Microstructure and microanalysis of hardened ordinary Portland cement pastes

Abstract

Hardened ordinary Portland cement pastes of various ages have been examined by analytical transmission electron microscopy (TEM) and electron microprobe analysis (EMPA). The stability of the various hydrate phases in the electron microscope is discussed. Although all are subject to damage in varying degrees, even the least stable phase, AFt, can be recognized in relict form in the TEM. The basic framework of the microstructure and the differentiation into inner and outer hydration product are well-established at 24 h hydration. Although the dominant inner product formed within the boundaries of the original anhydrous grains is C-S-H gel, particles of AFt, AFm, Ca(OH)₂, a magnesium-rich phase and an iron-rich phase are occasionally observed within the inner product. The Ca∶Si ratio of the C-S-H gel determined by TEM shows significant variation from one region to another in a given paste. There is no relationship between the average Ca∶Si ratio of the C-S-H and the maturity of the paste, although young pastes appear to show a bimodal distribution. Microanalysis by EMPA gives Ca∶Si ratios in substantial agreement with those found by TEM but it is essentially impossible to obtain by EMPA analyses of outer product C-S-H without admixture of other phases, particularly sulphoaluminate phases. Despite the presence of small amounts of embedded phases as revealed by TEM, single-phase inner product C-S-H can be analysed by EMPA. The compositions of AFt and AFm phases have been obtained by TEM and the results do not require the substitution of silicon in the formulae.