Chloride Binding Capacity and Its Effect on the Microstructure of Mortar Made with Marine Sand

Congtao Sun,Feng Qu,Peng Zhang,Tao Tao,Yantao Li,Gongxun Wang,Jizhou Duan,Ming Sun

doi:10.3390/su13084169

Abstract

Chloride binding capacity and its effect on the microstructure of mortar made with marine sand (MS), washed MS (WMS) and river sand (RS) were investigated in this study. The chloride contents, hydration products, micromorphology and pore structures of mortars were analyzed. The results showed that there was a diffusion trend for chloride ions from the surface of fine aggregate to cement hydrated products. During the whole curing period, the free chloride content in the mortars made by MS and WMS increased firstly, then decreased and stabilized finally with time. However, the total chloride content of three types of mortar hardly changed. The bound chloride content in the mortars made by MS and WMS slightly increased with time, and the bound chloride content included the MS, the WMS and the RS arranged from high to low. C3A·CaCl2·10H2O (Friedel’s salt) was formed at the early age and existed throughout the curing period. Moreover, the volume of fine capillary pore with a size of 10–100 nm increased in the MS and WMS mortar.

Highlights

As a primary raw material of concrete, river sand (RS) has been overly exploited as fine aggregate because of the rapid growth of the construction industry
The results indicate that the chloride ions introduced by fine aggregates were combined with hydration products to form Friedel’s salt at early curing ages, and that Friedel’s salt always exists in mortars throughout the whole curing period
The phase compositions of mortars made by three fine aggregates were analyzed by energy disperse spectroscopy (EDS), X-ray diffraction (XRD)

Summary

Introduction

As a primary raw material of concrete, river sand (RS) has been overly exploited as fine aggregate because of the rapid growth of the construction industry. The direct use of MS can increase the potential risk of steel bar corrosion and cause serious degradation of concrete structures [8,9,10,11]. To reduce the possibility of corrosion, MS desalination technology, such as washing, must be adopted, in which energy and fresh water could be consumed. Some chloride ions can still be found on the surface of the washed MS (WMS), and the application of WMS can increase the free chloride concentration in the pore solution of concrete. It is well known that free chloride can bond with the use of hydration products, which can retard the occurrence of steel corrosion behavior [12].

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