blended cement properties and hydration was studied. Calcareous diatomaceous rocks of Zakynthos Island, Ionian Sea, containing mainly CaCO₃ and amorphous silica of biogenic origin with the form of opal-A were used. Cement mortars and pastes, with 0%, 10%, 20% and 35% replacement of cement with the specific diatomite, were examined. Strength development, water demand and setting time were determined in all samples. In addition, XRD, SEM and weight loss at 350 °C were applied in order to study the hydration products and the hydration rate in the cement-diatomite pastes. Blended cements, having up to 10% diatomite content, develop the same compressive strength, as the corresponding Portland cement, while the presence of diatomite leads to an increase of the paste water demand. Diatomite is characterized as natural pozzolana, as it satisfies the requirements of EN 197 1 concerning the active silica content. The pozzolanic nature of the diatomite results to the formation of higher amounts of hydrated products, specifically at the age of 28 days.     

水泥-硅灰/粉煤灰体系强度、收缩性能与微观结构研究

为研究硅灰及粉煤灰对不同养护龄期的水泥浆体强度及收缩性能的影响,以水胶比为0.29的水泥浆体为基体,设计制备了五种硅灰及粉煤灰掺量的复合水泥浆体,借助量热仪和压汞仪测试表征了不同复合水泥浆体的水化放热特性以及孔结构组成,分析了水化放热量、孔隙率等参数随硅灰和粉煤灰掺量增加的变化规律,建立了复合浆体抗压强度与孔结构以及水化特性与收缩应变之间的量化关系。结果表明,掺入粉煤灰会大幅降低水泥净浆早期抗压强度,但对减小自收缩应变和干缩应变极为有利。掺入硅灰能明显提高净浆3 d抗压强度,但当硅灰掺量超过10%(质量分数)后,净浆3 d自收缩应变及28 d干缩应变增加极为明显。掺入硅灰会使水泥水化诱导期开始和结束的时间提前,还会增加水化反应级数和各阶段的反应速率常数值,导致水泥-硅灰复合浆体的水化放热总量和放热速率相较于水泥-粉煤灰体系大幅增加。粉煤灰和硅灰的掺入均能有效细化水泥浆体内部孔结构,提高凝胶孔比例,大幅降低大孔比例。复合浆体的72 h水化放热总量和3 d自收缩应变呈现正相关关系,而孔隙率和抗压强度呈现明显的负相关关系。     

Hydration and microstructure of ultra high performance concrete incorporating rice husk ash

Rice husk ash (RHA) and silica fume (SF) have a similar chemical composition and a very high specific surface area, but RHA is not an ultra-fine material like SF. The high specific surface area of RHA originates from its internal porosity. For this reason RHA can be expected to behave differently from SF in terms of the hydration and the resulting microstructure of concrete. This still remains unclear in Ultra High Performance Concrete (UHPC). The objective of this research was to study the effect of RHA on the hydration and microstructure development of UHPC. The results are compared to those obtained with a control sample and a sample made with SF.The results show that the addition of RHA can increase the degree of cement hydration in UHPC at later ages. RHA can also refine the pore structure of UHPC and reduce the Ca(OH)₂ content, but less significantly than SF. The thickness of the interface transition zone (ITZ) between sand particles and cement matrix of all samples is very small at the age of 28 days. The compressive strength of the sample made with RHA after 7 days was higher than that of the control sample and the sample made with SF. It is suggested that the porous structure of RHA and the uptake of water in this porous structure results in a kind of is attributable to the internal water curing of the RHA modified mixture.     

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.     

石膏、硅灰对硅酸盐胶凝材料早期抗压强度的影响

为了研究石膏、硅灰对硅酸盐胶凝材料早期强度的影响,分别测试了石膏、硅灰不同掺量下的胶凝材料的4 h、1 d、28 d的抗压强度。利用X射线衍射仪和扫描电子显微镜分析了水化产物的微观结构特征。研究表明,在一定的试验范围内,胶凝材料的抗压强度随石膏的增加而变大,掺量为0.75%时最佳,4 h和1 d的抗压强度分别达到5.8 MPa和63.4 MPa;硅灰掺量从0%增长到15%,胶凝材料的各龄期抗压强度均随掺量的增加而呈增长趋势;硬化浆体的微观结构特征表明,一定的试验范围内,石膏使体系中的AFt数量增加,硅灰使体系中的C-S-H凝胶增多,且硅灰未水化的细小颗粒体有效填充硬化浆体的孔隙。     

Effect of calcium formate as an accelerator on the physicochemical and mechanical properties of pozzolanic cement pastes

The aim of the present work is to study the effect of calcium formate (CF) as an accelerator on the properties of pozzolanic cement pastes. Three types of cements were used in this investigation. These cements were OPC and pozzolanic cements containing 80 mass% OPC and 20 mass% silica fume (SF) or 20 mass% ground clay bricks (GCB). The dosages of CF were 0.00, 0.25, 0.50, and 0.75 mass% of cement. The compressive strength, total porosity, and hydration kinetics such as free lime and combined water contents were investigated. The results obtained in this study showed that the addition of CF shortens the initial and final setting times and increases the compressive strength and combined water content as well as gel/space ratio at all ages of hydration. On the other hand, it decreases the total porosity. CF activates the liberation of Ca(OH)₂ of OPC pastes. The free lime content of pozzolanic cement in the presence of CF increases up to 7 days, then decreases at the later ages of hydration.     

Formation of magnesium silicate hydrate (M-S-H) cement pastes using sodium hexametaphosphate

Magnesium silicate hydrate (M-S-H) gel is formed by the reaction of brucite with amorphous silica during sulphate attack in concrete and M-S-H is therefore regarded as having limited cementing properties. The aim of this work was to form M-S-H pastes, characterise the hydration reactions and assess the resulting properties. It is shown that M-S-H pastes can be prepared by reacting magnesium oxide (MgO) and silica fume (SF) at low water to solid ratio using sodium hexametaphosphate (NaHMP) as a dispersant. Characterisation of the hydration reactions by x-ray diffraction and thermogravimetric analysis shows that brucite and M-S-H gel are formed and that for samples containing 60 wt.% SF and 40 wt.% MgO all of the brucites react with SF to form M-S-H gel. These M-S-H cement pastes were found to have compressive strengths in excess of 70 MPa.     

Physicochemical characteristics of acrylic-acid polymer-impregnated cement pastes

Various Portland cement pastes were made using water cement ratios of 0·20, 0·25, 0·35 or 0·40 and then cured for 1, 3, 7, 28, 90 or 180 days. These pastes were impregnated with acrylic acid monomer under vacuum and the monomer-impregnated samples were then treated at two different temperatures, 40 or 60°C, for the polymerization process, using benzoyl peroxide as initiator. Several physicochemical studies were carried out on each cement paste; these studies include compressive strength tests, bulk density, compressive strength versus gel/space ratio relationships, polymer load, X-ray diffraction analysis and differential thermal analysis. Results have indicated that compressive strength improvement in acrylic acid-polymer impregnated cement pastes is mainly dependent on initial water/cement ratio, curing time and gel/space ratio. The results of X-ray diffraction analysis and differential thermal analysis indicated that the intrusion of polymer into the cement paste matrix does not affect the phase composition of the Portland cement hydration products.     

海水和矿物掺合料对硫铝酸盐水泥砂浆性能的影响

本文研究了不同拌和水以及海水拌和时粉煤灰和硅灰掺量对硫铝酸盐水泥(SAC)砂浆力学性能和表观孔隙率以及净浆凝结时间、化学收缩、孔溶液pH值和氯离子结合能力等的影响,并通过XRD、SEM和EDS分析水泥水化产物和微观结构。结果表明,海水能加快SAC早期水化并提高其早期强度,但后期强度和淡水拌和时无明显差别。粉煤灰和硅灰均会延长SAC凝结时间,对早期抗压强度不利,而掺加质量分数为5.0%和7.5%的硅灰能提高SAC砂浆28 d抗压强度。硅灰掺量增加时会提高用水量和表观孔隙率,降低流动性,使水泥化学收缩增大,降低净浆pH值且减少氯离子结合量;粉煤灰能够提高砂浆流动性,减少水泥化学收缩,但掺量越大对SAC砂浆抗压强度和抗折强度越不利,掺质量分数为10%的粉煤灰可小幅提高氯离子结合量且减小表观孔隙率。     

高温条件下G级油井水泥原浆及加砂水泥的水化和硬化

利用X射线衍射仪和扫描电子显微镜研究了80～240℃温度范围内温度、硅砂对G级油井水泥水化硬化的影响,检测和分析了硬化体的水化产物、微观结构和强度,揭示了水化产物组成、微观结构及硬化体抗压强度的变化特点.结果表明:当养护温度超过110℃时,不添加硅砂的水泥原浆的主要水化产物由CSH(Ⅱ),C2SH2,C3S2H3转变为C2SH,硬化体微观结构由三维网络状结构转变为板快状或团块状结构,原浆水泥石抗压强度随温度升高而降低;在相对较高的温度条件下,添加硅砂的水泥主要水化产物则分别转变为C5S6H5,C6S6H(>150℃),C5S5A0.5H5.5,C3.2S2H0.8及其他类型的水化硅酸钙晶体,硬化体的微观结构相应地变为纤维网状、粗框架、短平行针状及团块状,在温度为100～150℃范围时,添加硅砂的水泥硬化体抗压强度随温度升高而增加,而在温度为150～240℃范围时.抗压强度随温度升高而降低.对于温度超过120℃的深井,合理的硅砂加量为30%～40%.     

辅助胶凝材料对预应力高强混凝土管桩强度及工艺的影响

为优化混凝土管桩生产工艺,以硅灰(SF)和偏高岭土(MK)作为辅助胶凝材料,研究硅灰和偏高岭土对不同蒸养时间下混凝土抗压强度的影响,并使用X射线衍射(XRD)和扫描式电子显微镜结合能量色散谱(SEM-EDS)分析其水化产物及微观结构。通过Design-Expert8.0软件设计Box-Behnken试验,以硅灰掺量、偏高岭土掺量和蒸养时间三个因素为自变量,蒸养混凝土抗压强度为响应值,构建多因素回归方程模型。结果表明:硅灰掺量为胶凝材料质量分数8%时,对抗压强度略有提高,提高幅度为6.2%,达到83.6 MPa;5%、8%和10%(质量分数)掺量的偏高岭土均可提高蒸养混凝土的抗压强度,蒸养4 h、8 h、12 h时,10%掺量的偏高岭土对混凝土抗压强度的提升幅度依次为15.6%、13.2%和13.6%,蒸养4 h、8 h和12 h对混凝土抗压强度影响不大。XRD和SEM-EDS结果表明,硅灰和偏高岭土均消耗了Ca(OH)₂,提升了水泥早期水化程度,可以改善内部孔结构。通过响应面法建立模型可以预测,当硅灰质量分数为6.6%、偏高岭土质量分数为10%、蒸养时间为8.6 h时,混凝土抗压强度最高,达到104.8 MPa,且具有较高置信度。     

Investigation of blended cement hydration by isothermal calorimetry and thermal analysis

Hydration of portland cement pastes containing three types of mineral additive; fly ash, ground-granulated slag, and silica fume was investigated using differential thermal analysis, thermogravimetric analysis (DTA/TGA) and isothermal calorimetry. It was shown that the chemically bound water obtained using DTA/TGA was proportional to heat of hydration and could be used as a measure of hydration. The weight loss due to Ca(OH)₂ decomposition of hydration products by DTA/TGA could be used to quantify the pozzolan reaction. A new method based on the composition of a hydrating cement was proposed and used to determine the degree of hydration of blended cements and the degree of pozzolan reaction. The results obtained suggested that the reactions of blended cements were slower than portland cement, and that silica fume reacted earlier than fly ash and slag.     

不同温度下矿渣对铝酸盐水泥早期水化行为的影响

通过凝结时间、抗压强度、电阻率、浆体内部温度测试和水化产物分析,研究了20 ℃、35 ℃和50 ℃下矿渣(GGBFS)对铝酸盐水泥(CAC)早期水化行为的影响。结果表明,掺入矿渣会逐渐减小CAC 72 h的化学收缩,降低化学收缩速率峰值。20 ℃时,电阻率变化曲线出现了明显的晶相转变期,化学收缩曲线存在明显的诱导期; 35 ℃时,凝结时间延长,掺入矿渣抑制了电阻率的发展;50 ℃时,电阻率在接近24 h时显著降低,凝结时间显著缩短,掺入矿渣缓解了24 h电阻率的减小。矿渣-铝酸盐水泥体系的水化产物和抗压强度受养护温度的影响较大。20 ℃时,掺入40%(质量分数)矿渣减少了CAH₁₀的生成量,降低了硬化浆体的强度;35 ℃和50 ℃时,1 d水化产物主要为C₂AH₈和少量C₃AH₆,掺入矿渣延缓了强度的倒缩。在28 d龄期时,不同养护温度下掺入矿渣均能促进C₂ASH₈的生成。