Rheological Characterization of Ultra-High Performance Concrete for 3d Printing

The authors recently developed a 3D-printable ultra-high performance fiber-reinforced concrete (3DP-UHPFRC) for additive construction of structural members with significantly reduced reliance on steel bars. This study investigates the rheological behavior of the developed 3DP-UHPFRC. The effects of two major factors affecting the performance of 3DP-UHPFRC, namely steel fiber volume (0, 1%, and 2%) and nano-clay (NC) content (0, 0.1%, and 0.2% by binder mass) on workability, static yield stress, dynamic yield stress, and apparent viscosity were determined. Test results showed that the inclusion of steel fibers and NC reduced the workability and led to a significant increase in the static yield stress, dynamic yield stress, and apparent viscosity. However, the effect of NC content on the rheological properties became negligible in the mixtures made with 2% fiber content. Similarly, the effect of steel fiber volume became negligible in the mixtures made with 0.2% NC. In addition, the influence of changes in rheology due to the addition of steel fiber and NC on the extrudability and buildability of the mixtures were investigated by 3D-printing of 500 mm high hollow columns with three different print speeds. The results showed all mixtures exhibited satisfactory extrudability (i.e., no blockage of extruder or tearing of filaments was observed). In addition, the buildability of the mixtures increased as the steel fiber and NC contents increased

Setting time, compressive strength and sulfuric acid resistance of a high calcium fly ash geopolymer containing borax

The objective of this research was to study the influence of borax on setting time and other properties viz., workability, compressive strength and sulfuric acid resistance of a high calcium fly ash (HCFA) geopolymer. Four borax types viz., deca-hydrate borax, deca-hydrate borax heat-treated for 5-minutes and 10-minutes and anhydrous borax were used to replace fly ash. A liquid to binder ratio (L/B) of 0.60, a 10M NaOH solution, and Na2SiO3/NaOH at a ratio of 1.0 were used in the formulation of the geopolymer. The results indicated that the replacement of fly ash with borax accelerated the initial setting time but retarded the final setting time of the geopolymer paste. The flow of mortar with deca-hydrate borax remained unaffected by the increase in the amount of borax. For other borax types, the mortar flow was reduced and this was related to the water molecules in the borax. The compressive strength of the mortar tended to decrease with increasing borax content. The resistance of the mortar to 3% sulfuric acid was reduced with increased borax content. The use of borax should be around 2.5% to control the setting of a high calcium fly ash geopolymer and to obtain good strength and resistance to acid

An investigation of the mechanisms for strength gain or loss of geopolymer mortar after exposure to elevated temperature

When fly ash-based geopolymer mortars were exposed to a temperature of 800 °C, it was found that the strength after the exposure sometimes decreased, but at other times increased. This paper shows that ductility of the mortars has a major correlation to this strength gain/loss behaviour. Specimens prepared with two different fly ashes, with strengths ranging from 5 to 60 MPa, were investigated. Results indicate that the strength losses decrease with increasing ductility, with even strength gains at high levels of ductility. This correlation is attributed to the fact that mortars with high ductility have high capacity to accommodate thermal incompatibilities. It is believed that the two opposing processes occur in mortars: (1) further geopolymerisation and/or sintering at elevated temperatures leading to strength gain; (2) the damage to the mortar because of thermal incompatibility arising from non-uniform temperature distribution. The strength gain or loss occurs depending on the dominant process

The hype and the reality of the role of multimedia in university education

Multimedia is being claimed as the technology that is going to revolutionize the way universities deliver courses. Traditionalists hope that it will simply go away like many other fads in the past while the education in universities remains predominantly face-to-face as it has for many centuries. This paper examines what the multimedia technology offers to university education, the hype associated with it, and the realistic figures predicted for the future. It also suggests areas where the use of multimedia and online education would be most effective in university education environments

Concretes without Portland cements: concrete of the future?

The paper shows that the portland cement based concretes are used in very large quantities without a major option for re-use or recycling method is unsustainable. The amount of virgin materials consumed for concrete making is unprecedented and rising very rapidly with the worldwide infrastructure boom. The paper presents geopolymer, alkali-activated-slag and waste-glass cements - all of which are made from industrial by-products or wastes and are emerging alternatives to conventional portland cement concrete. Portland cement is the major source of carbon emission in concrete making and is responsible for global carbon emissions second only to fossil fuels. The paper outlines the major technological advances in portland cement free concretes and discusses the future directions

Chemically graded geopolymer under flexural loading: experimental modelling

In this paper, a functionally graded solution is proposed for stress distribution in functionally graded geopolymers under flexural loading with the load direction parallel to the gradient direction. Functionally graded geopolymer specimen is made by sequential pouring and subsequent vibration of two different mixtures of alkali activated pastes into appropriate moulds. Diffusion of the two mixes into each other during vibration and early age curing process causes chemically graded distribution of the constituent oxides. Because of the variation of elastic modulus in functionally graded specimen, the stress distribution pattern is related to its pseudo-strain function through different equations. According to the considered functions for both modulus of elasticity and pseudo-strain, different equations are proposed to predict the flexural strength of functionally graded geopolymer. The best performance model is acquired by adopting inverse tangent for variation of strain in the deflected beam and the exponential function for variation of modulus of elasticity in functionally graded region

Hybrid effects of alumina and silica nanoparticles on water absorption of geopolymers: Application of Taguchi approach

The effect of amorphous nanosilica and crystalline nanoalumina on water absorption behaviour of geopolymers was considered at two and seven days of water curing regimes. Taguchi approach was used to design the experiments and the results were analysed by factorial analysis and analysis of variance (ANOVA). ANOVA results were used to find optimized mixture proportions. The results show that the degree of completion of geopolymeric reactions is the key factor affecting on the percentage of water absorption. While nanosilica particles support a denser geopolymeric paste, unreacted nanoalumina particles act as nanofillers and reduce water demands. The main important operational factor was oven curing temperature where increasing the temperature to 90 oC resulted in lower percentages of water absorption at both two and seven days of water curing regimes

Developing of non-linear weight functions for mix design optimization of cementitious systems

Mix design in production cementitious materials is of importance where selection the value of each parameter has a critical effect on final properties of the material. In the present work, a new method has been developed to determine the effect of each considered mix design factor on the output properties. A specific property can be related linearly to the factors of mix design through normalized nonlinear weight functions. The proposed procedure was applied on two different mix designs available in the literature. The first analysis was conducted on ordinary Portland cement based concrete specimens to analyse the importance of each factor on their compressive strength. The second one was conducted on a geopolymeric system to analyse the compressive strength. For both systems, the factors were divided into sensitive and non-sensitive where sensitive factors were suggested to be considered with more attention in mix design procedure

Johnson-Mehl-Avrami-Kolmogorov equation for prediction of compressive strength evolution of geopolymer

Geopolymers are cement-free constructional materials and for their production much lower greenhouse gases are emitted than traditional concrete. In this study, compressive strength of geopolymers was related to their degree of geopolymeric reactions. Then, the Johnson-Mehl-Avrami-Kolmogorov equation was applied to represent compressive strength evolution. The presented equation can be easily adopted for prediction of long-term compressive strength of geopolymers and even traditional cementitious systems. Johnson- Mehl-Avrami-Kolmogorov constants were shown to be higher for specimens with higher strengths while their corresponding exponents were smaller

The challenge of the cement industry towards the reduction of greenhouse emissions

The production of one tonne of Portland cement requires 1.5 tonnes of raw material. The production of Portland cement is highly energy intensive, consuming 4 to 7 MJ of fossil fuel energy per kg, and releases approximately one tonne of carbon dioxide for manufacture of each tonne of Portland cement. The production of cement contributes 5% of the global greenhouse gas emissions.Utilisation of unused industrial by-products such as slag, which otherwise would contribute to land pollution, as Portland cement substitute in concrete, results in less energy for the manufacture of cement while also reducing the emissions arising from the burning of fossil fuels. The paper discusses the greenhouse gas emission reductions which can be achieved by the use of slag as a partial replacement of Portland cement (blended cements) and as a full replacement of Portland cement by the use of alkali activated slag concrete