Fluid transport in high volume fly ash mixtures with and without internal curing

The transport of fluid and ions in concrete mixtures is central to many aspects of concrete deterioration. As a result, transport properties are frequently measured as an indication of the durability that a concrete mixture may be expected to have. This paper is the second in a series investigating the performance of high volume fly ash (HVFA) mixtures with low water-to-cementitious ratios (w/cm) that are internally cured. While the first paper focused on strength and shrinkage, this paper presents the evaluation of the transport properties of these mixtures. Specifically, the paper presents results from: rapid chloride migration (RCM), rapid chloride penetration test (RCPT), apparent chloride diffusion coefficient, surface electrical resistivity, and water absorption. The test matrix consisted of mortar samples with two levels of class C fly ash replacement (40% and 60% by volume) with and without internal curing provided with pre-wetted lightweight fine aggregates (LWA). These mixtures are compared to plain ordinary portland cement (OPC) mortars. The results indicate that HVFA mixtures with and without internal curing provide benefits in terms of reduced transport coefficients compared to the OPC mixtures.     

Influence of curing conditions on properties of high calcium fly ash geopolymer containing Portland cement as additive

This paper investigated the mechanical properties and microstructure of high calcium fly ash geopolymer containing ordinary Portland cement (OPC) as additive with different curing conditions. Fly ash (FA) was replaced with OPC at dosages of 0%, 5%, 10%, and 15% by weight of binders. Setting time and microstructure of geopolymer pastes, and flow, compressive strength, porosity and water absorption of geopolymer mortars were studied. Three curing methods viz., vapour-proof membrane curing, wet curing and temperature curing were used. The results showed that the use of OPC as additive improved the properties of high calcium fly ash geopolymer. The strength increased due to the formation of additional C–S–H and C–A–S–H gel. Curing methods also significantly affected the properties of geopolymers with OPC. Vapour-proof membrane curing and water curing resulted in additional OPC hydration and led to higher compressive strength. The temperature curing resulted in a high early compressive strength development.     

Mechanisms of hydration reactions in high volume fly ash pastes and mortars

This paper describes investigations of high-volume fly ash (HVFA)-Portland cement (PC) binders, the physical and chemical properties of which have been characterized up to 365 days of curing. Physical investigations were made of compressive strength development, pore structure by porosimetry, and morphology by scanning electron microscopy. Chemical examination was conducted for solid phase composition and degree of hydration by X-ray diffraction and thermal analysis, and for pore-fluid composition by high pressure extraction and analysis.

Up to 365d the cement in the HVFA pastes is not fully hydrated. However, the ash participates in both early (sulpho-pozzolanic) and late (alumino-silicate) hydration reactions. In addition to the usual products of cement hydration, ettringite (AFt) has been identified as a product of the early hydration of the fly ash. It has not been possible to identify long term hydration products of fly ash which appear to be non-crystalline. A two-step mechanism for pozzolanic reaction between fly ash and Portland cement has been proposed involving: (a) depolymerization/silanolation of the glassy constituents of the ash by the highly alkaline pore fluids, followed by (b) reaction between solubilized silicate and calcium ions in solution to form C---S---H.     

A study on the effect of nano silica on compressive strength of high volume fly ash mortars and concretes

This paper presents the effect of nano silica (NS) on the compressive strength of mortars and concretes containing different high volume fly ash (HVFA) contents ranging from 40% to 70% (by weight) as partial replacement of cement. The compressive strength of mortars is measured at 7 and 28 days and that for concretes is measured at 3, 7, 28, 56 and 90 days. The effects of NS in microstructure development and pozzolanic reaction of pastes containing above HVFA contents are also studied through backscattered electron (BSE) image and X-ray diffraction (XRD) analysis. Results show that among different NS contents ranging from 1% to 6%, cement mortar containing 2% NS exhibited highest 7 and 28 days compressive strength. This NS content (2%) is then added to the HVFA mortars and concretes and the results show that the addition of 2% NS improved the early age (7 days) compressive strength of mortars containing 40% and 50% fly ash by 5% and 7%, respectively. However, this improvement is not observed at high fly ash contents beyond 50%. On the other hand, all HVFA mortars exhibited improvement in 28 days compressive strength due to addition of 2% NS and the most significant improvement is noticed in mortars containing more than 50% fly ash. In HVFA concretes, the improvement of early age (3 days) compressive strength is also noticed due to addition of 2% NS. The BSE and XRD analysis results also support the above findings.     

Effects of different curing regimes on the compressive strength properties of self compacting concrete incorporating fly ash and silica fume

This paper presents the effect of air curing, water curing and steam curing on the compressive strength of Self Compacting Concrete (SCC). For experimental study, SCC is produced with using silica fume (SF) instead of cement by weight, by the ratios of 5%, 10% and 15%, and fly ash (FA) with the ratios of 25%, 40% and 55%. It is observed that mineral admixtures have positive effects on the self settlement properties. The highest compressive strength was observed in the concrete specimens with using 15% SF and for 28 days water curing. Air curing caused compressive strength losses in all groups. Relative strengths of concretes with mineral admixtures were determined higher than concretes without admixtures at steam curing conditions.     

Effect of specimen size and shape on compressive strength of concrete containing fly ash: Application of genetic programming for design

The use of fly ash as a mineral admixture in the manufacture of concrete has received considerable attention in recent years. For this reason, several experimental studies are carried out by using fly ash at different proportions replacement of cement in concrete. In the present study, the models are developed in genetic programming for predicting the compressive strength values of cube (100 and 150 mm) and cylinder (100 × 200 and 150 × 300 mm) concrete containing fly ash at different proportions. The experimental data of different mixtures are obtained by searching 36 different literatures to predict these models. In the set of the models, the age of specimen, cement, water, sand, aggregate, superplasticizers, fly ash and CaO are entered as input parameters, while the compressive strength values of concrete containing fly ash are used as output parameter. The training, testing and validation set results of the explicit formulations obtained by the genetic programming models show that artificial intelligent methods have strong potential and can be applied for the prediction of the compressive strength of concrete containing fly ash with different specimen size and shape.     

Effect of curing conditions and concrete mix design on the expansion generated by delayed ettringite formation

Many studies have shown the effect of different parameters on the expansion induced by delayed ettringite formation (DEF), but there is not yet a general agreement on their relative effects due to discrepancies on experimental results generally attributed to different experimental conditions. The aim of this study is to assess the coupling effects between some of the main parameters of this reaction using the experimental design method. The main interactions concern two factors: temperature and duration of heating during curing. Each interaction has been subjected to statistical tests in order to be validated. The resulting empirical models can reflect the experimental fields and therefore provide useful information about the risk of expansion related to␣DEF, knowing the temperature and the duration of curing, the alkali content of the concrete, the aggregate nature and the water to cement ratio.

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Résumé De nombreuses études ont montré l’influence de différents paramètres sur les expansions induites par la formation différée de l’ettringite (DEF). En raison des divergences de résultats attribuées à des conditions expérimentales différentes, il n’existe pas d’accord unanime sur l’effet de ces paramètres. L’objectif de cette étude consiste à étudier les couplages entre certains des principaux paramètres de cette réaction par la méthode des plans d’expériences. Les principales interactions concernent la température et la durée de l’échauffement. Chaque interaction a subi un test statistique afin d’être validée. Les modèles empiriques déterminés peuvent reproduire le champ expérimental étudié et ainsi fournir des informations pertinentes sur les risques d’expansion attribuée à la DEF, à partir de la connaissance de la température et de la durée de l’échauffement, la teneur en alcalins du béton, la nature des granulats et le rapport E/C.

     

Influence of recycled aggregates and high contents of fly ash on concrete fresh properties

This study presents the fresh properties of concrete with supplementary cementitious materials (SCM) and recycled concrete aggregates (RCA), with emphasis on the feasibility of using high volumes of fly ash (FA) in RCA concrete. For this purpose, two mix families (0% coarse RCA and 100% coarse RCA) were produced, both with and without superplasticizers (SP). The coarse natural aggregates (NA) were replaced with coarse RCA at 0% and 100%, respectively. For each of the mentioned families, three incorporation levels (0%, 50% and 100%) of fine RCA were used with 0%, 30% and 60% of FA, resulting in 28 compositions. Each mix was tested in the fresh state by means of slump, density and air content. The results of this study show that RCA decreased the slump of concrete mixes, but the required water content can be minimized by incorporation FA. Regardless of the water absorption of the aggregates, for a given fine RCA incorporation ratio and the same ratio of FA, no increase in water content is required to obtain the same target slump as in the reference concrete. On the other hand, for a given coarse RCA incorporation ratio, a five times lower FA ratio is enough to obtain the same target slump as in the reference concrete. Air voids in concrete mixes were more affected by the shape of the aggregates than by their water absorption. The air content of concrete mixes increased as the incorporation levels of FA and RCA increased. However, in comparison with the individual effects, the air content decreased by combining the incorporation of both FA and RCA. Moreover, the rate of reduction in fresh density by increasing the incorporation of RCA and FA was similar in concrete mixes with and without SP.