Orthogonal Study on Pore Structure and Compressive Strength of Densifier Modified Fly Ash Concrete

The effects of MATE-ADV densifier dosage on CT-resolvable pore characteristics and compressive strength in fly ash–slag concrete remain insufficiently quantified. An L9 orthogonal array was used to evaluate cement content, sand ratio, fly ash (FA) content, and densifier dosage. X-ray computed tomography (X-CT) characterized the specimens at 28 d, and compressive strength was measured at 7, 14, and 28 d. Across the nine specimen groups, mean slice porosity ranged from 0.75% to 2.50%, while 28 d compressive strength ranged from 59.49 to 68.37 MPa. Exploratory pooled-error ANOVA suggested main effects of cement content (F(2,4) = 8.05, p = 0.040) and sand ratio (F(2,4) = 7.71, p = 0.042) on mean slice porosity. Cement content and sand ratio showed the largest main effects on 28 d strength; FA content was also significant, whereas densifier dosage was not (F(2,18) = 2.79, p = 0.088). X-CT analysis showed variation in pore-section area and circularity among the groups. A second-order logarithmic model summarized the observed porosity–strength relationship (R2 = 0.948; leave-one-group-out RMSE = 1.422 MPa). Finally, a candidate for the optimal mix proportion was identified by jointly considering the evaluated 2D pore characteristics and 28 d compressive strength.