Relation between composition, structure and morphology in C-S-H

The aim of this study was to determine if there is a relationship between the morphology of C-S-H (calcium silicate hydrates) with its chemical composition and structure or the morphological change is kinetically driven. The morphology of C-S-H, the binding phase of cement, has been an open subject of debate for decades. C-S-H morphology affects the shape of the capillary pores, as the capillary porosity is defined by the outer product C-S-H. Thus, the morphology of C-S-H partially determines transport properties and the durability of cementitious materials. This underlines the importance of understanding it to model the degradation and predict the service life of such materials. The Op C-S-H (outer product) exhibits different morphologies from fibrillar to sheet-like foils in different cementitious systems. It is not clear whether the change in morphology is determined by the structure and chemical composition or it is kinetically driven [1]. Finding suitable synthetic analogues of materials formed under normal conditions remains a challenge. However, synthetic analogues are ideal systems for the aim of this project. Their fabrication is controlled under synthesis parameters which can affect the morphology and can be tailored. Therefore synthetic C-S-H, with Ca/Si ratios between 0.75 to ~1.7 (covering part of the range that commercial cements exhibit) were proposed as model systems to be compared with real cementitious systems in this study. TEM and NMR were the main techniques to analyze the morphology, chemical composition and structure of the samples. Other techniques such as STA, XRD, XRF, TG-FTIR-DSC and SEM were used to get complementary information.
The results obtained indicate that C-S-H morphology of samples fabricated via silica-lime reactions with bulk Ca/Si ratios from 0.75 to 1.5, and C-A-S-H samples with Ca/Si=1 and Al/Si=0-0.05 is foil-like. The morphology of C-S-H in samples hydrated via the controlled hydration of C3S at fixed lime concentrations was found to be dependent on the lime concentration in solution; being foil-like for lime concentrations from 12 to 20mmol/l (Ca/Si ratios from ~1.25 to ~1.4), a mixture of foils and fibrils for 22mmol/l (Ca/Si ratio of ~1.58) and fibrillar for concentrations ≥ 25mmol/l (Ca/Si ratios of ~1.60-1.65). For each lime concentration, the morphology was found to be independent of the growth rate, being the same for the acceleration period (fast growth) and the deceleration period (slow growth). This implies the morphology is composition dependent and not kinetically driven. However, a link between the silicate structure of C-S-H and its morphology was also found. Samples fabricated via the controlled hydration of C3S, with an ultrasound gun, at lime concentrations of 27-29mmol/l, were found to have higher percentages of Q2 silicate species and more flattened surfaces, than samples fabricated at the same lime concentrations but with the use of C-S-H seeds (Xseed). This agrees with the fact that flattened surfaces could accommodate longer silicate chains while surfaces with more features would accommodate more Q1 end-chains.