Ashish Kumer Saha

The effects of mono (single type) and hybrid (mixed types) fibres on the workability, compressive strength, flexural strength, and toughness parameters of fly ash geopolymer mortar were studied. The ratio of sand to geopolymer paste of the mortar was 2.75. It was found that workability of mortar decreased more with the use of PP fibres due to its higher dispersion into individual filaments in geopolymer mortar compared to the bundled ARG and PVA fibres. Compressive strength increased by 14% for using 1% steel with 0.5% PP fibres compared to that of the control mixture, which was 48 MPa. However, 25 to 30% decrease of compressive strength was observed in the mortars using the low-modulus fibres. Generally, flexural strength followed the trend of compressive strength. Deflection hardening behaviours in terms of the ASTM C1609 toughness indices, namely I5, I10 and I20 were exhibited by the mortars using 1% steel mono fibres, 0.5% ARG with 0.5% steel and 1% PVA with 0.5% steel hybrid fibres. The toughness indices and residual strength factors of the mortars using the other mono or hybrid fibres at 1 or 1.5% dosage were relatively low. Therefore, multiple cracking and deflection hardening behaviours could be achieved in fly ash geopolymer mortars of high sand to binder ratio by using steel fibres in mono or hybrid forms with ARG and PVA fibres.

Alkali–silica reactivity of granulated ferronickel slag (FNS) aggregate was evaluated by accelerated mortar bar tests. The test results showed noticeable expansions of the samples containing 50% FNS aggregate due to potential alkali–silica reaction (ASR). On the other hand, finely ground FNS (GFNS) exhibited pozzolanic property in the studies of compressive strength development, thermogravimetric analysis and X-ray diffraction analysis. Therefore, GFNS was used as a partial replacement of cement to study its effect on the reduction of ASR. It was found that the use of GFNS for at least 30% cement replacement reduced the 10 d and 21 d mortar bar expansions below the allowable limits. The consumption of portlandite by the pozzolanic reaction of GFNS is considered as the primary reason for the reduction of expansion. The ASR mitigation process of GFNS was further confirmed by scanning electron microscopy images and energy-dispersive X-ray spectroscopy of the reaction product.

Automatic construction systems have become the focus of the construction industry and research projects worldwide. Numerous technologies involving 3D printing (3DP) of concrete elements have been developed, and their application in construction projects has been growing. The 3DP in concrete construction is increasing due to its freedom in geometry, rapidness, formwork-less printing, low waste generation, eco-friendliness, cost-saving nature, and safety. Development of 3DP is not only limited to the earth but also gaining attention for building habitats in space. This study aims to present the technical, socio-economical, and environmental aspects related to 3DP of concrete structures for a systematic summation of the technology, guidelines, applications, challenges, and prospects of future research and market in the construction industry. This comprehensive review shows that challenges involved in 3D concrete printing should be analyzed further by researchers to enhance mechanical performance, durability, and sustainability and establish appropriate standard guidelines for printing structures.

The use of reactive aggregates in ordinary Portland cement (OPC) concrete is limited due to the potential expansion of alkali-silica reaction (ASR). This study evaluates the ASR expansion of ferronickel slag (FNS) fine aggregate in fly ash geopolymer mortar by the accelerated mortar bar test (AMBT) method. While FNS was found to be reactive in OPC mortar, it was found non-reactive with fly ash geopolymer. The OPC mortar showed extensive expansion and strength reduction after exposure to AMBT condition, whereas the geopolymer specimens showed low expansion and strength increment. Scanning electron microscopy (SEM) images of the OPC mortar specimens after AMBT showed the presence of some ASR gel around aggregate surface. However, a relatively firm bonding was observed between aggregate and binder in the geopolymer mortar specimens. The EDS results showed that the aluminosilicate consumed free alkali in geopolymer binder matrix that reduced the ASR of FNS aggregate in geopolymer mortar.

Summary: This study evaluates the effect of elevated temperature exposure on concrete incorporating ferronickel slag (FNS) as a replacement of natural sand. Concrete cylinders were exposed up to 800°C, and the changes in compressive strength, mass, ultrasonic pulse velocity (UPV), and microstructure were investigated. The concretes containing up to 100% FNS aggregate showed no spalling and similar cracking to that of the concrete using 100% natural sand. For exposures up to 600°C, the residual strengths of concretes containing 50% FNS were 7% to 10% smaller than the concrete with 100% sand. Use of 30% fly ash as cement replacement improved residual strength by pozzolanic reaction for exposures up to 600°C. An equation has been found from the correlation between residual strength and UPV. Therefore, UPV can be used as a nondestructive test to estimate the extent of postfire damage and residual strength of concrete incorporating FNS aggregate and fly ash.

This study investigates the potential alkali silica reaction (ASR) of ferronickel slag (FNS) aggregate, which is a by‐product of nickel production. A class F fly ash was used as a possible ASR mitigation in accelerated mortar bar test (AMBT) specimens containing 50% FNS. There were visible surface cracks on the specimens using no fly ash or 10% fly ash. Use of 20% fly ash reduced expansion by 45% as compared to that with 10% fly ash. In accordance with the expansion limits of Australian Standard, the mixtures using 20 and 30% fly ash were categorized as slowly reactive and nonreactive, respectively. Thermogravimetric analysis (TGA) and microstructural observations confirmed the effectiveness of fly ash in reducing Portlandite content that reduced the ASR expansion. Therefore, the use of 20–30% fly ash as cement replacement is considered as an adequate ASR mitigating measure of FNS fine aggregate.