Regulation on mechanical properties of fully recycled coarse aggregate concrete by alkali-modified rice straw ash and grey prediction of its pore structure parameters

To explore the regulatory effect of alkali-modified rice straw ash (RSA) on fully recycled coarse aggregate concrete, two test series were prepared: unmodified RSA replaces cement at equal-mass ratios of 0%, 5%, 10% and 15%, while 10% RSA fixed group was modified by NaOH with alkali–ash mass ratios from 1:100–8:100. Mechanical, water absorption, XRD and SEM multi-scale characterizations were performed, and a MATLAB pore quantification model and GM (1, 3) grey prediction model were established. Results show the optimal mix contains 10% RSA with an alkali-ash ratio of 2:100. Its 28-day compressive, splitting tensile and four-point bending strengths reach 53.61 MPa, 4.75 MPa and 7.11 MPa, increased by 20.63%, 26.67% and 29.74% compared with reference specimen S0, while water absorption falls to 3.08%, 26.8% lower than reference specimen S0. Moderate alkali modification promotes the transformation of crystalline SiO₂ into amorphous SiO₂ and generates abundant C-S-H gels, which optimizes the mesopore and micropore structure and densifies the interfacial transition zone of the concrete matrix. Excessive alkali, however, produces harmful hydrates and deteriorates the microstructure. Grey correlation analysis demonstrates that the mechanical properties of unmodified RSA concrete are dominated by mesopore fractal parameters, while micropore parameters become the controlling factors after alkali modification. The established GM (1, 3) model based on key pore fractal parameters exhibits excellent prediction accuracy, with an average relative error below 1.0% and all coefficients of determination R 2 greater than 0.99. This study provides theoretical support for the proportion optimization, engineering application, and performance prediction of recycled concrete incorporating alkali-modified agricultural solid waste ash.