Mortars were prepared using a low-alkali Portland cement with a water-to-solid binder (w/s − where solid binder is cement + silica fume) mass ratio of 0.35 and an 8 % mass fraction replacement of cement by silica fume. The cement has a Blaine fineness of 368 m2/kg and a Bogue phase composition of 58 % C3S, 25 % C2S, 4.0 % C3A, and 7.3 % C4AF, with a 3.4 % calcium sulfate content. The silica fume has a specific surface area of 23.2 m2/g. The mortars were prepared with CEN standard sand EN 196-1 [10]. The reference mortar is designated FSF and consisted of 1540 g cement, 134 g silica fume, 586 g water, and 4050 g CEN sand. 10 to 25 g of a superplasticizer was added to each mortar mixture to assure adequate workability. For two of the mortars, either 8 % or 20 % of the sand by mass was replaced by low-density aggregates smaller than 4 mm. For the final mortar, a 0.4 % addition (mass fraction of cement) of superabsorbent polymer particles [8, 9] was used.
A summary of the four mixtures tested is provided in Table 1, and particle size distributions for the CEN sand, the low-density aggregate, and the (expanded) superabsorbent polymer are given in Figure 1. All mortars were prepared by mixing in a 5 L epicyclic mixer. All mixtures were prepared using freshly boiled (then cooled), distilled water. The 'extra' water in Table 1 refers to the water contained in the low-density aggregates or absorbed by the polymer particles.
| Table 1 - Differences in the mortar mixtures. | ||||
|---|---|---|---|---|
|
Mixture characteristic |
FSF |
LWA20 |
LWA08 |
SAP |
|
Silica fume, 23.2 m2/g (w/w replacement) |
8 % |
8 % |
8 % |
8 % |
|
Low-density aggregate, 0 mm to 4 mm (w/w) |
20 % of sand |
8 % of sand |
||
|
Assumed absorption of LWA (by mass) |
25 % |
25 % |
||
|
Superabsorbent polymer |
0.4 % |
|||
|
Extra water/solid binder (internal curing water) |
0.126 |
0.046 |
0.046 |
Figure 1. Particle size distributions for the CEN sand, low-density aggregate, and the superabsorbent polymer.
The following measurements were performed: compressive strength of cylinders (diameter 60 mm, height 120 mm) after 7 d and 28 (or 27 or 29) d sealed curing, internal relative humidity (RH), and autogenous deformation using a custom-built dilatometer immersed in a constant temperature polyalkylene glycol bath [11, 12]. All curing and measurements were conducted at 30 ºC ± 0.5 ºC and under sealed conditions. Typical standard deviations measured among companion specimens were 6 MPa , 0.2 % RH, and 10 microstrain, for compressive strength, internal RH, and autogenous deformation, respectively [4].