مقاله انگلیسی رایگان در مورد نظارت بر انقباض داخلی در ملات سیمانی – الزویر 2018

1.1. Pore structure of cementitious materials

On its smallest scale, cement stone is made up of hydrated calciumsilicates (C-S-H), being responsible for cohesion and strength. C-S-H forms the structural backbone and largely defines the pore structure. In this, gel pores form with sizes as small as a few nanometres. These pores strongly interact with water. Drying of gel pores leads to a shrinkage of cement stone and conversely, if empty gel pores are filled again, a swelling of the cement matrix occurs. [1–5] A currently widely adopted model developed by Jennings describes C-S-H as a lammenar [6] colloid [7] structure. This model is based on a composition of so-called globules with sizes of about 8 nm. Water is found in four different positions inside the globular C-S-H structure: (i) as so-called interlayer water between C-S-H sheets, (ii) in intraglobular pores (IGP) smaller than 1 nm, both inside the C-S-H globules, as well as (iii) in small gel pores (SGP) with sizes of about 1–3 nm and (iv) large gel pores (LGP) with sizes of 3–12 nm in interstitial spaces between globules. Within this model, large gel pores show similarities to capillary pores to some extent. IGP originate in imperfections inside the C-S-H sheets. Though being similar in size to small gel pores between globules, they largely differ in their accessibility to e.g. nitrogen, leaving voids upon emptying. In Fig. 1 a schematic representation of the pore spaces according to Jennings colloid model CM-II is drawn. The detailed nature of the on-going processes while drying and rewetting in cement stone has been a point of discussion for some time. Mostly, two different processes at drying of hardened white cement paste are distinguished [3]: (i) mesopores grow between a relative humidity1 (RH) of 95% down to 40%, accessible to both, water vapour and nitrogen and (ii) drying below 40% RH leads to the further increase of mesopore volume, interpreted as a consolidation of C-S-H structure. In the latter, differences in nitrogen and water vapour absorption are explained by nitrogen inaccessible IGP formed inside the consolidated C-S-H globules. Maruyama et al. [3] showed a temporary reduction of drying shrinkage between these processes, explained as resolution of capillary tension inside the pore structure. Similar results were obtained by Setzer and his co-workers before [4, 5]. On a slightly larger scale, capillary pores are formed by an excess of liquid water at the making as well as water emptied pore space due to chemical shrinkage and self desiccation [8]. These pores are associated with transport processes such as capillary sorption. The equations developed by Washburn [9] suggest, that sorptivity S is proportional to the porosity Φ times the square root of the overall transport effective pore radius R [10]. For simplicity, the contact angle between the wetting liquid and the pore surface α usually is set to zero, thus its influence neglected [10, 11]. The overall curvature, described by tortuosity, and the interconnection of pores, described by restrictivity, are also expected to be important influences [10].