Bleeding of Portland composite cements

Abstract

Bleeding remains a critical issue in the performance and durability of cementitious composites, particularly in systems incorporating mineral powders (MPs). This study investigates the bleeding behaviour of Portland cement pastes modified with granite powder (GP), limestone powder (LP), and siliceous fly ash (FA) at varying substitution levels (10–30 wt.%). A comprehensive experimental programme was designed to correlate bleeding intensity with the physical and morphological properties of the powders, including particle size distribution, specific surface area (BET), shape descriptors (Roundness, Area ratio, Aspect ratio), and density. Advanced monitoring techniques—ultrasonic pulse velocity (UPV) and electrical conductivity measurements—were employed to track structural changes during the first 100 min of hydration. The results demonstrate that bleeding is primarily governed by powder morphology and density, rather than early-age reactivity. Powders with angular grains and high specific surface area (e.g., GP and LP) effectively reduce bleeding through enhanced particle packing and accelerated structure formation, whereas FA—with its lower density and spherical morphology—significantly increases bleeding capacity. Strong linear correlations were identified between bleeding volume and both UPV and electrical conductivity, supporting their application as non-destructive indicators of water migration and early matrix consolidation. A five-phase mechanistic model describing the evolution of bleeding is proposed, integrating gravitational settling, hydration kinetics, and structural stiffening. The study highlights that proper bleeding control in sustainable cement systems requires not only chemical compatibility but a targeted optimisation of particle shape, size, and density. These insights offer practical implications for the formulation of low-bleed, eco-efficient binders incorporating industrial by-products.