The effect of prehydration on the engineering properties of CEM I Portland cement

Abstract

Prehydration -the reaction between anhydrous cement and water vapour -has deleterious effects on engineering properties such as compressive strength, workability and setting time. This study assessed changes in the engineering properties of CEM I Portland cement exposed to relative humidities (RHs) of 60% or 85% for 7 and 28 days. Thin layers (no more than 2 mm thick) of CEM I 42.5R cement were exposed to controlled RHs of 60% and 85%, followed by assessment of compressive strength, setting time and workability. These measurements were complemented by characterising the prehydrated anhydrous cement using isothermal calorimetry, scanning electron microscopy, X-ray diffraction and thermal gravimetric analysis. Following prehydration at 60% RH, conventional hydration resulted in a negligible reduction in strength development plus a decrease in workability and increase in setting time. At 85% RH, compressive strength was greatly reduced, particularly at early ages. There was also a greater loss of workability and increase in setting time.

Dry powder alkali-activated slag cements

“…The fresh AAS cement does not form the conglomerates (Figure 3(a)) or large amounts of reaction products (Figure 3(b)) similar to that which cover particles of the aged AAS cement (Figures 2(c) and 2(d)). Similar formations of reaction products on the surface of prehydrated OPC particles were observed by Dubina et al (2010) and Whittaker et al (2013). These products effectively increase the surface area of the aged AAS cement and form a barrier between water and the unreacted part of the slag (Whittaker et al, 2013).…”

“…25, as the procedure of a standard paste consistency normally used for OPC cements (BSI, 2005) would influence the sodium carbonate/water ratio in the AAS pastes, changing the ionic and anionic concentration of the liquid media (Shi and Day, 1996;Shi and Li, 1989 To estimate the shelf life of dry powder AAS cements, the AAS cement SC45-390 was aged in an opened bag at 258C and 99% humidity for 9 weeks after manufacturing. A detailed study of prehydration of the AAS cements was not the main aim of this research and a rather practical approach was used, ageing the AAS cement in a bag instead of a thin layer of a few millimetres (Whittaker et al, 2013), assuming that AAS cement would be stored in bags as OPC normally is. Cement generally would not be stored in such conditions, but 99% humidity was used to intensify the ageing process, thus evaluating the performance of the cement if it would be stored for a long time (to replicate a worst-case scenario).…”

“…AAS concrete containing SC45-390 had a slump of 35 mm, while workability of concrete containing aged SC45-390 cement was 10 mm slump. The reduction in workability could be caused by the increase in surface area of cement due to the formation of hydration products during the ageing process (Figure 2(d)) and the interlocking effect caused by increased roughness of the cement particles (Whittaker et al, 2013). AAS concretes containing SF6-610 and SF6L-620 cements had Vebe workability of 18 and 10 s, respec-tively.…”

“…After the ageing procedure, dry powder AAS cement SC45-390 was used to produce concrete with a compressive strength of 49 MPa after 28 d (Figure 4) in comparison to the strength of AAS concrete containing fresh AAS cement SC45-390, which yielded 65 MPa after 28 d. Whittaker et al (2013) showed that prehydration of OPC for only 28 d at 85% relative humidity caused around 13% loss in compressive strength of mortars at 28 d. A loss of less than 25% in compressive strength during storage in extreme conditions (99% relative humidity) for 2 months could be a good indicator of sufficient shelf life for the dry powder AAS cements.…”

Editorial

“…The remaining paper considers future developments in cement science and looks into the possibility of using waste materials to produce hydraulic binders which would help to reduce the carbon footprint of construction. Whittaker et al (2013) have looked at the effect of prehydration, the interaction of anhydrous Portland cement with water vapour, and its effect on the early-age performance of cementitious systems. This phenomenon delays the onset of hydration and may, under extreme circumstances, have a detrimental impact on strength development.…”