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Introduction Chemical composition of rock phosphate (tricalciumphosphate) is Ca₃(PO₄)2. It is found in nature in yellowish white colour. Themineral is brittle. Its hardness and density are approx. 5 and 3.2 respectively. Literature is silent about the effect of this material on setting and bondingcharacteristics of Portland cement. Rock phosphate is sparingly soluble mineral. The author thought that finally powdered rock phosphate mayinteract through its residual surficialvalenies with active lime and othersiliceous materials to serve as an strength giving filler admixture. Hence it wastried as an admixture.

i. Soluble phosphorus and acidic pH in phosphogypsum lead to significant delays in Portland cement setting time.

ii. Alkaline phosphogypsum may be a substitute for natural gypsum in Portland cement.

iii. Cement setting times were extended with P2O5 soluble from 0.33% to 1.32%.

iv. Soluble fluorides provide hydration delays with lower contents than soluble phosphates.

1. Experimental

To evaluate rock phosphate an additive in Portland cementcomposition, following important investigations have been carried out.

1.1   Setting Characteristics

The effect of Rock Phosphate onsetting characteristics of Portland cement was studied by admixing dry and solid powder of Rock Phosphate invarying proportions (0%, 5%, 10%, 20%) by weight of the cement in dry-mix.Composition of dry-mix was kept uniform (1:4 weight by weight of Portland cement and siliceous dust). Thoroughly mixed dry-mix was gauged withoptimum amount of water to obtain a plastic mass of IS consistency. Settingperiods of wet-mixes were determined by adopting standard procedure as perspecifications. Experimental findings are summarised in the Table 1

Table 1 : Effect of Rock Phosphate on setting characteristics ofPortland cement,

Temperature – 27+1°C                      Composition of dry-mix -1:4(w/w)

Relative humidity - above 80%         (Portland Cement: dust)

Quantity of dry-mix- 200 gm.

S.No.	%Additive	Vol. of water cm2 and ml	Setting time (min)
			Initial	Final
1	0	56 ml	235	405
2	5	56 ml	340	680
3	10	56 ml	335	560
4	15	56 ml	225	510
5	20	56 ml	215	475

1.2 Weathering Effect –

The setting time blocks consisting varying proportions of the additive,prepared as above, were cured under identical conditions of temperature andrelative humidity. They were weighed at the time intervals of 10 days, 20 daysand 30 days. Weathering effects are reflected by observing the change inweight with time as per standard procedures. Experimental results areenumerated in the Table 2.

Table 2 : Effect of Rock Phosphate on weathering characteristics ofPortland cement.

Temperature – 27+1°C                                      Composition of dry-mix -1:4(w/w)

Relative humidity - above 80%         (Portland Cement: dust)

Quantity of dry-mix - 200 gm.

S. No.	% Additive	Weight of blocks (gm) after
		10 days	20 days	30days
1	0	229.920	224.640	220.730
2	5	236.100	227.850	225.500
3	10	233.100	228.350	226.250
4	15	237.020	228.400	225.900
5	20	240.200	231.850	229.400

1.3 Moisture Ingress Investigations:

All the setting investigation blocks after studying weathering effects were exposed to boiling water as per standard procedure. Relative water vapour transmission has been expressed as a function of time. The results are recordedin the Table 3.

Table3 : Effect of Rock Phosphate on moisture ingress characteristicsof portland cement.

Temperature - 27+1°C                       Composition of dry-mix -1:4(w/w)

Relative humidity - above 80%         (Portland Cement : dust)

Quantity of dry-mix- 200 gm.

S.No.	% Additive	Trial locks kept in boiling water for (hrs.)
		0-5	5-10	10-15	15-20	20-25	25-30
1	0	NE	NE	NE	NE	NE	NE
2	5	NE	NE	NE	NE	NE	NE
3	10	NE	NE	NE	NE	NE	NE
4	15	NE	NE	NE	NE	NE	NE
5	20	NE	NE	NE	NE	NE	NE

NE: No Effect

1.4 Compressive Strength Investigations:-

The effect of Rock phosphate on compressive strength of Portland Cement compositions was studied by preparing standard sized trial blocks (70.6mm x 70.6 mm x 70.6 mm). Solid form of magnesium oxide was added in dry-mix in different proportions (0%, 5%, 10%, 15%, 20%) by weight of Portland cement. Thoroughly mixed 1:4 dry-mix with Rock Phosphate was gauged withwater and filled into standard compressive strength testing moulds with surface area 50 cm. These moulds were kept under identical condition of temperatureand relative humidity. Compressive strength of these block were determined asper IS specifications with the help of compressive strength testing machine.Experimental findings are summarised in Table 4.

Table 4 : Effect t of Rock Phosphate on compressive strength of Portland cement.

Temperature - 27+1°                                          Composition of dry-mix -1:4(w/w)

Relative humidity - above 80%         (Portland Cement : dust)

Quantity of dry-mix - 1000 gm.

S.No.	% Additive	Compressive strength (kg/cm2)
1	0	200
2	5	210
3	10	160
4	15	140
5	20	120

1.5  Linear Change:

Investigation. amount just sufficient to obtain a plastic mass of IS consistency. Wet-mixes were then filled into standard sized moulds (200 mm x 25 mmx 25 mm) and kept under identical conditions of temperature and relative humidity. Linear changes in the beams so formed were determined as per IS specifications. Finding are summarised in the Table 5.

Table 5 : Effect of Rock Phosphate on linear change characteristics of Portland cement.

Temperature – 27+ 1°C                     Composition of dry-mix -1:4(w/w)

Relative humidity - above 80%         (Portland Cement : dust)

Quantity of dry-mix- 200 gm.

S.No.	% Additive	Lenghth of beams (mm)		Change in length (mm)
		Initial	Final
1	0	200	200.28	0.28
2	5	200	201.01	1.01
3	10	200	201.52	1.52
4	15	200	201.61	1.61
5	20	200	201.67	1.67

The Table 1summarises the effect of rock phosphate as a trialadditive in varying proportions in wet mix on setting characteristics of Portland cement. It is noted that initial incorporation of rock phosphate increases initial as well as final setting periods (up to about 50%). On further admixing theadditive setting periods decrease gradually, with increasing proportions of the additive. The plausible reason seems that rock phosphate has a tendency to absorb di or trivalent cations (Ca, AI etc.) to from insoluble crystallinephases. The removal of these active bond forming species in this way shouldhamper the kinetics of setting reactions. Accordingly, initial as well as finalsetting periods increase sharply. On further incorporation (beyond 5%) of theadditive, it appears that reactions involving bond formation (exothermic) andformation of interlacing crystals of phosphates and alluminasilicates assist thesetting process. Accordingly initial as well as final setting periods are found todecrease.

The Table 2 record the effect of rock phosphate on weatheringcharacteristic of Portland cement. The observed results suggest that someuncombined moisture is left even after the final setting period. Accordinglygradual loss in weight with time of the trial setting blocks due to slowevaporation of the uncombined moisture is quite expected.

The observed data of the Tables and the apparent look of the trial blocksafter 30 days suggest that the additive has not caused any damaging effects onthe blocks on ageing weathering. The above conclusions are substantiated fromthe moisture ingress characteristics of the trial blocks recorded in the Table3 It is found that the trial blocks are not affected even after 30 hours oftheir exposure to steam. The moisture ingress in also found to be veryinsignificant.

The Table 4 reflects the effect of rock phosphate on compressivestrength of the trial cubic block. It is observed that minoradditions of theadditive up to about 5% improve their structural or compressive strength. This appears to be due to inactivation of impurities of free active lime/alumina.Further incorporation of the additive is not favourable to the compressivestrength. It may be attributable to the fact that phosphate ion being a bigger andless complex trivalent anion distorts the structural chains of the strengthgiving compositions. Thus decreasing compressive strength with increasingamounts of the additive beyond a limit is quite expected. The trends are shownin the Table 4.

The data of the Table 5 pertaining to effect of rock phosphate onlinear change characteristics of the standard blocks are in quite harmony withthe observed compressive strength data. Thus it is noted that beyond 5%incorporation of the additive contributes to positive linear change for thereasons mentioned above. It is an established fact that linear change beyond1.5 mm is harmful of strength to the structure. Thus increasing linear changesbeyond 5%, incorporation of the additive results in decreasing compressivestrength of the trail blocks

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