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The preparation of cement-based supplementary cementitious materials is an important method for the efficient use of iron tailings and the reduction in CO2 emissions. The aim of this study is to improve the reactivity of iron tailings by mixing them with steel slag, slag, and fly ash through orthogonal tests to solve the problem that iron tailings cannot be utilised on a large scale. The compressive strength, hydration products, and microstructure of the iron tailings–cement-based supplementary cementitious materials were investigated using ICP-OES, XRD, TG, FTIR, and SEM. The results revealed that each solid waste raw material played a distinct role in the hydration reaction. In the iron tailings–cement-based supplementary cementitious materials system, steel slag provided Ca2+, OH−, and Si4+ ions, slag provided Ca2+ and Al3+ ions, fly ash contributed a significant amount of Ca2+ and Al3+ ions, and iron tailings offered more nucleation sites and some Si4+ ions for the hydration products. Moreover, there was a synergistic effect among these four materials, promoting the formation of hydration products such as ettringite, C-(A)-S-H gels, and others. When the proportion of IOTs:SS:FA:SL was 9:8:8:2, the highest 7 d compressive strength of cementitious material was 24.8 MPa. When the proportion of IOTs:SS:FA:SL was 9:6:8:4, the highest 28 d compressive strength of cementitious material was 35.0 MPa. This study provides a comprehensive solution for the utilisation of iron tailings and contributes to the high-value green utilisation of solid waste. [ABSTRACT FROM AUTHOR]

Utilizing a variety of solid wastes to prepare alkali-activated cementitious materials is one of the principal trends in the development of cementitious materials. Commonly used alkali activation precursors such as granulated blast furnace slag (GBFS) and fly ash (FA) will be less available due to resource pressures. Supply limitation is an important reason to research alternative precursors. To realize the high value-added utilization of copper–molybdenum tailings (CMTs), this study adopted the modified sodium silicate solution as an alkaline activator to activate GBFS-FA-CMTs cementitious system to prepare alkali-activated cementitious materials. The influence of CMTs content on the compressive strength of GBFS-FA-CMTs cementitious system was analyzed, and the mechanism of GBFS-FA-CMTs cementitious system was also analyzed through hydration product types, physical phase composition, and microscopic morphology. The results indicated that a paste with the incorporation of CMTs, S50F30C20 (50% GBFS, 30% FA, 20% CMTs), achieved the highest compressive strength of 79.14 MPa, which was due to the filling effect of the CMTs and the degree of participation in the reaction. Pastes with different contents of CMTs, while maintaining a constant CBFS content, exhibited similar strength development. Excessive amounts of CMTs could result in reduced compressive strength. Microstructural analysis revealed that the hydration products were structurally altered by the addition of CMTs. In addition to ettringite, quartz, C(-N)-S-H gel, and calcite, gaylussite was also formed; moreover, the mass of chemically bound water increased, and the microstructure of reaction products became denser. An excess of CMTs may restrict the growth of the hydration gel, leading to more microstructural defects. The study suggests that CMTs could enhance the compressive strength of hardened paste within an alkali-activated slag-fly ash system, possibly due to a filling effect and participation in the chemical reaction. This research confirms the feasibility of using CMTs in alkali-activated cementitious materials. [ABSTRACT FROM AUTHOR]

There are some limitations in the application of tuff powder as a supplementary cementitious material (SCM). Exploring its feasibility in new fields will consume a large amount of silica-alumina mine solid wastes. This study has investigated the mechanical properties and mechanism in contact-hardening of tuff powder with a method of compression molding. The compressive strength of specimens was tested, and the X-ray diffraction (XRD), thermogravimetric analysis (TG), scanning electron microscopy (SEM), and Mercury intrusion porosimetry (MIP) methods were used to reveal the mechanism of contact-hardening of tuff powder from a micro-perspective. The results indicated that the compressive strength of specimens was higher when activated by sodium hydroxide compared to calcium hydroxide. Compared to calcium hydroxide, the compressive strength of TFS20 and TFF20 activated by sodium hydroxide was improved by 20% and 23%, respectively. The hydration degree of tuff powder was very low, with a water–cement ratio (w/c) of 0.15, while the hydration degree of coal gangue powder was higher. The results of TGA and SEM indicated that the sodium hydroxide had a better activating effect on slag and fly ash. Therefore, more C-S-H gels were generated in those samples activated by sodium hydroxide. Furthermore, the structure of samples was more compacted, and there was a reduction of porosity by 10% and 11% for TFS20 and TFF20, respectively, especially the proportion of harmful pores. [ABSTRACT FROM AUTHOR]

Exposure of concrete to acidic environments can cause the degradation of concrete elements and seriously affect the durability of concrete. As solid wastes are produced during industrial activity, ITP (iron tailing powder), FA (fly ash), and LS (lithium slag) can be used as admixtures to produce concrete and improve its workability. This paper focuses on the preparation of concrete using a ternary mineral admixture system consisting of ITP, FA, and LS to investigate the acid erosion resistance of concrete in acetic acid solution at different cement replacement rates and different water–binder ratios. The tests were performed by compressive strength analysis, mass analysis, apparent deterioration analysis, and microstructure analysis by mercury intrusion porosimetry and scanning electron microscopy. The results show that when the water–binder ratio is certain and the cement replacement rate is greater than 16%; especially at 20%, the concrete shows strong resistance to acid erosion; when the cement replacement rate is certain and the water–binder ratio is less than 0.47; especially at 0.42, the concrete shows strong resistance to acid erosion. Microstructural analysis shows that the ternary mineral admixture system composed of ITP, FA, and LS promotes the formation of hydration products such as C-S-H and AFt, improves the compactness and compressive strength of concrete, and reduces the connected porosity of concrete, which can obtain good overall performance. In general, concrete prepared with a ternary mineral admixture system consisting of ITP, FA, and LS has better acid erosion resistance than ordinary concrete. The use of different kinds of solid waste powder to replace cement can effectively reduce carbon emissions and protect the environment. [ABSTRACT FROM AUTHOR]

In this study, a type of fly ash and iron tailing powder were used as a binary supplementary cementitious material (SCM) and two different mechanically activated procedures, named coalescent activation and separated activation, were carried out to improve the pozzolanic reactivity. Then, three binary supplementary cementitious materials were used to replace 30 wt% of ordinary Portland cement to develop cemented mortars, and the compressive strength, hydration products, and micro-structure were studied. The experimental results indicated that the activated supplementary cementitious materials increased the compressive strengths of the mortars by 5.4% and 13.2%, negligibly changed the hydration product types, and impacted the quantity only. The application of SCMs also prolonged the setting time and decreased the hydration rate, profiting the application. Simultaneously, the micro-pore structure was ameliorated, and the porosity decreased. Therefore, comprehensively considering the improved mechanical behavior and inexpensive cost, the activated binary SCM can be considered an ecological and economical admixture, especially for the coalescent activation procedure. [ABSTRACT FROM AUTHOR]

In order to control energy consumption and reduce pollution, the use of supplementary cementitious materials (SCMs) instead of cement to produce green cementitious materials can save energy, reduce emissions and achieve sustainable development. This study demonstrates the possibility of developing SCMs with iron tailings (IOTs), fly ash (FA) and ceramic powder (CP) ternary system, as well as the optimization and improvement scheme of gelation activation. The effects of activator dosage, mix ratio and substitution rate on mechanical properties of ternary SCMs system were investigated. The formation and evolution of hydration products were analyzed by differential thermogravimetric analysis (DTA) and scanning electron microscopy (SEM). The results of the study show that there is synergy in the system. The results show that there is synergy in the system and the hydration reaction is sufficient. At the substitution rate of 30%, the doping ratio of IOTs, CP and FA is 1:2:2 and the Ca(OH)2 is 0.6%, the strength reaches 39.9 MPa and the activity index is 91.5%, which can provide a basis for the application and more in-depth study of IOTs multi SCMs. [ABSTRACT FROM AUTHOR]

The use of shellfish waste as a raw material in cement and concrete production is recognized as a feasible approach. Similar to limestone, seashell powder (SP), which is primarily made up of calcium carbonate, has been identified as a promising supplementary cementitious material (SCM). However, the current understanding of SP's chemical interactions within aluminum-rich cement systems remains limited, often resulting in its classification as simply an inert filler. Therefore, this research seeks to clarify the contribution of SP to fly ash (FA) cement by investigating the hydration products. The investigation reveals that SP facilitated hydration albeit to a lesser extent compared to limestone. In cements incorporating SP, the AFm phases, characterized by carboaluminate formations, became predominant, and the calcium carbonate content diminished over time, indicating reactive engagement of SP. The addition of 5 % SP enhanced the polymerization of the C-(A)-S-H structure in FA cement. Moreover, the substitution of cement with SP elevated the effective water to cement ratio, leading to a reduction in the Ca/Si ratio of C-(A)-S-H, while the Al/Si ratio remained virtually unchanged. Analysis identified the presence of monocarboaluminate (Ca/Al/C atomic ratio of 4: 1.97: 4.39) and hemicarboaluminate (Ca/Al/C atomic ratio of 4: 2.25: 2.40) in FA cements with 5 % and 10 % SP additions, respectively. Nonetheless, incorporating SP negatively impacted the compressive strength of FA cement, underscoring the importance of using SP in moderation. Specifically, the 28-day compressive strengths of FA cements with 5 % and 10 % SP additions measured at 42.9 MPa and 40.2 MPa, respectively. Moreover, SP-FA cement offers significant environmental advantages. This research innovatively elucidates the reaction mechanism of SP within fly ash cement, offering vital insights into SP's impact on the hydration process and advancing the recycling of shellfish waste. [Display omitted] • Seashell powder facilitated cement hydration to a lesser extent than limestone. • Seashell powder exhibits significant reactivity in fly ash cement. • Carboaluminate phases were detected by various means such as XRD and SEM. • 5 % seashell powder inspired the C-(A)-S-H polymerization. • Synergistic effects between seashell powder and fly ash were demonstrated. [ABSTRACT FROM AUTHOR]

This research investigates the effects of Na2SO4 and Na2CO3 on the mechanical and hydration properties of high-volume fly ash (FA) cement (HFC) at three curing temperatures: 25°C, 40°C, and 60°C. Setting time, compressive strength, reaction kinetics, phase assembles, and pore structure were explored. The research found that the hydration of HFC was significantly promoted by the chemical activators at all temperatures. At 25°C, the incorporation of Na2CO3 and Na2SO4 decreased the initial setting time by 59% and 50%, respectively, compared to the control sample. This was accompanied by increased heat release from the isothermal calorimetry tests. At the same time, the introduction of Na2CO3 (HFC-C) increased the early compressive strength by 40%, higher than that of samples with Na2SO4 addition (HFC-S). While reduced compressive strength was noticed in HFC-C after curing for a long time, resulting from the increased porosity. Furthermore, temperature rise reduced the contribution index (Ci) to the strength development of both HFC-C and HFC-S. On the other hand, the presence of Na2SO4 contributed to the formation of more ettringite (AFt), while no AFt was detected after the addition of Na2CO3. Increasing the curing temperature did not strongly change the phase assembles of the hydration products, although the amount of AFt content was observed to increase. [ABSTRACT FROM AUTHOR]

• Introducing FA to adjust fresh behaviors of OPC-MK cementitious material system. • The synergistic effects of SCMs indicate the filling effect and pozzolanic reaction. • Hydration products of OPC-MK-FA system were beneficial to the Al-rich system. • Modification of gels and pores are the contributions of FA to OPC-MK system. The fresh behavior of Ordinary Portland cement (OPC)-Metakaolin (MK) cementitious material system is relatively undesirable and needs to be used in conjunction with other materials and comprehends interaction mechanism to meet practical application specifications. The present work investigated the feasibility of using fly ash (FA) to modify the properties of MK cement and reported the effects of the interaction between MK and FA on the microstructure and hydration products. The results showed that the fluidity loss behavior and premature setting time for the cement-MK system was improved by adding the appropriate amount of FA. Synergistic stacking and hydration effect of FA and MK in the ternary blend significantly yielded a more uniform mix, resulting in a matrix with a higher compressive strength and more compact structure. Besides, the addition of FA contributed to reduce the typical hydration product Ca/Si ratio and promoted the ability of gel products to absorb Al3+, which was beneficial to the Al-rich system such as OPC-MK system. This research provides the foundation for the long-term development of multi-based supplementary cementitious materials (SCMs). [ABSTRACT FROM AUTHOR]

• Both mechanical and mechanochemical activation methods are effective to improve the chemical activity of iron ore tailings. • Less surface binding energy reflects less crystallinity and higher reactivity. • Ternary IOTs-based SCMs resulted in the comressive strength enhancement and pore structure refinement. The increasing concern for decarbonization calls for sustainable construction materials since carbon dioxide emissions from cement industries account for 8% of the total emissions. Incorporating municipal solid waste into concrete as supplementary cementitious materials (SCMs) is one of the fundamental ways to achieve urban sustainability. Iron ore tailings (IOTs), a type of mining waste from the beneficiation process of iron ore concentrates, can be used as SCMs via various activation methods. While activation of IOTs alone has been a topic of intensive research for years, there is a lack of comprehensive study on mechanical-chemical coupling activation of IOT-based ternary SCMs systems (IOT, fly ash, and ground granulated blast furnace slag) considering the utilization of other industrial by-products and waste. This study investigates the effect of grinding time and chemical activators on the properties of IOT-based ternary SCMs. XRD, XPS, DTA-TG, compression tests, Mercury intrusion porosimetry, and SEM were employed to investigate the crystal structure and crystallinity of IOTs, surface energy, mechanical properties, pore structure, and morphology of the hydration products of the ternary system. The results obtained from this study revealed that an increase in grinding time and chemical activators improved the activity index of IOTs-based ternary SCMs. In addition, Na 2 SiO 3 contributed the most to the activity index among the three types of activators. This work has expanded the possibility of utilizing IOT-based ternary SCMs via proper activation methods for sustainable construction. [ABSTRACT FROM AUTHOR]

• MICP technology is used to cement iron tailings backfill. • The effects of microbial content, pH and curing time on the MIB are studied. • The strength of the MIB is greater than that of the untreated sample. • The optimal pH for microorganisms growth in the iron tailings backfill is 9. • The CaCO 3 generated by the MICP can fill the gaps in the backfill and smooth its surface. In the traditional backfill mining engineering, cement cementation method is generally adopted, but it has high cost and emits a large amount of CO 2 , which aggravates the greenhouse effect. So that it is important to find a green and environmentally friendly cementing materials to replace cement cementation. Thus, in this paper, microbial induced carbonate precipitation (MICP) is an emerging technology used to assist in cementing iron tailings, and the strength differences between the blank group (CG) and the microbial-cemented iron tailings-based backfill (MIB) are compared, and the purpose is to study the effect of microbial-cemented on mechanical properties of iron tailings backfill and its mechanism analysis. In this study, unconfined compressive strength test, analysis of XRD, TG, SEM and other methods are used, and it is found that pH, inoculated-microbial quantity and curing time all have effects on the MIB. The main conclusions of this study are as follows: The strength of the backfill treated by MICP technology is 5.45–63.33 % higher than that of the CG, further, the optimal pH for microorganisms growth in the iron tailings backfill is 9, the optimal inoculated-microbial quantity is 4 mL when the curing time is 7 d and so on. Finally, the toxicity of MIB leaching is evaluated. The microbial-cemented technology used in this study can provide a test and scientific basis for the application of MICP technology in mining and materials science. [ABSTRACT FROM AUTHOR]