Seeing Through Mortar: Rod Layout, Matrix Cure State and Failure Mechanisms in a Light-Transmitting White-Cement Composite with Pultruded Glass-Fibre Rods

Light-transmitting cementitious composites integrate daylighting functionality directly into the building envelope, potentially reducing dependence on artificial lighting and its associated operational energy demand and environmental impacts. Such multifunctional materials may therefore contribute to more energy- and resource-efficient buildings by combining the durability of cement-based materials with controlled light transmission. However, most studies rely on commercial polymer or silica optical fibres and treat the embedded light guides as passive inclusions, paying limited attention to their manufacturing quality and spatial stability during casting. In this study, glass-fibre/epoxy (GFRP) micro-rods (d ≈ 0.43 mm) were manufactured in-house by pultrusion from a single 300-tex E6-glass roving and embedded in a white CEM I 52.5R mortar using regular, alternating (staggered), and random layouts. The rods were characterised by flexural, tensile, and Charpy tests, differential scanning calorimetry (DSC), and digital microscopy, while the light-transmitting channels were quantified using automated spot detection combined with Clark–Evans nearest-neighbour statistics. The rods exhibited stiff, fibre-dominated tensile behaviour, with an indicative tensile modulus of about 55 GPa and a tensile strength of approximately 1.1 GPa (n = 2; indicative values only). In contrast, the matrix-sensitive properties were low and highly scattered, as indicated by a flexural modulus of 15.5 ± 3.8 GPa and a Weibull modulus of 3.5. Charpy tests on taped ten-rod bundles (n = 3) provided only a qualitative indication of a damage-tolerant, non-catastrophic failure mode. DSC revealed a residual cure exotherm of 19.8 J g−1, a matrix glass-transition temperature of 67 °C, and an estimated degree of cure of only 0.47–0.62. Microscopy additionally showed periodic resin beads spaced approximately 6 mm apart, consistent with insufficient resin gelation at the pultrusion die. The strong, although small-sample (n = 6), correlation between flexural modulus and strength (r = 0.94, p = 0.006) suggested that specimen-level cure heterogeneity, rather than randomly distributed flaws, was the principal source of mechanical scatter. The staggered layout produced approximately twice as many active light channels (N = 271 versus 133–134) and was the only arrangement that substantially preserved its intended geometry after casting (Clark–Evans R = 1.55, z = 17.4). By contrast, the nominally regular layout became statistically indistinguishable from complete spatial randomness (R = 1.07). Light transmission was assessed semi-quantitatively from back-illuminated images (channel counts and spatial statistics) on one specimen per layout; photometric transmittance and the mechanical properties of the composite element were outside the scope of this exploratory study. The results indicate that the cure state of in-house-manufactured light guides and their fixation during casting are as important as the nominal rod arrangement. Effective control of these parameters can improve manufacturing reliability, limit defective material, and support the development of durable, material-efficient cementitious elements for daylight-based, lower-energy building applications.