Table 1. The fractions of the tetrahedral sites that are present in tobermorites and C-A-S-H phases that are occupied by Si or Al or that are vacant.

Al substitutes for Si only at bridging sites and paired sites cannot be vacant. The theoretical maximum values of f, v and (f + v) are , (i.e. dimer) and , respectively, for double-chain structure and , and for single-chain structure. The actual minimum and maximum values of f are dependent on the fraction (v) and distribution of vacant sites.

Sites occupied by	Value† for double-chain tobermorite and cross-linked C-A-S-H	Value for single-chain tobermorite and C-A-S-H
Al	f	f
Vacancy	v	v
Q1 Si	2v	2v
Q2(0Al) Si
Q2(1Al) Si	2f	2f
Q3(0Al) Si
Q3(1Al) Si
Total	1	1
Paired Q2(0Al) Si
Bridging Q2(0Al) Si	pv
Paired Q2(0Al) Si adjacent to Q3
Paired Q2(0Al) Si adjacent to Q2B	2pv
Bridging Q2(0Al) Si	pv

^† p = the fraction of vacant sites that are adjacent to a bridging site that is occupied by Si; q = the fraction of vacant sites that are adjacent to a bridging site that is occupied by Al; it follows that the fraction of vacant sites that are opposite other vacant sites = 1p q. Vacant sites that are opposite a bridging site that is occupied by Al or that is vacant are illustrated in Fig. 3 ▸ of Myers et al. (2013 ▸).