Effect of ilmenite and barite addition on gamma rays attenuation of cement pastes: study of the pore structure by positron annihilation technique

Designing cementitious composites that both withstand mechanical loads and provide effective γ-ray shielding requires understanding how high-density mineral fillers alter hydration, pore structure, and radiation attenuation—knowledge that is currently fragmented for ilmenite and barite additions. This study therefore systematically evaluates Type I OPC pastes containing 5–15 wt% ilmenite or barite (with sodium lignosulfonate at 0–0.3 wt% as a water-reducing admixture). Cubic specimens were water-cured to 28 days and characterized by compressive strength and bulk density measurements, X-ray diffraction (XRD), FTIR, thermogravimetry (TG), and scanning electron microscopy (SEM) for phase and microstructure, positron annihilation lifetime spectroscopy (PALS) (interpreted via the Tao–Eldrup model) for nanoscale pore sizing, and γ-ray attenuation tests (mass and linear attenuation coefficients, and half-value layers) across relevant energies. Increasing filler content raised chemically bound water but reduced bulk density and compressive strength; ilmenite-bearing composites consistently retained higher density and strength than equivalent barite mixes. XRD/FTIR/TG/SEM show ilmenite (poorly crystalline) supplies Fe²⁺/Fe³⁺/Ti⁴⁺ that substitute into ettringite/monosulfate and suppress their formation, whereas crystalline barite (Ba²⁺) promotes hydration phase development. Lignosulfonate altered water demand and setting times (accelerating up to 0.2% then retarding) and influenced ettringite and C–S–H morphology. SEM indicated 10% barite densifies the matrix and reduces microporosity (but higher barite reverses this), while ilmenite improves packing at 20% yet coarsens pores at 30%. PALS revealed distinct nanoscale signatures: control OPC τ₃ ≈ 2.9 ns (R ≈ 0.2–0.3 nm), I₃ ≈ 0.95; barite at 15% τ₃ ≈ 2.1–2.5 ns (R ≈ 0.35 nm) with I₃ ≈ 1.35 (many small accessible pores); ilmenite at 15% produced larger nanopores (R ≈ 0.57 nm) and reduced I₃ ≈ 0.80 (fewer accessible sites). γ-attenuation tests show neat OPC is relatively better against low-energy photons; both fillers increase porosity and generally lower mass attenuation coefficients, but ilmenite exhibits a modest improvement in attenuation with increasing loading. Barite and ilmenite impart clearly different microstructural and functional trade-offs: barite can be used to engineer a dense network of small pores (beneficial for certain durability profiles), while ilmenite produces coarser voids that preserve mechanical performance and can selectively enhance γ-ray attenuation. These findings provide a basis for tailoring radiation-attenuating cementitious composites to balance shielding efficacy, porosity, and mechanical durability. Barite addition enhances the rate of hydration than ilmenite whereas ilmenite addition retains higher strength than barite at the same loading. Fe²⁺/Fe³⁺/Ti⁴⁺ leached from ilmenite substitute for Al³⁺ in ettringite/monosulfate, reducing their formation, whereas Ba²⁺ from barite promotes those phases. PALS proves that barite filler produces small pores while ilmenite produces large pores. Ilmenite additions slightly boost attenuation coefficient with increasing content, making it marginally more beneficial than barite.