Showing papers in "Cement & Concrete Composites in 2015"
TL;DR: Graphene oxide (GO) is the product of chemical exfoliation of graphite and is a potential candidate for use as nanoreinforcements in cement-based materials as discussed by the authors.
Abstract: Graphene oxide (GO) is the product of chemical exfoliation of graphite. Due to its good dispersibility in water, high aspect ratio and excellent mechanical properties, GO is a potential candidate for use as nanoreinforcements in cement-based materials. In this paper, GO was used to enhance the mechanical properties of ordinary Portland cement paste. The introduction of 0.05 wt% GO can increase the GO-cement composite compressive strength by 15-33% and the flexural strength by 41-59%, respectively. Scanning electron microscope imaging of the GO-cement composite shows the high crack tortuosity, indicating that the two-dimensional GO sheet may form a barrier to crack propagation. Consequently, the GO-cement composite shows a broader stress-strain curve within the post-peak zone, leading to a less sudden failure. The addition of GO also increases the surface area of the GO-cement composite. This is attributed to increasing the production of calcium silicate hydrate. The results obtained in this investigation suggest that GO has potential for being used as nano-reinforcements in cement-based composite materials.
574 citations
TL;DR: In this paper, a modified Andreasen & Andersen particle packing model is used to achieve a densely compacted cementitious matrix, and the results show that the influence of FA, ground granulated blast-furnace slag (GGBS) and limestone powder (LP) on the early hydration kinetics of UHPC is very similar during the initial five days, while the hydration rate of the blends with GGBS is mostly accelerated afterwards.
Abstract: This paper addresses the development of an eco-friendly Ultra-High Performance Concrete (UHPC) with efficient cement and mineral admixtures uses are investigated. The modified Andreasen & Andersen particle packing model is utilized to achieve a densely compacted cementitious matrix. Fly ash (FA), ground granulated blast-furnace slag (GGBS) and limestone powder (LP) are used to replace cement, and their effects on the properties of the designed UHPC are analyzed. The results show that the influence of FA, GGBS or LP on the early hydration kinetics of the UHPC is very similar during the initial five days, while the hydration rate of the blends with GGBS is mostly accelerated afterwards. Moreover, the mechanical properties of the mixture with GGBS are superior, compared to that with FA or LP at both 28 and 91 days. Due to the very low water amount and relatively large superplasticizer dosage in UHPC, the pozzolanic reaction of FA is significantly retarded. Additionally, the calculations of the embedded CO2 emission demonstrate that the cement and mineral admixtures are efficiently used in the developed UHPC, which reduce its environmental impact compared to other UHPCs found in the literature.
408 citations
TL;DR: In this paper, a fly ash with partial slag substitution was used for GFC synthesis by mechanical mixing of preformed foam, and the GFCs exhibited 28 d compressive strengths ranging from 3 to 48 MPa with demolded densities from 720 to 1600 kg/m3 (105 °C oven-dried densities ranging from 585 to 1370 kg /m3), with the different densities achieved through alteration of the foam content.
Abstract: This study reports the synthesis and characterization of geopolymer foam concrete (GFC). A Class F fly ash with partial slag substitution was used for GFC synthesis by mechanical mixing of preformed foam. The GFCs exhibited 28 d compressive strengths ranging from 3 to 48 MPa with demolded densities from 720 to 1600 kg/m3 (105 °C oven-dried densities from 585 to 1370 kg/m3), with the different densities achieved through alteration of the foam content. The thermal conductivity of GFCs was in the range 0.15–0.48 W/m K, showing better thermal insulation properties than normal Portland cement foam concrete at the same density and/or at the same strength. The GFC derived from alkali activation of fly ash as a sole precursor showed excellent strength retention after heating to temperatures from 100 to 800 °C, and the post-cooling compressive strength increased by as much as 100% after exposure at 800 °C due to densification and phase transformations. Partial substitution of slag for fly ash increased the strength of GFC at room temperature, but led to notable shrinkage and strength loss at high temperature. Thin GFC panels (20–25 mm) exhibited acoustic absorption coefficients of 0.7–1.0 at 40–150 Hz, and 0.1–0.3 at 800–1600 Hz.
356 citations
TL;DR: In this article, the composition and properties of highly flowable self-consolidating concrete (SCC) mixtures made of high proportions of cement replacement materials such as fly ash and pulverized limestone instead of high dosage of a plasticizing agent or viscosity-modifying chemical admixtures were reported.
Abstract: This paper reports the composition and properties of highly flowable self-consolidating concrete (SCC) mixtures made of high proportions of cement replacement materials such as fly ash and pulverized limestone instead of high dosage of a plasticizing agent or viscosity-modifying chemical admixtures. Self-consolidating concrete mixtures are being increasingly used for the construction of highly reinforced complex concrete elements and for massive concrete structures such as dams and thick foundation. In this study, by varying the proportion of portland cement (OPC), Class F-fly ash (F), and limestone powder (L), SCC mixtures with different strength values were produced, and the properties of both fresh and hardened concrete were determined. For a comprehensive analysis and quantification of emissions and global warming potential (GWP) from concrete production, life-cycle assessment (LCA) was employed. We find that high volume, up to 55% by weight replacement of OPC with F, or F and L produces highly workable concrete that has high 28-day and 365-day strength, and extremely high to very high resistance to chloride penetration along with low GWP for concrete production.
322 citations
TL;DR: In this article, a small proportion of ordinary Portland cement (OPC) was added with low calcium fly ash to accelerate the curing of geopolymer concrete instead of using elevated heat.
Abstract: Most previous works on fly ash based geopolymer concrete focused on concretes subjected to heat curing. Development of geopolymer concrete that can set and harden at normal temperature will widen its application beyond precast concrete. This paper has focused on a study of fly ash based geopolymer concrete suitable for ambient curing condition. A small proportion of ordinary Portland cement (OPC) was added with low calcium fly ash to accelerate the curing of geopolymer concrete instead of using elevated heat. Samples were cured in room environment (about 23 °C and RH 65 ± 10%) until tested. Inclusion of OPC as little as 5% of total binder reduced the setting time to acceptable ranges and caused slight decrease of workability. The early-age compressive strength improved significantly with higher strength at the age of 28 days. Geopolymer microstructure showed considerable portion of calcium-rich aluminosilicate gel resulting from the addition of OPC.
307 citations
TL;DR: In this article, the hydration of quaternary Portland cements containing blast-furnace slag, type V fly ash and limestone and the relationship between the types and contents of supplementary cementitious materials and the hydrate assemblage were investigated at ages of up to 182 days.
Abstract: In this study the hydration of quaternary Portland cements containing blast-furnace slag, type V fly ash and limestone and the relationship between the types and contents of supplementary cementitious materials and the hydrate assemblage were investigated at ages of up to 182 days using X-ray diffraction and thermogravimetric analysis. In addition thermodynamic modeling was used to calculate the total volume of hydrates. Two blast-furnace slag contents of 20 and 30 wt.% were studied in blends containing fly ash and/or limestone at a cement replacement of 50 wt.%. In all cases the experiments showed the presence of C–S–H, portlandite and ettringite. In samples without limestone, monosulfate was formed; in the presence of limestone monocarbonate was present instead. The addition of 5 wt.% of limestone resulted in a higher compressive strength after 28 days than observed for cements with lower or higher limestone content. Overall the presence of fly ash exerts little influence on the hydrate assemblage. The strength development reveals that amounts of up to 30 wt.% fly ash can be used in quaternary cements without significant loss in compressive strength.
254 citations
TL;DR: In this paper, the performance of concrete with sugarcane bagasse ash (SCBA) based blended cements with different replacement levels of SCBA, and the performance in terms of compressive strength, heat of hydration, drying shrinkage and durability are discussed.
Abstract: By-products from a number of industrial processes are used as alternative supplementary cementitious materials in concrete. Sugarcane bagasse ash is mainly composed of amorphous silica and can be used as a pozzolanic material in concrete. Production of sugarcane bagasse ash (SCBA) based blended cements with different replacement levels of SCBA, and the performance of concrete with these cements in terms of compressive strength, heat of hydration, drying shrinkage and durability are discussed in this paper. Durability performance was investigated by five different methods in this study, namely oxygen permeability test, rapid chloride penetration test, chloride conductivity test, water sorptivity test, DIN water permeability test and Torrent air permeability test. The results from this study show that use of sugarcane bagasse ash in concrete prominently enhances its performance. Low heat of hydration, additional strength gain due to pozzolanic reaction, significant reduction in permeability because of pore refinement and similar drying shrinkage behavior were observed for bagasse ash blended concrete compared to control concrete.
233 citations
TL;DR: In this paper, the effect of incorporating recycled aggregates, sourced from construction and demolition waste, on the carbonation behavior of concrete is reviewed and a statistical analysis is presented on the effect that introducing increasing amounts of recycled aggregate concrete on carbonation depth and coefficient of accelerated carbonation is presented.
Abstract: This paper reviews the effect of incorporating recycled aggregates, sourced from construction and demolition waste, on the carbonation behaviour of concrete. It identifies various influencing aspects related to the use of recycled aggregates, such as replacement level, size and origin, as well as the influence of curing conditions, use of chemical admixtures and additions, on carbonation over a long period of time. A statistical analysis on the effect of introducing increasing amounts of recycled aggregates on the carbonation depth and coefficient of accelerated carbonation is presented. This paper also presents the use of existing methodologies to estimate the required accelerated carbonation resistance of a reinforced recycled aggregate concrete exposed to natural carbonation conditions with the use of accelerated carbonation tests. Results show clear increasing carbonation depths with increasing replacement levels when recycled aggregate concrete mixes are made with a similar mix design to that of the control natural aggregate concrete. The relationship between the compressive strength and coefficients of accelerated carbonation is similar between the control concrete and the recycled aggregate concrete mixes.
228 citations
TL;DR: In this article, the early age reaction kinetics and microstructural development in alkali-activated slag binder are discussed, and in-situ isothermal calorimetric data for sodium hydroxide-activated systems exhibited only one major heat evolution peak with no dormant period.
Abstract: The early age reaction kinetics and microstructural development in alkali-activated slag binder are discussed. In-situ isothermal calorimetry was used to characterize the reaction progression in sodium hydroxide and sodium silicate-activated slag binders cured at ambient temperature. Microstructure and strength development were monitored to correlate the heat evolution with the property development. In-situ isothermal calorimetric data for sodium hydroxide-activated systems exhibited only one major heat evolution peak with no dormant period. Sodium silicate-activated pastes exhibited multiple peaks and extended dormant periods. Microstructural evolution, monitored using BSE–SEM, showed rapid product formation on the surface of slag grains in sodium hydroxide-activated systems, forming thin reaction shells—the thickness of which was related to the activator concentration—and leading to diffusion controlled hydration at a very early stage. Sodium silicate-activated systems exhibited slow and progressive product formation, predominately nucleated from the solution. These results are supported by electron mapping and electron dispersive X-ray spectroscopy.
212 citations
TL;DR: In this article, the influence of cellulose nanocrystals (CNCs) on the performance of cement paste was investigated, and the results indicate that short circuit diffusion is more dominant than steric stabilization.
Abstract: The influence of cellulose nanocrystals (CNCs) addition on the performance of cement paste was investigated. Our mechanical tests show an increase in the flexural strength of approximately 30% with only 0.2% volume of CNCs with respect to cement. Isothermal calorimetry (IC) and thermogravimetric analysis (TGA) show that the degree of hydration (DOH) of the cement paste is increased when CNCs are used. The first mechanism that may explain the increased hydration is the steric stabilization, which is the same mechanism by which many water reducing agents (WRAs) disperse the cement particles. Rheological, heat flow rate measurements, and microscopic imaging support this mechanism. A second mechanism also appears to support the increased hydration. The second mechanism that is proposed is referred to as short circuit diffusion. Short circuit diffusion appears to increase cement hydration by increasing the transport of water from outside the hydration product shell (i.e., through the high density CSH) on a cement grain to the unhydrated cement cores. The DOH and flexural strength were measured for cement paste with WRA and CNC to evaluate this hypothesis. Our results indicate that short circuit diffusion is more dominant than steric stabilization.
208 citations
TL;DR: In this paper, the effects of nano-SiO 2 particles on the mechanical performance, hydration process and microstructure evolution of ultra-high performance cementitious composites were investigated by different methods.
Abstract: In this research the effects of nano-SiO 2 particles on the mechanical performance, hydration process and microstructure evolution of ultra-high performance cementitious composites were investigated by different methods. The results showed that the compressive and flexural strength increased with the increase of the nano-SiO 2 content up to 3% and due to agglomeration of nano-SiO 2 particles, the mechanical properties decreased slightly when the nano-SiO 2 content was more than 3%. The hydration process was accelerated by the addition of nano-SiO 2 . The porosity and the average pore diameter decreased with the increase of the nano-SiO 2 content and aging. The microstructure was more homogenous and dense for nano-SiO 2 specimens as compared to the control specimen. All of these improvements could be mainly attributed to the pozzolanic and filler effects of nano-SiO 2 .
TL;DR: In this article, ground granulated blastfurnace slag (GGBS) is activated by two commercial reactive MgOs, with contents ranging from 2.5% to 20% up to 90 days.
Abstract: Ground granulated blastfurnace slag (GGBS) is widely used as a partial replacement for Portland cement or as the major component in the alkali-activated cement to give a clinker-free binder. In this study, reactive MgO is investigated as a potentially more practical and greener alternative as a GGBS activator. This paper focuses on of the hydration of GGBS, activated by two commercial reactive MgOs, with contents ranging from 2.5% to 20% up to 90 days. The hydration kinetics and products of MgO–GGBS blends were investigated by selective dissolution, thermogravimetric analysis, X-ray diffraction and scanning electron microscopy techniques. It was found that reactive MgO was more effective than hydrated lime in activating the GGBS based on unconfined compressive strength and the efficiency increased with the reactivity and the content of the MgO. It is hence proposed that reactive MgO has the potential to serve as an effective and economical activator for GGBS.
TL;DR: In this paper, the effect of these particles on the cement hydration was followed by heat flow calorimetry to evaluate their acceleration potential as a function of the particles material, particles size and their percentage in the cement paste.
Abstract: Nanoparticles have recently become a focus in the development of new accelerators for cement hydration. We have produced nanoparticles of different materials like Al2O3, C–S–H-phase, and quartz by different top-down and bottom-up production methods. The effect of these particles on the cement hydration was followed by heat flow calorimetry to evaluate their acceleration potential as a function of the particles material, particles size and their percentage in the cement paste. In this work we will demonstrate which particles have the best potential as accelerators for cement hydration and therefore are most suitable for further investigations. Interestingly, nanoparticles which have a retarding effect on cement hydration were also found.
TL;DR: In this paper, a comprehensive evaluation of pozzolanic activity of sugarcane bagasse ash based on different processing methods including burning, grinding, complete removal of coarse fibrous particles by sieving and combinations of these methods were examined in this study.
Abstract: Sugarcane bagasse ash is obtained as a by-product from cogeneration combustion boilers in sugar industries. Previous studies have reported that the use of sugarcane bagasse ash as supplementary cementitious material in the concrete can improve its properties. The utilization of bagasse ash has been constrained because of inadequate understanding of the material and lack of suitable processing methodology for use in a large scale. Processing methods significantly influence the pozzolanic activity of any supplementary cementitious material. Proper assessment of pozzolanic activity and processing methodology of bagasse ash were not investigated in earlier research studies. This paper describes a study that involves pozzolanic performance evaluation and microstructural characterization of sugarcane bagasse ash for use as pozzolanic material in concrete. A comprehensive evaluation of pozzolanic activity of sugarcane bagasse ash based on different processing methods including burning, grinding, complete removal of coarse fibrous particles by sieving and combinations of these methods were examined in this study. Suitable processing methodology to attain maximum pozzolanic activity of sugarcane bagasse ash with minimum level of processing is described in this paper.
TL;DR: In this paper, the flexural behavior of steel-fiber-reinforced concrete (SFRC) beams under quasi-static and impact loads was investigated, and the impact tests were performed using a drop-weight impact test machine for two different incident potential energies of 40 and 100 J.
Abstract: This study aims to investigate the flexural behavior of steel-fiber-reinforced concrete (SFRC) beams under quasi-static and impact loads. For this, a number of SFRC beams with three different compressive strengths ( f c ' of approximately 49, 90, and 180 MPa) and four different fiber volume contents (vf of 0, 0.5, 1.0, and 2.0%) were fabricated and tested. The quasi-static tests were carried out according to ASTM standards, while the impact tests were performed using a drop-weight impact test machine for two different incident potential energies of 40 and 100 J. For the case of quasi-static load, enhancements in the flexural strength and deflection capacity were obtained by increasing the fiber content and strength, and higher toughness was observed with an increase in the fiber content. For the case of impact load, an increase in the load carrying capacity was obtained by increasing the potential energy and strength, and an improvement in the post-peak behavior was observed by increasing the fiber content. The increases in fiber content and strength also led to enhancements in residual flexural performance after impact damage. Finally, the flexural strength became less sensitive to the strain-rate (or stress-rate) as the strength of concrete increased.
TL;DR: In this paper, the effects of raw materials' composition on fresh behavior, reaction kinetics, mechanical properties and microstructure of alkali activated slag-fly ash-limestone blends are investigated.
Abstract: In this article, the effects of raw materials’ composition on fresh behavior, reaction kinetics, mechanical properties and microstructure of alkali activated slag–fly ash–limestone blends are investigated. The results indicate that, with the increasing content of fly ash and limestone, the slump flow increases. The setting times are shortened when increasing the slag content, while both fly ash and limestone show a negligible influence. The reaction process is slightly accelerated by the presence of limestone due to the extra provided nucleation sites, but the reaction process is mainly governed by the slag. The slag content exhibits a dominant role on strength in this ternary system, while for a constant slag content, the compressive strength increases with the increasing limestone content up to 30%. The microstructure analysis shows that the gel characteristics are independent of the limestone powder content. The presence of limestone in initially high Ca and Al conditions does not lead to the formation of additional crystalline phases, which is different from Portland cement systems. Both physically and chemically bound water contents are slightly increased when limestone powder is incorporated.
TL;DR: In this article, the compressive strength and hydration products of three blends (85-15, 70-30 and 40-60% of OPC-CSA cement) using two different OPC.
Abstract: Ordinary Portland cement (OPC) and calcium sulfoaluminate (CSA) cement are two chemically different hydraulic binders. OPC and CSA cement blends can be used to adjust the binder properties for specific applications. The first part of this article compares the compressive strength and hydration products of three blends (85–15, 70–30 and 40–60% of OPC–CSA cement) using two different OPC. CSA cement percentage modifies the hardening speed as well as the hydration mechanisms (hydrates nature and quantity). The composition of OPC has also a significant influence even for the lowest OPC proportion (40% of OPC). In the second part, investigations based on compressive strength and calorimetry analysis indicate that OPC free lime is a key parameter.
TL;DR: In this paper, a process was developed to quickly screen SFS aggregates for free oxide contents and expansion potential using complexometric titration, thermogravimetric analysis, and an autoclave expansion test.
Abstract: Steel furnace slag (SFS) is an industrial by-product that is not commonly utilized in bound applications because of its potential to contain high contents of free calcium and magnesium oxides, which expand when hydrated. In this study, a process was developed to quickly screen SFS aggregates for free oxide contents and expansion potential using complexometric titration, thermogravimetric analysis, and an autoclave expansion test. Two of the three SFS aggregate sources (high and low expansion) were selected for testing as a coarse aggregate in concrete. It was confirmed that SFS aggregates in concrete can produce acceptable strength properties, suitable freeze/thaw durability, and exceptional fracture properties. However, these SFS aggregates produced greater free drying shrinkage than concrete with dolomite aggregates. For SFS aggregates having low expansion potential, the hardened property tests indicate that SFS may be a suitable aggregate for concrete.
TL;DR: In this article, nano-TiO2 was added to AAS, and its mechanical properties and shrinkage were tested to examine its effect on hardened alkali-activated slag paste (AASP).
Abstract: For alkali-activated slag (AAS), high drying shrinkage is an obstacle which impedes its application as a construction material. In this investigation, nano-TiO2 was added to AAS, and its mechanical properties and shrinkage were tested to examine its effect on hardened alkali-activated slag paste (AASP). To understand the impact of nano-TiO2 on AASP at micro scale, FTIR, MIP and SEM were carried out. Experimental results indicate that the addition of nano-TiO2 to AAS enhances the mechanical strength, and decreases the shrinkage of AASP. FTIR and SEM results demonstrated that the addition of nano-TiO2 into the AASP accelerates its hydration process, resulting in more hydration products and denser structure. MIP results showed that the addition of nano-TiO2 reduces the total porosity of AASP and changes the pore structure. The porosity of 1.25–25 nm mesopores, which is believed to be responsible for the high shrinkage of AASP, is remarkably reduced due to the addition of nano-TiO2.
TL;DR: In this paper, three different polymers with a low glass transition temperature Tg have been extruded: Poly(lactic acid) (PLA), Polystyrene (PS), and Poly(methyl methacrylate/n-butyl methACrylate) (P(MMA/n)-BMA)) (Tg = 59°C).
Abstract: Up to now, glass capsules, which cannot resist the mixing process of concrete, have been mostly used in lab-scale proof-of-concept to encapsulate polymeric agents in self-healing concrete. This study presents the design of polymeric capsules which are able to resist the concrete mixing process and which can break when cracks appear. Three different polymers with a low glass transition temperature Tg have been extruded: Poly(lactic acid) (PLA) (Tg = 59 °C), Polystyrene (PS) (Tg = 102 °C) and Poly(methyl methacrylate/n-butyl methacrylate) (P(MMA/n-BMA)) (Tg = 59 °C). After heating the capsules prior to mixing with other components of the mix, to shift from a brittle state to a rubbery state, their survival ratio considerably increased. Moreover, a part of the capsules, which previously survived the concrete mixing process, broke with crack appearance. Although some optimization is still necessary concerning functional life of encapsulated adhesives, this seems to be a promising route.
TL;DR: In this article, the effects of the addition of minerals of various compositions (i.e., silica-based materials, chemical expansive agents, swelling minerals and crystalline components) on the self-healing performances of cementitious materials were investigated.
Abstract: The effects of the addition of minerals of various compositions (i.e., silica-based materials, chemical expansive agents, swelling minerals and crystalline components) on the self-healing performances of cementitious materials were investigated. The self-healing capabilities were assessed by performing water permeability tests, quantifying the widths of the surface cracks and studying the water absorption of mortars that were pre-cracked by either splitting or compression. The results showed that the cracks that appeared early on healed more efficiently when they were cured in still rather than flowing water. High pHs and high temperatures accelerate crack healing. The healing efficiency can be further improved by utilizing a combination of minerals rather than a single mineral. A self-healing mechanism was discussed by combining these results with micro-observations. The precipitation of calcium carbonate, which is aided by higher pH values and higher calcium ion contents, was found to be the main contributor to the healing of surface cracks.
TL;DR: In this article, the results of an experimental investigation of UHP-FRC tensile response under a range of low strain rates are presented, and the rate sensitivity of the composite material in tension is evaluated in terms of its first cracking strength, post-cracking strength, energy absorption capacity, strain capacity, elastic modulus, fiber tensile stress and number of cracks.
Abstract: The results of an experimental investigation of UHP-FRC tensile response under a range of low strain rates are presented. The strain rate dependent tests are conducted on dogbone specimens using a hydraulic servo-controlled testing machine. The experimental variables are strain rate, which ranges from 0.0001 1/s to 0.1 1/s, fiber type, and fiber volume fraction. Five different types of fibers are considered including straight and twisted fibers with different geometric properties. The rate sensitivity of the composite material in tension is evaluated in terms of its first cracking strength, post-cracking strength, energy absorption capacity, strain capacity, elastic modulus, fiber tensile stress and number of cracks. The test results show pronounced rate effects on post-cracking strength and energy absorption capacity. Further, post cracking strength varies linearly with the fiber reinforcing index and energy absorption capacity varies linearly with the product of the fiber length and the reinforcing index, as predicted from the theory for fiber reinforced concrete.
TL;DR: In this article, the characteristics of the pozzolanic reactivity of nanoSiO2 from studies of its reaction kinetics, morphology and structure of the hydrates and the influences of these features on the properties of cement-based materials were explored.
Abstract: The aim of this work is to understand the characteristics of the pozzolanic reactivity of nanoSiO2 from studies of its pozzolanic reaction kinetics, morphology and structure of the hydrates and the influences of these features on the properties of cement-based materials, so as to explore a more targeted way of using nanoSiO2 in cement or concrete. It revealed that the pozzolanic reaction of nanoSiO2 is of the first-order and the apparent reaction rate constant of nanoSiO2-4 nm is about one order of magnitude bigger than that of silica fume, but the specific reaction rate constant is about one half to that of silica fume. A compacter gel structure and poorer crystallinity of the hydrates of nanoSiO2 to those of silica fume are found, as well. The rate of hydration of cement at very early ages is enhanced by nanoSiO2, but the rate slows down with aging due to the compact gel structure. To make the use of the high pozzolanic reactivity and ultrafine particle size of nanoSiO2, as well as its resulting compact gel structure, colloidal nanoSiO2 was applied onto the hardened cement mortar by brushing technique and a less permeable surface was resulted, which shows the potential of using nanoSiO2 as a surface treatment material for cement-based materials.
TL;DR: In this paper, the authors report a full study of the hydration of calcium sulfoaluminate eco-cements with different sulfate sources (gypsum, bassanite and anhydrite) and two water/cement ratios (0.50 and 0.65).
Abstract: The availability of cements, including eco-cements, with tailored mechanical properties is very important for special applications in the building industry. Here we report a full study of the hydration of calcium sulfoaluminate eco-cements with different sulfate sources (gypsum, bassanite and anhydrite) and two water/cement ratios (0.50 and 0.65). These parameters have been chosen because they are known to strongly modify the mechanical properties of the resulting mortars and concretes. The applied multi-technique characterization includes: phase assemblage by Rietveld method, evolved heat, conductivity, rheology, compressive strength and expansion/retraction measurements. The dissolution rate of the sulfate sources is key to control the hydration reactions. Bassanite dissolves very fast and hence the initial setting time of the pastes and mortars is too short (20 min) to produce homogeneous samples. Anhydrite dissolves slowly so, at 1 hydration-day, the amount of ettringite formed (20 wt%) is lower than that in gypsum pastes (26 wt%) (w/c = 0.50), producing mortars with lower compressive strengths. After 3 hydration-days, anhydrite pastes showed slightly larger ettringite contents and hence, mortars with slightly higher compressive strengths. Ettringite content is the chief parameter to explain the strength development in these eco-cements.
TL;DR: In this paper, the authors studied the dynamic behavior of concrete by testing annular and solid concrete specimens using split Hopkinson pressure bar (SHPB) and found that the dynamic increase factor (DIF) of annular samples is relatively lower than the DIF of solid samples.
Abstract: The dynamic behavior of concrete is studied experimentally by testing annular and solid concrete specimens using split Hopkinson pressure bar (SHPB). The dynamic increase factor (DIF) of annular samples is relatively lower than the DIF of solid samples. The dynamic behavior of concrete seems to be independent of the quasi-static strength of concrete. The mode of failure of concrete was a typical ductile failure at high strain-rates and brittle at low strain-rates. No significant influence of strain-rate on the initial elastic modulus of concrete was observed. An empirical equation is proposed for the estimation of DIF of concrete based on the experiments. A model is developed for the prediction of stress–strain curve of concrete under dynamic loading which shows good agreement with the experiments.
TL;DR: In this article, the authors developed a prediction model of the chloride diffusion coefficient for silica fume concrete under long-term exposure to a durability site located in the southern region of Iran.
Abstract: Chloride-induced corrosion is the main factor in determining the durability and service life of the reinforced concrete structures exposed to marine environments. Recognition of chloride diffusion phenomenon in concrete and developing a prediction model that can estimate the service life of the concrete structures subject to long-term exposure is vital for aggressive marine environments. The present study focuses on developing such a prediction model of chloride diffusion coefficient for silica fume concrete under long-term exposure to a durability site located in the southern region of Iran. All investigations are based on 16 concrete mix designs containing silica fume with variable water-to-binder ratios exposed to sea water for maximum period of 60 months. This empirical model is developed by applying regression analysis based on Fick’s second law on the experimental results and is compared with previous studies in this area. This comparison indicates that the predicted chloride diffusion coefficient level is within a ±25% error margin in the specimens. The results indicate that reducing the water-to-binder ratio and adding the silica fume to a dosage of 10% reduces the chloride diffusion coefficient in concrete. This study also confirms that the chloride diffusion coefficient increases with temperature and decreases over time.
TL;DR: In this paper, the properties of RHA samples obtained at different conditions in muffle furnace and their effects on the mechanical behavior of cement have been studied, the properties were characterized by SEM, XRF, XRD and specific surface area.
Abstract: The properties of RHA samples obtained at different conditions in muffle furnace and their effects on the mechanical behavior of cement have been studied. The properties were characterized by SEM, XRF, XRD and specific surface area. The effect of using RHA as a partial replacement for cement has been investigated by compressive and flexural strengths test. It is showed that in muffle furnace 600 °C is the appropriate temperature for RHA preparation with large specific surface area due to the existences of nanoscale and amorphous SiO 2 . While, long combustion time could increase pozzolanic activity of RHA. The presence of K could decrease the specific surface area and increase the residual carbon content which content should be control strictly. RHA obtained in proper conditions can be used as cement additive to increase compressive and flexural strengths of cement mortar specimens. It is verified that RHA replacement ratio of 10% (by weight) has the best enhancement effect on cement strength.
TL;DR: In this paper, a broad study of Portland cement mortar mixtures containing silica fume, plain or silica-functionalized carbon nanotubes, and carbon fibers was performed to characterize changes in fracture properties.
Abstract: Fiber reinforcements provide many benefits to cementitious composites, including reduction of crack widths and increases in ductility. However, the interfacial transition zone between fibers and hydrated cement can contain a high proportion of calcium hydroxide and porosity. With their high moduli of elasticity, carbon nanotubes and carbon fibers could provide substantial mechanical reinforcement at multiple length scales, but only if their bond to the matrix can be controlled. Surface treatments of fibers and addition of supplemental materials in the matrix can influence both the mechanical interaction at the interface and the dispersion of these relatively small reinforcements. We performed a broad study of Portland cement mortar mixtures containing silica fume, plain or silica-functionalized carbon nanotubes, and carbon fibers to characterize changes in fracture properties. The early age hydration kinetics of cement pastes containing carbon nanotubes were compared using isothermal calorimetry. Early age fracture surfaces of cement pastes containing carbon fibers were observed using a scanning electron microscope. The notched beam test method of the Two Parameter Fracture Model was used to determine the fracture properties of each mix. We observed that silica fume and silica functional groups improved the fracture performance of mixtures containing carbon nanotubes and carbon fibers. Further optimization of dosage, size, and interface strength is required to fully utilize carbon nanotubes in cementitious composites.
TL;DR: In this article, the authors investigated the flowability, compressive strength, heat of hydration, porosity and calcium hydroxide content of ultra-high-strength concrete (UHSC) with cement-silica fume-slag binder at 20°C.
Abstract: This study investigated the flowability, compressive strength, heat of hydration, porosity and calcium hydroxide content of ultra-high-strength concrete (UHSC) with cement–silica fume–slag binder at 20 °C The composition of the binder was designed using seven-batch factorial design method The relationships between the binder composition and the properties were expressed in contours Results showed that proper silica fume content could improve the flowability and compressive strength of UHSC, reduce the porosity and calcium hydroxide content of UHSC Slag reduced the flowability, compressive strength, porosity, and calcium hydroxide content of UHSC to certain extent The silica fume and slag demonstrated positive synergistic effects on the flowability and 3 d compressive strength, but have negative synergistic effects on the total heat of hydration, hydration heat when the time is infinitely long(P0), 56 d compressive strength, porosity and calcium hydroxide content of UHSC
TL;DR: In this paper, the carbonation resistance of mortar and concrete is measured by the ratio between water added during production and the amount of reactive CaO present in the binder (w/CaO active).
Abstract: When cement with mineral additions is employed, the carbonation resistance of mortar and concrete may be decreased. In this study, mortars containing mineral additions are exposed both to accelerated carbonation (1% and 4% CO2) and to natural carbonation. Additionally, concrete mixtures produced with different cements, water-to-cement ratios and paste volumes are exposed to natural carbonation. The comparison of the carbonation coefficients determined in the different exposure conditions indicates that mortar and concrete containing slag and microsilica underperform in the accelerated carbonation test compared to field conditions. The carbonation resistance in mortar and concrete is mainly governed by the CO2 buffer capacity per volume of cement paste. It can be expressed by the ratio between water added during production and the amount of reactive CaO present in the binder (w/CaOreactive) resulting in a novel parameter to assess carbonation resistance of mortar and concrete containing mineral additions.