Long-term properties and microstructural characterization of natural clinoptilolite zeolite-based cementitious materials

The pozzolanic reaction of clinoptilolite zeolite is often considered a slow chemical process at early ages, and the interaction with portland cement can alter the chemical and microstructural condition due to the cation exchange capability of zeolite particles beyond the early ages. Therefore, the present study investigates the long-term properties and microstructural characterization of clinoptilolite zeolite-blended cementitious materials, replacing the portland cement up to 20% by mass. For this, pozzolanic activity via thermal gravimetric analysis, degree of hydration by non-evaporable water, estimated gel/space ratio, compressive strength, and drying shrinkage of mortar bars were conducted to assess the influence of zeolite addition up to 360 days. The volume of hydration reaction products for the cementitious pastes was measured by the chemical shrinkage. The microstructure and pore size distribution were observed by scanning electron microscopy and small-angle X-ray scattering, respectively. The chloride ion penetrability of concrete was measured via surface resistivity. Results revealed that the use of clinoptilolite zeolite decreased the compressive strength in the early ages likely due to the slow pozzolanic activity (i.e., particles were not properly reactive until 28 days), but the zeolite samples obtained a relatively higher compressive strength at later ages due to the continued pozzolanic action, increasing degree of hydration, and the gel/space ratio. Results also suggested that the zeolite particles attributed to the internal curing capacity in the later ages. The lower chemical shrinkage of zeolite samples indicated the lower hydration reaction products in the early ages, but the higher pozzolanic consumption of CH in the early ages produced more gel/space ratio of microstructure leading to an apparently increased fibrous C-S-H network in the long term. The finer pore size distribution of zeolite-blended samples led to lower drying shrinkage by up to 25% compared to the control sample in 360 days. Overall, the use of to 20% clinoptilolite zeolite substantially increased the surface resistivity (by up to 230% higher compared to the control) in 90 days and subsequent resistance to chloride ion penetrability by reducing the permeability due to the refinement of pore microstructure.