SULFATE ATTACK RESISTANCE OF ECO-EFFICIENT CEMENT-BASED MORTAR MIXTURES

Abstract

Ordinary Portland cement (OPC) is one of the most widely used construction materials in civil engineering infrastructure construction but it is susceptible to sulfate attack. One of the ways to improve the sulfate resistance of an OPC mortar/concrete is to replace a certain amount of OPC with different pozzolanic materials such as ground granulated blast furnace slag (GGBFS) and metakaolin. The use of pozzolanic materials to mortar/concrete not only enhances durability but also reduces carbon dioxide (CO2) emission due to the less usage of OPC at the initial construction state. As considering these aspects, limestone calcined clay cement (LC3) has been developed in recent decades. However, the influence of LC3 on sulfate attack resistance has not been fully evaluated. Therefore, this study investigated the efficiency of LC3 mortar mixtures against sulfate attack in normal and wet-dry cycle exposure conditions up to approximately 6 months. To evaluate the synergistic effect of a combination of LC3 and GGBFS on the sulfate resistance, the LC3 and OPC mixtures containing 25% and 50% GGBFS were also assessed. Three different initial curing periods, namely 1-day, 3-day, and 16-d warm curing, were applied to both LC3 and GGBFS-incorporated LC3 mixtures in order to achieve similar sulfate testing strength to OPC mixtures. Therefore, the initial compressive strength of LC3 mixtures with/without GGBFS for measuring the volumetric expansion of mixtures submersed into 5% sodium sulfate (Na2SO4) solution was categorized into 3 MPa, 7 MPa, and 14 MPa. To further examine the properties of LC3 mixtures, density, porosity, compressive strength, weight changes, and dielectric constant tests were conducted. The experiment results show that the expansion of the LC3 mixture with initial strength of under 7 MPa regardless of the addition of GGBFS and initial curing made a plateau after a rapid increase up to 7 days, while the expansion of the OPC mixture kept increasing throughout the period. The LC3 mixtures demonstrated negligible expansions with the initial high strength of approximately 14 MPa under fully submersed test condition. However, in the wet-dry cycle condition, the LC3 mixture is discovered to be more susceptible than OPC at both low and high strength. Furthermore, the addition of GGBFS to OPC or LC3 mixture provides the synergistic effect on reducing the expansion due to sulfate attack. Therefore, if the LC3 mixture has high initial strength (min. 15 MPa) and dense microstructure to minimize the penetration of sulfate ion into the mixture, it is expected that the LC3 mixture is more efficient than the OPC mixture against the sulfate attack.