Influence of MgO on the Hydration and Shrinkage Behavior of Low Heat Portland Cement-Based Materials via Pore Structural and Fractal Analysis

Lei Wang,Xiao Lu,Ge Zhang,Fanxing Guo,Jie Xiao,Lisheng Liu,Li Li

doi:10.3390/fractalfract6010040

Abstract

Currently, low heat Portland (LHP) cement is widely used in mass concrete structures. The magnesia expansion agent (MgO) can be adopted to reduce the shrinkage of conventional Portland cement-based materials, but very few studies can be found that investigate the influence of MgO on the properties of LHP cement-based materials. In this study, the influences of two types of MgO on the hydration, as well as the shrinkage behavior of LHP cement-based materials, were studied via pore structural and fractal analysis. The results indicate: (1) The addition of reactive MgO (with a reactivity of 50 s and shortened as M50 thereafter) not only extends the induction stage of LHP cement by about 1–2 h, but also slightly increases the hydration heat. In contrast, the addition of weak reactive MgO (with a reactivity of 300 s and shortened as M300 thereafter) could not prolong the induction stage of LHP cement. (2) The addition of 4 wt.%–8 wt.% MgO (by weight of binder) lowers the mechanical property of LHP concrete. Higher dosages of MgO and stronger reactivity lead to a larger reduction in mechanical properties at all of the hydration times studied. M300 favors the strength improvement of LHP concrete at later ages. (3) M50 effectively compensates the shrinkage of LHP concrete at a much earlier time than M300, whereas M300 compensates the long-term shrinkage more effectively than M50. Thus, M300 with an optimal dosage of 8 wt.% is suggested to be applied in mass LHP concrete structures. (4) The addition of M50 obviously refines the pore structures of LHP concrete at 7 days, whereas M300 starts to refine the pore structure at around 60 days. At 360 days, the concretes containing M300 exhibits much finer pore structures than those containing M50. (5) Fractal dimension is closely correlated with the pore structure of LHP concrete. Both pore structure and fractal dimension exhibit weak (or no) correlations with shrinkage of LHP concrete.

Highlights

Low heat Portland (LHP) cement or high-belite cement is featured by a significantly larger proportion of belite (C2 S) and a smaller proportion of alite (C3 S) compared with conventional ordinary Portland cement [1,2]
Hydration Heat Results of LHP Cement Paste Added with magnesia expansion agent (MgO)
7.2–8.4% lower and LC8M5 presents 14.2–16.7% lower 360-day strength than the LC0 concrete. This is consistent with the results reported by Choi et al [93], who found that the compressive strength of concrete added with 5% MgO (265 s) began to increase after the hydration time of 56 days and achieved a comparable strength with the control at 180 days

Summary

Introduction

Low heat Portland (LHP) cement or high-belite cement is featured by a significantly larger proportion of belite (C2 S) and a smaller proportion of alite (C3 S) compared with conventional ordinary Portland cement (or ASTM Type I and CEM I) [1,2]. 2022, 6, 40 the peak adiabatic temperature of LHP cement concrete is about 5.5–6.2 ◦ C lower than. The conditions in mass concrete are nearly adiabatic and the peak temperature can achieve as high as 50–80 ◦ C, and the large temperature difference between inside and outside of structures can lead to thermal stress development and cracking [8]. LHP cement is a perfect cement to be used in mass concrete structures, which could obviously reduce the potential thermal-stress-induced cracking risk [1,4]. The thermal stress of LHP cement-based materials cannot be considerably avoided since the hydration of LHP cement still releases a large amount of heat, even though its peak value is declined and postponed to some extent compared with OPC. How to reduce the shrinkage of LHP cement concrete remains a challenging issue for engineers

Methods

Results

Conclusion