Alkali-silica reactivity of large silica fume-derived particles

Pozzolanic materials, including silica fume, are commonly added to concrete to reduce expansion due to alkali-silica reaction (ASR). It has been noted, however, that commercial silica fume is not always adequately dispersed, and large agglomerates may be present. These large particles have been hypothesized to act as amorphous silica aggregates, thereby participating in an expansive reaction with the alkalis present in cement paste pore solution. If such were the case, some silica fume particles would actually aggravate expansion due to ASR rather than suppress it. The present investigation characterizes the microstructure and morphology of agglomerated and sintered silica fume particles and compares their effects on alkali-silica-related expansion. While a 5% replacement of moderately reactive sand with sintered silica fume aggregates caused significant expansion under accelerated testing conditions (modified ASTM C1260), the replacement with large agglomerates of densified silica fume decreased expansion compared with control mortar bars containing only sand. Both the sintered aggregates and the agglomerates reacted with the pore solution; one reaction was expansive, while the other was not.     

Reaction products of densified silica fume agglomerates in concrete

Most silica fume currently used in concrete is in the dry densified form and consists of agglomerates of sizes between 10 μm and several millimeters. Many of these agglomerates may break down only partially in normal concrete mixing. Examination of various mature silica-fume-bearing concretes using backscatter mode scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis shows that such agglomerates have reacted in situ and given rise to recognizable types of reaction products filling the space within the original outline of the agglomerate. One type is “quiescent,” and usually shows no evidence of volume instability. EDX spectra indicate that the product formed within such grains is C-S-H of very low Ca/Si ratio, with modest alkali contents. Other silica fume agglomerates may undergo a distinct alkali-silica-type reaction (ASR), with the reaction product found within the original outline of the agglomerate having significantly less calcium and usually much higher alkali contents than the quiescent type. Such reacted agglomerates show evidence of local expansion, shrinkage cracking (on drying), and other features common to ASR. Both types may be found within the same concrete, sometimes in close proximity. It further appears that exposure to seawater may convert previously formed reaction products of silica fume agglomerates to magnesium silicate hydrates.     

Silica fume-basic blast furnace slag systems activated by an alkali silica fume activator

This paper deals with the results of research on binder systems based on the use of silica fume. The data obtained show that the optimal proportion of blast furnace slag and alkali activator prepared from silica fume permits the obtaining of interesting materials from the point of view of their technical properties as well as from the possibility of utilizing industrial wastes. The composite developed is hydraulic. It is possible to state that these kinds of binding systems have shown promise of future use.     

硅粉抑制碱集料反应的试验研究

通过对青海地区的集料调研分析,发现青海大部分地区的集料存在高的碱活性.采用西宁城北地区典型的砂石集料以及大通地区的集料进行了试验研究,对比分析了不同掺量的硅粉对碱集料反应的抑制效果.结果表明:对于城北石、大通石两种碱活性较低的集料,选择掺量10％、15％、20％硅粉时,其14d膨胀率均小于0.1％,均能有效抑制碱集料反应的膨胀.而对于城北砂碱活性较高的集料,选择掺量20％硅粉时,其14d膨胀率小于0.1％,可以有效抑制碱集料反应的膨胀.     

Multi-scale analysis of alkali–silica reaction (ASR): Impact of alkali leaching on scale effects affecting expansion tests

Alkali–silica reaction expansions are disturbed by a variety of mechanisms (alkali leaching, ASR-gel permeation through cracks, chemical conditions in pore solution water and its dependence on temperature). An important consequence is the difficulty of using the expansion test on specimens to analyse the behaviour of ASR-damaged structures. The paper focuses on the influence of leaching: alkali transport and consumption are modelled using a multi-scale approach (aggregate and concrete scales). The evaluation of the alkali concentration below which expansion stops is needed to perform relevant analysis in various alkali conditions and this alkali threshold is quantified according to calcium concentration and temperature. The impact of the coupling between alkali transport in aggregate and silica reactivity is also studied. Lastly, the consequences of leaching on ASR-expansion are analysed in two case studies drawn from the literature.     

Occurrence of large silica fume-derived paticles in hydrated cement paste

In an examination of a 1-year old hydrated silica fume bearing cement paste a number of large (35–80 μm) rounded siliceous particles were found that had apparently been derived from the coarse fraction of the silica fume. Calcium had diffused inward from the periphery of the grains, and in most cases had reached the centet. The Ca:Si mole ratio near the outer zone was almost the same as that in the surrounding matrix CSH gel, but decreased continuously with distance toward the center. Potassium and sodium were also found within the particles, but were distributed inversely to the calcium, i.e. increasing toward the center. The reaction product generated here appears to be CSH and not potentially expansive alkali silica reaction product gel, although such gel might be produced with high alkali cements.     

Effect of silica fume and fly ash on heat of hydration of Portland cement

Results of calorimeter tests on Portland cement-silica fume-fly ash mixtures are presented. Data indicate that silica fume accelerates cement hydration at high water/cementitious ratios and retards hydration at low water/cementitious ratios. On the other hand, fly ash retards cement hydration more significantly at high water/cementitious ratios. When silica fume and fly ash are added together with cement, the reactivity of the silica fume is hampered and the hydration of the cementitious system is significantly retarded.     

Alkali–silica reaction (ASR)—performance testing: Influence of specimen pre-treatment,exposure conditions and prism size on concrete porosity,moisture state and transport properties

Whether or not concrete prism tests (CPTs) developed for assessment of alkali–silica reactivity of aggregates might be suitable for general ASR performance testing of concrete has been evaluated. This paper presents the background for the choice of test procedures and results on how variations in specimen pre-treatment, ASR exposure conditions and prism size influence concrete porosity, moisture state and transport properties. Results from measurements of alkali leaching and prism expansions during the ASR exposure are presented in a separate paper, together with discussion of consequences for ASR test procedures.For ordinary Portland cements and with water-to-cementitious-materials ratio (w/cm) 0.45 and higher it was found that the internal moisture state is sufficiently high in all the assessed procedures to produce ASR expansion. However, for less permeable concretes lack of internal moisture and lower rate of diffusion can significantly reduce the rate and extent of ASR expansion during laboratory performance testing.     

The influence of aluminium on the dissolution of amorphous silica and its relation to alkali silica reaction

It is known that the addition of supplementary cementitious materials (SCMs) in concretes reduces or even stops expansion due to alkali silica reaction (ASR). It has been widely shown that the main mechanism controlling ASR in blends is the alkali fixation capacity of silica rich C–S–H, which lowers the pH of the pore solution. The role of alumina additions is less clear. It was shown in a previous paper [1] that the alumina present in certain SCMs does not further reduce the alkalinity of the paste pore solution. It is proposed here that aluminium acts directly on the reactive phases of the aggregates. Aluminium species, present in the pore solution, are absorbed on the silica surface and limit the dissolution of amorphous silica of the aggregates, restricting ASR. The effect of aluminium was demonstrated through a study of mortar expansion and SEM image analysis of reactive aggregates in simulated pore solutions.     

Effects of densified silica fume on microstructure and compressive strength of blended cement pastes

Some experimental investigations on the microstructure and compressive strength development of silica fume blended cement pastes are presented in this paper. The silica fume replacement varies from 0% to 20% by weight and the water/binder ratio (w/b) is 0.4. The pore structure by mercury intrusion porosimetry (MIP), the micromorphology by scanning electron microscopy (SEM) and the compressive strength at 3, 7, 14, 28, 56 and 90 days have been studied. The test results indicate that the improvements on both microstructure and mechanical properties of hardened cement pastes by silica fume replacement are not effective due to the agglomeration of silica fume particles. The unreacted silica fume remained in cement pastes, the threshold diameter was not reduced and the increase in compressive strength was insignificant up to 28 days. It is suggested that the proper measures should be taken to disperse silica fume agglomeration to make it more effective on improving the properties of materials.     

Effect of silica fume on steel fiber bond characteristics in reactive powder concrete

In this paper, the effect of silica fume on the bond characteristics of steel fiber in matrix of reactive powder concrete (RPC), including bond strength, pullout energy, etc., are presented. The experimental results on steel fiber pullout test of different conditions are reported. Various silica fume contents ranging from 0% to 40% are used in the mix proportions. Fiber pullout tests are conducted to measure the bond characteristics of steel fiber from RPC matrix. It is found that the incorporation of silica fume can effectively enhance the fiber-matrix interfacial properties, especially in fiber pullout energy. It is also concluded that in terms of the bond characteristics, the optimal silica fume content is between 20% and 30%, given the conditions of the experimental program. The microstructural observation confirms the findings on the interfacial-toughening mechanism drawn from the fiber pullout test results.     

The mechanism of cesium immobilization in densified silica-fume blended cement pastes

The role of silica-fume agglomerates, found in densified silica-fume (DSF) pastes, in the immobilization mechanism of Cs ions was studied. Samples of cementitious pastes containing two different forms of silica fume - DSF and raw silica fume (RSF) - were prepared. Leaching experiments showed that both additives reduced the leachability of the metal ion, but the effect of the DSF paste was much stronger. Scanning Electron Microscopy, together with Differential Thermal Analysis, proved that no agglomerated particles were present in the RSF pastes and that the extent of pozzolanic reactivity was higher. We therefore believe that unreacted silica within the DSF agglomerates adsorbs Cs ions and consequently increases their immobilization. Furthermore, this work suggests that during the pozzolanic reaction, a hydrated rim develops around the agglomerate that acts as an additional diffusion barrier for the Cs ions, resulting in an increased efficiency of Cs immobilization.     

Influence of silica fume on the desiccation cracks of compacted clayey soils

Clayey soils containing smectites are widely used for construction of liner and cover systems to reduce the hydraulic conductivity in geotechnical applications because of their low permeability and high cation exchange capacity. However, the compacted clayey soils crack on drying because of their high swelling potential, and their hydraulic conductivities increase. To solve this problem, it is essential to stabilize the clayey soils using additive materials. The aim of this study is to examine the suitability of silica fume as a stabilization material to reduce the development of desiccation cracks in compacted clayey liner and cover systems. Natural clayey soil and clayey soil–silica fume mixtures were compacted at the optimum moisture content and subjected to laboratory tests. The results show that silica fume decreases the development of desiccation cracks on the surface of compacted samples. We concluded that silica fume waste material can be successfully used to reduce the development of desiccation cracks in compacted clayey liner and cover systems.     

Pozzolanic activity of silica dust

Highly reactive aggregates and unusually alkali-rich cement call for special precautions in connection with concrete making in Iceland. Search in the past for efficient ways to counteract alkali-aggregate reactions is cited. High hopes are now connected with the pozzolanic use of silica fume from a ferrosilicon plant now under erection. The influence of this dust on cement strength and mortarbar expansion is described.     

不同养护条件下含石英玻璃地质聚合物砂浆的变形行为

为阐明典型活性组分（无定型SiO2）在地质聚合物中的作用行为和效应,探索地质聚合物体系中碱-集料反应评价方法,研究常温（23℃）和38℃湿气养护（相对湿度〉95%）、80℃在1 mol/L NaOH溶液浸泡及150℃在10%（质量分数）的KOH溶液压蒸下,含石英玻璃集料地质聚合物砂浆的变形行为,采用扫描电镜、电子散射能谱研究产物的组成和微观结构。结果表明：4种养护条件下,特别是在传统普通硅酸盐水泥（OPC）体系所规定的养护条件和龄期内,含石英玻璃集料地质聚合物砂浆没有发生有害膨胀;但是随养护条件不同,地质聚合物基体和石英玻璃可能经历不同的化学反应过程,进而导致不同的变形行为,特别是在高温且有外碱介入时,地质聚合物基体在后期会产生膨胀效应。不宜采用单一的适于OPC体系的高温、高碱快速检测混凝土碱-集料反应的检测方法来评价地质聚合物体系中的碱-集料反应行为。     

Preparation of scrap tire rubber fiber–silica fume mixtures for modification of clayey soils

This experimental work was performed to investigate the influence of silica fume–scrap tire rubber fiber mixture inclusion on the geotechnical properties of clayey soils. The natural and modified clayey soil samples were subjected to the unconfined compression, the shear box, the odometer and the falling-head permeability tests after compaction at optimum moisture content. The results of experimental research indicated that silica fume, fiber and silica fume–fiber mixture modification enhanced both the unconfined compression strength and strength parameters. Although, the fiber modification increased in the hydraulic conductivity, it decreased in the swelling pressure. It was observed also that the silica fume and silica fume–fiber modification decreased both the hydraulic conductivity and swelling pressure. Consequently, it is concluded that the silica fume–fiber mixture materials can be successfully used for the modifications of clayey soils in the geotechnical applications.     

Effects of silica fume on cemented sand using ultrasonic pulse velocity

When the geotechnical engineering projects have to be built on weak soils, problems related to bearing capacity and settlement arise. Chemical stabilization of soil using cement is a popular and effective technique that can significantly improve workability and shear strength of soil. However, very limited studies have been conducted to reveal the effect of silica fume on the engineering properties of cement-stabilized sandy soil. For this purpose, in the present study, a series of laboratory tests including standard Proctor compaction, unconfined compression and ultrasonic pulse velocity tests were carried out on sand-cement-silica fume samples. Moreover, it has been attempted to investigate the relation between ultrasonic pulse velocity and unconfined compressive strength of cemented sand containing silica fume. The cement contents were 3, 5 and 7% and silica fume contents were 0, 0.25, 0.5 and 1% by weight of dry sand. The cylindrical specimens were prepared and cured for 3, 7, 14, 28, 42 and 56?days and tested. The results show that inclusion of silica fume to cemented sand increases maximum dry unit weight, unconfined compressive strength and ultrasonic pulse velocity while decreases optimum moisture content. In addition, according to the obtained results, acceptable correlations exist between ultrasonic pulse velocity, unconfined compressive strength and stiffness.     

Electrical conductivity of concrete containing silica fume

The influence of silica fume on concrete properties represents an important technical research. In general, silica fume tends to improve both mechanical characteristics and durability of concrete. Thus the electrical properties of concrete containing silica fume can be studied to clarify its physical performance during hydration. The electrical conductivity of neat cement, mortar and concrete pastes was measured during setting and hardening. The ordinary Portland cement was partially replaced by different amounts of silica fume by weight. The changes in the electrical conductivity were reported during setting and hardening after gauging with water. The results of this study showed that the electrical conductivity can be used as an indication for the setting characteristics as well as the structural changes of the hardened pastes made with and without silica fume.     

Alkali–silica reaction (ASR)—performance testing: Influence of specimen pre-treatment,exposure conditions and prism size on alkali leaching and prism expansion

Whether or not concrete prism tests developed for assessment of alkali–silica reactivity of aggregates might be suitable for general ASR performance testing of concrete has been evaluated. This paper discusses how variations in specimen pre-treatment, ASR exposure conditions and prism size influence the rate and amount of alkali leaching and prism expansion, together with a discussion of consequences for ASR test procedures. Furthermore, results from some complementary tests are included.Generally, a remarkably high proportion of the in-mixed alkalis were leached out of the concrete prisms during the ASR exposure. For prisms exposed to 60 °C, the rate and amount of alkali leaching is the main controlling factor for the prism expansion. For less permeable concretes exposed to 38 °C, lack of internal moisture and lower rate of diffusion contributes to reduce the rate and extent of ASR expansion (reported in a separate paper).