Page 18 - Open-Access-Sep-2019

TECHNICAL PAPER

from the mid-top to mid-bottom of the cube (see Figure 6). The been repeatedly observed, even at 91 days, for cement mortar
nature of the failure and cracking in the specimen are clues to at temperatures of 60°C [23] .
the stiffness of the printed scaffold. The large specimen would
not crack and break easily through the middle of the specimen, Raised temperature increases the rate of reaction and reduces
whereas, the small printed structure cracked at the corner of the the setting time [10] since it accelerates the dissolving of alumina
specimen and broke easily in the middle of the specimen. and silica particles from the unreacted particles of the powder
and a larger amount of Alumina (Al 2O 3) and Silica(SiO 2) becomes
available for the reaction process. The modified powder for
3DP contains a high proportion of Alumina due to the high
levels of CAC in the main powder. In relation to the total mass,
Al 2O 3 comprises approximately 70% in CAC whereas, in general
purpose cement (OPC), it comprises 5%.

The possibility of delayed ettringite formation is a risk that
should be considered due to early hydration. Ettringite
(3CaO•Al 2O 3•3CaSO 4•32H 2O) is a mineral called calcium
sulfoaluminate [24] . It results from the reaction between gypsum
Figure 6: After cracking under uniaxial compressive strength and sulfate compounds with calcium aluminate in Portland
(a) 50×50×50 mm 3DP cement mortar cube; (b) 20×20×20 mm 3DP
cement mortar cube. cement. The ettringite is formed in the first few hours after
blending with water. It is obvious in Table (1) that the chemical
This section interprets the results of the tested specimens. composition of CAC, which comprises approximately (70%) of
Figure 7 shows the results of the compressive strength tests of the main component of the printed structure, contains minimal
printed mortar specimens. These results relate to the specimens or no sulfate. Therefore, the formation of the ettringite in the
that were cured under different conditions such as tap water printed scaffold is not a concern and we could not observe
only (3-hour 40°C, 28-day water, 3-hour 40°C), (3-hour 60°C, 28- any cracking of the printed structure after post-processing.
day water, 3-hour 60°C). However, after a specimen is printed, The layer-over-layer form of the printed structures differs from
the proper post-processing consists of (a) curing in the oven for conventional structures.
3-hours; (b) curing for 28-days in tap water then; (c) drying in the
oven for 3-hours. These basic post-processing sequences (a, b Binder (water) has a significant effect on the result of
& c) are used for all specimens but at various temperatures until compressive strength at high temperatures due to the isopropyl
the optimized maximum compressive strength of the printed alcohol or polyvinyl alcohol content of the binder. The binder
specimen is found. consists of humectant and water, where the humectant is
2-pyrrolidone [25] (see Table 3).
Figure 7 shows the compressive strength test outcomes for the
sets of printed specimens that were cured for 28-days at five table 3: physical and chemical properties of zb®63
different temperatures. The graph bars with indicated values binders
on top are the actual strengths, and error bars indicate the DEscrIPtIon DEnsIty sUrfAcE vIscosIty ph(20°c)
standard deviations of the results. As shown in Figure 7, an (g.cm ) tEnsIon (cPs)
-3
increase in the curing temperature from 40°C to 80°C resulted (dynes/cm)
in a near-linear increase in compressive strength. This increased zb®63 binder 1 45 1.35 9.8
strength in the cement mortar, which is proportional to the
increase in temperatures, could be due to the greater reaction Despite the trend observed up to 80°C, a contrary trend was
level of cement mortar at elevated temperatures. Curing in an observed when the temperature rose beyond 80°C up to 90°C,
oven accelerates the reaction of the cementitious process. Fast as shown in Figure 7. According to previous studies (Altan
hydration and a high early compressive strength were observed and Erdoğan [26] , a threshold temperature for the cementitious
to occur as the temperature increased [21] . reaction process will occur when temperature-controlled kinetics
is inhibited. Extra Al 2O 3 and SiO 2 particles react when the
The experimental results are consistent with the study curing temperature is above the threshold point. Mortar slurry
conducted by Abd elaty [22] , which demonstrated that the forms rapidly and is deposited on the surface of the unreacted
compressive strength of Portland cement concrete with a low powder, which will constrain further dissolution. Consequently,
w/c ratio at 50°C is higher than at lower temperatures (e.g. the compressive strength declines significantly. Hence, 80°C
10°C and 23°C). Early mechanical strength development for was nominated as the optimum curing temperature for cement
compressive strength and a trend of increased strength have mortar specimens.

The IndIan ConCreTe Journal | SepTember 2019 69