Exploring the mechanism of TEA in improving the performance of coal gangue concrete with high replacement of admixtures

The low early-age strength of concrete containing high volumes of fly ash/slag severely limits the utilization of industrial by-products in coal gangue concrete. To address this, the present study employed Response Surface Methodology (RSM) to systematically optimize the mix design for enhanced early strength. The key variables investigated were the ratio of ultra-fine fly ash to slag (UFA:SL), the cement substitution rate, and the content of triethanolamine (TEA) as an early-strength agent. The results demonstrated that quadratic polynomial models excellently fitted the experimental data (R² > 0.9). For the 3d and 7d compressive strength, the cement substitution rate was the most influential factor, followed by TEA content and the UFA:SL ratio. By 28 d, the synergistic effect of UFA and SL became predominant. The optimized mixture (UFA:SL = 3.4:6.6, 30.01% cement replaced, 0.131% TEA) achieved compressive strengths of 25.1 MPa, 28.6 MPa, and 35.1 MPa at 3, 7, and 28 days, respectively. Microstructural analysis via FT-IR, XRD, and SEM confirmed that TEA promoted the formation of hydration products, refined the pore structure, and enhanced the density of the matrix, thereby effectively improving early-age strength. This work provides a validated RSM-based strategy for developing high-performance, sustainable coal gangue concrete with satisfactory early-age properties.