The Effect And Mechanism Of Construction Solid Waste Powder On The Properties Of Cement-based Materials At The Micro-nano Scale

Concrete is the most widely used building material so far,and cement is the main component of concrete.The production of cement causes the depletion of mineral resource and a huge amout of CO₂emissions.To reduce the consumption of cement,the concrete industry has for some time been seeking supplementary cementious materials that can partially replace cement.In recent years,waste clay brick,waste ceramic brick and waste concrete have been grinded and used as cement replacement.The effect of these powders on the mechanical properties of cement-based materials has been extensively studied.However,few studies have been carried out to investigate the mechanism of the effect at the micro-nano scale.In this study,the effect of clay brick powder（CBP）,ceramic brick powder（PP）,recycled concrete powder（RP）and carbonated recycled concrete powder（CRP）on the rheological,hydration and mechanical properties of cement-based materials and the mechanism were studied at different scale.Firstly,XRD,TGA,²⁹Si RNM and other test methods were used to study the physical and chemical properties of the solid waste powders.Secondly,the interaction between the surface of the powders and the ions in the simulated solution was studied using Zeta potential test.Then,the rheological properties of cement paste with different building solid waste powders were investigated.After that,the nucleation and growth of C-S-H on the surface of these powder particles were observed by scanning electron microscope（SEM）to further clarify the mechanism of the influence of the surface characteristics on hydration.Finally,the interface properties between building solid waste powder particles and cement hydration products were invsetigated by means of the microscopic observation of the fracture path and fracture surface of the loaded cement paste.In this study,the limestone powder（LP）and quartz powder（QP）were used as the references.The results obtained in this study are as follows:（1）The surface chemical properties of building solid waste powder particles determine their surface charge properties.Particles with Ca CO₃-rich surface have a stronger adsorption force towards Ca²⁺in cement paste,such as LP.On the other hand,the particles with silicon-rich surface have a weak adsorption capacity towards Ca²⁺,such as QP,CBP,PP,RP and CRP.（2）LP particles have a strong ability to absorb calcium ions.It makes the electrostatic repulsion between particles strong in cement paste.Therefore,the rheological properties of cement paste with LP are better than pure cement paste.The fluidity of cement paste mixed with QP is similar as the pure cement paste.It is due to that QP and cement particles have similar surface charge properties.Building solid waste powder particles have similar surface charge characteristics as QP,so the influence of electrostatic repulsion between these particles on rheological properties is similar.However,due to the different physical properties of these powders,the rheologogy of the cement paste with these powders was performed differently.（3）The nucleus of C-S-H on LP surface are densier,and the growth of C-S-H on LP surface is directional,while the distribution of C-S-H on QP surface is less and sparse.CBP and PP have similar surface charge characteristics as QP,the nucleation and growth mode of C-S-H on the surface of these particles are similar as that of QP.However,due to the large amount of Ca（OH）₂on the surface of RP,C-S-H hardly grows on the surface of RP.Silica gel on the CRP surface rapidly dissolves and reacts with calcium hydroxide to generate more C-S-H.Afterwards,the calcium carbonate is exposed leading to a large amount of C-S-H growing on the CRP surface.（4）The microcracks tended to propagate along the QP interface compared with that in LP cement paste.This is due to the difference of surface charge characteristics.The development pattern of microcracks around CBP and PP particles is similar as that of QP,which is attributed to the fact that all three particles are connected to hydration products by electrostatic force.However,the development of microcracks along the particle interface is different possible due to the chemical reactivity of the powders in cement paste.Compared with RP,the micro cracks propagated along the interface of CRP particles was very few,instead most of it progagted through the CRP particles.It is due to the stronger bond formed between Ca CO₃and C-S-H,and increased C-S-H resulted from the reactions of silica gel with CH at the interface.