蒸压加气混凝土的孔结构及表征方法研究进展

蒸压加气混凝土因其质轻、保温性好、环保等优点而受到人们的重视。作为一种典型的宏观多孔建筑材料,蒸压加气混凝土的孔结构特征与其微观结构和性能有密切的关系。钙硅比、水料比、铝粉及工艺参数是影响蒸压加气混凝土孔结构的重要因素,孔隙率、孔径分布等孔结构特征与蒸压加气混凝土的强度、吸水性、干燥收缩、导热性能及耐久性等性能有紧密的联系。本文重点阐述了蒸压加气混凝土的孔结构特征及影响孔结构的主要因素,孔结构对蒸压加气混凝土性能的影响,并简述了蒸压加气混凝土孔结构现阶段主要采用的表征方法。     

冲击载荷作用下蒸压加气混凝土动态力学性能研究

为了研究蒸压加气混凝土(AAC)在冲击载荷作用下的动态力学性能,对密度为425 kg/m~3和625 kg/m~3的试件进行了准静态力学测试和分离式Hopkinson压杆(SHPB)试验。针对蒸压加气混凝土这类低阻抗多孔介质材料试件内部应力不均匀和透射信号难采集的问题,采用波形整形器和半导体应变片对SHPB装置进行了改进,保证了试验的有效性。试验得到了不同应变速率下的力学参数,并在此基础上研究了蒸压加气混凝土的强度性能。结果表明:蒸压加气混凝土的抗压强度随应变速率和密度的增加而增大;蒸压加气混凝土的动态强度增长因子与平均应变率的自然对数成线性关系;在冲压状态下蒸压加气混凝土存在应变率敏感阈值,当应变率超过这个值时,试件的抗压强度将明显增加;随着平均应变率和密度的增加,蒸压加气混凝土的冲击韧性不断增加且冲击韧性指标与平均应变率的对数呈线性关系。     

多孔介质湿物理性质预测方法综述

多孔建筑材料的湿物理性质是分析建筑围护结构热湿传递的重要基本参数。自20世纪80年代开始,欧美发达国家已针对常见建筑材料的湿物理性质开展多个大型测试项目,建立了涵盖上百种多孔建筑材料湿物理性质的基础数据库。我国学者从20世纪60年代开始研究多孔建筑材料的湿物理性质,至今已有一定积累,但在湿物理性质研究的广度和深度上尚需进一步探索;近年来,我国许多学者积极开展多孔建筑材料湿物理性质的研究工作,并取得明显进展。由于原材料及工艺等方面的差别,国际上的湿物理性质数据库在我国并非完全适用。因此,针对我国常见建筑材料,需要建立自主、完备、可靠的数据库。为获得材料的湿物理性质,最常用的是实验测试方法,但部分测试方法存在耗时长、操作复杂、仪器昂贵等问题。例如,等温吸放湿曲线可能需要数月甚至数年才能完成测试,压力平板实验操作流程复杂且对压力控制精度要求较高,而用于液态水扩散系数测试的X射线仪或核磁共振仪等设备价格较高,不利于常规实验室普及。多孔建筑材料与土壤、纤维、岩石、食品等都属于多孔介质,并且湿分的储存和传递性质都是其研究的关键参数。例如,保水曲线是研究旱地土壤的必要指标,湿扩散系数是评价织物热舒适...     

多壁碳纳米管增强混凝土的抗氯离子渗透性能

采用快速氯离子迁移系数(RCM)法和自然浸泡法对多壁碳纳米管(MWCNTs)增强混凝土的抗氯离子渗透性能进行研究,测量混凝土试件纵向断面上的氯离子扩散深度,据此计算氯离子扩散系数。试验结果表明：当MWCNTs掺量为0.15wt%时,混凝土28天的氯离子扩散深度、氯离子扩散系数分别降低了25.7%、19.1%;在4种不同侵蚀龄期的自然浸泡下,掺入MWCNTs的混凝土,内部氯离子浓度始终低于对照组。结合两种方法分析得出：混凝土内部各深度的自由氯离子浓度随着MWCNTs掺量的增加而降低,致使氯离子扩散系数随着MWCNTs掺量的增加而变小,MWCNTs的掺入提高了混凝土的抗氯离子渗透性能。此外,通过SEM和压汞(MIP)测试进一步探究MWCNTs对混凝土抗氯离子渗透性能的微观增强机制,分析结果表明,MWCNTs具有一定的桥接和填充效应,这可能使混凝土裂缝扩展受到抑制、孔隙更加细化,从而改善混凝土的微观结构,提高混凝土的抗氯离子渗透性能。     

加气混凝土用铝粉的应用与制备

用干式球磨法生产加气混凝土用铝粉。根据加气混凝土的生产工艺,确定其对铝粉的性能要求。在加气混凝土用铝粉的生产工艺中,通过调整原料铝粉的粒度、助磨剂加入量和磨内滞留时间的工艺参数来控制铝粉的性能。通过实践及数据分析,确定最佳工艺参数:原料铝粉粒度d97=630￣280μm;助磨剂加入量在3.0%;铝粉磨内滞留时间为16.6￣19h。     

钙硅比对水热合成镍矿渣加气混凝土性能的影响

废弃的镍矿渣与水泥、石灰等按一定比例混掺,通过水热合成的方法可制备出镍矿渣加气混凝土。研究了钙硅比对镍矿渣加气混凝土力学性能的影响及其机理,对镍矿渣加气混凝土进行了XRD、FT-IR、SEM微观测试。结果表明:随着钙硅比的降低,镍矿渣加气混凝土的干密度逐渐增加,强度先增大后减小,钙硅比为0.65时达到峰值;托勃莫来石和硬石膏的X射线衍射峰逐渐增强;红外图谱中硅氧四面体及含硅基团的聚合程度增加;水化产物的形貌由针棒状结构向片状结构过渡。     

建筑材料颗粒粒径及填装密度对γ能谱仪测量放射性核素的影响

为了探讨不同粒径和填装密度对γ能谱仪测量建筑材料放射性核素比活度的影响,本文选用蒸压加气混凝土砌块产品作为研究对象,筛分至不同粒径级别和选取粒径级别小于0.048mm的试样以两种密度填装后参照GB 6566-2010《建筑材料放射性核素限量》标准测量镭-226、钍-232和钾-40三种放射性核素的比活度,结果表明:不同粒径级别和两种填装密度对镭钍钾三种放射性核素比活度的测量值影响较小。     

聚酰亚胺渗透汽化膜的研究(Ⅲ)水蒸气小分子在PI膜中的传质行为

以2种二酐、2种二胺为单体,制备了4种不同结构的聚酰亚胺(PI)均质膜;用建立的动态吸附／脱附实验装置测定水蒸气在PI膜中的吸附／脱附行为;考察了不同结构的PI膜对水蒸气吸附／脱附的影响;计算了反常扩散指数;研究了温度对水蒸气在聚酰亚胺膜(PI)中扩散系数的影响．分析了扩散系数与温度和聚合物重复单元的摩尔自由体积之间的关系．研究结果表明:水蒸气小分子在PI膜中的扩散系数随着温度增加而增加,随着聚合物重复单元摩尔自由体积的增加,扩散系数也呈上升的趋势．     

COMSOL的加气混凝土热湿耦合传质模拟分析

采用COMSOL Multiphysics软件进行多孔介质加气混凝土的热湿耦合数值模拟研究.采用双驱动势和优化参数结合的模拟方案,对控制方程进行推导.通过对文献结果的对比,验证了该模型的可行性.模拟分析了环境温度、时间等因素对加气混凝土样品的湿迁移影响,随时间的增加湿传递速率减缓;5~15℃时湿传递速率缓慢,温度的影响较大,15~30℃时湿传递加快且速率保持一致.因此,在干燥环境下,加气混凝土墙体温度低于15℃以下可有效减少外界湿传递.     

高性能低密度加气混凝土的研究

研究了高性能低密度加气混凝土的凝结时间、抗压强度、干密度、吸水率、导热系数。实验结果表明,掺入7.5%的硫铝酸盐水泥和0.03%的碳酸锂,可以显著缩短加气混凝土料浆的凝结时间,水温也可调节料浆凝结时间;高锰酸钾可以提高双氧水发泡量;同密度等级的加气混凝土,在低水灰比条件下抗压强度明显较高,导热系数和吸水率较低。     

Determination of moisture content of concrete by microwave-resonance method

The moisture content of concrete is one of its important properties. A new method of measuring the moisture content of concrete and other building materials is described. The method is based on the measurement of the dielectric constant at microwave frequencies. Studies on concrete have shown that a linear relation exists between the evaporable water content of concrete and the instrument reading. The main advantages of this method are i) it is nondestructive, ii) rapid response of the instrument, iii) humidity gradients in the specimen can be studied iv) measurements are independent of the dielectric losses and the conductivity of the material, v) sharp resonance peak of the microwaves allows very accurate measurements.     

Time lag in measuring pore humidity in concrete by a gage in finite cavity

Measurements of the relative humidity of water vapor in concrete are important for understanding, predicting and controlling the shrinkage and creep of concrete, as well as other processes such as the ASR. A sufficient time is required for the pore water to diffuse into (or out of) the gage cavity. To determine this time, the nonlinear partial differential equation of diffusion of moisture through concrete is solved numerically. The cavity is simplified as spherical and its volume is considered to be far smaller than the volume of surrounding concrete, which permits assuming spherical symmetry of the pore humidity field. The body of the surrounding concrete is assumed to be so large that the water escape into (or intake from) the cavity does not change the pore humidity in concrete appreciably. For the current measurement practice, the humidity difference between the concrete pores and the cavity is found to drop below the inevitable error of the gage after the lapse of about 24 h, and an acceptable error of about 1% is achieved in about 10 h. It is shown that extrapolation by a decaying power function of time can shorten this time to 2 h. A further shortening could be achieved by reducing the volume of the cavity, although any contact of the gage with the concrete must be avoided.     

Nonlinear Effect of Moisture Content on Effective Thermal Conductivity of Building Materials with Different Pore Size Distributions

Understanding the quantitative relationship between the effective thermal conductivity and the moisture content of a material is required to accurately calculate the envelope heat and mass transfer and, subsequently, the building energy consumption. We experimentally analyzed the pore size distributions and porosities of common building materials and the influence of the moisture content on the effective thermal conductivity of building materials. We determined the quantitative relationship between the effective thermal conductivity and moisture content of building materials. The results showed that a larger porosity led to a more significant effect of the moisture content on the effective thermal conductivity. When the volumetric moisture content reached 10 %, the thermal conductivities of foam concrete and aerated concrete increased by approximately 200 % and 100 %, respectively. The effective thermal conductivity increased rapidly in the low moisture content range and increased slowly in the high moisture content range. The effective thermal conductivity is related to the moisture content of the materials through an approximate power function. As the moisture content in the walls of a new building stabilizes, the effective thermal conductivity of normal concrete varies only slightly, whereas that of aerated concrete varies more significantly. The effective thermal conductivity of the material is proportional to the relative humidity of the environment. This trend is most noticeable when the wall material is aerated concrete.     

An approach to determine diffusivity in hardening concrete based on measured humidity profiles

A novel method to numerically determine the diffusion coefficient of hardening concrete from measured pore relative humidity profiles is presented, which is based upon an inverse nonlinear diffusion analysis. A system for nondestructively measuring the development of pore relative humidity required for this analysis in a uniaxially drying concrete specimen is described. The diffusion equation is solved by the method of lines, with collocation in the space variable to reduce the problem to a system of ordinary differential equations. Based on the solution of the diffusion equation, an auxiliary formulation is derived by searching the calibration factor in diffusivity that minimizes the residual between the calculated relative humidity and the measured data at each measured point in each small time interval. A working algorithm and numerical results are presented in this paper.     

基于图像法分析孔隙特征对加气充填砼性能的影响

通过对不同孔隙特征的加气充填混凝土试样沿发气方向进行切割和截面图像提取,采用Matlab对试样截面图像进行分析和统计计算,分别对各组试样的孔隙特征进行了表征;测试了试样各养护龄期的比强度、吸水率、体积膨胀率、冻融后的抗压强度损失率和质量损失率,以分析孔隙特征对加气充填混凝土力学性能和抗冻性的影响。结果表明:采用图像法对加气充填混凝土截面照片进行信息采集,通过Matlab的统计计算,可实现对该加气充填混凝土宏观孔和细观孔的信息提取与表征;大于5.00mm的孔隙占比越大,平均孔径差值c越大,反映出联通孔的相对数量越多,则试样的比强度越低,吸水率越大,体积膨胀率越小,抗冻性越差,反之,试样的比强度越高,吸水率越小,体积膨胀率越大,抗冻性越好;以两种细度混合的铝粉膏作为引气剂可以有效降低该加气充填混凝土连通孔的相对数量。     

Low-velocity flexural impact response of fiber-reinforced aerated concrete

Impact response of fiber-reinforced aerated concrete was investigated under a three-point bending configuration based on free-fall of an instrumented impact device. Two types of aerated concrete: plain autoclaved aerated concrete (AAC) and polymeric fiber-reinforced aerated concrete (FRAC) were tested. Comparisons were made in terms of stiffness, flexural strength, deformation capacity and energy absorption capacity. The effect of impact energy on the mechanical properties was investigated for various drop heights and different specimen sizes. It was observed that dynamic flexural strength under impact was more than 1.5 times higher than the static flexural strength. Both materials showed similar flexural load carrying capacity under impact, however, use of 0.5% volume fraction of polypropylene fibers resulted in more than three times higher flexural toughness. The performed instrumented impact test was found to be a good method for quantifying the impact resistance of cement-based materials such as aerated concrete masonry products.     

Determination of length changes due to moisture variations in autoclaved aerated concrete

Length changes of autoclaved aerated concrete due to moisture variations have been investigated by tests on a Danish brand of autoclaved aerated concrete. The investigation have shown that autoclaved aerated concrete with delivery moisture content exhibits swelling when initially conditioned at relative humidities above 43%, whereas initial conditioning below 43% relative humidity leads to shrinkage. After the initial conditioning has been performed, length changes are shown to be almost proportional to the variation in relative humidity. The investigation has shown that the present Danish Standard in this area is misleading, and the principle of a new NORDTEST method, NT Build 444(1996), is presented, Furthermore this test method is compared with the European Standard EN 680,Determination of the drying shrinkage of autoclaved aerated concrete, and a draft for another European Standard prEN 772-12:1992,Determination of length change during moisture movements in autoclaved aerated concrete masonry units.     

Engineering properties of concrete containing natural zeolite as supplementary cementitious material: Strength,toughness, durability,and hygrothermal performance

A complex analysis of engineering properties of concrete containing natural zeolite as supplementary cementitious material in the blended Portland-cement based binder in an amount of up to 60% by mass is presented. The studied parameters include basic physical characteristics, mechanical and fracture–mechanics properties, durability characteristics, and hygric and thermal properties. Experimental results show that 20% zeolite content in the blended binder is the most suitable option. For this cement replacement level the compressive strength, bending strength, effective fracture toughness, effective toughness, and specific fracture energy are only slightly worse than for the reference Portland-cement concrete. The frost resistance, de-icing salt resistance, and chemical resistance to MgCl₂, NH₄Cl, Na₂SO₄, and HCl are improved. The hygrothermal performance of hardened mixes containing 20% natural zeolite, as assessed using the measured values of water absorption coefficient, water vapor diffusion coefficient, water vapor sorption isotherms, thermal conductivity, and specific heat capacity, is satisfactory.     

Simultaneous heat and moisture transport in porous building materials: evaluation of nonisothermal moisture transport properties

The paper presents a mathematical model for calculating the nonisothermal moisture transfer in porous building materials. The simultaneous heat and moisture transfer problem was modeled. Vapor content and temperature were chosen as principal driving potentials. The coupled equations were solved by a numerical method. An experimental methodology for determining the temperature gradient coefficient for building materials was also proposed. Both the moisture diffusion coefficient and the temperature gradient coefficient for building material were experimentally evaluated. Using the measured moisture transport coefficients, the temperature and vapor content distribution inside building materials were predicted by the new model. The results were compared with experimental data. A good agreement was obtained.     

Effect of pore structure and moisture content on gas diffusion and permeability in porous building stones

Hazardous gases in buildings are a concern for public health and security. These gases can be released from the building materials to indoor air and their concentration may become critical where ventilation is hindered, as such in hypogean or more energetically efficient airtight constructions. Furthermore, the gas ventilation and the indoor gas concentration can considerably increase by the vapour condensation on the ceiling and walls of buildings. In this paper, we characterise the CO₂ gas diffusion for a representative range of building porous stones with the aim of establishing the effect of the water content in the gaseous diffusion coefficient. We propose a new methodology to measure gas diffusion with a laboratory device that works under different hygrometric conditions. Results reveal water pore condensation reduces both connected porosity and pore size and therefore, the CO₂ diffusion coefficient. This variation occurs in all the studied porous building stones although it is especially important in stones with small pores. Thus, the reduction of CO₂ diffusion coefficient for the stone with thinnest pores is by 50% when relative humidity varies from 20 to 90%. Permeability and gas diffusion coefficients present similar trends. Porous stones with larger pores and higher porosity values present the highest CO₂ diffusion, water and gas permeability coefficients. Pore size is the conclusive parameter within the transport coefficients. It greatly affects both the tortuosity factor of the CO₂ gaseous diffusion and the slip parameter of the Klinkenberg’s model for gas permeability coefficient. Finally, for studied samples, we establish a power regression, which correlates thoroughly both coefficients.