Influence of MgO on Water Distribution in Paste Based on Low-Field NMR

Abstract:

This study utilized low-field nuclear magnetic resonance （LF-NMR） to in situ investigate the water distribution， water migration， and pore structure evolution during setting and hardening of cement paste incorporating MgO expansive agent （MEA） at different temperatures （5，20，60 ℃） and water-binder ratios （m_w/m_b=0.35，0.45）. Results show that MEA accelerates total signal decay， improves internal water distribution， reduces free water content， and increases the proportion of micro capillary water. Those effects can be enhanced with the increased temperatures. After hydrating for 6 h of cement paste prepared with m_w/m_b of 0.35， the addition of MEA increases the content of micro-capillary water at 60 ℃ by 6.4 and 2.9 times compared to that at 5 ℃ and 20 ℃， respectively. The presence of MEA raises the weighted average relaxation time （T¯₂） and reduces the fractal dimension， which shows an enhanced influence with the increased temperature. At 6 h， T¯₂ increases by 0.19， 0.22， 0.29 ms at 5， 20， 60 ℃， respectively， and is more pronounced at m_w/m_b=0.45.