Water-bearing porous rocks can be greatly strengthened by freezing; its microscopic mechanisms lie in how subzero temperature modifies pore structure but have not yet been clearly defined. In this study, we link the mechanical properties of frozen sandstone to phase composition of pore water (i.e., relative amount of unfrozen water and ice). Both the strength (uniaxial compressive strength and tensile strength) and the phase composition of pore water (tested through the nuclear magnetic resonance (NMR) method) of frozen sandstone at nine subzero temperatures were measured. The results showed the following: (1) subzero temperature strengthened saturated sandstone significantly, both the strengths increased in a three-stage mode and each stage showed an approximately linear trend as temperature decreased from 0 to – 20 °C; and (2) change of phase composition with freezing displayed two distinct stages: the rapid decreasing stage of unfrozen water content (0 to – 4 °C), when bulk water and capillary water froze almost completely, and the slow decreasing stage (− 4 to – 20 °C), when adsorbed water froze partially. Based on direct observations on microscopic pore structure of sandstone and detailed discussion on the mechanical interactions between water, ice, and pore wall, phase composition may modify the mechanical properties of frozen rocks through the following effects: the supporting effect of ice under compression, the crack-filling effect of ice, the cementing effect of unfrozen water film under tension or under shearing load, and the frost damage effect. The former three are strengthening effects, while the latter one is weakening effect.

含水的多孔岩石可以通过冷冻而大大增强。其微观机制在于零度以下温度如何改变孔隙结构，但尚未明确定义。在这项研究中，我们将冷冻砂岩的力学性能与孔隙水的相组成（即，未冷冻水和冰的相对含量）联系起来。在9个零度以下的温度下，测量了冷冻砂岩的强度（单轴抗压强度和拉伸强度）和孔隙水的相组成（通过核磁共振（NMR）测试）。结果表明：（1）零以下温度显着增强了饱和砂岩的强度，二者强度均以三阶段模式增加，并且随着温度从0降低至– 20°C，每一阶段都呈现出近似线性的趋势；（2）冻结时相组成的变化表现出两个明显的阶段：未冻结水含量的快速下降阶段（0至– 4°C），即散装水和毛细管水几乎完全冻结时；以及缓慢下降阶段（− 4）到– 20°C），当吸附的水部分冻结时。在直接观察砂岩微观孔隙结构并详细讨论水，冰与孔壁之间的机械相互作用的基础上，相组成可能会通过以下作用来改变冻岩的力学性能：压缩下的冰的支撑作用，冰的裂缝填充作用，在拉伸或剪切载荷下未冻结的水膜的固结作用以及霜冻破坏作用。前三个是增强作用，而后三个是减弱作用。