Increasing concrete durability with high-reactivity metakaolin

High-reactivity metakaolin (HRM) is a manufactured pozzolan produced by thermal processing of purified kaolinitic clay. Field performance and laboratory research of concrete containing HRM have demonstrated its value for bridge decks, bridge deck overlays, industrial flooring, high-strength concrete and masonry products. This paper discusses laboratory evaluations to assess the long-term performance of concrete containing HRM produced in North America for resistance to chloride penetration and reduction in expansion due to alkali-silica reactivity. Bulk diffusion testing indicated that HRM substantially reduced chloride ion penetration in concrete with w/cm of 0.30 or 0.40. Reductions in diffusion coefficients compared to control specimens were of the order of 50% and 60% for concrete with 8% and 12% HRM, respectively. Also, the performance of the concrete containing 8% or 12% cement replacement with HRM showed improved performance versus merely reducing the w/c from 0.4 to 0.3. Such reductions can be expected to have a substantial impact on the service life of reinforced concrete in chloride environments. Expansion tests on concrete prisms containing reactive aggregates showed that 15% HRM can prevent deleterious expansion due to alkali-silica reactivity (ASR). The mechanism of control is likely linked to the substantial reduction in pore solution alkalinity seen in pastes containing 20% HRM in comparison to the control specimen which contained no supplementary cementing materials. However, the reduction was not large enough to depassivate steel reinforcement.     

Interface-durability correlations in concrete

The performance of concrete is dictated mostly by the characteristics of the transition zone. Internal bleeding leads to a higher effective water/cement ratio and hence a more open, porous structure at the cement/aggregate interface. Besides, more of AFt, AFm and CH phases and halides, sulphate, alkalis and other admixture components and less of C-S-H are found in this zone. These cause the deterioration of the reinforcing materials like steel, silicious aggregates, glass and natural fibres. The orientation of the crystalline phases parallel to the interface also weakens the mortar. The porosity and related effects are considerably modified by the use of plasticizers, fine pozzolanic additives and reduced water cement ratio. The mechanism of interfacial structure formation and their resulting effects on the reinforcement are discussed together with the possible precautionary measures.     

Alteration of a cement matrix subjected to biolixiviation test

This paper reports on the assessment of durability and long-term performance of a cement matrix subjected to heterotrophic microbial mediated degradation. In near surface disposal facilities for hazardous and radioactive wastes, microbial activities may likely, in a long-term perspective threaten the integrity of cement-solidified wastes. To investigate the detrimental impact of heterotrophic microorganisms on cement matrices, Aspergillus niger reputed as versatile and prevalent fungus in soil flora was selected as candidate. It was shown that this fungus has the potential of severely degrading ordinary Portland cement pastes through organic acids production. Cement pastes experienced chemical alterations such as substantial leaching of calcium, and mechanical degradation was evident as highlighted by the drastic decline in Young’s modulus. Their poor behaviour with respect to heterotrophic biodeterioration and susceptibility to failure were therefore demonstrated. Consequently, biolixiviation scenario should be seriously considered in order to ensure safe long-term disposal for cement-solidified wastes.

岩土类材料的动弹性模量机理

本文从两个方面考虑岩土类材料(包括岩石和砼材料)动弹模机理:一是从连续力学的粘弹性材料模型——线性A型出发,分别讨论在不同恒应变率、正弦波激励和突加载三种不同加载方式下所表现出的动态弹性模量;二是从非线弹性(不包括粘性)材料模型出发,讨论非线弹性材料悬梁作轴向振动(前面加集中质量块)时,自振频率随振幅而变化。结果表明:振幅越小,则自振频率越大,从而表现的动弹模越大,这是影响材料动弹模实验结果的重要原因之一。文中还讨论了理想线弹性梁各阶振型动弹模意义。     

浅谈生态建筑材料的发展

建筑材料占据着建筑的主要资源和能源消耗,对其绿色化的处理,对实现建筑的可持续发展具有重要意义。本文介绍了生态建材在国内外的发展现状,并以集成木材、混凝土、循环再生建材为例简要说明了传统建材的生态化过程。     

Durability of concrete incorporating high-volume of low-calcium (ASTM Class F) fly ash

Research on structural concrete incorporating high volumes of low-calcium (ASTM Class F) fly ash has been in progress at CANMET since 1985. In this type of concrete, the cement content is kept at about 150 kg/m³. The water-to-cementitious materials ratio is of the order of 0·30, and fly ash varies from 54 to 58% of the total cementitious material. A large dosage of a superplasticizer is used to achieve high workability.

This paper presents data on the durability of this new type of concrete. The durability aspects considered are: freezing and thawing cycling; resistance to chloride ion permeability; and the expansion of concrete specimens when highly reactive aggregates are used in the concrete.

The investigations performed at CANMET indicate that concrete incorporating high volumes of low-calcium fly ash has excellent durability with regard to frost action, has very low permeability to chloride ions and shows no adverse expansion when highly reactive aggregates are incorporated into the concrete.     

Coal fly ash-slag-based geopolymers: Microstructure and metal leaching

This study deals with the use of fly ash as a starting material for geopolymeric matrices. The leachable concentrations of geopolymers were compared with those of the starting fly ash to evaluate the retention of potentially harmful elements within the geopolymer matrix. Geopolymer matrices give rise to a leaching scenario characterised by a highly alkaline environment, which inhibits the leaching of heavy metals but may enhance the mobilization of certain oxyanionic species. Thus, fly ash-based geopolymers were found to immobilise a number of trace pollutants such as Be, Bi, Cd, Co, Cr, Cu, Nb, Ni, Pb, Sn, Th, U, Y, Zr and rare earth elements. However, the leachable levels of elements occurring in their oxyanionic form such as As, B, Mo, Se, V and W were increased after geopolymerization. This suggests that an optimal dosage, synthesis and curing conditions are essential in order to obtain a long-term stable final product that ensures an efficient physical encapsulation.     

A study on durability properties of high-performance concretes incorporating high replacement levels of slag

This paper presents an experimental study of combined effects of curing method and high replacement levels of blast furnace slag on the mechanical and durability properties of high performance concrete. Two different curing methods were simulated as follows: wet cured (in water) and air cured (at 20°C and 65% RH). The concretes with slag were produced by partial substitution of cement with slag at varying amounts of 50–80%. The water to cementitious material ratio was maintained at 0.40 for all mixes. Properties that include compressive and splitting tensile strengths, water absorption by total immersion and by capillary rise, chloride penetration, and resistance of concrete against damage due to corrosion of the embedded reinforcement were measured at different ages up to 90 days. It was found that the incorporation of slag at 50% and above-replacement levels caused a reduction in strength, especially for the early age of air cured specimens. However, the strength increases with the presence of slag up to 60% replacement for the 90 day wet cured specimens. Test results also indicated that curing condition and replacement level had significant effects on the durability characteristics; in particular the most prominent effects were observed on slag blended cement concrete, which performed extremely well when the amount of slag used in the mixture increased up to 80%.     

包装材料表面超疏液性能耐久性及其提高方法

海量油墨、涂料、油漆、胶粘剂、沥青等的包装金属桶罐由于内部粘附属于危险固废且难于处理。酸奶、果酱、牙膏等的包装由于粘附不便于分类回收。材料表面超疏液性能源于表面微纳多级粗糙结构和低表面能。制约超疏液材料应用的瓶颈是表面微纳结构制备工艺复杂和力学强度低导致的耐久性差。本文总结提出了增强超疏液性能耐久性的四类技术方法:具有自我修复功能的超疏表面,SLIPS表面,具有形状记忆的微纳结构和提高表面微纳结构本身的力学强度。具有良好耐久性的包装材料有广阔的应用空间,可以解决包装废弃物回收和利用的关键难题。     

Structure and properties of clay-based geopolymer cements: A review

This paper reviews about geopolymers fundamental focusing in clay materials. The review includes the chemistry and structure of geopolymers, reaction mechanism and prime materials involved in geopolymer formation. The characterization of geopolymers in term of microstructural, crystallographic and functional groups is elucidated. Besides, the important factors (such as alkali concentration, mixing parameters, curing conditions, and water contents) affecting properties (such as setting time, bulk density, strength, thermal properties, and stability) of clay-based geopolymers are critically reviewed. Finally, the paper also includes the previous geopolymer applications that have been successfully marketed and the potential current and future areas or focuses of study and application.     

Microstructural characterisation of alkali-activated PFA matrices for waste immobilisation

A variety of natural and industrial waste product materials rich in SiO₂ and Al₂O₃ may be activated with alkalis to produce cementitious systems which when cured under mild temperature conditions, set and harden to give a very compact paste. When fly ash is the activated material, fast setting and rapid strength development have been reported. In addition to their application in construction, they may have a role in the immobilisation of toxic waste.

Alkali-activated fly ash matrices containing arsenic were studied by a combination of scanning and transmission electron microscopy along with energy-dispersive X-ray analysis of argon-milled sections. Little arsenic was incorporated in the matrix of hydration product but it was apparently associated with iron derived from the hydration of the fly ash. There was no association of As with added Fe₂O₃.     

Flight service environmental effects on composite materials and structures

NASA Langley and the U.S. Army have jointly sponsored programs to assess the effects of realistic flight environments and ground-based exposure on advanced composite materials and structures. Composite secondary structural components were initially installed on commercial transport aircraft in 1973; secondary and primary structural components were installed on commercial helicopters in 1979; and primary structural components were installed on commercial aircraft in the mid-to-late 1980's. Over 5.3 million total component flight hours have been accumulated on 350 composite components since 1973. Service performance, maintenance characteristics, and residual strength of numerous composite components are reported. In addition to data on flight components; 10-year ground-based exposure test results on material coupons are reported. Comparisons between flight and ground-based environmental effects for several composite material systems are also presented. Test results indicate excellent in-service performance with the composite components during the 15 year evaluation period. Good correlation between ground-based material performance and operational structural performance has been achieved.