Very high early strength cement

A combination of a hydroxy polycarboxylic acid and sucrose improves the water-demand, setting, and compressive strength properties of a calcium sulfoaluminate cement to a greater degree than would be the case if the hypothetical cumulative effect of two admixtures were operative.

This invention relates to high aluminate cements which produce upon 
hydration a substantial amount of tricalcium aluminosulfate hydrates. It 
particularly relates to cement compositions of this type which contain a 
synergistic combination of a saccharide and a hydroxy polycarboxylic acid. 
Further, it relates to high aluminate cements which may be handled and 
transported for a longer period of time before setting than is usual with 
such cements. More particularly, it relates to cements containing the 
ternary compound, 3CaO.3Al.sub.2 O.sub.3.CaSO.sub.4, hereinafter referred 
to as CSA. 
The economy and versatility of Portland cements, along with their high 
ultimate strengths, have made them pre-eminent among hydraulic cements 
despite their practical limitations of being slow to set and slow to 
develop the strength necessary to be self-supporting. The development of 
Type III Portland cement was an early response to the need for a faster 
setting, early strength cement. Calcium halo-aluminates have been 
incorporated into Portland cement compositions to achieve shortened but 
controllable setting times. Mixtures of a halo-aluminate cement and a 
calcium sulfate anhydrite have been offered as early strength cements. 
Cement mortars containing CSA, calcium sulfate and dicalcium silicate set 
quickly and develop compressive strengths of about 2900 psi and higher 
within 24 hours after mixing with water. 
The setting times for many of these early strength cements however, is too 
fast--not enough time is allowed for mixing, transporting, and proper 
placing. This problem was addressed by Nakagawa et al in U.S. Pat. No. 
3,973,978. The solution proposed therein was to prepare two separate 
mixtures--a Portland cement paste and a quick hardening agent--and then 
mixing the two at the job site. The need for special equipment such as a 
Y-tube, metering apparatus and an additional mixer is apparent from the 
description of the patented method. 
The quick hardening agent of Nakagawa et al optionally contains a setting 
retarder and/or a quick hardening accelerator. The setting retarders 
listed are the organic carboxylic acids conventionally used, such as 
gluconic, tartaric salicylic, citric, and malic acid. The hydroxides and 
carbonates of alkaline earth metals and alkali metals are described as 
quick hardening accelerators. 
In U.S. Pat. No. 3,860,433, Ost et al teach that very high early strength 
cements containing CSA, calcium sulfate and dicalcium silicate usually can 
be produced to have an initial set in about 20 minutes but that 
conventional retarders such as sucrose, boric acid, and mucic acid (i.e. 
tetrahydroxyadipic acid) may be added. 
A water-repelling and set-retarding admixture for incorporation in Portland 
cements and other hydraulic cements is taught by Serafin et al in U.S. 
Pat. No. 3,885,985. Serafin et al teach the use of many various materials 
and mixtures thereof as set-retarding agents. Included among these are 
polyhydroxy polycarboxylic compounds and saccharides such as glucose, 
fructose, lactose, sucrose, starch and cellulose. 
According to U.S. Pat. No. 4,058,407, combinations of admixtures are 
frequently used in hydraulic cements to achieve certain results or 
overcome inefficiencies, such as where an admixture does not produce a 
sufficient improvement in the compressive strength or does not effect the 
desired degree of retardation. Several admixtures, such as 
lignosulfonates, salts of hydroxycarboxylic acids, sugars, and 
polysaccharides are listed as having the multiple effects of water 
reduction, set retardation and compressive strength improvement. 
Now, it has been discovered, however, that sucrose accelerates the setting 
of the very high early strength cements containing CSA taught by Ost et 
al. It also have been discovered that a combination of sucrose and a 
hydroxypolycarboylic acid reverses the effect of the sucrose and that the 
setting time is longer than would have been expected from a consideration 
of the sum of the individual effects. In further contrast to the teachings 
of the prior art, the hypothetical cumulative effect of the 
sucrose/hydroxy polycarboxylic acid combination would be a lower 
compressive strength of the cement than that of the cement with neither 
admixture whereas the observed effect of the combination is a greater 
compressive strength. Finally, the sucrose/acid combination synergises the 
water-reducing effect of each agent. 
It is an object of this invention, therefore, to provide a hydraulic cement 
composition having a high early strength when hydrated but whose initial 
setting time is retarded sufficiently to allow proper placing after mixing 
it with water and transporting the mixture to a job site. 
It is another object of this invention to provide such a cement composition 
having a compressive strength which is synergistically increased by a 
combination of admixtures whose hypothetical cumulative effect would be to 
decrease the compressive strength. 
It is a further object of this invention to provide a cement composition 
which is very fluid and essentially self-leveling without sacrificing 
strength. 
These and other objects which will become apparent from the following 
disclosure are achieved by a cement composition which comprises, on a dry 
weight basis, from about 3.75% to about 40% of CSA, from about 0.1% to 
about 2% of a hydroxy polycarboxylic acid, from about 0.25% to about 4% of 
sucrose, from about 0.1% to about 5% of lime, from about 3% to about 35% 
of calcium sulfate, and dicalcium silicate to make up the substantial 
remainder. 
The dicalcium silicate generally is present in amounts ranging from about 
20% to about 90% of the total dry weight of the composition. 
A preferred composition comprises from about 10% to about 30% of CSA. 
Particularly preferred is a composition comprising from about 15% to about 
25% CSA. 
The hydroxy polycarboxylic acid contemplated in the invention is 
exemplified by citric acid, tartaric acid, malic and mucic acid. It 
contains up to about 6 carbon atoms and up to about 4 hydroxyl groups. 
Citric acid is preferred. The admixtures may be used in their solid forms 
but also as aqueous solutions. Dilute aqueous solutions of the admixtures 
may be used as all or part of the mix water when the cement is used to 
make a paste, grout, mortar or concrete. 
It is evident that the introduction of the acid into a system containing 
basic calcium will result in the in situ formation of the calcium salt. 
Therefore, other soluble sources of the carboxylate ion are contemplated 
as an admixture in this invention. 
The setting of the cement compositions is retarded by the presence of from 
about 0.25% to about 1% of the hydroxy polycarboxylic acid and from about 
0.25% to about 4% of sucrose when the weight ratio of sucrose to the acid 
is from about 1:1 to about 6:1. Among the preferred compositions having a 
retarded set are those comprising about 0.25% citric acid and from about 
0.25% to about 0.5% sucrose, those comprising about 0.5% citric acid and 
from about 0.5% to about 3% sucrose, those comprising about 1% citric acid 
and from about 1% to about 2% sucrose, and those comprising about 1% 
citric acid and about 4% sucrose. 
The water reducing effect of the sucrose/hydroxy polycarboxylic acid 
combination is greater than the sum of the individual effects at three 
different levels of concentration: (1) at about 0.5% of the acid and a 
sucrose/acid ratio of from about 1:1 to about 8:1; (2) at about 1% of the 
acid and a sucrose/acid ratio of from about 2:1 to about 4:1; and (3) at 
about 2% each of the aid and sucrose. A composition containing about 0.5% 
of citric acid is preferred. 
The early and the ultimate compressive strengths of a hydrated cement 
composition of this invention are increased by the presence of from about 
0.1% to about 2% of the hydroxy polycarboxylic acid and from bout 0.25% to 
about 4% of sucrose regardless of the ratio of one to the other. Preferred 
from this purpose are compositions comprising from about 0.1% to about 
0.25% citric acid and from about 0.25% to about 0.5% sucrose, compositions 
comprising about 0.5% citric acid and from about 0.5% to about 4% sucrose, 
and compositions comprising about 1% citric acid and from about 1% to 
about 3% sucrose. Expecially preferred are compositions comprising from 
about 0.1% to about 0.25% citric and from about 0.25% to about 0.5% 
sucrose. 
The composition contemplated in the invention includes the dry cement 
composition, neat pastes thereof, grouts, mortars, and concrete mixes. The 
addition of the admixture may be made, according to whether the admixture 
is in its solid form or in solution, at the time of preparing the dry 
cement or when the composition is mixed with water at a mixing plant or at 
the job site.