Hydration mechanism and microstructure characteristics of modified magnesium slag alkali-activated coal-fired slag based cementitious materials

Abstract

As the second most important solid waste produced by coal-fired power plants, the improper management of coal-fired slag has the potential to result in environmental pollution. It is therefore imperative that high-value utilization pathways for coal-fired slag should be developed. In this study, modified magnesium slag (MMS), produced by a magnesium smelter, was selected as the alkali activator. The activated silica-aluminum solid wastes, namely coal-fired slag (CFS) and mineral powder (MP), were employed as pozzolanic materials in the preparation of alkali-activated cementitious materials. The alkali-activated cementitious materials prepared with 50 wt% MMS, 40 wt% CFS and 10 wt% MP exhibited favorable mechanical properties, with a compressive strength of 32.804 MPa in the paste sample cured for 28 d. Then, the activated silica-aluminum solid waste consisting of CFS-MP generated a significant amount of C-S(A)-H gels, AFt, and other products, which were observed to occupy the pore structure of the specimen. In addition, the secondary hydration reaction of CFS-MP occurs in high alkalinity environments, resulting in the formation of a mutually stimulated and promoted reaction system between CFS-MP and MMS, this will subsequently accelerate the hydrolysis reaction of MMS. It is important to emphasize that the amount of MMS in alkali-activated cementitious materials must be strictly regulated to avert the potential issue of incomplete depolymerization-repolymerization of active silica-aluminum solid waste containing CFS-MP. This in turn could have a deleterious impact on the late strength of the cementitious materials. The aim of this work is to improve the joint disposal of MMS, CFS and MP and thereby provide a scientific basis for the development of environmentally friendly and low-carbon modified magnesium slag alkali-activated coal-fired slag based cementitious materials for mine backfilling.

Keywords

coal-fired slag / alkali-activated / hydration characteristics / pore structure / composite cementitious material

References

[1] ZhangQ-l, LiuB, FengY, et al.. Mechanism development of strength contributed by CPB with rice husk ash. Journal of Central South University, 2024, 31(5): 1608-1618[J]

[2] LiZ-x, YinD-w, LiuH-m, et al.. Mechanical properties of cemented coal gangue – flyash backfill under coupled effects of water pressure and chloride salt erosion. Journal of Central South University, 2024, 31(5): 1574-1591[J]

[3] ZhaoQ-h, MaC-z, HuangB-s, et al.. Development of alkali activated cementitious material from sewage sludge ash: Two-part and one-part geopolymer. Journal of Cleaner Production, 2023, 384135547[J]

[4] MikulčićH, KlemešJ J, VujanovićM, et al.. Reducing greenhouse gasses emissions by fostering the deployment of alternative raw materials and energy sources in the cleaner cement manufacturing process. Journal of Cleaner Production, 2016, 136: 119-132[J]

[5] ZhangJ-h, DingL, LiF, et al.. Recycled aggregates from construction and demolition wastes as alternative filling materials for highway subgrades in China. Journal of Cleaner Production, 2020, 255120223[J]

[6] ChenX, ShiX-z, ZhouJ, et al.. Effect of overflow tailings properties on cemented paste backfill. Journal of Environmental Management, 2019, 235: 133-144[J]

[7] YuanQ, HuangY-l, HuangT-j, et al.. Effect of activator on rheological properties of alkali-activated slag-fly ash pastes. Journal of Central South University, 2022, 29(1): 282-295[J]

[8] AlomayriT, RazaA, ElhadiK M, et al.. Mechanical, microstructural, and thermal characterization of geopolymer composites with nano-alumina particles and micro steel fibers. Structural Concrete, 2025, 26(2): 1540-1559[J]

[9] SinghN, Shehnazdeep, BhardwajA. Reviewing the role of coal bottom ash as an alternative of cement. Construction and Building Materials, 2020, 233117276[J]

[10] SunW-j, LiuL, ZhouJ, et al.. Effect of inorganic salt composition on strength, microstructure and leaching toxicity of coal-based solid waste backfill materials. Process Safety and Environmental Protection, 2024, 192: 1110-1126[J]

[11] YaoZ T, JiX S, SarkerP K, et al.. A comprehensive review on the applications of coal fly ash. Earth-Science Reviews, 2015, 141: 105-121[J]

[12] KuramaH, KayaM. Usage of coal combustion bottom ash in concrete mixture. Construction and Building Materials, 2008, 22(9): 1922-1928[J]

[13] ZhangB-y, PoonC S. Use of furnace bottom ash for producing lightweight aggregate concrete with thermal insulation properties. Journal of Cleaner Production, 2015, 99: 94-100[J]

[14] LuukkonenT, AbdollahnejadZ, YliniemiJ, et al.. One-part alkali-activated materials: A review. Cement and Concrete Research, 2018, 103: 21-34[J]

[15] KahlaN B, RazaA, ElhagA B, et al.. Synergistic effects of hybrid microfibers on mechanical, thermal, and microstructural characterization of nanocomposites. Environmental Science and Pollution Research International, 2024, 31(17): 25991-26005[J]

[16] PanZ-h, LiD-x, YuJ, et al.. Properties and microstructure of the hardened alkali-activated red mud – slag cementitious material. Cement and Concrete Research, 2003, 33(9): 1437-1441[J]

[17] LuoX, XuJ-y, BaiE-l, et al.. Systematic study on the basic characteristics of alkali-activated slag-fly ash cementitious material system. Construction and Building Materials, 2012, 29: 482-486[J]

[18] ZhangQ-l, JiT, YangZ-x, et al.. Influence of different activators on microstructure and strength of alkali-activated nickel slag cementitious materials. Construction and Building Materials, 2020, 235117449[J]

[19] LiuJ, XieG-m, WangZ-d, et al.. Manufacture of alkali-activated cementitious materials using municipal solid waste incineration (MSWI) ash: Immobilization of heavy metals in MSWI fly ash by MSWI bottom ash. Construction and Building Materials, 2023, 392131848[J]

[20] LuanY-c, WangJ-h, MaT, et al.. Modification mechanism of flue gas desulfurization gypsum on fly ash and ground granulated blast-furnace slag alkali-activated materials: Promoting green cementitious material. Construction and Building Materials, 2023, 396132400[J]

[21] ZhangG, YangH-l, JuC, et al.. Novel selection of environment-friendly cementitious materials for winter construction: Alkali-activated slag/Portland cement. Journal of Cleaner Production, 2020, 258120592[J]

[22] LuoL, YaoW, LiangG-w, et al.. Workability, autogenous shrinkage and microstructure of alkali-activated slag/fly ash slurries: Effect of precursor composition and sodium silicate modulus. Journal of Building Engineering, 2023, 73106712[J]

[23] GuoW-c, ZhangZ-y, BaiY-y, et al.. Development and characterization of a new multi-strength level binder system using soda residue-carbide slag as composite activator. Construction and Building Materials, 2021, 291123367[J]

[24] JiaL, FanB-g, HuoR-p, et al.. Study on quenching hydration reaction kinetics and desulfurization characteristics of magnesium slag. Journal of Cleaner Production, 2018, 190: 12-23[J]

[25] LiuL, RuanS-s, QiC-c, et al.. Co-disposal of magnesium slag and high-calcium fly ash as cementitious materials in backfill. Journal of Cleaner Production, 2021, 279123684[J]

[26] RuanS-s, LiuL, ZhuM-b, et al.. Development and field application of a modified magnesium slag-based mine filling cementitious material. Journal of Cleaner Production, 2023, 419138269[J]

[27] RazaA, AhmedB, El OuniM H, et al.. Microstructural and thermal characterization of polyethylene fiber-reinforced geopolymer composites. Journal of Building Engineering, 2024, 94109904[J]

[28] RazaA, AhmedB, El OuniM H, et al.. Mechanical, durability and microstructural characterization of cost-effective polyethylene fiber-reinforced geopolymer concrete. Construction and Building Materials, 2024, 432136661[J]

[29] ZhanR, ZhangB, LiuL, et al.. An improved equivalent heat capacity method to simulate and optimize latent thermal energy storage units. International Journal of Heat and Mass Transfer, 2024, 235126135[J]

[30] TraynorB, UvegiH, OlivettiE, et al.. Methodology for pH measurement in high alkali cementitious systems. Cement and Concrete Research, 2020, 135106122[J]

[31] L’HôpitalE, LothenbachB, ScrivenerK, et al.. Alkali uptake in calcium alumina silicate hydrate (C-A-S-H). Cement and Concrete Research, 2016, 85: 122-136[J]

[32] PintoR C A, HobbsS V, HoverK C. Relationship between nonevaporable water content and hardened properties of high-performance mixtures. International Concrete Abstracts Portal, 2000351366[C]

[33] WuJ-d, GuoL-p, QinY-y. Preparation and characterization of ultra-high-strength and ultra-high-ductility cementitious composites incorporating waste clay brick powder. Journal of Cleaner Production, 2021, 312127813[J]

[34] ZhaoY-s, GaoJ-m, ChenG-f, et al.. Early-age hydration characteristics and kinetics model of blended cement containing waste clay brick and slag. Journal of Building Engineering, 2022, 51104360[J]

[35] OuyangS-y, HuangY-l, WuL-w, et al.. Effects of chlorides on setting time, hydration heat and hydration products of fresh slurry of cemented paste backfill. Case Studies in Construction Materials, 2022, 17e01462[J]

[36] BondarD, LynsdaleC J, MilestoneN B, et al.. Effect of adding mineral additives to alkali-activated natural pozzolan paste. Construction and Building Materials, 2011, 25(6): 2906-2910[J]

[37] EtcheverryJ M, Villagran-ZaccardiY A, van Den HeedeP, et al.. Effect of sodium sulfate activation on the early age behaviour and microstructure development of hybrid cementitious systems containing Portland cement, and blast furnace slag. Cement and Concrete Composites, 2023, 141105101[J]

[38] ZhangJ-t, ZhangW-s, YeJ-y, et al.. Influence of alkaline carbonates on the hydration characteristics of β-C₂S. Construction and Building Materials, 2021, 296123661[J]

[39] HewlettP C, LiskaMLea’s chemistry of cement and concrete, 2019[M]

[40] ChiM, HuangR. Binding mechanism and properties of alkali-activated fly ash/slag mortars. Construction and Building Materials, 2013, 40: 291-298[J]

[41] XieZ-h, XiY-p. Hardening mechanisms of an alkaline-activated class F fly ash. Cement and Concrete Research, 2001, 31(9): 1245-1249[J]

[42] YangP, LiuL, SuoY-l, et al.. Investigating the synergistic effects of magnesia-coal slag based solid waste cementitious materials and its basic characteristics as a backfill material. Science of the Total Environment, 2023, 880163209[J]

[43] van DeventerJ S J, ProvisJ L, DuxsonP, et al.. Reaction mechanisms in the geopolymeric conversion of inorganic waste to useful products. Journal of Hazardous Materials, 2007, 139(3): 506-513[J]

[44] Fernández-JiménezA, PalomoA, CriadoM. Microstructure development of alkali-activated fly ash cement: A descriptive model. Cement and Concrete Research, 2005, 35(6): 1204-1209[J]

[45] Escalante-GarciaJ I. Nonevaporable water from neat OPC and replacement materials in composite cements hydrated at different temperatures. Cement and Concrete Research, 2003, 33(11): 1883-1888[J]

[46] ChengY-h, YangS-h, ZhangJ-y, et al.. Test research on hydration process of cement-iron tailings powder composite cementitious materials. Powder Technology, 2022, 399117215[J]

[47] WangJ-x, LyuX-j, WangL-y, et al.. Influence of the combination of calcium oxide and sodium carbonate on the hydration reactivity of alkali-activated slag binders. Journal of Cleaner Production, 2018, 171: 622-629[J]

[48] García CalvoJ L, HidalgoA, AlonsoC, et al.. Development of low-pH cementitious materials for HLRW repositories Resistance against ground waters aggression. Cement and Concrete Research, 2010, 40(8): 1290-1297[J]

[49] SongS, JenningsH M. Pore solution chemistry of alkali-activated ground granulated blast-furnace slag. Cement and Concrete Research, 1999, 29: 159-170[J]

[50] RichardsonI G. The nature of the hydration products in hardened cement pastes. Cement and Concrete Composites, 2000, 22(2): 97-113[J]

[51] PlusquellecG, GeikerM R, LindgárdJ, et al.. Determination of the pH and the free alkali metal content in the pore solution of concrete: Review and experimental comparison. Cement and Concrete Research, 2017, 96: 13-26[J]

[52] AndersenM D, JakobsenH J, SkibstedJ. A new aluminium-hydrate species in hydrated Portland cements characterized by ²⁷Al and ²⁹Si MAS NMR spectroscopy. Cement and Concrete Research, 2006, 36(1): 3-17[J]

[53] DengY-y, WangX-c, ZhangY, et al.. Preparation, mechanism, and application of silica fume-steel slag composite cementitious material based on the D-optimal mixture method. Construction and Building Materials, 2023, 408133638[J]

[54] KhanM, CaoM-l, HussainA, et al.. Effect of silica-fume content on performance of CaCO₃ whisker and basalt fiber at matrix interface in cement-based composites. Construction and Building Materials, 2021, 300124046[J]

[55] ShiC, ZhangG, HeT-s, et al.. Effects of superplasticizers on the stability and morphology of ettringite. Construction and Building Materials, 2016, 112: 261-266[J]

[56] KongD L Y, SanjayanJ G. Effect of elevated temperatures on geopolymer paste, mortar and concrete. Cement and Concrete Research, 2010, 40(2): 334-339[J]

[57] ZhangS-f, NiuD-t. Hydration and mechanical properties of cement-steel slag system incorporating different activators. Construction and Building Materials, 2023, 363129981[J]

[58] ChenW-w, LiB, WangJ, et al.. Effects of alkali dosage and silicate modulus on autogenous shrinkage of alkali-activated slag cement paste. Cement and Concrete Research, 2021, 141106322[J]

[59] RenB-k, ChaiL-j, LiuY-z, et al.. Performance optimization design of high ductility cement-based alkali-activated municipal solid waste incineration fly ash composite for rapid repair material. Construction and Building Materials, 2023, 404133301[J]

[60] HamidiR M, ManZ, AzizliK A. Concentration of NaOH and the effect on the properties of fly ash based geopolymer. Procedia Engineering, 2016, 148: 189-193[J]

[61] TaylorR, RichardsonI G, BrydsonR M D. Nature of C-S-H in 20 year old neat ordinary Portland cement and 10% Portland cement–90% ground granulated blast furnace slag pastes. Advances in Applied Ceramics, 2007, 106(6): 294-301[J]

RIGHTS & PERMISSIONS

Central South University