The effects of carbonation and drying during intermittent leaching on the release of inorganic constituents from a cement-based matrix

A Portland cement mortar was submitted to cycles of intermittent wetting (IW) in which tank leaching was interspersed with periods of storage in either an inert or a reactive atmosphere. Relative humidity (RH) (23%, 48% and 98%) was maintained during storage to control the drying process. The effects of IW were qualified by comparing flux and cumulative release of matrix constituents (Ca, OH, Na, K and Cl) to that of continuous water saturation. The carbonation process was associated with the degree of drying occurring due to storage. Cumulative release of most major constituents was suppressed in samples storage under 100% CO₂ in comparison to the inert atmosphere (100% N₂). Results suggest that accurate long-term performance assessment must account for the potential impact of phenomena associated with IW.     

二氧化碳养护混凝土活性组分固碳率评价方法

CO₂养护混凝土技术作为一种新型的CO₂利用技术,既能实现工业固废的资源化利用,又能实现温室气体的永久封存,近年来受到越来越多的关注。由于混凝土原料组分复杂、养护工艺路线多样,关键指标固碳率的评价方法也各不相同,因此亟需形成适用于不同工业固废混凝土产品体系的固碳率通用评价新方法。在对已有诸多评价方法进行比较后,本文提出一种适用性和可操作性较好的增重法-烘至绝干法的固碳量评价新方法和基于修正后活性组分质量的折算基准,在Steinour公式基础上,通过对工业固废中常见的钙、镁组分中惰性部分进一步辨别、扣除,可以直观地反映原料中活性组分的有效反应程度。水泥净浆体系实验表明,烘至绝干法可以更合理地补偿水蒸发带来的误差,适用于工业规模的固碳率评价。多种固废混凝土体系实验表明,以修正后的活性组分质量为基准可以体现试件中活性组分的碳酸化程度,排除了试件中含水率和惰性组分的影响。最后,利用本文提出的评价方法,对河南焦作CO₂养护混凝土工业单釜试验产品的固碳率进行评价,为CO₂养护技术的指标评价提供了更完善的科学依据。     

Microstructure of glass fibre reinforced supersulphated cement

The microstructure of one and two year old grc made from Cem-FIL alkali-resistant glass fibres and supersulphated cement has been examined using the scanning electron microscope. The structure and composition of two year old samples of supersulphated cement hydrated in a CO₂ free atmosphere is also reported. Differences in the microstructure of these cement samples and the matrix phase of the fibre composites are related to the carbonation of the composite specimens.     

Influence of initial curing on sulphate resistance of blended cement concrete

The paper presents results of an investigation on the effect of initial curing conditions on the sulphate resistance of concrete made with ordinary portland cement and using pfa, silica fume and ground granulated blast furnace slag for partial replacement of cement. In addition, porosity and pore structure analysis of representative pastes was carried out to examine the relationship between these properties and sulphate resistance of concrete. The depth of carbonation in specimens of pastes was also determined.

Three different initial curing conditions immediately after casting of specimens were adopted, namely: WET/AIR CURED at 45°C, 25% RH; AIR CURED at 45°C, 25% RH; AIR CURED at 20°C, 55% RH. The results show that pore volume and pore structure of the paste bear no direct relationship with the sulphate resistance of concrete. The presence of a carbonated layer on the surface is generally accompanied by superior sulphate resistance—there are, however, important exceptions. Low humidity curing at high temperature (45°C) results in higher depths of carbonation but lower sulphate resistance than similar curing at 20°C.

The sulphate resistance of concrete increases with the replacement of cement with 22% pfa, 9% silica fume and 80% ggb slag. The sulphate resistance also increases due to drying out of concrete during early curing at low relative humidity and due to carbonation. The possible common factor which leads to this improved sulphate resistance is the reduced Ca(OH)₂ content which leads to smaller volume of the expansive reaction products with sulphate ions. The effect of initial curing at high temperature (45°C) is significantly harmful to the sulphate resistance of plain concrete but much less so to the blended cement concretes.     

Strength and shrinkage properties of mortar containing a nonstandard high-calcium fly ash

A laboratory study was undertaken to assess the compressive and flexural tensile strength and drying shrinkage properties of mortar mixtures containing high-calcium nonstandard Afsin-Elbistan fly ash (FA). Possibility of using Afsin-Elbistan FA in cement-based materials as shrinkage-reducing or compensation agent was also discussed. Five mortar mixtures including control Portland cement (PC) and FA mortar mixtures were prepared. FA replaced the cement on mass basis at the replacement ratios of 10%, 20%, 30% and 40%. Water-cementitious materials ratio was 0.4 for all mixtures. The mixtures were cured at 65% relative humidity and 20±2 °C. The compressive and flexural tensile strength and drying shrinkage values of the mortar mixtures were measured. The results show that Afsin-Elbistan FA reduced drying shrinkage of the mortar by 40%. Therefore, it was concluded that Afsin-Elbistan FA can be used as a shrinkage-reducing agent. The mortar containing 40% FA expanded. This indicates that Afsin-Elbistan FA may be utilized to compensate drying shrinkage of cement-based materials.     

CO2 capture by double metal modified CaO-based sorbents from pyrolysis gases

High-temperature pyrolysis technology can effectively solve the problem of municipal solid waste pollution. However, the pyrolysis gas contains a large amount of CO₂, which would adversely affect the subsequent utilization. To address this problem, a novel method of co-precipitation modification with Ca, Mg and Zr metals was proposed to improve the CO₂ capture performance. X-ray diffraction (XRD) patterns and energy dispersive X-ray spectroscopy analysis showed that the two inert supports MgO and CaZrO₃ were uniformly distributed in the modified calcium-based sorbents. In addition, the XRD results indicated that CaZrO₃ was produced by the reaction of ZrO₂ and CaO at high temperatures. The effects of doping ratios, adsorption temperature, calcination temperature, CO₂ concentration and calcination atmosphere on the adsorption capacity and cycle stability of the modified calcium-based sorbent were studied. The modified calcium-based sorbent achieved the best CO₂ capture performance when the doping ratio was 10:1:1 with carbonation at 700 ℃ under 20% CO₂/80% N₂ atmosphere and calcination at 900 ℃ under 100% N₂ atmosphere. After ten cycles, the average carbonation conversion rate of Ca-10 sorbent was 72%. Finally, the modified calcium-based sorbents successfully reduced the CO₂ concentration of the pyrolysis gas from 37% to 5%.     

粉煤灰水泥石碳化性能的化学分析

开发了一种自制测量装置。利用该装置,沿碳化深度方向,分层测定了碳化粉煤灰水泥石中CaCO3的含量。绘制了＂CaCO3含量–深度＂曲线。根据曲线特点,定义了完全碳化区、部分碳化区、未碳化区等概念和碳化程度、碳化速度等特征指标。研究了粉煤灰掺量、水胶比、龄期、养护条件、碳化前烘干处理、碳化时间、碳化湿度等对粉煤灰水泥石碳化性能的影响。结果表明：粉煤灰掺量越大,水胶比越高,龄期越短,养护环境越干燥,碳化前试件越干燥,粉煤灰水泥石的抗碳化性能越差;碳化时间越长,碳化湿度越低,粉煤灰水泥石的碳化程度越高;但碳化速度随碳化时间的增长而减慢。这些结论与文献报道的用其他方法得到的公认的结论一致。本方法还得到了更加深入的研究结果,即：粉煤灰掺量越大、粉煤灰水泥石中可碳化物质的相对含量越低;水胶比、龄期、养护条件、碳化时间、碳化湿度基本不改变粉煤灰水泥石中可碳化物质的相对含量;龄期、养护条件可改变未碳化粉煤灰水泥石中碳酸盐的相对含量。     

Carbonation and porosity of mortar specimens with pozzolanic and hydraulic cement admixtures

The effect of the addition of lignite fly ash, natural greek pozzolan and blastfurnace slag to ordinary portland cement on the carbonation of mortar specimens was studied in a programme of long-term exposure to the atmosphere in relation to the evolution of their porous structure. Additions of 4%, 15% and 30% or 50% of the admixtures to the cement were tested. The carbonation depth and the specific pore volume of the cover were measured. The additions of 4% pozzolan, and 4% and 15% fly ash, and 4% and 15% slag did not influence the porosity and the carbonation of the specimens compared to reference specimens without admixtures. Higher additions resulted in an increase of the above parameters.     

Mechanical and physical properties of flyash concrete after hydrothermal storage

In order to study the effect of fly-ash addition on the hydrothermal reaction in concrete an experimental program was conducted. Specimens of normal weight concrete made with ordinary Portland cement and fly-ash were exposed to 95 °C and relative humidities (RH) in between 95 % and 100 %. After storing the samples for 3, 6 and 12 months the following residual properties were studied: compressive strength, splitting tensile strength, content of calcium hydroxide, pH-value of concrete suspensions and carbonation depth. Alterations of the concrete microstructure were investigated by mercury porosimetry, DTA and X-ray technique. The results indicated that hydrothermal reactions proceed incomplete if environmental humidities are below 100 % RH. Because most of the pores dry out the liquid phase reactions cannot progress any more. However, during the drying process the hydrothermal reactions can take place to a considerable extent, if fly-ash is added to the concrete. At 100 % RH also concretes made without fly-ash indicate a nearly complete consumption of the free Ca(OH)₂. However, in the course of the tests none of the investigated specimens showed pH-values below the critical value of 9.5.     

新旧水泥砂浆界面粘结性能试验研究

在荷载和环境因素作用下,混凝土结构产生不同程度的劣化。为了保证结构的安全性和耐久性,需要对损伤水泥基材料进行修复。基体的含水饱和度、界面粗糙度、修补砂浆的水灰比以及试件的养护条件都会影响修补砂浆与基体间的粘结强度。选取四种含水饱和度(0%、30%、70%、100%)的旧砂浆作为基体,浇筑水灰比为0.4和0.6的新砂浆,试件密封养护28 d,剪切试验结果表明:当新砂浆水灰比为0.6,旧砂浆含水饱和度按照70%、30%、100%、0%的顺序变化时,界面的剪切强度逐渐减小;当新砂浆水灰比为0.4,旧砂浆含水饱和度按照30%、0%、70%、100%的顺序变化时,界面的剪切强度逐渐减小。同时发现,新砂浆水灰比为0.4时的界面剪切强度普遍大于水灰比为0.6的数值。通过切槽法改变旧砂浆的界面粗糙度,然后浇筑水灰比为0.6的新砂浆,试件标准养护。剪切试验结果表明:当旧砂浆界面粗糙时,界面间的剪切强度是旧砂浆光滑时的1.26倍。选取两种含水饱和度(0%、100%)的旧砂浆作为基体,浇筑水灰比为0.4和0.6的新砂浆,分别进行标准养护和密封养护,剪切试验结果表明:在旧砂浆含水饱和度和新砂浆水灰比相同的情况下,标准养护下的界面剪切强度明显大于密封养护下的界面剪切强度。     

Investigation of moisture transport properties of cementitious materials

This paper is focused on two ways to enhance our understanding of moisture transport properties of cementitious materials. The first one utilizes a multi-phasic moisture transport model to adjust the permeability through the fitting of measured mass loss curves during a drying experiment. In the second one, measured apparent diffusion coefficient curves vs. relative humidity (RH) are fitted by a general expression which includes both liquid transport and vapour diffusion. Due to different RH ranges of predominance of liquid and vapour transport, the permeability and vapour diffusion coefficient can be determined separately. Three cement pastes, whose properties are collected from the literature, are analysed by means of those two methods. Discussions on relative permeability and Knudsen diffusion show a significant influence on the modelling of moisture transport. A further comparison with measured permeability data found in the literature is able to provide a deeper understanding of moisture transport properties.     

Forced and natural carbonation of lime-based mortars with and without additives: Mineralogical and textural changes

We have studied the carbonation process in different types of mortars, with and without pozzolana or air-entraining additives, subject to a CO₂-rich atmosphere and compared the results with those of similar naturally carbonated mortars. We used X-ray diffraction technique to demonstrate that high CO₂ concentrations favour a faster, more complete carbonation process with 8 days being sufficient to convert portlandite into 90 wt.% calcite. Full carbonation, however, is not reached during the life-span of the tests, not even in forced carbonation experiments. This could be due to at least one of the following phenomena: a premature drying of samples during carbonation reaction, the temperature at which the carbonation process was carried out or the reduction of pore volume occupied by newly formed calcite crystals. This last option seems to be the least probable. We observed a more prolific development of calcite crystals in the pores and fissures through which the carbonic anhydride flows. Under natural conditions, carbonation is much slower and similar levels are not reached for 6 months. These differences suggest that the carbonation process is influenced by the amount of CO₂ used.

Both the mineralogy and texture of mortars vary depending on the type of additive used but the speed of the portlandite–calcite transformation does not change significantly. Pozzolana produces hydraulic mortars although the quantity of calcium aluminosilicate crystals is low. The air-entraining agent significantly alters the texture of the mortars creating rounded pores and eliminating or reducing the drying cracks.     

Petrographical analysis of calcium aluminate cement mortars: Scanning electron microscopy and transmitted light microscopy

The samples studied for this paper were obtained from experimentally induced carbonation and alkaline hydrolysis of calcium aluminate cement (CAC) mortars. The mineralogical composition of phases present was studied from X-ray diffraction (XRD) spectra. Scanning electron microscopy (SEM) and transmitted light microscopy (TLM) were used to obtain some morphological (size and shape) and fabric data of mineralogical phases to improve the interpretation of damage processes. SEM is a useful tool to examine the evolution of mineral phases in cavities and discontinuity zones. The interpretation of the degree of carbonation and the distribution of carbonated phases in the samples, the presence of C₃AH₆ and CAH₁₀, as well as possible alterations of aggregates are well differentiated and defined using TLM method. The main objective of the present paper is to validate the use of both microscopic techniques (TLM and SEM) for the study of phase evolution in the CAC mortar during curing and degenerative processes.     

Effect of humidity and porosity on fracture of hardened portland cement

The dependence of fracture mechanics terms (γ_T, K_c, G_c and J_c) on relative humidity and porosity is determined for hardened portland cement paste (HCP). The cardinal features of the fracture term/humidity curves and fracture term/porosity curves are discussed. Large hysteresis in fracture term/relative humidity curves is observed, the magnitude depending on the drying history of HCP. Meniscus effects, aging and moisture-assisted stress corrosion processes are thought to contribute to fracture of HCP. Unhydrated portland cement, Ca(OH)₂ particles, and pores with specific size distributions in the HCP system possibly act as crack arrestors.     

Influence of Grain Size and Humidity on the Nanotribological Properties of Wear-Resistant Nanostructured ZrO2 Coatings: An Atomic Force Microscopy Study

The nanotribological properties of nanostructured thin films of tetragonal ZrO₂ on oxidized Si (100) were studied as a function of grain size and relative humidity (RH) by atomic force microscopy. The nanostructured ZrO₂ showed a 50% decrease in friction coefficient μ compared with oxidized Si (100) in dry nitrogen atmosphere and 40% RH. A maximum of μ was observed at ca. 40% RH for both samples, while there were insignificant differences in μ for ZrO₂ samples with grain sizes between 12 and 30 nm. The good tribological properties open the possibility for nanostructured zirconia to be applied as wear-resistant, low friction coatings on various materials.     

Study of stabilized phases in high alumina cement mortars Part I Hydration at elevated temperatures followed by carbonation

The use of temperatures higher than 30°C during the hydration of 1:3 high alumina cement mortar leads to the formation of the cubic hydrate C₃AH₆, hence avoiding the conversion of CAH₁₀ to C₃AH₆. The subsequent carbonation of C₃AH₆ with CO₂ and thermal treatment leads to the formation of stable carbonated phases. The optimum hydration ages are the minimum necessary times to get C₃AH₆: 3 hours at 80°C, 6 hours at 60°C. The product is in a state we call “semiplastic” in which the next structural transformation - carbonation - occurs more easily than when the specimens have a rigid structure. The effect of a super-water-reducing admixture has also been studied.     

Moisture diffusion of concrete considering self-desiccation at early ages

In concrete structures exposed to the ambient air at early ages, the moisture content in concrete decreases due to moisture diffusion. In addition, self-desiccation due to hydration of cement causes an additional decrease of moisture content in concrete at early ages, especially for high-strength concrete. In this study, the internal relative humidity in drying concrete specimens was measured at early ages. Furthermore, the variation of relative humidity due to self-desiccation in sealed specimen was measured. The moisture distribution in low-strength concrete with high water/cement ratio was mostly influenced by moisture diffusion due to drying rather than self-desiccation. In high-strength concrete with low water/cement ratio, however, self-desiccation had a considerable influence on moisture distribution. The results obtained from the moisture diffusion theory were in good agreement with experimental results.     

胡萝卜对流干燥过程理论分析——临界水分质量比

收缩和非收缩条件下,对胡萝卜对流干燥特征进行了理论分析。发现临界水分质量比是试样几何形状、尺寸大小以及热空气温度、速度、相对湿度的函数。在收缩条件下,只有薄片状试样对流干燥过程包含恒速干燥阶段和降速干燥阶段;而圆柱状和球状试样干燥速率从干燥起始就随试样水分质量比的下降降低。数值实验结果表明,根据物料临界水分质量比很难确定对流干燥过程处于外部干燥条件控制的第1干燥阶段还是物料内部条件因素控制的第2干燥阶段。临界水分质量比不是表明物料干燥特性的特征参数。