Influence Of Limestone And Calcined Clay On The Rheological Properties Of Limestone Calcined Clay Cement

Clinker substitution using supplementary cementitious materials（SCMs）is becoming increasingly prominent with the increased demand to cut down on CO₂emissions in the cement industry.In order for clinker substitution to remain practical and effective,it is imperative that we use SCMs with the potential of meeting high demand while yielding desired chemical and mechanical properties.Among the most used SCMs,limestone and calcined clay have been shown to be available in quantities that can match the demand while yielding the desired mechanical and durability properties.This has led to the development of limestone calcined clay cement（LC3）.As of today,critical to the practical applications of LC3 is a thorough knowledge of its rheological properties and the influence of limestone and calcined clay on these properties.It is thus the focus here to add to the limited knowledge in this area.In this study,the rheological properties of pastes made from ordinary Portland cement blended with different amounts of calcined clay and limestone were investigated in order to understand the combined and independent effects of limestone and calcined clay on the rheological properties of limestone calcined clay cement（LC3）.Large Amplitude Oscillation Strain was applied and the subsequent harmonic distortion was used to evaluate nonlinear response for the first time for cementitious materials.The results showed that calcined clay leads to increased static and dynamic yield stress,initial thixotropic index,plastic viscosity and cohesion,as well as decreased harmonic distortion,while limestone has an opposite effect.It is believed that these results will aid in understanding the viscoelasticity of this blended cement and that harmonic distortion offers an innovative,simple technique to studying the viscoelastic properties of cementitious materials.Furthermore,based on the first part of the study,questions as to the mechanisms governing the thixotropy of LC3 in comparison to ordinary Portland cement were raised.It was observed that compared with ordinary Portland cement,the thixotropy index of LC3 is higher in the early stage of hydration（10 min）,while after 1 hour of hydration,the thixotropy index increase is significantly lower than that of Portland cement.A careful combination of leading techniques–focused beam reflectance measurement,zeta potential,¹H nuclear magnetic resonance relaxometry（¹H NMR）and micro X-ray computed tomography–was then used to track the colloidal interaction and hydration extent within LC3 to probe the mechanisms governing rheological behaviors,especially thixotropy.A new approach to understanding water distribution in cement pastes is proposed based on the ¹H NMR results.Overall,results obtained through all the methods used showed that flocculation due to the negative surface charge and water affinity of calcined clay introduced by its physiochemical features appears to be the factor dominating thixotropy in LC3.This effect of calcined clay leads to a reduction of water available to contribute to the fluidity of paste and,in turn,governs the development of thixotropy over time.In addition,the dilution effect due to high clinker substitution diminishes thixotropy growth with time.The conclusions reached in this study pave way for the tailoring of the rheological properties of LC3 for different applications and also provide needed insights for the development of suitable superplasticizers.