Effect of Excess Water on Microstructural Development

connectivity of solid volume, a series of virtual microstructure simulations were performed on the plain OPC pastes at two different states: OWD and MWD. The simulated degree of hydration and volumetric phase assemblage for the OPC systems at

OWD and MWD states are shown in Figure 5-11. The difference in the degree of

hydration for these two systems was found to further magnify at later ages. While both OPC systems exhibited initially similar hydration kinetics at early age, OPC system at flow onset (i.e., MWD state), wherein excess water is present, developed higher degree of hydration at later ages. Comparison between simulated phase assemblage in Figure 5-11(b) and (c) shows that while OPC paste at OWD state have lower capillary porosity than that of similar system at MWD state, a larger portion of cement remains unhydrated after 56 of hydration in the former system due to the lack of water for cement hydration. The simulated results are in agreement with previous studies. Several studies have reported that although w/c has little or no effect on early-age hydration kinetics of cement pastes (Masoero et al. 2014; Oey et al. 2013; Thomas 2007), higher w/c cement pastes generally exhibit higher degree of hydration than lower w/c cement pastes at later ages (Bentz 1997, 2006a; Kirby and Biernacki 2012). This is likely due to the availability of more space for the dissolution of reactants and higher nucleation/precipitation of hydration products in system with higher w/c (Bentz 1997, 2006a; Kirby and Biernacki 2012).

(a) (b)

(c)

Figure 5-11For the plain OPC system: (a) simulated degree of hydration as a function of time, (b) simulated phase assemblage at concentrated state (i.e., OWD), and (c) simulated

phase assemblage at flow onset (i.e., MWD) as a function of degree of hydration occurring during 56 days. All results were extracted from the virtual microstructure simulations. Based on six replicate simulations, the uncertainty was quantified to be less

than 5%.

Figure 5-12(a) depicts the relationship between evolution of connectivity of solid phase and compressive strength development for OPC systems at OWD and MWD states. The evolution of connected solid volume was calculated as the ratio of volume fraction of connected solid phase to the volume fraction of total solid phase present in system as a function of time. An example of continuous formation of connected solid network in

OPC systems at both OWD and MWD states is shown in Figure 5-12(b). While OPC

strengths than that of MWD state, the latter system was shown to have larger rate in connected solid volume.

(a) (b)

Figure 5-12 (a) Correlation between compressive strength and connected solid volume

(calculated from HYMOSTRUC model) for the plain OPC systems at OWD and MWD states. Based on six replicate simulations, the uncertainty was quantified to be less than 4%. (b) Non-evaporable water contents for the plain OPC systems at OWD and MWD

states.

The larger rate of connectivity of solid phase for OPC system at MWD state results in higher rate of strength gain at later ages as compared to the similar system at OWD state. Therefore, the difference in compressive strength results between OWD and

MWD states decreases with progressing cement hydration, as indicated in Figure 5-12.

The normalized non-evaporable water contents (determined from TGA data) for the plain

OPC system at OWD and MWD states are shown in Figure 5-12(b) as a function of time.

The non-evaporable water content is typically considered as an indicator of hydration reaction progress in cement. Although, the OPC mixture at OWD exhibited higher early- age non-evaporable water content than that of the corresponding mixture at MWD state, this trend was not propagated at later ages, and two systems had equivalent non- evaporable water contents at later ages. The increase in non-evaporable water content was faster in the OPC system at OWD state, as identified by the larger slope of non- evaporable water content-time results in Figure 5-12(b). This suggests a higher degree of

hydration of cement at later ages for the system at MWD state than that of OWD state. The enhanced hydration kinetics of cement can manifest progressive increase in the compressive strength of system at later ages, as the cement hydration progresses and solid-to-solid connectivity increases. These results are consistent with simulation observations in Figure 5-11 and Figure 5-12(a).

5.4. SUMMARY

A series of experiments coupled with virtual microstructural simulations were carried out to evaluate the combined effects of SCMs and w/cm on physico-mechanical characteristics of cement paste over transition from concentrated state to flow onset. The material properties of the investigated binary and ternary OPC-SCM systems were characterized at two different states: (i) concentrated state as identified at w/cm = OWD and (ii) flow onset state as identified at w/cm = MWD. Based on the obtained results from this study, the following conclusions can be drawn:

• Irrespective of the nature of cement paste, ternary systems of SF and SL or FA have markedly higher particle packing and lower inter-particle spacing compared to the plain OPC system. However, there exists a saturation surface area effect of SF, beyond which the increased SSA does not necessarily lead to enhanced particle packing. This is due to the loosening effect and agglomeration of SF particles, which becomes progressively dominant with increasing SF content. • Although transition of cement paste from concentrated to flow onset state reduces

particle packing and elevates solid-to-solid distances, these effects becomes less pronounced for OPC-SCM systems than pure OPC system. The ternary system of 40% SL and 10% SF exhibited merely 2% reduction in solid concentration compared to 13% for the plain OPC mixture over concentrated-flow onset transition. This is attributed to the smaller thickness of water film (i.e., smaller inter-particle spacing) required to initiate flow for OPC-SCM systems compared to the pure OPC mixture.

• The volume of solid connectivity and mechanical property development are strongly related to the cement paste state due to changes in initial water content. While all OPC-SCM mixtures at MWD state exhibited consistently lower

compressive strength than corresponding systems at OWD state, the former mixtures developed larger rate of strength gain than latter systems. This is on account of the higher degree of hydration at later ages and larger rate of connected solid network formation for mixtures at flow state (i.e., MWD) than that of the concentrated state (i.e., OWD).

6. OPTIMIZATION OF GRANULAR SKELETON TO ACHIEVE DENSE