An Experimental Investigation on the Effects of Limestone Fines in Manufactured Sands on the Performance of Magnesia Ammonium Phosphate Mortar

Wenting Mao,Jueshi Qian,Yudong Dang,Xincheng Li,Chunpeng Cao

doi:10.3390/buildings12020249

Abstract

Magnesium ammonium phosphate cement (MAPC) prepared with ammonium dihydrogen phosphate (NH4H2PO4, ADP) and dead-burned Magnesium oxide (MgO) is a new type of rapid patch repair material for concrete structures. In order to reduce the material costs of MAPC mortar, manufactured limestone sands, being a more widely-available resource with lower cost, was investigated in this study as an alternative to quartz sands for the preparation of MAPC mortar. The limestone fines in manufactured sands were found to be the key factor that influences properties of MAPC mortar by causing bubbling and volume expansion before hardening. As a result, the mechanical strength of MAPC mortar decreased with the increasing content of limestone fines due to increased porosity. According to microstructure analysis, the mechanism of these negative effects can be inferred as the reaction between limestone fines and ADP with the gas generation of CO2 and NH3. This reaction mainly occurred during a short period before setting while most limestone fines remained unreactive in the hardened MAPC mortar. Based on the above detailed experimental findings on the effects of limestone fines in manufactured sand on the properties of MAPC mortar, this paper pointed out that effective defoaming methods for inhibiting bubbling was the key to the utilization of manufactured sands in preparation of high performance MAPC mortar.

Highlights

Maintenance work has always been the focus of concrete structures during longterm service life after construction
This paper focused on the effects of limestone fines on the properties of Magnesium ammonium phosphate cement (MAPC) mortar prepared with limestone sand, including the setting time, workability, mechanical strength, and long-term volume stability
This paper presented experimental findings on the effects of limestone fines of manufactured sand on properties of MAPC mortar

Summary

Introduction

Maintenance work has always been the focus of concrete structures during longterm service life after construction. High-quality quartz sand usually costs several times higher than natural river sand or manufactured limestone sand This further increases the material cost of MAPC mortar. Yang et al [19] investigated the properties of magnesium phosphate cement with limestone fines They found that the addition of limestone fines improved the early hydration degree of MKPC with an increased crystallization degree and the amount of hydration products, resulting in high mechanical strength and low shrinkage. This paper focused on the effects of limestone fines on the properties of MAPC mortar prepared with limestone sand, including the setting time, workability, mechanical strength, and long-term volume stability. Based on the above results, this paper attempted to provide useful references for the prospective application of limestone sand in the production of cost-effective MAPC mortar

Materials and Methods

Preparation of MAPC Mortar

Microstructure Analysis MIP

Conclusions

Findings