•Factors influencing the inter-layer strength in concrete 3D printing are studied.•The moisture level at the surface is identified as an important factor.•Low moisture level is linked to low ...inter-layer strength in 3D concrete printing.•The surface moisture level is affected by bleeding and evaporation rates.•Strengths of 3D printed concrete were different in x, y and z directions.
The extrusion-based 3D concrete printing is a new technology under development for construction of buildings and structures of complex geometries without the use of expensive formwork. The weak inter-layer strength between printed layers is one of the limitations of this technology when compared with cast-in-the-mold concrete. This study investigates effects of print-time interval on the inter-layer strength, along with compressive and flexural strengths of extrusion-based 3D printed concrete in different directions. Specimens were printed with 10, 20 and 30 min delay times (print-time intervals). Compressive, flexural and inter-layer strengths of the 3D printed concretes were measured. The inter-layer strengths of the specimens printed with 10 and 30 min delay times were comparable but higher than that of the specimens printed with 20 min delay time. A correlation was found between the results of inter-layer strength and the surface moisture content at the interface of the layers. The surface moisture content, in turn, depends on the bleed rate of the concrete and the rate of drying of moisture from the surface among other factors. The results also indicated that the compressive and flexural strengths of 3D printed concrete depended on the testing direction. The orthotropic phenomenon was more pronounced in compressive than flexural strength.
Geopolymers are generally believed to provide good fire resistance due to their ceramic-like properties. Previous experimental studies on geopolymer under elevated temperatures have mainly focused on ...metakaolin-based geopolymers. This paper presents the results of a study on the effect of elevated temperature on geopolymer paste, mortar and concrete made using fly ash as a precursor. The geopolymer was synthesized with sodium silicate and potassium hydroxide solutions. Various experimental parameters have been examined such as specimen sizing, aggregate sizing, aggregate type and superplasticizer type. The study identifies specimen size and aggregate size as the two main factors that govern geopolymer behavior at elevated temperatures (800
°C). Aggregate sizes larger than 10
mm resulted in good strength performances in both ambient and elevated temperatures. Strength loss in geopolymer concrete at elevated temperatures is attributed to the thermal mismatch between the geopolymer matrix and the aggregates.
•Summarizing the latest literature on CNTs and GO reinforced cement composites.•Cement fabrication focuses on separating and avoiding re-agglomeration of nanomaterials.•Preserving the workability of ...nanocomposites is accomplished by using admixtures.•The seeding of C–S–H gel occurs on the large surface area of nanomaterials.•Reinforcement and pore refinement by 1D and 2D nanomaterials strengthen cement.
Progress in the field of nanomaterials presents an invaluable opportunity to develop cementitious composites at the nanoscale. Engineered nanomaterials exist in three principal shapes, namely 0D nanoparticle, 1D nanofiber and 2D nanosheet. The application of 0D nanoparticle and 1D nanofiber, such as nanosilica and carbon nanotubes (CNTs), respectively, has been reported in literature. The discovery of 2D nanosheet known as graphene oxide (GO) provides an extra dimension to interact with cement and concrete matrix and has yet to gain widespread attention. In this paper, recent research studies in developing cement and concrete nanocomposites are comprehensively reviewed. Also highlighted herein are the effect of incorporating nanomaterials in low dosages to the fabrication, workability, hydration, microstructure, and mechanical properties of cement-based composites.
Alkali activated binders (geopolymer) is an emerging technology to produce concrete without the use of any Ordinary Portland Cement (OPC) by alkali activation of alumino-silicate source materials ...such as fly ash and/or slag. Limited knowledge on durability issues like corrosion behaviour of reinforced geopolymer concrete impedes the usage of this technology in structural applications.
This study explored the chloride permeability and initiation of chloride induced corrosion of geopolymer concrete in accelerated chloride environment using longer test period. Corrosion initiation was also monitored in embedded rebar in 2% chloride contaminated concrete. Corrosion state of the rebar was monitored using non-destructive test method using Cu/CuSO4 reference electrode.
The results showed that the apparent chloride diffusion coefficient of blended fly ash and slag geopolymer concrete is lower than that of OPC concrete. The diffusion coefficient also decreased with the increase of slag content in the binder. Blended fly ash and slag geopolymer concrete also exhibited higher aging factor than OPC concrete indicating improved resistance to chloride ingress with time. The study also showed that the embedded rebar in fly ash and slag based geopolymer concrete has higher protection against corrosion than a rebar in OPC concrete even when the concrete is contaminated with significant levels of chloride.
•Chloride ingress rate in geopolymers containing slag is low.•Age factor of geopolymer is significantly higher than OPC.•Geopolymer can hinder corrosion in steel rebar.
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•The dispersion of GO in water is independent of the sonication degree with only small amount of GO reagglomeration.•High alkalinity and calcium ions are key factors inducing the agglomeration of GO ...in cement system.•Polycarboxylate-based superplasticisers exhibits the most promising results to disperse GO in cement alkaline environment.•ADVA 210 can disperse GO better in the pore solution environment, which was preferable to preparing GO-ADVA 210 suspension.•The inclusion of 0.03% GO protected by cement weight increased the GO-cement composite flexural strength up to 67%.
Graphene oxide (GO) is a novel class of two-dimensional nanoscale sheet material due to its excellent dispersibility in water, high aspect ratio and good intrinsic strengths. In order to obtain a well-distributed GO-reinforced cement composites, the dispersion of GO in water, alkali and several ionic species are investigated with the aid of UV–vis spectroscopy. High alkalinity and calcium ions are key factors inducing the agglomeration of GO in cement system. Dispersion of GO in simulated pore solution is the culmination of the alkali and salt experiments. Agglomeration of GO occurred when GO contacted with the simulated pore solution, highlighting the necessity to protect GO against such aggressive media. The test on surfactant compatibility was then carried out to ensure GO was effectively dispersed in polycarboxylate, air-entrainment and Gum Arabic admixtures within the pore solution. Polycarboxylate-based superplasticisers gave the most promising results to disperse GO in cement alkaline environment. Flexural experiments was performed to highlight the importance of fabrication protocol on the mechanical properties of GO-cement composites. The result shows that the amount of 0.03% GO by weight of cement can increase the flexural strength of GO-cement composite up to 67%.
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•Novel post-processing can increase strength to 30 MPa for 3D Printed Geopolymer.•This is a major improvement from the previously attained strength of 16 MPa.•Optimum cure temperature ...is 60 °C for post-processing.•Water is an ineffective post-processing medium for strength gain.•A specially formulated is found to be the most effective post-processing medium.
A previous study presented a method of formulating geopolymer for powder-based 3D concrete printing (3DCP) for construction applications. The highest strength achieved in using this method was 16 MPa. While this is sufficient for some special construction applications, a higher strength is necessary for the majority of the construction applications. The present study presents a number of curing methods to enhance the strength of 3D printed geopolymer parts. The effects of curing temperatures and curing mediums on the compressive strength of geopolymer samples produced via the 3DCP process have been evaluated. Curing of the printed samples has been carried out in tap water, three alkaline solutions and three fly ash-based geopolymer slurries at different temperatures (25 °C, 40 °C, 60 °C and 80 °C). Results showed that printed geopolymer samples cured in a combination solution of N Grade sodium silicate solution and sodium hydroxide solution (8.0 M) at 60 °C gained the highest compressive strength of 30 MPa. This strength is nearly double the strength achieved in the previous study. The higher strength achieved in this newly developed post-processing method opens up wider range of possible construction applications due to the significantly improved strengths.
In this study, an experimental program was conducted to investigate the effect of microwave curing on the strength of geopolymer. Volumetric heating provided by microwave curing results in faster ...strength development as compared to conventional heat curing that depends on heat transmission from the outside to the core of a specimen. Initially, the effect of different traditional curing methods (ambient air, water, and oven) on the strength development of Class F fly ash activated by sodium silicate and sodium hydroxide was investigated. Later, microwave curing and oven curing were introduced at higher temperatures in short periods of time (≤ 60 min) as alternative curing methods. Results showed that the compressive strength of geopolymer pastes cured in a microwave oven was superior to those of the control cured in a conventional oven at 90 and 120 °C for the same period. When the size of specimen size reduced from 50 mm to 25mm, the strength development rate of the geopolymer decreased as the microwave curing time increased. A 56 MPa compressive strength was obtained for 25 mm size geopolymer paste cured in a microwave oven for five minutes.
Despite the growing interest in 3D concrete printing, its current progress is limited by reinforcing methods. Inclusion of steel fibers is a potential reinforcing solution; however, the effect of ...printing process on orientation of the fibers is still unknown. This study aims to quantitatively investigate the orientation distribution of steel fibers in 3D printed ultra-high performance concrete. The effects of extrusion nozzle size, Cartesian print speed, and fiber volume fraction on the orientation of fibers were evaluated using digital image analysis. The consequent effects of the fiber orientation on the mechanical properties of the 3D-printed specimens were also determined. The results were compared with those of the conventionally mold-cast specimens. The results revealed that the smaller nozzle size and higher fiber volume fraction significantly enhanced the fiber alignment parallel to the printing direction. This preferential fiber alignment led to superior mechanical performance of the printed specimens to the mold-cast specimens.
The strength and transient creep of geopolymer and ordinary Portland cement (OPC)-based material (paste and concrete) were compared at elevated temperatures up to 550°C. The strength properties were ...determined using an unstressed hot strength test and unstressed residual strength test for paste and concrete, respectively. At 550°C, compared with the original strength, the strength of geopolymer was increased by 192% while the strength of OPC paste showed little change. However, after exposure to 550°C, the residual strength percentage of both geopolymer and OPC concretes was similar. Transient creep data show that geopolymer had little change in transitional thermal creep (TTc) between 250 and 550°C while OPC paste developed significant TTc in this temperature range. In comparison with OPC concrete, a higher strength loss of geopolymer concrete is thus believed to be due to the absence of TTc to accommodate nonuniform deformation during thermal exposure.
Aluminium and grey cast iron slags are produced in small industries and in spite of blastfurnace slag are considered as wastes. In this work, synthesis possibility of geopolymeric paste and concrete ...through alkali activation of different mixes of these slags is studied. Workability of the mixtures was achieved immediately after mixing and compressive strength tests were conducted on specimens cured at room temperature. Workability of all pastes and concrete specimens were in acceptable range for use in constructions. Maximum compressive strength for both paste and concrete specimens was achieved by using silica/alumina weight ratio of 3.0. By increasing the value of parameters of this study including sodium hydroxide concentration, age of curing and silica/alumina weight ratio, more silicon and aluminium ions dissolve from slag mixture into alkali activator and hence, compressive strength increases. It was concluded that among the considered parameters, silica/alumina weight ratio is the most important factor. The results indicated the possibility of production of geopolymers by utilizing appropriate ratios of aluminium slag (alumina source) and grey cast iron slag (silica source). In most related works reported in the literature, an aluminosilicate source or a mixture of two aluminosilicate sources with defined silica/alumina weight ratio are evaluated for possibility of geopolymerization. In the current study, each alumina and silica is provided from an individual source, and geopolymerization is performed by manipulating silica/alumina weight ratio.
•Geopolymers were synthesised by different mixtures of aluminium and grey cast iron slags.•Compressive strength and workability were conducted to evaluate performance criteria.•Green geopolymers with suitable properties are achievable by using these waste materials.
