Structural kinetics of cement carbonation examined by neutron interferometric imaging (INFER) and SESANS

Cements and concretes are hierarchical porous solids that form the basis of most modern infrastructures, and growing interest exists to sequester atmospheric CO2 in their alkaline structure to offset emissions during cement production. Cement structures vary from nm to cm, and it remains a significant challenge to capture this structural hierarchy as it reacts. NIST is developing a three-grating far-field neutron interferometric imaging instrument (INFER) that aims to quantify hierarchical structures by combining neutron interferometry, tomography, and small-angle scattering. Despite promising results, INFER lacks quantitative validation from an independent method. The SESANS technique is ideal to compare with recent INFER data of carbonated cements. SESANS will also extend the range of comparison beyond SANS limitations, toward length scales of 1000 nm, which is useful to identify granular regions in carbonated cements. We propose to measure the structural kinetics of carbonated cement formulations with different water-to-cement ratios, limestone-to-cement ratios, and carbonation curing times, which are known to influence cement rheological properties and water/CO2 permeabilities. SESANS data will be modeled by a summed mass-surface fractal by a Hankel transformation shown to agree with previous SANS and INFER data of the same carbonated cements. Together, these measurements will improve understanding of cement structural evolution from nanometer to centimeters.

水泥与混凝土属于分级多孔固体，是绝大多数现代基础设施的核心基材。当前学界日益关注如何利用其碱性结构固存大气二氧化碳，以抵消水泥生产过程中的碳排放。水泥基结构的尺度跨度从纳米至厘米级，而在其发生反应时完整捕捉这一结构层级特征仍是一项重大挑战。美国国家标准与技术研究院（NIST）正在研发一款三光栅远场中子干涉成像仪器（INFER），该仪器旨在结合中子干涉测量、断层成像与小角中子散射（SANS）技术，实现分级结构的定量表征。尽管已取得颇具前景的研究成果，但INFER仍缺乏来自独立方法的定量验证。自旋回波小角中子散射（SESANS）技术十分适合与近期针对碳酸化水泥的INFER测试数据进行对比。此外，SESANS可突破小角中子散射（SANS）的量程限制，将对比范围拓展至1000纳米的长度尺度，这有助于识别碳酸化水泥中的颗粒区域。本研究拟针对不同水灰比、石灰石-水泥比以及碳化养护时长的碳酸化水泥配方，测量其结构动力学过程——上述参数均已知会影响水泥的流变特性与水/二氧化碳渗透性能。SESANS测试数据将通过汉克尔变换得到的叠加质量表面分形模型进行拟合，该模型已被证实与此前针对同款碳酸化水泥的小角中子散射（SANS）与INFER测试数据相符。上述一系列测试将共同加深对水泥基材料从纳米至厘米尺度的结构演化过程的认知。