Poly(carboxylate ether)-based superplasticizer achieves workability retention in calcium aluminate cement

Calcium aluminate cement (CAC) suffers from loss of workability in less than an hour (~15 minutes) after first touch of water. Current superplasticizers that are utilized to modify the viscosity of cement admixtures are designed to target ordinary Portland cement (OPC). The high affinity between these superplasticizers and cement particles were found to be detrimental in CAC systems. Utilization of a monomer that, instead, facilitates gradual adsorption of a superplasticizer provides workability retention. For the first time in literature, we report a superplasticizer that caters to the properties of CAC such as high rate of surface development and surface charge. While neat CAC was almost unworkable after 1 hour, with the addition of only 0.4% of the optimized superplasticizer, 90% fluidity retention was achieved.

(1) Determined by H 1 -NMR (2) Calculated from direct addition of NaOH to polymer solution by titration (3) Calculated from direct addition of Ca(OH) 2 to polymer solution by titration

Materials proportioning:
Different amounts of PCEs (0.5-10 mg/g cement) were first dissolved in water and then 100 g of cement grains were charged into these polymer solutions to make mixtures with water to cement ratio (w/c) of 0.3 in CAC and 0.4 in OPC system. All mixtures were prepared using high-shear mixer (Heidolph RZR 2102 equipped with a 4-blade stainless steel propeller) rotating at 1000 rpm for 5 min. Such a high-shear mixing regime was used to ensure good dispersion of cement grains and homogeneous suspensions. To perform the slump retention test, four different batches were prepared and then sealed to prevent the evaporation of water.
In the intervals of 15, 30, 45, and 60 min after the first touch of water, labeled mixture was stirred at 300 rpm for 30 sec and slump test was conducted after 15 sec resting time of the mixture in the slump cone.

NaOH and Ca(OH) 2 titration:
Titration was conducted with a HI-2211 bench top pH meter on 50 ml solution of 1 mg/ml PCEs/water at 22 ± 2 °C. PCEs solutions were titrated with a 0.1 M NaOH and 0.02 M Ca(OH) 2 .

Determination of specific surface:
The Brunauer-Emmett-Teller (BET) specific surface area of samples was determined by analyzing the standard nitrogen adsorption isotherms at 77 K using nova 2200e, Quantachrome instruments. Suspensions with w/c of 0.4 (OPC) and 0.3 (CAC) were mixed for 1-5 min followed by centrifugation at 5000 rpm for 5 min. OPC and CAC particles were then redispersed in 2-propanol and acetone, respectively and centrifuged at 5000 rpm for 15 min. This process was repeated for 3 times following by vacuum drying at 40 °C for 3 days.

Characterization of dispersing state of cement suspensions:
Dispersing state of cement particles in aqueous medium was determined by dynamic light scattering (DLS) and measuring the Z-average (average particle size) of cement suspensions with and without polymers (Zetasizer nanoseries, Malvern Instruments, Ltd.). The suspensions with w/c of 40 and PCEs/cement of 0-10 mg/g were mixed by a magnetic stirrer for 10 min and then diluted by distilled water with a ratio of 1 to 20 (v/v). Each result represents the average value of 6 measurements, with 15 to 20 runs for each measurement.

Rheological measurements:
Rheological behavior of cement pastes was characterized by determining the plastic viscosity and yield stress using Anton-Paar MCR 302 rheometer equipped with cone-plate geometry of 50 mm/2° and a gap size of 0.208 mm at 25 ± 0.1 °C. All cement pastes were loaded immediately after cessation of the mixing process. After loading each sample, a thin layer of low viscosity paraffin oil (~150 mPa.s at 20 °C) was employed around the outer edge of the plates to protect the sample from evaporation. Rheological measurements were carried out by the measurement sequence as following; cement pastes were first kept under constant shear rate of 100 s -1 for 60 sec to ensure the structural breakdown. An increasing shear rate ramp from 10 to 100 s -1 in 150 sec was then applied followed by a decreasing ramp rate from 100 s -