The mechanical performance evaluation revealed significant variations in strength development patterns among different mix designs, with optimal performance achieved through balanced incorporation of industrial waste materials. Table 4 presents the comprehensive mechanical performance data demonstrating the evolution of compressive strength, flexural strength, and elastic modulus over extended curing periods. The results indicate that Mix 5, containing 15% fly ash, 7.5% steel slag, and 22.5% blast furnace slag replacement, achieved the highest 28-day compressive strength of 52.3 MPa, representing a 16.2% improvement over the reference cement mixture.
The strength development patterns can be quantitatively described using the modified Abrams' law, which correlates compressive strength with hydration degree and phase composition:
where fc(t) represents compressive strength at time t, fc∞ is the ultimate strength, k is a rate constant related to material composition, and n is an empirical parameter typically ranging from 0.5 to 0.8 for waste-incorporated mixtures. The analysis reveals that waste incorporation influences both the ultimate strength potential and the kinetics of strength development, with pozzolanic materials contributing to enhanced long-term performance through secondary hydration reactions.
Figure 5 illustrates the comprehensive comparison of mechanical properties among different mix designs, highlighting the superior performance of optimized waste-incorporated mixtures compared to conventional cement-based materials. The comparative analysis demonstrates that strategic combination of fly ash, blast furnace slag, and steel slag can achieve mechanical properties exceeding those of ordinary Portland cement while utilizing substantial quantities of industrial waste materials.
The flexural strength to compressive strength ratio provides insights into material ductility and failure characteristics, with optimal mixtures maintaining ratios between 0.12 and 0.15, indicating balanced mechanical behavior suitable for structural applications. The elastic modulus development follows a similar trend to compressive strength, with the relationship expressed through the empirical correlation:
where E represents elastic modulus, fc is compressive strength, α is a material-dependent coefficient (typically 3500-4500 for waste-incorporated mixtures), and β is an exponent ranging from 0.4 to 0.6.
The durability evaluation encompassed freeze-thaw resistance, carbonation resistance, and chloride permeability testing to assess long-term performance characteristics under aggressive environmental conditions. The freeze-thaw resistance testing after 300 cycles revealed that Mix 5 exhibited only 2.1% mass loss and 3.8% strength reduction, significantly outperforming the reference mixture which showed 4.7% mass loss and 8.2% strength reduction. This enhanced freeze-thaw resistance is attributed to the refined pore structure and improved interfacial bonding achieved through optimized waste material incorporation.
Carbonation depth measurements after 90 days of accelerated exposure indicated that waste-incorporated mixtures demonstrated superior carbonation resistance compared to conventional cement, with Mix 5 showing 40% reduction in carbonation depth relative to the reference mixture. The chloride permeability testing revealed charge passed values ranging from 1850 to 3200 Coulombs for waste-incorporated mixtures, with optimal formulations achieving "low" permeability classification according to ASTM C1202 standards.
Additional durability testing of the optimal Mix 5 revealed enhanced sulfate resistance with 6.2% ± 0.8% strength loss after 180 days exposure to 5% sodium sulfate solution compared to 11.8% ± 1.2% for reference cement (Mix 1). Accelerated carbonation testing (1% CO₂, 65% RH, 20°C) showed 38% reduction in carbonation depth for optimized mixtures after 90 days exposure. Life cycle assessment conducted according to ISO 14040/14044 standards indicates potential reductions of 35% in CO₂ emissions and 42% in natural resource consumption compared to conventional cement production, based on cradle-to-gate system boundaries and assuming 100 km average transportation distance for waste materials.
The comprehensive durability assessment confirms that properly designed ecological building materials incorporating industrial waste can achieve superior long-term performance characteristics while contributing to environmental sustainability through resource conservation and waste reduction.
The comprehensive characterization of chemical microscopic phase composition through X-ray diffraction and thermogravimetric analysis revealed significant variations in hydration product formation and evolution patterns among different waste-incorporated mixtures. The primary hydration products identified include calcium silicate hydrate (C-S-H) gel, portlandite (Ca(OH)₂), ettringite (AFt), monosulfate (AFm), and various unreacted phases from both cement and industrial waste materials. The quantitative phase analysis utilizing Rietveld refinement methodology with internal standard calibration provided precise determination of phase contents and their temporal evolution during hydration processes.
The formation kinetics of major hydration products can be described through the modified Avrami equation, which correlates the degree of hydration with time and material composition:
where α(t) represents the degree of hydration at time t, k₁ and k₂ are rate constants for primary and secondary hydration reactions respectively, and n₁ and n₂ are empirical exponents reflecting nucleation and growth mechanisms. The dual-term expression accounts for the distinct kinetics of cement hydration and pozzolanic reactions from industrial waste materials.
The quantitative analysis of microscopic phase composition demonstrates the progressive evolution of hydration products and the influence of industrial waste incorporation on phase assemblage development. Table 5 presents the detailed quantitative analysis results showing the temporal changes in major phase contents across different mix designs and curing periods. The results indicate that waste-incorporated mixtures exhibit enhanced C-S-H gel formation at extended curing periods, with Mix 5 achieving 58.2% C-S-H content at 90 days compared to 52.1% for the reference mixture, demonstrating the beneficial effects of pozzolanic reactions.
The portlandite content exhibits a decreasing trend in waste-incorporated mixtures due to consumption through pozzolanic reactions, with the reduction rate following first-order kinetics according to the relationship:
where [Ca(OH)₂] represents portlandite concentration at time t, [Ca(OH)₂]₀ is the initial concentration, and kₚ is the pozzolanic reaction rate constant dependent on waste material reactivity and fineness.
Figure 6 presents the comparative XRD diffraction patterns illustrating the distinct phase assemblages and crystalline structure variations among different mix designs at 28-day curing period. The C-(A)-S-H gel exhibits broad diffraction humps around 29° and 50° 2θ, indicating its poorly crystalline nature, while sharp peaks corresponding to portlandite (18.1°, 34.1°, 47.1° 2θ) and ettringite (9.5°, 15.8°, 22.9° 2θ) demonstrate their well-defined crystalline structures. The main ettringite peak identification was confirmed using PDF card 00-041-1451 with the characteristic (100) reflection at 9.5° 2θ. The diffraction peak at 26.6° 2θ corresponds to unreacted quartz, which originates primarily from fly ash rather than river sand based on peak intensity analysis and comparison with raw material patterns.
The waste-incorporated mixtures exhibit additional diffraction peaks corresponding to unreacted quartz (26.6° 2θ) from fly ash, gehlenite (31.0° 2θ) from steel slag, and merwinite (35.2° 2θ) from blast furnace slag, indicating partial reaction of waste materials and their integration into the hydrated cement matrix. The systematic shift in peak positions and intensity variations provide quantitative insights into the extent of pozzolanic reactions and the formation of secondary C-S-H gel with modified chemical composition.
The incorporation of industrial waste materials significantly influences hydration mechanisms through multiple pathways including physical effects (filler action, nucleation sites), chemical effects (pozzolanic reactions), and synergistic interactions among different waste components. The pozzolanic reactivity of waste materials follows the dissolution-precipitation mechanism, where amorphous silica and alumina phases dissolve in the alkaline pore solution and react with available calcium ions to form additional C-S-H gel with lower Ca/Si ratios compared to primary hydration products.
The degree of pozzolanic reaction can be quantified using the Chapelle test results and correlated with the consumption of portlandite according to:
where CH₀ and CHₜ represent the portlandite content at initial and time t respectively. The analysis reveals that optimal waste combinations achieve pozzolanic reaction degrees exceeding 45% at 90 days, contributing significantly to strength development and microstructural refinement through the formation of additional binding phases and pore structure modification.
The scanning electron microscopy and energy-dispersive X-ray spectroscopy analysis revealed distinct microstructural features and chemical composition variations within the interfacial transition zone between industrial waste particles and cement matrix. The ITZ exhibited heterogeneous characteristics with thickness ranging from 15 to 35 µm, depending on waste particle type, size, and surface reactivity. The SEM investigations demonstrated that fly ash particles, with their smooth spherical surfaces, developed relatively uniform ITZ regions characterized by reduced porosity compared to angular steel slag particles, which created more complex interfacial geometries with enhanced mechanical interlocking potential.
Figure 7 presents SEM-EDS analysis showing interfacial transition zone characteristics and elemental distribution maps for waste-incorporated mixtures. The ITZ thickness ranges from 15 to 35 μm with distinct Ca/Si ratio gradients from 1.8 to 2.1 in bulk matrix to 1.2-1.5 near waste particles, indicating C-(A)-S-H gel formation through pozzolanic reactions.
The EDS elemental mapping analysis provided quantitative insights into the chemical composition gradients across the ITZ, revealing systematic variations in calcium, silicon, and aluminum distributions that directly correlate with hydration product formation and interfacial bonding characteristics. The calcium-to-silicon ratios exhibited gradual transitions from 1.8 to 2.1 in the bulk cement matrix to 1.2-1.5 near waste particle surfaces, indicating the formation of modified C-S-H gel with enhanced binding properties through pozzolanic reactions. The aluminum-rich zones adjacent to fly ash and blast furnace slag particles demonstrated the incorporation of aluminate phases into the C-S-H structure, contributing to improved mechanical properties and reduced permeability.
The interface bonding mechanisms between waste particles and cement matrix involve multiple concurrent processes including physical adhesion, chemical bonding through hydration reactions, and mechanical interlocking at particle-matrix interfaces. The chemical bonding mechanisms are primarily governed by pozzolanic reactions where amorphous silica and alumina phases from waste materials react with calcium hydroxide produced during cement hydration, forming additional C-S-H gel with modified stoichiometry and enhanced binding capacity. The dissolution kinetics of waste particle surfaces create localized chemical environments with elevated pH values and ionic concentrations that promote accelerated hydration product formation and improved interfacial bonding.
The surface reactivity of different waste materials significantly influences the extent and nature of interfacial interactions, with blast furnace slag exhibiting the highest reactivity due to its glassy structure and favorable chemical composition. Steel slag particles demonstrated selective reactivity with certain mineral phases contributing to C-S-H formation while iron-rich phases remained largely unreacted, creating heterogeneous interfacial characteristics that require optimization through particle size control and surface treatment strategies. The synergistic effects of combined waste materials resulted in enhanced interfacial bonding through complementary chemical reactions and improved particle packing efficiency.
The establishment of quantitative relationships between microscopic phase composition and macroscopic mechanical properties required comprehensive correlation analysis incorporating multiple microstructural parameters and their weighted contributions to overall performance.
The ITZ quality index was calculated as a composite parameter incorporating three normalized components: porosity (P), microhardness (H), and chemical bonding strength (C), according to the following equation:
where P₀, H₀, and C₀ represent reference values from ordinary Portland cement. Porosity measurements were obtained through quantitative image analysis of SEM micrographs using ImageJ software with threshold segmentation, analyzing a minimum of 20 fields per sample at 5000 × magnification. Microhardness was measured using nanoindentation with a Berkovich tip at 2 mN load, with 15 indentations per ITZ region at 5 μm intervals from the particle surface. Chemical bonding strength was assessed through EDS point analysis measuring the Ca/Si ratio gradient across the ITZ, with values normalized to the bulk matrix composition.
Figure 8 illustrates the quantitative structure-property relationships between microstructural parameters and compressive strength for validated mix designs. The ITZ quality index represents a composite parameter calculated using Eq. 9, combining normalized porosity, microhardness, and chemical bonding strength measurements. The correlation analysis includes 8 data points, excluding Mix 4 due to incomplete hydration that compromised accurate C-(A)-S-H gel quantification. The relationship demonstrates strong correlation (R = 0.92) within the investigated parameter space, though extrapolation beyond these boundaries requires additional validation.
The predictive model development incorporated multiple regression analysis with nonlinear terms to capture the complex interactions between different microstructural parameters and their cumulative effects on mechanical performance. The optimized model achieved correlation coefficients exceeding 0.92 for compressive strength prediction and 0.88 for flexural strength estimation, demonstrating excellent predictive capability for material design optimization and quality control applications. The model validation through independent test datasets confirmed the robustness and reliability of the structure-property relationships across different waste material combinations and processing conditions.
The validation of theoretical predictions through experimental verification demonstrated excellent agreement between model predictions and measured performance characteristics, with prediction errors typically below 8% for compressive strength and 12% for flexural strength across the investigated parameter ranges. The predictive accuracy was highest for mixtures with moderate waste replacement levels (15-25%) where interfacial interactions remained within the optimized parameter space used for model development. The systematic analysis of prediction errors revealed that deviations were primarily associated with extreme waste replacement levels or unusual waste material combinations that exceeded the model calibration boundaries.
The model refinement through iterative optimization procedures and expanded experimental databases improved prediction accuracy and extended the applicable parameter ranges for reliable performance estimation. The validated structure-property relationship models provide essential tools for rational material design, enabling optimization of waste material combinations and processing parameters to achieve target performance characteristics while maximizing waste utilization rates. The successful validation of theoretical predictions confirms the fundamental understanding of interface interaction mechanisms and their quantitative influence on macroscopic properties, supporting the development of high-performance ecological building materials through science-based design approaches.