White sand - poor mortar bond

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Author Message Anonymous   Posted on Monday, March 19, 2007 - 06:31 pm:    This is a new one to me. The mason on one of our jobs is claiming that white sand in the mortar (because it is high in silica content) bonds poorly to the brick and they cannot get the appropriate bond strength with it. My immediate reaction is.... this is a bunch of hooey. Anyone ever heard of white sand causing mortar not to bond to brick? John Bufford, PE Member Username: jbufford Post Number: 3 Registered: 04-2006 Posted on Monday, March 19, 2007 - 06:40 pm:    all sand is "high" in silica content. that's what sand is. white sand should not be a problem unless gradation is wrong and/or too fine. J. Peter Jordan Senior Member Username: jpjordan Post Number: 224 Registered: 05-2004 Posted on Monday, March 19, 2007 - 08:08 pm:    I was under the impression that it was the lime in the "cement" that provided the bond. It may be possible that the mason is using this as an excuse to substitute a more "normal" color sand (normal for your area) in order to cut cost. Naaa... no one would even think of doing that would they? (Unregistered Guest) Unregistered guest Posted on Monday, March 19, 2007 - 08:13 pm:    I vote HOOEY. There could be and is a difference between natural sand and "crushed" material (round edges ve sharp angles) I have speced natrual sand for historic work when I want the mortar to match existing material. If the product meets the ASTM for sand then it's sand. You have three mineral choices Quartz SIO2 FeldSpar (probably Potassium with an AL and 3 (I think) Si and a bunch of O. or Marble/limestone (often used in precast) CaCo3 All of which should be fine (i think) with the cement. Beacuse the cement is a bunch of CA and SI and O. once again I say Hooey. But not all sand is silica Marc C Chavez Senior Member Username: mchavez Post Number: 188 Registered: 07-2002 Posted on Monday, March 19, 2007 - 08:30 pm:    oops. that last unregistered guest was me. I take all blame for any bad chemistry John Bunzick, CCS, CCCA Senior Member Username: bunzick Post Number: 684 Registered: 03-2002 Posted on Tuesday, March 20, 2007 - 08:38 am:    Could the problem be with the gradation? Marc C Chavez Senior Member Username: mchavez Post Number: 189 Registered: 07-2002 Posted on Tuesday, March 20, 2007 - 05:17 pm:    From a colleague ...The white or buff colored sands that are used in Western Washington usually yield fairly high compressive strengths for CMU and mortar even though the sand is more uniform in the aggregate size mix than our normal gray sands. It is the variety of aggregate sizes that helps our mortars and grouts reach higher compressive strengths with gray sand than the more uniform white sands. In other areas of the country, the lighter color sands could have more rounded aggregate which can reduce strength too. I have been told by people in other parts of the country, that their white sands generally produce lower strengths because of the composition and lack of “sharp” aggregates like we have here in Washington. Some Florida sands in particular are said to have high coral content and are not good for construction. (I suspect the sand also contained salts like beach sand.)... So maybe roundness DOES have something to do with it. Gradation is noted as well. Anonymous   Posted on Wednesday, March 21, 2007 - 11:26 am:    Well, the above comments are all pretty much in line with what I was thinking. And upon further investigation, there IS a cost difference (surprise, surprise). I got another theory from a local masonry expert: Perhaps the sand is more rounded in contrast to the sharp angular river sand we normally use, and it "feels" different to the mason coming off the trowel. Think "round sand = ball bearings". So to the mason it may seem that the bond strength isn't the same. I thought this was a charitable explanation, more charitable than I would naturally be. At any rate, thanks to all for your help and comments. I am still firmly on the side of "hooey". The cement in the mortar determines the bond, and as long as the sand meets ASTM C 144, it qualifies "masonry sand". Anonymous   Posted on Monday, June 04, 2007 - 11:00 am:    As a follow-up, we performed a bond wrench test on brick samples with white sand mortar and with "normal" river sand mortar, and in fact the samples with river sand were stronger, but not significantly. All samples tested, white and river sand, were well within expected values for bond strength. So, at least in our case, river sand mortar does have a somewhat stronger bond than white sand, but the mason's original contention that white sand causes a "poor" bond was untrue. It was a case of them missing that we specified white sand, and trying to not spend the extra bucks by making an unsubstantiated claim in front of the owner that white sand would somehow compromise the quality of the work. Thanks for all your insight and comments while we worked through this issue.