Shrinkage reducing agents (SRAs) are admixtures, first developed in Japan [1], that are used to reduce the drying shrinkage and cracking of concrete. Shrinkage-induced cracking is unsightly and can also reduce concrete durability by various mechanisms, e.g. by allowing intrusion of chloride ions and carbon dioxide gas, which promote steel corrosion in reinforced concrete. SRAs are often used to mitigate cracking, but the mechanism of shrinkage reduction is still poorly understood. The first generation of SRAs were moderately water-soluble nonionic organic solutes known to reduce the surface tension of water [2]. However, it was noted that it was usually necessary to use them at aqueous concentrations well above their apparent “critical micelle concentration” (CMC) in order to obtain the maximum effect on reducing free drying shrinkage of hardened cement pastes (hcp) [3,4]. These experimental results threw doubt on the idea that SRAs work solely via a capillary tension mechanism [5]; despite intensive additional research [5–۱۲], their mechanism remains poorly understood. Recently, we showed that SRAs reduce the unrestrained drying shrinkage of hcp mainly during the first desorption, but have little impact on further length changes during rewetting and further drying after sufficiently long process times for altering the calcium silicate hydrate (C-S-H)1 [13]. In cases of short term drying and wetting cycles of the mortar with lower water to cement ratio, the continuous impact was confirmed to show contrary behavior [12]. The difference might be explained by a difference of drying period for C-S-H and a difference in amount of outer C-S-H which has large impact on the irreversible shrinkage. It was also shown that portlandite crystals in hcp can be strongly modified by the use of certain SRAs [7,12,13], which seems consistent with observations that simple alcohols modify the morphology and increase the specific surface area of portlandite produced by slaking quicklime, apparently by preferential adsorption on the 001 crystal face, where alkoxide (-OR) groups can partially replace hydroxide (-OH) groups [14,15]. It is thus conceivable that certain SRAs might also modify the nanostructure of calcium silicate hydrate (C-S-H), possibly by interaction with some of its –OH groups [16]. C-S-H is a highly basic solidsolution phase of widely-varying composition, thought to be composed mainly of tobermorite-like anionic basal layers, with associated calcium cations for charge balance [17–۱۹]. The C-S-H produced during cement or C3S hydration is formed at a high pH from solutions slightly supersaturated with respect to portlandite. It has a very disordered structure [20,21] with many Ca-OH groups [16], which could be partly alkylated to give some Ca-OR groups. Based on many such prior studies, a new model for the structure of the “wet” C-S-H initially formed in Portland cement hydration was recently proposed by Gartner et al. [22]. We will refer to it here as the “GMC” model. In this model, freshly-precipitated C-S-H is composed of tobermorite-like basal layers containing primarily dimeric silicate anions bonded to both sides of basal CaO sheets. The negative charge of the simplest basal sheet repeating unit, ([Ca2Si2O7] 2−), is initially balanced by fully-hydrated calcium cations, either divalent (Ca2+) or univalent (CaOH+), and it is assumed that each such cation holds at least 6 oxygen atoms in their inner coordination shell. Thus, fresh wet C-S-H can be represented as a solid solution between the end members ([Ca2Si2O7] 2−·[Ca(H2O)6] 2+) (=C3S2H6) and ([Ca2Si2O7] 2−·۲[Ca(OH) (H2O)5] +) (=C4S2H11). If γ represents the fraction of the low Ca/Si end members of this solid solution, it can readily be seen that for γ = ۰٫۴, the composition of the solid solution becomes C1.7SH4. This composition is the same as that assumed by Young and Hansen [23] to account for the minimum amount of water required for complete cement hydration.