The effect of desiccation on the porosity and pore structure of freeze dried hardened portland cement and slag-blended pastes

Mercury porosimetry studies of hardened cement pastes of Portland cement and blast furnace slag blended cements have been conducted after freeze drying cured samples. The samples have been aged after freeze drying in a desiccating atmosphere so as to evaluate the effect of aging on the pore structure of freeze dried hardened Portland cement paste prepared with and without admixtures. Results indicate that after 24 hours of freeze drying the hardened paste shows negligible change in porosity and pore structure with aging in a desiccating atmosphere.     

Effects of elevated temperature on pumice based geopolymer composites

Abstract

Aluminosilicate type materials can be activated in alkaline environment and can produce geopolymer cements with low environmental impacts. Geopolymers are believed to provide good fire resistance so the effects of elevated temperatures on mechanical and microstructural properties of pumice based geopolymer were investigated in this study. Pumice based geopolymer was exposed to elevated temperatures of 100, 200, 300, 400, 500, 600, 700 and 800°C for 3?h. The residual strength of these specimens were determined after cooling at room temperature as well as ultrasonic pulse velocity, and the density of pumice based geopolymer pastes before and after exposing to high temperature was determined. Microstructures of these samples were investigated by Fourier transform infrared for all temperatures and SEM analyses for samples that were exposed to 200, 400, 600 and 800°C. Specimens, which were initially grey, turned whitish accompanied by the appearance of cracks as temperatures increased to 600 and 800°C. Consequently, compressive strength losses in geopolymer paste were increased with increasing temperature level. On the other hand, compressive strength of geopolymer paste was less affected by high temperature in comparison with the ordinary Portland cement. As a result of this study, it is concluded that pumice based geopolymer is useful in compressive strength losses exposed to elevated temperatures.     

The effect of w/c ratio and curing temperature on the permeability of hardened cement paste

The importance of concrete permeability is discussed relative to its effect on durability. Experimental studies were made of the effect of water/cement ratio and curing temperature on the porosity, pore size distribution and permeability of cement pastes. Although total porosities of samples cured at 60°C are smaller than in those cured at 27°C, the pore volume larger than 750Å radius is greater in the 60°C samples and is related to higher permeabilities also in the latter.     

Time-dependent behaviour of hardened cement paste under isotropic loading

The experimental results of isotropic compression tests performed at 20 °C and 90 °C on a class G hardened cement paste hydrated at 90 °C (Ghabezloo et al., 2008, Cem. Conc. Res. 38, 1424–1437) have been revisited considering time-dependent response. Within the frame of a viscoplastic model, the non-linear responses of the volumetric strains as observed in drained and undrained tests and of the pore pressure in undrained tests are analysed. The calibration of model parameters based on experimental data allows to study the effect of the test temperature on the viscous response of hardened cement paste showing that the creep is more pronounced for a higher test temperature. The effect of the hydration temperature on the time dependent behaviour is also studied by evaluating the model parameters for a cement paste hydrated at 60 °C. The time-dependent deformations are more pronounced for hydration at a higher temperature.     

Effect of Curing Conditions on the Capillary Porosity of Hardened Portland Cement Pastes

The capillary pore structure of hardened portland cement pastes cured by high-pressure steam, chemical acceleration, high-pressure steam with reactive SiO₂, water immersion, water immersion and high-pressure steam, and hot-pressing was measured using mercury porosimetry to 50,000 psi. Differences of > 2 orders of magnitude exist in the average capillary pore diameters of the cement pastes studied. The largest pores (∼1 to 3 μm in diameter) are associated with high-pressure steam-cured pastes. The smallest average capillaries observed were 0.02 μm for pastes steam-cured with reactive SiO₂. Hot-pressed pastes had essentially no porosity accessible to mercury. The application of pore size control to problems of polymer-impregnated concrete is discussed.     

Effect of curing and heat treatment history on the dynamic mechanical response and the pore structure of hardened cement paste

Results for the dynamic mechanical response of hcp (hardened cement paste) specimens as a function of the curing-heat treatment history are reported. The temperature range investigated is from ?160°C to +100°C. In the temperature range from +25°C to +100°C, specimens cured at room temperature (R-cured) show a partially irreversible transition (reduction) in E. There are two low temperature transitions: the “adsorbate transition” between ?160°C and ?6-°C, and the “capillary transition” between ?50°C and 0°C. Both of these transitions are significantly affected by the curing-heat treatment history. Furthermore, both the E-modulus and the BET (water) surface area decrease as the severity of heat treatment increases.     

Variations of water absorption rate and porosity with depth from an exposed concrete surface: Effects of exposure conditions and cement type

Small prisms of hardened cement paste were conditioned for 4 years within cavities in a uniaxially exposed, companion concrete cube. Four exposure conditions and three types of cement were used. The concretes and cement pastes were made with a water/cement ratio of 0.59 and were cured for three days prior to exposure. Water absorption rate, capillary porosity and gel porosity of the prisms were investigated as functions of the depth from the exposed concrete surface. The average water absorption rate and capillary porosity of the prisms in the 20 mm surface zone were generally greater than those of the underlying material. Carbonation reduced water absorption rates in prisms made with Portland cement due to associated reductions of capillary pore volume and continuity : carbonation increased the water absorption rate and capillary porosity of prisms made with OPC/fly ash or OPC/ground granulated blastfurance slag. When the capillary porosity reduces below about 0.2 the water absorption rate reduces markedly, suggesting that the pores are becoming less continuous.     

Strength development and pore size distribution of cement paste cured with high hydrostatic pressure

Compressive strength, pore size distribution and total pore volume of cement pastes cured with high hydrostatic pressure were examined. Compressive strength (S) of paste cured with high hydrostatic pressure (P) is shown in the following formula: S = a · logP + b. Compressive strength of paste having total pore volume (V) is calculated from the following:formula: S = c · V + S_o. When hardened paste cured at atmospheric pressure is placed in high hydrostatic pressure, the larger the strength is, the larger the lowering it becomes.     

The effect of undrained heating on a fluid-saturated hardened cement paste

The effect of undrained heating on volume change and induced pore pressure increase is an important point to properly understand the behaviour and evaluate the integrity of an oil well cement sheath submitted to rapid temperature changes. This thermal pressurization of the pore fluid is due to the discrepancy between the thermal expansion coefficients of the pore fluid and of the solid matrix. The equations governing the undrained thermo-hydro-mechanical response of a porous material are presented and the effect of undrained heating is studied experimentally for a saturated hardened cement paste. The measured value of the thermal pressurization coefficient is equal to 0.6 MPa/°C. The drained and undrained thermal expansion coefficients of the hardened cement paste are also measured in the heating tests. The anomalous thermal behaviour of cement paste pore fluid is back analysed from the results of the undrained heating test.     

The effect of the lowering of initial curing temperature on the strength of steam cured mortar

Studies were made of the compressive strength and bending strength of mortars of standard composition. The period of initial curing was 1, 3 or 6 hours and the temperature of steam curing was 65°C and 80°C. The initial curing temperature was 5° or 20° or lowered from 20° to 5°C. The strength of mortar cooled in the course of initial curing showed larger strength than the specimens cured at 20°C and 5°C. The strength increase is related with the ill-crystallized hydrates and the homogenous distribution of pore size in cement paste.     

Residual compressive behavior of alkali‐activated concrete exposed to elevated temperatures

This paper reports the effect of elevated temperature exposures, up to 1200^°C , on the residual compressive strengths of alkali‐activated slag concrete (AASC) activated by sodium silicate and hydrated lime; such temperatures can occur in a fire. The strength performance of AASC in the temperature range of 400–800^°C was similar to ordinary Portland cement concrete and blended slag cement concrete, despite the finding that the AASC did not contain Ca(OH)₂ , which contributes to the strength deterioration at elevated temperatures for Ordinary Portland Cement and blended slag cement concretes. Dilatometry studies showed that the alkali‐activated slag (AAS) paste had significantly higher thermal shrinkage than the other pastes while the basalt aggregate gradually expanded. This led to a higher thermal incompatibility between the AAS paste and aggregate compared with the other concretes. This is likely to be the governing factor behind the strength loss of AASC at elevated temperatures. Copyright © 2008 John Wiley & Sons, Ltd.     

CoFeMgAl-LDHs/CNTs复合材料对水泥水化及微观结构的影响

通过进行水化热测试、非蒸发水含量测试、X射线衍射分析、扫描电镜分析、压汞测试及强度试验,研究了钴铁镁铝水滑石-碳纳米管复合材料(CoFeMgAl-LDHs/CNTs)对水泥水化过程、硬化水泥净浆孔结构及强度的影响。结果表明,CoFeMgAl- LDHs/CNTs复合材料通过成核作用显著提升了水泥3 d内的水化程度,从而显著提高了水泥净浆3 d内的抗压强度,3 d后强度提高效果逐渐减小,7 d后复合材料对硬化水泥浆体强度没有明显的提高作用。掺入CoFeMgAl-LDHs/CNTs复合材料的水泥净浆与空白净浆相比,胶凝孔和毛细孔含量明显增多,大孔含量有所降低,同时复合材料的桥联作用进一步优化了水泥净浆的微观结构,从而提高了水泥基体薄弱部位的应力承载能力。因此在同一龄期,复合材料掺入后硬化水泥净浆的强度有所增大。由于复合材料掺量的变化对孔径分布没有明显的影响,改变复合材料的掺量对同龄期硬化水泥净浆强度提升影响较小。     

Water sorption properties of hardened cement paste cured or stored at elevated temperatures

Structural changes take place when mature hardened cement paste cured at room temperature is heated. BET V_m values were found to decrease as a function of the time the specimens had been stored in water either at 60°C or at 100°C. The decrease is greatest at 100°C. The complete water adsorption isotherms show that heating also makes the pore structure more coarse. The development of BET surface areas for pastes cured near 100°C depend strongly on the temperature history before the curing temperature is reached.     

Recycled granulate obtained from waste alumina-rich refractory powder by the cold bonding process

Within the scope of the present study the cold-bonding process was used for the recycling of waste filter powder which was mixed with two different binders in different concentrations; alumino-silicate cement and potassium water glass, and combinations of these two materials, and hardened at room temperature. Selected samples were also fired at 1200 °C. Tests to determine tensile and compressive strength, density and porosity, as well as dilatometry and SEM analyses, were performed. As expected, compressive strength increased as a function of the concentration of the potassium water glass. When combinations of both binders were used, compressive strengths were higher, but a significant increase in strength was also achieved by firing the samples. The compressive strengths of the non-fired samples were in the range from 0.8 to 2.4 MPa, whereas after firing strengths of up to 36 MPa were obtained. During the firing density increased, and porosity was reduced, while the average pore size increased. The results of dilatometric analysis showed that the granulate produced with cement shrink upon firing up to 300 °C, but then start slowly to expand, whereas the granulate produced by water glass first expanded on firing up to 800 °C, and then began to shrink swiftly. In the case of combinations of the two binders, shrinkage as well as expansion on firing was less pronounced. Selected granulate prepared using potassium water glass were also tested in a refractory concrete matrix in order to verify their usability.     

Cost‐effective porous ceramic tubes fabricated through a phase inversion/casting process using calcined bauxite

Cost‐effective ceramic tubes based on low‐price commercial calcined bauxite for economical separation were fabricated by a new phase‐inversion casting method. The thermal shrinkage and weight loss during heating of the green tubes were characterized by dilatometric analysis and TG, respectively. Three shrinkage stages appear successively, corresponding to the viscous deformation of polymeric binder at 200‐300°C, significant combustion loss of ~5.2 wt% at 500‐620°C and sintering shrinkage over 800°C, respectively. However, due to high enough viscosity of the casting suspension that can guarantee the green tube against collapse or deformation during the phase inversion/casting process, the sintered tubes display nearly uniform microstructure instead of characteristic asymmetrical structure of the phase inversion process. The influence of sintering temperature on the pore property (including pore size and porosity) and mechanical strength was investigated. As the sintering temperature increases from 1200 to 1400°C, the porosity and average pore size decrease from 46.4% to 37.0% and from 0.98 to 0.81 μm, respectively, and the flexural strength increases from 25.8 to 65.1 MPa. The cost‐effective ceramic tube sintering at the range of 1250‐1400°C can be capable of functioning as a microfiltration membrane or an ultrafiltration membrane support.     

Ice formation in hardened cement paste,Part I — room temperature cured pastes with variable moisture contents

Ice formation in room temperature cured mature hardened Portland cement paste with different moisture contents have been measured. The measurements were carried out continuoslly in the temperature range from +20° C to −60° C.For moisture contents higher than that corresponding to approximately three equivalent BET-monolayers, the initial freezing temperature increases with increasing moisture contents. Moisture contents below approximately three monolayers is not freezable. For specimens containing freezable water, there is an increase in the amount of non-frozen water content with increasing moisture contents.Further, there is a slight increase in the amount of non-frozen water with increasing water/cement ratios for specimens in equilibrium with the same relative water vapor pressure.It is postulated that frost problems only can be expected in specimens containing more water than corresponding to a relative water vapor pressure of about 0.9.     

Porous cenosphere ceramic microfiltration membrane: Preparation,characterization, permeability evaluation,and cost estimation

Ceramic membranes are now receiving greater attention and are regarded as the best alternative option for reducing energy use. There are currently a number of limitations on the use of ceramic membranes, including high raw material costs and high sintering temperatures during synthesis. Cost-effective raw materials were employed in the synthesis of ceramic membranes to get around these restrictions. Utilizing a straightforward pressing technique, circular disc-type membranes were prepared. To assess the membrane properties, the impact of sintering temperatures between 700 and 900°C was examined. By varying the sintering temperature, the average membrane pore diameter was observed. The membrane, which was sintered at 800°C, had pores that were on average 110 nm in size. Furthermore, the porosity of these synthesized membranes ranged from 22% to 35% with an average pore diameter of 74–121 nm. These manufactured membranes showed very good chemical stability when both acidic and basic solutions were used. Various characterization methods, including thermogravimetric analysis (TGA), x-ray diffraction analysis (XRD), and scanning electron microscopy (SEM), were used. Thermo-gravimetric investigation revealed that the synthesized cenosphere membrane required a minimum sintering temperature of about 700°C. The flux measured with deionized water and the applied transmembrane pressure showed an upward trend. The impact of sintering temperature on permeate flux was investigated. The results showed that as the sintering temperature increased from 700°C to 900°C, the flux reduced. It was determined that the synthesized membrane cost ₹1618.80/m².     

粉煤灰-水泥浆体的孔体积分形维数及其与孔结构和强度的关系

采用压汞法对不同龄期粉煤灰-水泥浆体的孔分形结构进行了实验研究，测定了复合浆体孔体积分形维数，探讨了孔体积分形维数与孔隙率，孔表面积、平均孔径、孔分布及宏观力学性能的关系。实验结果表明：粉煤灰-水泥浆体的孔结构具有明显的分形特征，孔体积分形维数在3．3～3．5之间；孔体积分形维数越大，浆体的孔径越小、孔隙率越低，孔表面积越大，小于20nm的微孔越多，大于100nm的大孔越少，而且复合体系的抗压及抗折强度也越高。     

Properties of hardened highly refractory basic cements as a function of in relation to their phase composition

The phase, composition of the previously prepared three hardened highly refractory basic cements (dolomitic magnesite, periclase and dolomitic magnesite chrome) was established by the aid of d.t.a., t.g.a., microscopic examination, electron microscopy and X-ray analysis. The following properties of these cements were determined: compressive strength, apparent porosity, linear shrinkage and ignition loss. These parameters were measured for each cement after 3 days air curing at ～ 60% r.h. and after heating these cements to 110°, 450°, 600°, 800° and 1200°C. The variation in these parameters as a function of temperature was correlated in the light of their component phases.     

Temperature influence on water transport in hardened cement pastes

Describing water transport in concrete is an important issue for the durability assessment of radioactive waste management reinforced concrete structures. Due to the waste thermal output such structures would be submitted to moderate temperatures (up to 80 °C). We have then studied the influence of temperature on water transport within hardened cement pastes of four different formulations. Using a simplified approach (describing only the permeation of liquid water) we characterized the properties needed to describe water transport (up to 80 °C) using dedicated experiments. For each hardened cement paste the results are presented and discussed.