饱水水泥砂浆孔结构的低场磁共振测试分析

Pore Structure of Water-Saturated Cement Mortars by Low-Field Nuclear Magnetic Resonance

由于水化硅酸钙(C-S-H)凝胶干缩湿胀,水泥基材料在干燥与含水状态下的孔结构差异显著且受干燥制度的影响,干燥状态下测量所得孔结构特征及其演化规律的代表性和准确性存疑。为深入揭示孔结构随水灰比演化的基础规律,以典型白水泥砂浆为研究对象,利用低场磁共振和压汞法分别测试饱水和干燥时的孔结构特征,同时测量得到水分渗透率。结果表明:利用饱水时的离散孔径分布,基于平行毛细管束假设的经典Kozeny-Carman模型,可准确预测水分渗透率;水灰比对饱水砂浆孔结构和水分渗透率的影响规律基本一致。压汞测试所得孔径分布离散性较大,且难以反映水灰比影响规律,干燥状态下测试所得孔结构特征及据此建立的孔径分级方式具有明显局限性。由于孔结构全面影响硬化水泥基材料宏观性能,且低场磁共振技术能避免干燥预处理的显著影响,利用该技术实现饱水状态孔结构的测试分析对水泥基材料科学研究具有重要价值。

Calcium silicate hydrate(C-S-H)gels shrink upon drying and swell upon on wetting due to the specific water sensitivity,making that the pore structure of cement-based material depends on its water content.The representativeness and accuracy of the pore structure characteristics and its development obtained in a totally dry state are questionable because of the influence of drying preconditioning.To explore the evolution of pore structure of cement-based materials with respect to water to cement(w/c)ratio,several typical white cement mortars were investigated by low-field nuclear magnetic resonance(LF-NMR)and mercury intrusion porosimetry(MIP).Meanwhile,their permeability coefficients to water were also measured.The results show that the discrete pore size distribution(PSD)achieved by LF-NMR in a water-saturated state can be used to predict the water permeability through a classical Kozeny-Carman model based on the parallel cylindrical pores.The effect of w/c ratio on the pore structure of water-saturated mortars is consistent with that on the water permeability.However,the PSD curves measured by MIP scatter remarkably,which cannot reflect the effect of w/c ratio on the pore structure of mortars.As a result,the pore structural characteristics obtained in a dry state are limited,which makes the typical pore classifications questionable.Since the pore structure plays a fundamental role in interpreting almost all the properties of hardened cement-based materials,it is essential to detect the more relevant pore structure in a water-saturated state through LF-NMR to avoid the effect of drying preparation.