Magnesium silicate hydrate (M-S-H) attracts significant attention as an alternative to traditional portland cement due to its unique properties, including reduced energy consumption during preparation. However, its synthesis typically requires MgO from calcined magnesite, which undermines the environmental benefits it provides. This study addresses these limitations by proposing a waste valorization strategy: recycling waste acid to dissolve naturally abundant magnesium silicate minerals (serpentine, olivine, and sepiolite) for M-S-H production. By combining acid waste management with mineral resource utilization, this approach not only avoids the energy-intensive MgO production process but also eases the ecological hazards caused by acid disposal.

The experimental involved three key stages: acid leaching, material preparation, and characterization. First, serpentine, olivine, and sepiolite were subjected to dissolution in a mixed waste acid solution (HCl: 6.20 mol/L, H₂SO₄: 7.77 mol/L) at 90 ℃ for 3 h under continuous mechanical agitation. The solid-to-liquid ratio was maintained at 1∶5 for acid leaching. After leaching, the insoluble residues (mainly amorphous silica) were separated with a centrifuge (Thermo Scientific ST 16R). The obtained residues were washed ultrasonically with deionized water 5 times to remove residual acid. Subsequently, the residue was dried in a vacuum drying oven at 50 ℃ for 24 h. The acid-leached filtrate was added into a 10 mol/L NaOH solution, with real-time pH monitoring until reaching pH 11.5. The obtained suspension was left to settle at room temperature for 24 h. The white flocculent precipitate was collected by vacuum filtration and dried at 100 ℃ for 24 h to obtain Mg(OH)₂-rich powder. The dried residue was mixed with Mg(OH)₂ powder at a ratio of n(SiO₂) ∶ n(MgO)=1.0∶1.4, and blended with water with a water-to-solid ratio of 1 ∶ 2 to form a paste. The paste was sealed in glass tubes and cured in water baths at 20, 50 ℃, and 80 ℃ for 3, 7, 14 d, and 28 d, followed by vacuum drying at 50 ℃ for 24 h prior to characterization. Mortar samples with the same residue-Mg(OH)₂ ratio were tested for compressive strength, fluidity, and drying shrinkage according to Chinese national standards.

The residues contain 89%-96% amorphous SiO₂, and Mg(OH)₂ precipitates exhibit 96% purity. Promoting curing temperature significantly accelerates M-S-H formation. At the same age, the diffraction peak of Mg(OH)₂ is lower in samples cured at higher temperature. TG curves reveal 3 distinct mass loss: 50-350 ℃ (loss of physically bound water, Δw₁), 350-450 ℃ (Mg(OH)₂ dehydroxylation, Δw₂), and 450-750 ℃ (Si-OH dehydroxylation, Δw₃). The Δm₂ content decreases with age and elevated temperatures, indicating accelerated Mg(OH)₂ reaction. A linear relationship between Δm₂ and (Δw₂+Δw₃) is established. A decline is observed in the slopes of fitting line, from 0.96 (20 ℃) to 0.80 (50 ℃) and 0.76 (80 ℃), which indicates that the contribution from unreacted Mg(OH)₂ decreases while the formation of M-S-H is promoted with increasing temperature. The broadening and intensification of the exothermic peak near 850 ℃ (M-S-H recrystallization) in the DSC curve indicate enhanced maturation of M-S-H with increasing curing temperature. ²⁹Si NMR spectra indicate elevated curing temperatures reduced the relative content of Q¹, confirming enhanced polymerization of silicon-oxygen tetrahedra in M-S-H phases, though infinite chain structures were not formed. The diminished Q³_c signal (corresponding to talc-type 2∶1 layered structures) indicated a gradual decline in "transitional" M-S-H (Mg/Si = 0.9-1.1), while increased Q³_a (serpentine-like 1∶1 layers) reflected accelerated alignment of Mg/Si ratios with the raw material's composition (1.4). M-S-H gels form via Mg²⁺/H₃SiO₄^- polymerization into Q²/Q³ networks with minor Q¹, accelerated by elevated temperatures to yield serpentine-like layer structure. The M-S-H mortars demonstrated remarkable early strength (12-18 MPa at 3 d). This was attributed to the high reactivity of amorphous SiO₂ and the rapid dissolution of Mg(OH)₂. After 28 d, compressive strength reached 40 MPa at 80 ℃. The mortar exhibited superior fluidity (157-181 mm flow diameter) without any superplasticizer, enabled by 28%-32% moisture in residues. However, excessive drying shrinkage was observed.

The mixed waste acid effectively dissolved serpentine, olivine, and sepiolite, yielding Mg(OH)₂ (≥96% purity) and SiO₂-rich residues (>89% SiO₂). The polymerization degree of silicon-oxygen tetrahedra in the resulting M-S-H phases increased with rising curing temperature (from 20 ℃ to 80 ℃) and extended curing time (from 3 d to 28 d), leading to the formation of serpentine-like layered structures. The mortar exhibited high early strength (12-18 MPa at 3 d) and reached 40 MPa at 28 d, which is attributed to the high reactivity of silica residues and rapid dissolution of Mg(OH)₂. Although the inherent moisture in the residues and the absence of water consumption by Mg(OH)₂ contributed to excellent fluidity without adding superplasticizers, this combination still causes significant shrinkage.