The effect of metakaolin (MK) and magnesium oxide (MgO) on the flexural strength of ultra-high performance concrete (UHPC) was investigated. The 0–15 wt% ordinary Portland cement in UHPC was replaced with MK and MgO (MM) with a weight ratio of 3:2. The fluidity and strength of UHPC, the pullout performance of a single steel fibre in UHPC matrix and the microstructure (X-ray diffraction, thermogravimetry , 29 Si and 27 Al nuclear magnetic resonance, microhardness , scanning electron microscopy–energy-dispersive X-ray spectroscopy and mercury intrusion porosimetry) of UHPC were studied. Results show that the flexural strength of UHPC initially increases and then decreases with an increase in MM replacement ratios. UHPC with 10% MM at the age of 60 days have the highest flexural strength, which is 33.8% higher than that of UHPC without MM. When MM replacement ratios increases from 0 to 10%, MM promotes the formation of C-(A-)S–H and LDHs . The addition of MgO increases the pH value, resulting in more Al leaching from MK, which favours Al uptake by C-(A-)S–H. MM also improves the densification of UHPC matrix and the bond properties between fibres and UHPC matrix. However, excessive MM (＞10%) exhibits a stronger clinker dilution effect, and some of the cement particles are covered by MM. Therefore, the flexural strength of UHPC decreases. 

Effect of metakaolin and magnesium oxide on flexural strength of ultra-high performance concrete

In recent years, 3D concrete printing technology has been developing dynamically. Intensive research is still being carried out on the composition of the materials dedicated to innovative 3D printing solutions. Here, for the first time, concrete-geopolymer hybrids produced with 3D printing technology and dedicated environmentally friendly building construction are presented. The concrete-geopolymer hybrids consisting of 95% concrete and 5% geopolymer based on fly ash or metakaolin were compared to standard concrete. Moreover, 3D printed samples were compared with the samples of the same composition but prepared by the conventional method of casting into molds. The phase composition, water leachability, compressive, and flexural strength in the parallel and perpendicular directions to the printing direction, and fire resistance followed by compressive strength were evaluated. Concrete-geopolymer hybrids were shown to contain a lower content of hazardous compounds in leaches than concrete samples. The concentration of toxic metals did not exceed the limit values indicated in the Council Decision 2003/33/EC; therefore, the materials were classified as environmentally neutral. The different forms of Si/Al in fly ash and metakaolin resulted in the various potentials for geopolymerization processes, and finally influenced the densification of the hybrids and the potential for immobilization of toxic elements. Although the compressive strength of concrete was approximately 40% higher for cast samples than for 3D printed ones, for the hybrids, the trend was the opposite. The addition of fly ash to concrete resulted in a 20% higher compressive strength compared to an analogous hybrid containing the addition of metakaolin. The compressive strength was 7–10% higher provided the samples were tested in the parallel direction to the Z-axis of the printout. The sample compressive strength of 24–43 MPa decreased to 8–19 MPa after the fire resistance tests as a result of moisture evaporation, weight loss, thermal deformation, and crack development. Importantly, the residual compressive strength of the hybrid samples was 1.5- to 2- fold higher than the concrete samples. Therefore, it can be concluded that the addition of geopolymer to the concrete improved the fire resistance of the samples.

/pdf/3d-printing-of-concrete-geopolymer-hybrids-ut9ull0w.pdf

3D Printing of Concrete-Geopolymer Hybrids

With the increasing quality of concrete construction, the requirements for concrete performance have also gradually increased. In this paper, the effect of nanomaterials on concrete performance was investigated by incorporating nanomaterials (nano-SiO2 and nano-Fe2O3) into concrete to prepare high performance concrete (HPC). The optimum content of nanomaterials was determined by response surface method (RSM), which revealed the influence mechanism of nanomaterial on concrete performance. Results showed that adding nanomaterials remarkably enhanced the mechanical properties of concrete. When the dosage of nano-SiO2 was 3.54% and the dosage of nano-Fe2O3 was 4.24%, which was the optimum dosage of nanomaterials. Concrete with this dosage had compressive, splitting tensile, and flexural strength of 67.27 MPa, 8.95 MPa, and 9.56 MPa, respectively. The two types of nanomaterials can achieve excellent coupling effect. Nano-SiO2 acted as an auxiliary cementing material, and nano-Fe2O3 accelerated the hydration reaction rate of cementing materials. According to the microstructure of nanoconcrete, energy dispersive spectrometer (EDS) elemental analysis and thermogravimetry analysis (TGA), it was concluded that the incorporation of nanomaterials cannot produce new kinds of hydration products in concrete but can produce more hydration products. These results can serve as theoretical reference for HPC preparation. 

Modification mechanism of hybrid nano-materials on high performance concrete and optimization of nano-material content

Improvement in flexural strength of Portland cement by lamellar structured montmorillonite

Increasing the infrastructure in developing and non-developing countries requires a huge amount of cement which leads to cause significant impact on the environment and human health, such as the emission of greenhouse gases (SOx, NOx, and particulate matter) and severe acute respiratory issues (asthma and silicosis), respectively. To reduce such problems, the consumption of cement in construction materials needs to be reduced. In recent times, various advanced supplementary materials have been used to improve the performance of mortar and concrete. Metakaolin (MK) is a cementitious material that is currently in demand because of its easy availability and low production cost. The current article highlights the positive effects of metakaolin on the mechanical and durability properties of cementitious materials. Metakaolin reacts with calcium hydroxide get produced additional cementitious compounds and enhance the strength. It improves the microstructure by reducing porosity and improving the interfacial transition zone. 

Effect of metakaolin on mechanical characteristics of the mortar and concrete: A critical review

Mechanical properties and acid attack test of hybrid fiber reinforced high performance concrete for fly ash based mineral admixture

Combined effect of metakaolin and hybrid fibers on the strength properties of high performance concrete

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Mechanical properties and acid attack test of hybrid fiber reinforced high performance concrete for fly ash based mineral admixture

Combined effect of metakaolin and hybrid fibers on the strength properties of high performance concrete