Effects of Erosion Form and Admixture on Cement Mortar Performances Exposed to Sulfate Environment

Peng Liu,Zhiwu Yu,Ying Chen

doi:10.3390/cryst10090774

Abstract

The effects of the admixtures, erosion age, concentration of sulfate solution, and erosion form of sulfate attack on the mechanical properties of mortar were investigated. Simultaneously, the microstructure, pore characteristics, kinds and morphologies of erosion products of mortar before and after sulfate attacks were performed by Mercury Intrusion Porosimetry (MIP), Environment Scanning Electronic Microscope and Energy Dispersive Spectrometer (ESEM-EDS). In addition, the crystal form and morphology characteristics of crystallization on mortar surfaces attacked by partial immersion form were studied. The results showed that the compressive and flexural strengths of mortar attacked by sulfate for four months decreased with the increase of the replacement of cement with fly ash, and the corresponding strength of mortar containing slag first increased and then decreased. The admixtures can improve the microstructure and mechanical properties of mortar within the replacement ratio of 10%. Although the change laws of the mortar specimens containing different admixtures were similar, the mortar containing slag had an excellent sulfate resistance under the same condition. Compared with the complete immersion form, the strength variation of the mortar containing fly ash attacked by semi-immersion form was less. The porosity and average pore diameter of mortar attacked by sulfate for four months increased, and the percentage of micropore with the pore diameter less than 200 nm increased. Plenty of rod-like and plate-like erosion products were generated in mortar attacked by a sulfate solution with a high concentration. A larger number of fibrous and flocculent crystallization covered the mortar’s surface containing fly ash, but it was a granular and dense crystallization formed on the mortar’s surface containing slag. Much dendritic erosion product was generated in the mortar attacked by semi-immersion form, and ESEM-EDS analysis revealed that it may be scawtite, spurrite, and residue of the decomposed calcium silicate hydrate (CSH) in the inner mortar; however, the crystallization sodium sulfate was crystallized on mortar surface.

Highlights

IntroductionConcrete and mortar are the most commonly used construction materials in civil engineering [1,2,3].Many environmental influencing factors affect the performance variation of concrete structures, and the sulfate attack is one of the most important factors in the deterioration of concrete structures [4].The performance deterioration of cement-based materials is because of the expansion effect of Crystals 2020, 10, 774; doi:10.3390/cryst10090774 www.mdpi.com/journal/crystalsCrystals 2020, 10, 774 sulfate erosion products and due to the sulfate attack, which causes a reduction in the adhesion forces of the structure by destroying the major cementitious products of the hydrated cement, i.e., calcium aluminate hydrate (CAH), calcium hydroxide (CH), and calcium silicate hydrate (CSH) [5,6]
Crystals 2020, 10, 774 sulfate erosion products and due to the sulfate attack, which causes a reduction in the adhesion forces of the structure by destroying the major cementitious products of the hydrated cement, i.e., calcium aluminate hydrate (CAH), calcium hydroxide (CH), and calcium silicate hydrate (CSH) [5,6]
The concrete can be treated as the combination of mortar and coarse aggregates, so the performance variation of concrete can be indirectly characterized by the property evolution of the mortar

Summary

Introduction

Concrete and mortar are the most commonly used construction materials in civil engineering [1,2,3].Many environmental influencing factors affect the performance variation of concrete structures, and the sulfate attack is one of the most important factors in the deterioration of concrete structures [4].The performance deterioration of cement-based materials is because of the expansion effect of Crystals 2020, 10, 774; doi:10.3390/cryst10090774 www.mdpi.com/journal/crystalsCrystals 2020, 10, 774 sulfate erosion products and due to the sulfate attack, which causes a reduction in the adhesion forces of the structure by destroying the major cementitious products of the hydrated cement, i.e., calcium aluminate hydrate (CAH), calcium hydroxide (CH), and calcium silicate hydrate (CSH) [5,6]. Many environmental influencing factors affect the performance variation of concrete structures, and the sulfate attack is one of the most important factors in the deterioration of concrete structures [4]. The performance deterioration of cement-based materials is because of the expansion effect of Crystals 2020, 10, 774; doi:10.3390/cryst10090774 www.mdpi.com/journal/crystals. The sulfate attack of cement-based materials is a complex physical and chemical process including crystallization and reaction. Sulfate attack can affect the various performance of the mortar, such as strength, length change, long-term behavior, and durability [7,8,9,10,11]. It is very significant to investigate the performance variation of the mortar under sulfate environment

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Conclusion