Title: Investigation of Pore Structure of Lightweight Ultra-High- Performance Concrete under Curing Regimes

Author(s): Yang Li, Gaozhan Zhang, Jun Yang, Jian Zhang, Qingjun Ding

Publication: Materials Journal

Volume: 119

Issue: 6

Appears on pages(s): 133-148

Keywords: curing regime; hydration; lightweight aggregate; pore structure; products; ultra-high-performance concrete

DOI: 10.14359/51737188

Date: 11/1/2022

Abstract:
The apparent density of lightweight aggregate (LWA)-modified ultra-high-performance concrete composite is 2080 kg/m3, and the compressive strength is not less than 110 MPa at 28 days. Lightweight ultra-high-performance concrete (LUHPC) not only has light weight and high strength, but also reduces the consumption of raw materials and the section size of the structure, thus reducing the cost. The macroscopic properties are closely related to the pore structure characteristics, but the structural nature of LUHPC under different curing regimes and the LWA on their pore structure remain unclear. To comprehensively understand the pore structure of LUHPC and then control its properties, capillary absorption method, low-field nuclear magnetic resonance (LF-NMR), computed tomography (CT), and nitrogen adsorption (BET) technologies were used to characterize the pore structure characteristics of LUHPC. The experimental results show that there are many nanoscale pores (mainly harmful and more-harmful pores) in LUHPC. With the increase of water absorption of the added LWA, the porosity of LUHPC and the proportion of less-harmful pores increase, thus changing the pore structure of LUHPC. With the increase of temperature and pressure, the internal curing effect of LWA is accelerated. Heat treatment promotes the formation of dense additional hydrates such as tobermorite and xonotlite, and the average chain length of the hydrates and the pozzolanic reaction between supplementary cementitious material and Ca(OH)2. Steam curing increases the total porosity and coarsens the pore size while accelerating the hydration of cementitious paste. Autoclaved curing can stimulate the pozzolanic activity of inert SiO2, promote the formation of secondary hydration products, and fill the pores in the matrix. The evolution of the pore structure of LUHPC plays a key role in improving its performance due to the curing regimes and presence of LWA.