Influence of lightweight aggregate on the microstructure and durability of mortar

The results of an investigation on the effect of dry and prewetted lightweight aggregates on the microstructure and durability of mortar are presented in this paper. The results are compared with those obtained for normal aggregate mortar. There appears to be only a small difference in the microstructure of the interfacial transition zone (ITZ) between dry and prewetted lightweight aggregate mortars. The porous ITZ surrounding lightweight aggregate appears to extend for about 10 and 15 μm from the aggregate surface for dry and prewetted lightweight aggregates, respectively. The ITZ for dry and prewetted lightweight aggregates seems to be surrounded by dense paste that extends from 10 to about 50 μm from the aggregate surface. This dense paste has lower porosity than that observed in the bulk paste located 50 μm and farther from aggregate surface. The normal aggregate mortar prepared with the same water/cement ratio appears to have porous ITZ that extends beyond 35 μm from the aggregate surface. The dry and prewetted lightweight aggregate mortars seem to have a lower sorptivity and electrical conductivity than does the normal aggregate mortar. Lightweight aggregate mortars also appear to have excellent resistance to sulfate attack as compared with normal aggregate mortar.     

The pozzolanic reactivity of monodispersed nanosilica hydrosols and their influence on the hydration characteristics of Portland cement

This study reveals that the nanosilica hydrosols with higher specific surface areas had faster pozzolanic reactivity, especially at early ages; moreover, the results are indicative of the accelerating influence of nanosilicas and silica fume on the hydration of cement. Faster initial and final setting times observed for cement pastes containing nanosilicas are consequence of these mechanisms. However, less hydration degree of cement compared to the plain paste was observed at age of 7 days and after. This can be attributed to the entrapment of some of mix water in the aggregates of nanosilicas formed in cement paste environment, making less water available for the progress of cement hydration. The same mechanism is believed to be responsible for the reduction of flowability of cement pastes.     

几种火山灰质掺合料的火山灰活性研究

采用酸溶法测定并比较几种典型的火山灰质掺合料的火山灰活性,探讨其火山灰反应程度对浆体强度的影响。试验结果表明,硅灰的火山灰活性最大,明显高于煤矸石和粉煤灰;煤矸石和Ⅱ级磨细粉煤灰的火山灰活性比Ⅰ级分选粉煤灰较大;酸溶法测定的煤矸石或硅灰的火山灰活性误差较大,活性偏低;掺合料火山灰的活性与其浆体强度有一定相关性。     

Value-added utilisation of waste glass in concrete

A large proportion of the postconsumer glass is recycled into the packaging stream again, and some smaller proportions are used for a variety of purposes, including concrete aggregate. However, a significant proportion, which does not meet the strict criteria for packaging glass, is sent to landfill, taking the space that could be allocated to more urgent uses. Glass is unstable in the alkaline environment of concrete and could cause deleterious alkali-silica reaction (ASR) problems. This property has been used to advantage by grinding it into a fine glass powder (GLP) for incorporation into concrete as a pozzolanic material. In laboratory experiments, it can suppress the alkali reactivity of coarser glass particles as well as that of natural reactive aggregates. It undergoes beneficial pozzolanic reactions in the concrete and could replace up to 30% of cement in some concrete mixes with satisfactory strength development. The drying shrinkage of the concrete containing GLP was acceptable.     

A computational linear elastic fracture mechanics-based model for alkali–silica reaction

A fracture mechanics model for alkali–silica reaction (ASR) is presented that deals with the case of a concrete made up of dense spherical aggregates. Chemistry and diffusion (of ions and gel) are not modelled. The focus is put on the mechanical consequences of the progressive replacement of the aggregates by a less dense gel. A ring-shaped crack then appears in the cement paste depending on the pressure build-up, according to an incremental energy criterion. The stored elastic energy and deformation of each configuration are determined assuming that each aggregate is embedded in an infinite cement paste matrix, through Finite Element Analysis. We note a very different behaviour of aggregates of different sizes. Adding the contributions of different aggregates leads to an estimate of the free expansion of a concrete of given aggregate size distribution. Parameters of the model are identified, providing a good fit to experiments taken from Multon's work.     

Alkali-silica reaction: Suppression of expansion using porous aggregate

Concretes containing porous natural aggregate or lightweight aggregate have been monitored for expansion due to alkali-silica reaction. Very much reduced expansions were found when these aggregates replaced a dense limestone in a reactive mix containing Thames Valley sand. These results are discussed in terms of aggregate absorption, concrete porosity, and potential reactivity of some of the porous aggregates.     

State of the art for fly ash uses in concrete

An overview of accomplishments in recent research and of attractive further work for improvements of the uses of fly ash in concrete is presented.Improved workability of fresh concrete, reduction of peak curing temperatures and increase of the denseness of hardened concrete are being recognized as attainable advantages in practice. Also pozzolanic reactivity as protection against reactions with alkali-susceptible aggregates is of increasing interest.There is in research a trend towards more work about the characterisation of fly ashes in relation to chemical composition and mineralogy structure. Also their impact on the rheology of fresh cement paste, on the energetics of the hydrating system and on the microstructure of hardened cement paste is increasingly being studied. More interaction with other branches of materials science is still desirable.The practice-research relations are demonstrated by experience with use of fly ash in the concrete of a major bridge construction in Denmark. Coherent research and development efforts are found commendable.     

粉煤灰和磨细矿渣对高强轻骨料混凝土抗渗及抗冻性能的影响

研究了粉煤灰和磨细矿渣对高强轻骨料混凝土抗渗及抗冻性能的影响。结果表明：粉煤灰和磨细矿渣的复合掺入能显著提高高强轻骨料混凝土的强度、抗渗及抗冻性能。在不掺入引气剂的情况下,轻骨料混凝土的抗冻性达F200以上。轻骨料混凝土的扫描电镜照片表明：粉煤灰和磨细矿渣的综合效应,使火山灰反应更加充分,Ca(OH)2含量降低,轻骨料与水泥石的界面过渡区得到强化,混凝土结构更加密实,其抗渗、抗冻性能得到进一步提高。     

透水混凝土冻融剥蚀成因分析

为探究透水混凝土冻融剥蚀是否来自水泥浆体劣化,选取2.5~10.0 mm粒径骨料,制备了水灰比为0.31的透水混凝土及同水灰比的水泥石,测量二者冻融循环下质量、强度及透水混凝土相对动弹性模量变化。采用压汞法(MIP)测量水泥石冻融循环下孔结构特征参数及孔径分布变化,并通过扫描电镜(SEM)观察透水混凝土骨料水泥界面形貌演变。结果表明,在水冻与盐冻环境下透水混凝土宏观性能指标均有不同程度下降,骨料水泥界面产生裂缝并随冻融次数增加不断扩展,而水泥石强度、质量及微观孔隙结构均无明显变化。这表明透水混凝土冻融劣化与骨料水泥界面劣化相关。     

ITZ microstructure of concrete containing GGBS

The interfacial transition zone (ITZ) of aggregate-cement paste and the morphology of hydrates in concrete containing ground granulated blast-furnace slag (GGBS) have been investigated using XRD, SEM and microhardness measurements. The experimental results demonstrated that GGBS significantly decreases both the quantity and the orientating arrangement of CH crystals at the ITZ. The CH crystal size becomes smaller because of the addition of GGBS. The weak ITZ between aggregate and cement paste was strengthened as a result of the pozzolanic reaction of GGBS. The above improvements become much more significant with the decrease of particle sizes of GGBS.     

Effect of aggregates and microcracks on the drying rate of cementitious composites

We investigated the effect of aggregate concentration on the drying rate of cementitious composites with glass beads, sand grains, or expanded polystyrene (EPS) beads as aggregates. The drying rate of composites with non-diffusive aggregates (glass beads and sand grains) decreased with aggregate concentration. Composites with 60% glass beads dried a factor 1.3 slower than plain cement paste with the same w/c-ratio. Composites with EPS-beads showed the opposite trend: an increased drying rate for composites with higher aggregate concentrations. However, the effective diffusion coefficients of the EPS-composites decreased with increasing aggregate concentration. A higher aggregate concentration means that less water needs to diffuse out of the material to reach a specific degree of drying, and this effect mainly determined the drying rate of the composites with EPS-beads. The development of drying shrinkage microcracks had a small effect on the drying rate of the studied composites.     

粉煤灰-水泥水化的核磁共振定量分析

利用高分辨固体核磁共振仪结合去卷积技术,定量分析了粉煤灰水泥浆体中水泥和粉煤灰的水化程度以及C-S-H凝胶中硅氧-铝氧链平均长度,同时研究了粉煤灰火山灰反应对C-S-H结构的影响。结果表明:水化3 d时,系统中约47%的水泥和14%的粉煤灰参与了水化反应,C-S-H平均链长为3.2;水化120d时,水泥和粉煤灰的水化程度分别为89%和33%,C-S-H平均链长约为3.8,远大于纯水泥浆体中C-S-H的平均链长(为2.7)。水化3 d时粉煤灰玻璃相结构中的Si—O—Si,Si—O—和Al—Al共价键断裂,形成了单体硅酸根和单体铝酸根,这些单体结构桥连体系中的二聚体单元进而提高了C-S-H平均链长。粉煤灰掺入并不会因为C-S-H聚合度提高以及ACL增加就能促进粉煤灰水泥浆体强度。     

煤矸石对水泥熟料水化促进作用及机理(英文)

通过测试化学结合水量及用背散射电子图像分析法测试水泥熟料水化程度,研究了煤矸石–水泥混合体系中煤矸石对水泥熟料水化的促进作用,并通过对Ca(OH)2含量测试分析了其作用机理。结果表明:煤矸石的掺入促进了混合体系中水泥熟料的水化进程,且活性越高,掺量越大,促进水泥效果越明显;水化早期煤矸石对水泥熟料水化的促进作用主要来源于简单"稀释"作用(物理作用),水化中后期具有较高活性的活化煤矸石通过自身反应活性发挥吸收体系中Ca(OH)2促进水泥熟料水化(化学作用)。     

A novel approach to lightweight alumina-carbon refractories for flow control of molten steel

Functional refractory materials for flow control devices of molten steel in continuous casting used to be prepared from Al₂O₃–C refractories containing dense corundum aggregates. According to the traditional concept, the denser the refractories, the higher the strength of refractories. However, we prepared a new lightweight Al₂O₃–C refractory material using microporous corundum aggregates instead of dense corundum aggregates, which were reinforced by in situ formed SiC whiskers. A comparative analysis of microstructures and properties was carried out for conventional and lightweight Al₂O₃–C refractories with and without Si powder addition. We showed that microporous aggregates formed a better aggregate/matrix interface bonding and an improved distribution of SiC whiskers. The SiC whiskers formed not only in the matrix, but also inside of the microporous aggregates and at the aggregate/matrix interface by a vapor-solid reaction mechanism. Due to the formation of a microporous aggregate/matrix interface reinforced by SiC whiskers, the crack propagation along the aggregate/matrix interface was suppressed, whereas the percentage of cracks propagating within the aggregates was enhanced. Thus, the synergy between in situ formed SiC whiskers and microporous aggregates resulted in a significant higher strength of lightweight Al₂O₃–C refractories compared to conventional ones. The results therefore provide an original strategy to strengthen Al₂O₃–C refractories.     

Evolution microstructurale des composites du systeme ciment alumineux-granulat calcaire. I. Mode de propagation de la fissure

The transitional layer, formed at the interface between a calcareous aggregate and an aluminous cement paste, changes versus the hydration time and leads to a zone rich in hydrated carboaluminate, which is formed by epitaxic growth on calcium carbonate. This phenomenon is responsible for a noticeable change in the crack propagation process in the solid. In the system calcium aluminate - siliceous aggregates, the process is always intergranular. In the case of aluminate - calcareous aggregate it is intergranular at the beginning of hydration and becomes transgranular after some months. Then the crack propagation concerns the carbonate grain itself but neither the surrounding zone nor the interface aggregate - transitional zone.     

Effect of aggregate size on transition zone properties at the portland cement paste interface

Structural features of the transition zone between granular aggregate and portland cement paste were studied. Two types of aggregates were used, quartz and limestone. The transition zones are structurally characterized by a parameter referred to as “interfacial excess conductance” based on electrical conductivity methods. The experimental results indicate that the transition zone between quartz particles and portland cement paste is always less dense than bulk paste, regardless of the aggregate size, and that the thickness of this transition zone decreases with the decreasing of the aggregate size. The same general features occur for the transition zone between larger limestone particles and portland cement paste. A transition zone denser than bulk paste occurs, however, for much smaller limestone particles due possibly to chemical interaction between the limestone particles and portland cement paste.     

粉煤灰在复合胶凝材料水化过程中的作用机理

分析了粉煤灰在复合胶凝材料水化硬化过程中的作用。探讨了化学激发、物理激发和热激发对于粉煤灰的活性的促进作用以及确定粉煤灰反应程度的方法。粉煤灰的火山灰活性可以用碱性物质或硫酸盐来激发,但是,这种化学激发措施不适合在商品混凝土生产过程中使用。物理激发和热激发是两种简单实用的提高粉煤灰使用效能的技术路线。热激发特别适用于大体积混凝土结构。在复合胶凝材料中,粉煤灰的反应程度很低,不会大量消耗Ca(OH)_2,在水化硬化初期,矿物掺和料主要以物理填充作用参与复合胶凝材料的水化硬化过程;随龄期延长,粉煤灰的火山灰活性作用逐渐明显。     

Enhanced mechanical properties of lightweight Al2O3-C refractories reinforced by combined one-dimensional ceramic phases

We prepared a new lightweight Al₂O₃-C refractory material with a higher strength by using microporous corundum aggregates instead of dense tabular corundum aggregates, which was reinforced by in situ formed SiC whiskers, multi-walled carbon nanotubes (MWCNTs), and mullite rods. A comparative study of the microstructure, mechanical properties, and fracture behavior was carried out for dense and lightweight Al₂O₃-C refractories coked at 1200°C and 1400°C, respectively. By using the microporous corundum aggregates, a better aggregate/matrix interface bonding and an optimized distribution of SiC whiskers were obtained. The SiC whiskers formed inside the microporous corundum aggregates and simultaneously in the matrix by a vapor-solid reaction mechanism, resulting in an enhancement at the microporous aggregate/matrix interface. Furthermore, the in situ formed MWCNTs and well-developed mullite rods at 1200°C in the matrix also contributed to the better interface structure. Thus, due to the improved microporous aggregate/matrix interface, the crack propagation along the aggregate/matrix interface was suppressed, resulting in an increased crack propagation within the aggregates. Consequently, the synergy between microporous corundum aggregates and combined one-dimensional ceramic phases caused a lower bulk density but a markedly higher strength, a higher fracture energy, and a higher toughness of lightweight Al₂O₃-C refractories compared to the dense ones. Overall, our study allows to overcome the traditional concept that a higher strength of refractories is reached by a higher density.     

Investigations about the effect of aggregates on strength and microstructure of geopolymeric mine waste mud binders

Geopolymeric binders appear to be an alternative to traditional Portland cement, due to high mechanical performances and environmental advantages. Some aspects related to the effect of aggregates in the microstructure and mechanical behaviour of geopolymeric mine waste mud (GMWM) binders are reported in the present study. Compressive and tensile strength of mine waste mud binders were analyzed. The factors investigated were the aggregate/binder ratio, the aggregate dimension and aggregate type, schist, granite and limestone.Test results showed that GMWM binders have a very high strength at early ages and also possess a very high tensile strength. It's suggested that behaviour may be due to the dissolution of quartz and alumina in the presence of alkalis enhancing bonding between paste and aggregates.The aggregate dimension showed only significant effect on tensile strength. Limestone aggregates showed a chemical bond to the alkali-activated paste but presented higher shrinkage. It was also found that no traditional porous ITZ was detected in GMWM binders.     

The contact zone between portland cement paste and glass “aggregate” surfaces

The morphology of the contact zone developed between Portland cement paste and glass slide “aggregates” has been explored using SEM and other techniques. A duplex film of about 1 μm total thickness is rapidly deposited on the glass surface. This is a continuous film of Ca(OH)₂ overlain by a parallel array of rod-shaped CSH gel particles projecting normal to the interface. The nearby cement paste exhibits high porosity, but after a few days becomes partly filled with a secondary deposit of stacked platelets of relatively pure Ca(OH)₂. Cement particles near the interface hydrate in a peculiar manner. A hydration product shell is quickly formed, but the encapsulated cement particles dissolve away to leave partly or completely empty shells. This behavior occurs with various Portland cement types and presumably occurs near aggregate surfaces in concrete.