# Crossover Effect in Cement-Based Materials: A Review
## 5 Effect of Cemertitious Materials on COE Phenomenon for Prolonged Curing
### Blast Furnace Slag, Fly Ash and Silica Fume  
[PERSON] and [PERSON] [30] used different OPC-slag mortars in their research with 0%, 40% and 50% slag replacement with OPC by mass. They studied the influence of temperatures of 50 \({}^{\circ}\)C, 60 \({}^{\circ}\)C and 70 \({}^{\circ}\)C on compressive strengths of these mortars for up to 90 days. They found that hot water curing at a temperature of 60 \({}^{\circ}\)C for 20 h period is the most effective to enhance the early age strengths of all the mortars. Therefore, they selected 60 \({}^{\circ}\)C as the optimum temperature. They observed COE of 2.2% in OPC-slag mortar with 50% replacement of slag with OPC by mass at 56 and 28 days. This COE appeared as a result of 60 \({}^{\circ}\)C hot water curing for 20 h. However, they did not observe any COE in other two Portland cement slag mortars with 0% and 40% replacement of slag with OPC by mass for up to 90 days. [PERSON] et al. [63] used Portland cement-slag mortars with various percentages of slag as 0%, 30% and 50% in their study. They observed a COE of about 3.7% in Portland cement slag mortar with 30% slag at 28 days when samples were initially water cured at 60 \({}^{\circ}\)C for 20 h. [PERSON] [64] suggested that the addition of higher percentages of slag as replacement with OPC in mortars tends to reduce the occurrence of COE. [PERSON] [46] explained that curing at elevated temperature resulted in a non-homogeneous scattering of the hydration products in microstructure. However, at lower temperature, the hydration products have adequate time period to precipitate and diffuse more homogeneously throughout the bulk cement matrix. [PERSON] et al. [65] used 13 concrete mixtures to examine the effect of OPC type I and III, water to CMs ratios of 0.37, 0.41, 0.44 and 0.48 and various types and dosage of SCMs. They used class F and C fly ashes at replacement values of 20% and 30%, respectively. They used GGBFS at replacement levels of 30% and 50% and silica fume at a replacement level of 10% in a ternary mix with 20% class F fly ash. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Number** & **Name of Mix** & **Curing Condition** & **Age at Which** & **COE** \\  & & & **COE Occurred** & **(\%)** \\ \hline
1 & OM-WH-ac &
\begin{tabular}{c} Water bath heated followed by air \\ curing under room temperature \\ \end{tabular} & 90 days & 7.14 \\ \hline
2 & OM-WH-wc &
\begin{tabular}{c} curing at water bath heated \\ and then water curing \\ \end{tabular} & 90 days & 5.13 \\ \hline
3 & OM-OH-ac &
\begin{tabular}{c} Over heated curing and then air \\ curing under room temperature \\ \end{tabular} & 56 days & 4.30 \\ \hline
4 & OM-OH-wc &
\begin{tabular}{c} Over heated curing \\ and then water curing \\ \end{tabular} & 90 days & 4.70 \\ \hline
5 & OM-G6/WH-ac &
\begin{tabular}{c} Water bath heated followed by air \\ curing under room temperature \\ \end{tabular} & 56 days & 5.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Mix proportions of ordinary Portland cement (OPC) mortars in one batch, curing conditions and crossover effect (COE) records. They made three batches of each mix as cold batch (4 \({}^{\circ}\)C and 13 \({}^{\circ}\)C for 24 h), hot batch (32 \({}^{\circ}\)C and 41 \({}^{\circ}\)C for 24 h) and control batch (20 \({}^{\circ}\)C and 23 \({}^{\circ}\)C). They measured compressive strengths at 24 and 48 h and seven, 14 and 28 days. They observed that hot batches achieved strength rapidly, but they had lower later-age strengths compared with control and cold batches. The cold batches achieved strength slowly but sustained to achieve strength at later ages. They concluded that all control mixtures exhibited COE ranging from 7-12% at seven and 16 days. No COE was observed in case of Portland cement type I mixtures with 20% and 30% class F fly ash having w/b of 0.41. The COE for mixtures with 20% class C fly ash for Portland cement type I mixture with w/b ratio of 0.41 was delayed but not completely removed.