A novel kind of concrete superplasticizer based on aryl isocyanate polycondensates

A novel superplasticizer was synthesized by polycondensation of Alkyl phenol phosphate and toluene diisocyanate grafting with methoxy polyoxyethylene ether (MPEG). The chemical structure and molecular weight of polycondensates molecules were determined by 1H nuclear magnetic resonance (NMR) and gel permeation chromatography respectively. The experimental results indicated that the polycondensates of Alkyl phenol phosphate and toluene diisocyanate grafting with MPEG not only exhibited good water-reducing properties but also demonstrated effective anti-clay abilities alone. Furthermore, the polycondensates showed good fluidity maintaining abilities within 1–3 h and good workability of concrete. The results of T500 Time experiments show that lower plastic viscosity of the polycondensates leads to fresh concrete much “looser” than conventional PCEs in self-compacting concrete.


Introduction
Polycarboxylate superplasticizers (PCEs) are widely used in different industrial fields to improve the performance of concrete. With the development of the construction industry and requirements of environmental protection, modern concrete technology aims towards minimizing the needs for Portland cement without sacrificing the important engineering properties of hardened concrete [1][2][3]. PCEs reduce the dosage of water to increase the strength of hardened concrete. However, it can be observed that some PCEs lead to fresh concrete with a very "sticky" and cohesive consistency [4]. In china, there has been a growing dependency on manufactured and dramatic decline of availability of naturally local sands as fine aggregates for concrete and mortar because of environmental and resource pressures. The sands used in concrete and mortar are getting less and less pure. The presence of clay as impurities increasing the water demand to provide a concrete or mortar of given workability, compromising the rheological property of concrete or mortar after absorbing water and swelling, weakening the bond between the cement and aggregates, and dramatic increasing the dosage of PCEs especially [5][6][7][8].
In this work, we synthesized and characterized a novel superplasticizer by polycondensation of Alkyl phenol phosphate and toluene diisocyanate grafting with methoxy polyoxyethylene ether (MPEG) and compare it with commercially available conventional PCEs. The backbone of this superplasticizer consists of benzene ring structure which is the same as poly (β-naphthalene sulfonates) (BNS).

Measurements
NMR spectra were performed on Bruker Avance DPX 400 MHz for 1 H NMR. The residual solvent signals were uses as reference; chemical shifts are given on the TMS scale (D 2 O: δ=4.79 ppm). Molecular weight was determined by gel permeation chromatography (GPC) measurements. The samples were analyzed using a 0.1mol/L NaNO 3 aqueous solution as eluent at a flow of rate of 0.5 mL/min.

Cement paste tests
An ordinary Portland cement (52.5, Jiangnan Onoda Co., China), which meets the requirements of GB8076 standard was used throughout all the experiments. The cement fluidity tests were conducted with the following procedure. The fresh cement pastes were prepared at 20℃～25℃, the water to cement ratio (w/c) is 0.29. The amount of 87g water containing the defined amount of polymer was added to 300g cement and mixed for 2 min at low speed and a further 2 min at high speed. The cement fluidity took the average of two measurements.

T 500 Time of self-compacting concrete [9]
This test is used to assess the flow rate of self-compacting concrete (SCC). The slump cone with above described dimensions was filled with fresh concrete. Upon lifting the cone, the time for the concrete to flow to diameter of 500 mm is recorded. The concrete mixture proportions are shown in table 1.  [10]. Phenoxyethanol was esterified azeotropically in the presence of polyphosphoric acids in toluene on a water separator at 120℃～125℃. The reaction product which precipitated on cooling was filtered off, washed with petroleum ether and dried to give a white powder. The structure of Phenoxyethanol Phosphate was shown as figure 1.

Results and discussion
Polycondensates of phenoxyenthanol phosphate and MPEG-TDI were synthesized and characterized by 1 H-NMR. The performance of polycondensates was investigated by cement paste and T500 time of fresh concrete. Figure 4 shows the 1H NMR spectrum of the typical polycondensates of phenoxyenthanol phosphate and MPEG-TDI.

Cement dispersion test
Cement dispersion test evaluates the dispersing performance of synthesized compounds. In figure 6, the results indicated that polycondensates demonstrated good water-reducing effects compared with conventional PCEs, despite the dosage of polycondensates is higher about 30% than conventional PCEs. The fluidity of polycondensates reached 200 mm at a 0.2 wt.% and 240 mm at a 0.25wt.% dosage. Impressively, the spread flow decreased only from 200 to 165 mm for polycondensates after 1 h, even maintained at 130 mm after 3 h. The loss of polycondensates was obviously less than conventional PCE used here (from 205 to 100 mm after 3 h). The polycondensates showed good fluidity maintaining abilities within 1～3 h here.

T 500 time of concrete
Concrete viscosity can be assessed by the T 500 time during the slump-flow test. The time value of concrete fluidity with 500 mm is recorded to describe the rate of flow. Concrete with a low viscosity will have a very quick initial flow and then stop. Concrete with a high viscosity may continue to creep forward over an extended time. T 500 time test results were described in table 2. There were the same slump and spread of fresh concrete with PCEs and polycondensates by adjusting the dosage of admixture. T 500 time of polycondensates was only 7.8s. Compared with 11.4s of PCEs at the same spread flow, the spread flow velocity of polycondensates increased by 36 %. It means fresh concrete with polycondensates has lower plastic viscosity than concrete with PCEs at the same plastic stress.

Conclusions
A novel kind of concrete superplasticizer has been synthesized based on aryl isocyanate polycondensates. The chemical structure and molecular weight of desired products have been determined by 1 H NMR and GPC, respectively. The results of cement fluidity tests indicate that the polycondensates of Alkyl phenol phosphate and toluene diisocyanate grafting with MPEG not only exhibite good water-reducing properties but also demonstrate effective anti-clay abilities alone. Furthermore, the polycondensates show good fluidity maintaining abilities within 1-3 h and good